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Timing and extent of Mg and Al isotopic homogenization in the early inner Solar System
... wt.%), but overlap with lunar feldspathic highland rocks (Fig. 6d). 65 Compositional field of meteoritic anorthosites (dashed oval field) is based on data reported by Gross et al. 28 and Xu et al. 30 Highland clasts in this study form a continuous trend from highly magnesian to slightly ferroan with a restricted An composition. Mineral compositions of highland crusts are within the meteoritic anorthosite field. ...
... They differ from typical FANs in the Apollo collection, 43,[47][48][49] but are similar to the MANs from lunar highland meteorites. 28,30,50,51 The Mg # of bulk clasts 1-4 vary from 75.7 to 78.2, suggesting that Mg-rich mafic minerals were abundant in the protoliths. The covariation between bulk Al2O3 and Mg # implies a possible mixing scenario between anorthosites with a high-Mg mantle component (Fig. 7). ...
... Since the lunar meteorites sample randomly distributed sites across the lunar surface, the common occurrence of MANs in lunar meteorites suggests that the MANs may be an important constituent of the lunar highland crust. 26,28,50 MANs could also be the precursor lithology of magnesian granulites. 26,50,52 Arai et al. 26 proposed that magnesian granulites may originate from a mixture of MANs and other pristine rocks that are more mafic (e.g., noritic anorthosite) to account for their lower plagioclase content (70-80%) than feldspathic rocks. ...
In this study, we identify for the first time four lunar highland clasts from the breccias (CE5C0800YJYX132GP) returned by the Chang’E-5 (CE-5) mission by combining tescan integrated mineral analysis (TIMA), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) techniques. The chemical compositions of plagioclases (An93.9–97.6) and mafic minerals (Fo71.4–87.9 for olivine and Mg#65.1–84.6 for pyroxene) in these clasts are remarkably distinct from the more abundant mare basalts in the CE-5 landing site. They are in noritic anorthositic, troctolitic anorthositic, magnesian anorthositic and troctolitic compositions, which represent lunar highland crustal materials. Additionally, the three anorthositic clasts in the CE-5 samples are more magnesian than the Apollo ferroan anorthosites (FANs), but are similar to the magnesian anorthosites (MANs) commonly found in lunar highland meteorites. These newly found MANs in the CE-5 breccia are among the few reported in lunar returned samples, and may represent an important component of the lunar highland crust. Placing MANs in the rock lithologies of lunar feldspathic crust requires a more complex crust-mantle process than that predicted by the classic Lunar Magma Ocean (LMO) hypothesis. Therefore, future research and characterization of Mg-suite and magnesian anorthositic rocks in CE-5 samples may help elucidate early lunar crustal evolution and crust-mantle interaction. © 2022, Atomic Spectroscopy Press Limited. All rights reserved.
... The discovery of correlated 26 Mg/ 24 Mg with Al/Mg in refractory inclusions in primitive meteorites (1)-chondrites-bore witness to the previous presence of live 26 Al ( 26 Al→ 26 Mg; t 1/2 = ~0.730 million years (Ma); see the Supplementary Materials) in the nascent solar system, in abundances sufficient to drive melting and metamorphism in planetesimals (2), and provide a valuable high-resolution chronometer of early solar system processes (2,3). Moreover, the inferred ( 26 Al/ 27 Al) 0 was sufficiently high to place important constraints on the birth environment of the solar system and the processes that mixed recently synthesized nuclides into the pre-solar nebula and protoplanetary disc [see (4)]. ...
... The most recent, highest-precision analyses of bulk refractory inclusions in chondrites define an isochron slope in keeping with previous studies, ( 26 Al/ 27 Al) 0 of (5.26 ± 0.01) × 10 −5 , but ′ 26 Mg 0 of −15.8 ± 1.2 ppm (9); this initial ′ 26 Mg 0 implies that bulk CI chondrites (Ivuna-like carbonaceous chondrites, which are the chondrite group thought to best represent the bulk solar system composition) had a reduced ( 26 Al/ 27 Al) 0 of (2.71 ± 0.21) × 10 −5 to evolve to their modern ′ 26 Mg (Fig. 1). Calculations of ( 26 Al/ 27 Al) 0 for bulk ordinary and bulk enstatite chondrites based on their modern ′ 26 Mg and 27 Al/ 24 Mg, assuming that they each had ′ 26 Mg 0 of −15.8 ppm ( fig. S1), also yield similarly subcanonical ( 26 Al/ 27 Al) 0 . ...
... If this is the case, the key assumption of spatial ( 26 Al/ 27 Al) 0 homogeneity is invalid, and a substantial reinterpretation of Al-Mg chronometry of early solar system objects is required (19). Yet, the use of so-called amoeboid olivine aggregates [AOAs: aggregates of forsteritic olivine with an oxygen isotopic composition similar to CAIs (23)] alongside CAIs in the construction of the isochron that yields the intercept ′ 26 Mg 0 of −15.8 ppm (9) has been a matter of considerable debate as the AOAs strongly influence the value of the intercept, but the temporal and genetic relationship between CAIs and AOAs is still unclear (24). Hence, we refer to the model with lower bulk chondritic ( 26 Al/ 27 Al) 0 , deduced from the isochron of (9), as the "AOA-CAI model" (Fig. 1). ...
Understanding the spatial variability of initial ²⁶ Al/ ²⁷ Al in the solar system, i.e., ( ²⁶ Al/ ²⁷ Al) 0 , is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The ( ²⁶ Al/ ²⁷ Al) 0 of calcium-aluminum–rich inclusions (CAIs) in primitive meteorites (~5 × 10 ⁻⁵ ) is frequently assumed to reflect the ( ²⁶ Al/ ²⁷ Al) 0 of the entire protoplanetary disc, and predicts its initial ²⁶ Mg/ ²⁴ Mg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ′ ²⁶ Mg 0 = −35 ppm). Others argue for spatially heterogeneous ( ²⁶ Al/ ²⁷ Al) 0 , where the source reservoirs of most primitive meteorite components have lower ( ²⁶ Al/ ²⁷ Al) 0 at ~2.7 × 10 ⁻⁵ and Δ′ ²⁶ Mg 0 of −16 ppm. We measured the magnesium isotope compositions of primitive meteoritic olivine, which originated outside of the CAI-forming reservoir(s), and report five grains whose Δ′ ²⁶ Mg 0 are within uncertainty of −35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with ( ²⁶ Al/ ²⁷ Al) 0 of ~5 × 10 ⁻⁵ , supporting the accuracy of the ²⁶ Al→ ²⁶ Mg chronometer.
... Since the discovery of 26 Al in 1976 (8), it has been established by numerous studies, especially high-precision in situ and bulk-inclusion analyses in the past 15 years, that large (>5 mm) CAIs in CV3 chondrites are characterized by well-constrained 26 Al- 26 Mg isochrons with slopes corresponding to 26 Al/ 27 Al of 5.2 (±0.1) × 10 −5 , and intercepts suggesting that the initial (pre- 26 Al decay) 26 Mg/ 24 Mg ratio (≡D 26 Mg 0 *; see the Supplementary Materials) of CAIs varies from −0.13 to −0.014‰ relative to a terrestrial standard value (9)(10)(11)(12)(13)(14)(15). It is noteworthy that in situ measurements are, in general, more sensitive to subsequent thermal reprocessing that disturbed the original magnesium isotopes than are bulk-inclusion analyses and, therefore, provide more information about the timing of the last melting/disturbance event and isotope reequilibration [interested readers are referred to (16) for more detailed discussions]. The fact that bulk-sample and some in situ work yielded 26 Al/ 27 Al of 5.2 (±0.1) × 10 −5 but variable D 26 Mg 0 * implies a <30,000-year time scale (inferred from the analytical error of 26 Al/ 27 Al) for the formation of large CAIs in a reservoir with uniformly distributed 26 Al at this abundance level but slightly heterogeneous initial 26 Mg/ 24 Mg. ...
... Inclusions having 26 Al/ 27 Al lower than 4 × 10 −5 are associated with more positive D 26 Mg 0 * (up to 1.8‰) and make up two small peaks at 3.5 × 10 −5 and 2.5 × 10 −5 . Such an 26 Al/ 27 Al−D 26 Mg 0 * relationship can be best understood in the context of postformation thermal processing, similar to that suggested to account for the 26 Al/ 27 Al differences between pristine (unmelted) and thermally reprocessed (igneous) CV3 CAIs [e.g., (35,13,36,16)]. Therefore, inclusions having 26 Al/ 27 Al = 5.4 × 10 −5 and D 26 Mg 0 * = −0.14‰ could be considered the most pristine among those analyzed here and should most faithfully record the isotopic signatures of the formation region. ...
... These two values are in good agreement with the esti-mates for "true" solar system 26 Al/ 27 Al = (5.62 ± 0.42) × 10 −5 and D 26 Mg 0,i * = −(0.052 ± 0.013‰) based on CV CAI data (16). The peaks at lower 26 Al/ 27 Al (along with more positive D 26 Mg 0 *) would have been a consequence of late thermal processing of these early formed inclusions that had led to (partial) isotopic reequilibration. ...
Dust condensation and coagulation in the early solar system are the first steps toward forming the terrestrial planets, but the time scales of these processes remain poorly constrained. Through isotopic analysis of small Ca-Al–rich inclusions (CAIs) (30 to 100 μm in size) found in one of the most pristine chondrites, Allan Hills A77307 (CO3.0), for the short-lived ²⁶ Al- ²⁶ Mg [ t1/2 = 0.72 million years (Ma)] system, we have identified two main populations of samples characterized by well-defined ²⁶ Al/ ²⁷ Al = 5.40 (±0.13) × 10 ⁻⁵ and 4.89 (±0.10) × 10 ⁻⁵ . The result of the first population suggests a 50,000-year time scale between the condensation of micrometer-sized dust and formation of inclusions tens of micrometers in size. The 100,000-year time gap calculated from the above two ²⁶ Al/ ²⁷ Al ratios could also represent the duration for the Sun being a class I source.
... In contrast, the formation ages of chondrules in CV3 chondrites including errors are 2.1-2.5 Ma after CAI formation for Efremovka, 2.8-4.6 Ma after CAI formation for Vigarano, and 1.8-3.9 Ma after CAI formation for Kaba based on their initial 26 Al/ 27 Al ratios with 2r errors (e.g., Mishra and Chaussidon 2014;Nagashima et al. 2017) (gray squares in Fig. 8). ...
... The clasts and CV3 chondrites could be originated from the same parent body with a peak Fig. 8. Accretion age of the clast parent body (black square) and its comparison with formation age of chondrules in the CV3 Vigarano, Efremovka, and Kaba chondrites (gray square). The error on the accretion age of the clast parent body is~0.1 Myr based on variations of the modeling parameters (see the Thermal Modeling of the CV3-Like Body section); the errors on the chondrule formation ages are estimated from 2r errors of the initial 26 Al/ 27 Al ratios of chondrules (Mishra and Chaussidon 2014;Nagashima et al. 2017). temperature of 800-1100°C. ...
... Al-Mg formation ages of CV3 chondrules (~2.0-3.4 Ma after CAI formation;Mishra and Chaussidon 2014;Nagashima et al. 2017) and Mn-Cr ages of fayalite and Ca,Fesilicates (~3.2-4.2 Ma after CAI formation;Doyle et al. 2015;Jogo et al. 2017;MacPherson et al. 2017) are also shown.clasts" ...
We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato‐86009. These clasts are coarse‐grained polymineralic rocks composed of Ca‐bearing ferroan olivine (Fa24–40, up to 0.6 wt% CaO), diopside (Fs7–12Wo44–50), plagioclase (An52–75), Cr‐spinel (Cr/[Cr + Al] = 0.4, Fe/[Fe + Mg] = 0.7), sulfide and rare grains of Fe‐Ni metal, phosphate, and Ca‐poor pyroxene (Fs24Wo4). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE ~3.80–13.83 × CI) and plagioclase (Eu ~12.31–14.67 × CI) but are low in olivine (REE ~0.01–1.44 × CI) and spinel (REE ~0.25–0.49 × CI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato‐86009 breccias (Jogo et al. 2012) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 °C, the clast parent body should have accreted by ~2.5–2.6 Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800–1100 °C. If the body has a peak temperature of >1100 °C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.
... The nodules typically have perovksite in the core or at the edge of an anhedral spinel that is surrounded by melilite which in turn are mostly surrounded by a thin layer of pyroxene. These structures are quite similar to the previously described fine-grained spinel (pyroxene) rich CAIs in Efremovka and Vigarano except for the enrichment in Fe in some cases of Vigarano and Efremovka 19,20 . Several spinel-rich inclusions in pristine CVs have additional layers of melilite along with anorthite and pyroxene in these fine-grained CAIs 19 . ...
... These structures are quite similar to the previously described fine-grained spinel (pyroxene) rich CAIs in Efremovka and Vigarano except for the enrichment in Fe in some cases of Vigarano and Efremovka 19,20 . Several spinel-rich inclusions in pristine CVs have additional layers of melilite along with anorthite and pyroxene in these fine-grained CAIs 19 . The relative high abundance and compact texture of this type in Y 81020 is another differentiating factor compared to CV types in Efremovka and Vigarano. ...
... Characterization and comparison of refractory inclusions and salient features of each of the different groups of meteorites particularly in CVs have been done earlier 5,7,[11][12][13]19 . Succinctly, in general, large (up to a few cm in size) CAIs with abundant mineral phases of melilite, spinel, anorthite and pyroxene have been found in CV types that have been altered to different degrees depending on the individual meteorite. ...
Petrographic and quantitative analyses of more than 70 refractory inclusions found in the studied thin section of Yamato 81020 (CO3.05) showed diverse objects that can be grouped into five distinct types based on morphology and mineralogy. Mineralogical, textural similarities to the pristine carbonaceous vigarano (3.1-3.4) types of Efremovka, Vigarano were observed despite their smaller size (~100 micron diameter). Wark-Lovering rims were found predominantly in melilite-rich calcium, aluminum-rich inclusions. Comparison of mineralogy, morphology at macro and micro scale of Y 81020 with ALH A77307 and other carbonaceous chondrites is suggestive of the unaltered characteristics of the refractory inclusions in agreement with previous studies.
... The analysis of bulk Ca-, Al-rich inclusions (CAIs) allowed to determine an initial Mg and Al isotopic composition ( 26 Al / 27 Al 0 = 5.25 × 10 −5 and d 26 Mg * 0 =-0.034 ± 0.032‰) for the Solar system (Jacobsen et al. 2008;Larsen et al. 2011) with a much higher precision than from previous data acquired with mono-collector SIMS (see review by MacPherson et al. 1995). These new data of unprecedented precision made it possible to look for the scale of heterogeneity of Al and Mg isotopes in the early accretion disk (Villeneuve et al. 2009;Larsen et al. 2011;Mishra and Chaussidon 2014) and for the implications on the dynamic of the disk, the origin of 26 Al and the significance of 26 Al relative ages MacPherson et al. 2012;Wasserburg et al. 2012;Holst et al. 2013). ...
... The major results obtained for CAIs and chondrules, which are key components of chondritic meteorites, are the following. (i) CAIs formed over a very short time interval of ≈ 10,000-40,000 years (Thrane et al. 2006;Jacobsen et al. 2008;Larsen et al. 2011) from refractory precursors formed at the same time or at maximum 100,000 years before (Mishra and Chaussidon 2014). (ii) Most CAIs experienced a complex history in the accretion disk with episodes of reheating (or remelting) over a period of ≈ 300,000years after their formation, a few of them being reheated much later, up to ≈ 2 My after their formation (MacPherson et al. 2012;Kita et al. 2013;Mishra and Chaussidon 2014). ...
... (i) CAIs formed over a very short time interval of ≈ 10,000-40,000 years (Thrane et al. 2006;Jacobsen et al. 2008;Larsen et al. 2011) from refractory precursors formed at the same time or at maximum 100,000 years before (Mishra and Chaussidon 2014). (ii) Most CAIs experienced a complex history in the accretion disk with episodes of reheating (or remelting) over a period of ≈ 300,000years after their formation, a few of them being reheated much later, up to ≈ 2 My after their formation (MacPherson et al. 2012;Kita et al. 2013;Mishra and Chaussidon 2014). (iii) Most chondrules have 26 Al relative formation ages that extend from ≈ 1 to ≈ 3 Myr after CAIs (Villeneuve et al. 2009;Kita et al. 2013) but their precursors formed over an interval from ≈ 0 to ≈ 1.5 Myr after CAIs (Villeneuve et al. 2009;Luu et al. 2015). ...
... The analysis of bulk Ca-, Al-rich inclusions (CAIs) allowed to determine an initial Mg and Al isotopic composition ( 26 Al / 27 Al 0 = 5.25 × 10 −5 and d 26 Mg * 0 =-0.034 ± 0.032‰) for the Solar system (Jacobsen et al. 2008;Larsen et al. 2011) with a much higher precision than from previous data acquired with mono-collector SIMS (see review by MacPherson et al. 1995). These new data of unprecedented precision made it possible to look for the scale of heterogeneity of Al and Mg isotopes in the early accretion disk (Villeneuve et al. 2009;Larsen et al. 2011;Mishra and Chaussidon 2014) and for the implications on the dynamic of the disk, the origin of 26 Al and the significance of 26 Al relative ages MacPherson et al. 2012;Wasserburg et al. 2012;Holst et al. 2013). ...
... The major results obtained for CAIs and chondrules, which are key components of chondritic meteorites, are the following. (i) CAIs formed over a very short time interval of ≈ 10,000-40,000 years (Thrane et al. 2006;Jacobsen et al. 2008;Larsen et al. 2011) from refractory precursors formed at the same time or at maximum 100,000 years before (Mishra and Chaussidon 2014). (ii) Most CAIs experienced a complex history in the accretion disk with episodes of reheating (or remelting) over a period of ≈ 300,000years after their formation, a few of them being reheated much later, up to ≈ 2 My after their formation (MacPherson et al. 2012;Kita et al. 2013;Mishra and Chaussidon 2014). ...
... (i) CAIs formed over a very short time interval of ≈ 10,000-40,000 years (Thrane et al. 2006;Jacobsen et al. 2008;Larsen et al. 2011) from refractory precursors formed at the same time or at maximum 100,000 years before (Mishra and Chaussidon 2014). (ii) Most CAIs experienced a complex history in the accretion disk with episodes of reheating (or remelting) over a period of ≈ 300,000years after their formation, a few of them being reheated much later, up to ≈ 2 My after their formation (MacPherson et al. 2012;Kita et al. 2013;Mishra and Chaussidon 2014). (iii) Most chondrules have 26 Al relative formation ages that extend from ≈ 1 to ≈ 3 Myr after CAIs (Villeneuve et al. 2009;Kita et al. 2013) but their precursors formed over an interval from ≈ 0 to ≈ 1.5 Myr after CAIs (Villeneuve et al. 2009;Luu et al. 2015). ...
Isotopic variation for traditional elements (H, C, N, O and S) has been widely used in the past 40 years in Earth and planetary sciences to study many processes with an emphasis on environments where fluids are present (e.g., Valley and Cole 2011). More recent developments have allowed high-precision measurements of isotope ratios of what has been called non-traditional elements (i.e., Mg, Si, Fe, Zn, Cu, Mo), which are usually less fractionated than traditional elements by at least an order of magnitude (see this volume). These non-traditional stable isotopes can give insights on processes where fluids are not present (e.g., metal–silicate fractionation, e.g., Georg et al. 2007 and review by Poitrasson et al. 2017, this volume), evaporation processes during planetary formation (e.g., Paniello et al. 2012, Wang and Jacobsen 2016, and review by Moynier et al. 2017 this volume), igneous differentiation (e.g., Williams et al. 2009; Sossi et al. 2012; and review by Dauphas et al. 2017, this volume), and on biological processes (e.g., Walczyk and von Blanckenburg 2002, and review by Albarede et al. 2017 this volume).
Among all these non-traditional isotopic systems, Mg isotopes are of major importance because (i) Mg is a major constituent of the silicate portion of planetary bodies, (ii) Mg has more than two isotopes (24Mg, 25Mg and 26Mg) allowing to study processes leading to various types of mass fractionation (Young et al. 2002; Young and Galy 2004; Davis et al. 2015) and (iii) 26Mg excesses produced by the radioactive decay of short-lived 26Al (T1/2=0.73 Ma) (Lee et al. 1976) are a key tool for early Solar system chronology (see reviews by Dauphas and Chaussidon 2011; Chaussidon and Liu 2015). Note that in addition, significant Mg isotopic …
... 9, 19, and 20). However, this conclusion relies on the assumption of homogeneity of the 26 Al distribution at time 0 in the inner region of the disk [see discussions on 26 Al distribution in Villeneuve et al. (9), Larsen et al. (11), and Mishra and Chaussidon (21)]. In addition, mineral 26 Al isochrons in chondrules date the last melting event, thus giving no clue on the age of formation of chondrule precursors. ...
... The simplest interpretation of a bulk composition of a chondrule is to consider that it reflects the average composition of its solid precursors. Assuming that 26 Al and Mg isotopes were homogenized to ∼±10% at time 0 in the regions of the SPD where the precursors of CAIs and chondrules formed (9,21), the present bulk chondrule 26 Al isochron with ( 26 Al/ 27 Al) bulk initial = 1.23(±0.21) × 10 −5 would measure the average age of the precursors of the 14 Allende chondrules studied relative to CAIs under the assumption that chondrules and CAIs are averages of the same materials sampled at different times. ...
... Arguments For and Against 26 Al Homogeneity in the CAIsand Chondrules-Forming Region of the Disk All of the present Mg isotopic data can be understood in the context of homogeneous, or nearly homogeneous, 26 (9) and CAIs (21). However, other observations exist that are used to suggest significant heterogeneity in these isotope ratios from ultra-high-precision (a few parts per million level) Mg isotopic composition of CAIs and AOAs (11) or from apparent age discrepancies between the Pb−Pb and 26 Al chondrule age dates (16). ...
Significance
Combining bulk and in situ Mg isotope measurements allows dating the formation of chondrule precursors relative to that of chondrules themselves. This has never been achieved before and has profound implications for the origin of chondrules, the origin of their precursors, and the evolution of the solar protoplanetary disk. There is a minimum bulk chondrule isochron, analogous in some ways to the bulk calcium−aluminum-rich inclusion isochron, which dates either the cessation of condensation of chondrule precursors or the cessation of their transport to the regions where chondrules formed.
... Plus l' 26 Al sera bas, plus la CAI se sera formée tard. Certaines études montrent ainsi que les CAIs se sont formées dans un intervalle de quelques centaines de milliers d'années (Mishra and Chaussidon, 2014). 10 Be McKeegan et al. (2000) ont mis en évidence que les CAIs comportait du 10 Be lors de leur formation. ...
... La classe 0 ne durant que quelques dizaines de milliers d'années (Lada, 1999) est cohérent avec l'environnement de formation des CAIs (Figure 1.4). L' 26 Al suggère que les CAIs se sont formées dans la classe I (Mishra and Chaussidon, 2014). ...
Les inclusions réfractaires riches en calcium et en aluminium (CAIs) sont les premiers objets solides du système solaire à s'être formés. Malgré 4,568 Ga d'évolution, on peut remonter à leurs conditions de formation et dire qu'elles se sont formées à plus de 1200 °C dans des conditions très réductrices près du Soleil jeune. Les phases secondaires présentes dans les CAIs suggèrent quant à elles une formation dans des conditions plus oxydantes et/ou à plus basse température. La plupart de ces phases ont été interprétées comme provenant d'évènements tardifs. Néanmoins, une origine nébulaire de certaines phases secondaires reste débattue. L'objectif de cette thèse est de vérifier en couplant différentes techniques si certaines phases secondaires se seraient formées lors de la formation des CAIs dans la nébuleuse. Pour cela une CAI composée, nommée E101.1, provenant de la météorite CV3 réduite Efremovka a été étudiée. Cette CAI a été choisie car comportant des phases riches en FeO incluses dans des diopsides eux-mêmes inclus dans la CAI hôte. Ces phases ont été caractérisées comme de la Fe-åkermanite, des assemblages à grains fins associés à de la kirschsteinite et de la wollastonite. L'étude pétrologique et texturale de ces phases réalisée pendant cette thèse a permis de suggérer que la kirschsteinite et la wollastonite s'étaient formées dans la nébuleuse au sein de précurseurs riches en diopside et anorthite. La Fe-åkermanite résulterait de l'incorporation de ces précurseurs dans une CAI de type A partiellement fondue. Cela est cohérent avec des expériences en pétrologie expérimentale qui ont été entamées. Après avoir développé une méthode d'imagerie du rapport D/H dans des sections ultraminces de minéraux peu hydratés en NanoSIMS, les δD des différents minéraux d'E101.1 ont été mesurés. Les valeurs les plus basses jamais mesurées dans un échantillon météoritique ont ainsi été répertoriées au sein de l'anorthite avec un δD de -817 ± 185 ‰ (2σ). Cette valeur est en accord avec une formation près du Soleil jeune. Les assemblages à grains fins ont des valeurs allant jusque 1250 ± 516 ‰ (2σ). La kirschsteinite a quant à elle un δD chondritique de 163 ± 201 ‰ (2σ). Les valeurs élevées ont été attribuées, en accord avec les observations pétrologiques, à la capture des xénolithes. La kirschsteinite et la wollastonite se sont donc formées dans la nébuleuse dans un réservoir avec une composition isotopique en H chondritique. Cela signifie que le D/H de l'eau dans la nébuleuse serait passé d'une valeur solaire à une valeur presque terrestre en quelques centaines de milliers d'années maximum. Ces approches complémentaires nous ont ainsi permis de montrer la présence de phases d’altération nébulaires dans une CAI et qu’un épisode oxydant non prédit par la thermodynamique a eu lieu dans la nébuleuse.
... This undoubtedly led to chemical and isotopic variations at multiple geometric and temporal scales. Some materials in the most primitive meteorites were created as these reservoirs were forming and evolving and therefore offer a window into the conditions and processes dominating the early evolution of the protoplanetary disk at the time soon after its formation (<1 Myr, Mishra and Chaussidon, 2014). Calcium-Aluminium-rich Inclusions (CAIs) formed in the very earliest times of solar system history, and chondrules mainly formed up to a few million years later; although the formation age of some of them overlap with that of CAIs (e.g., Connelly et al., 2012;Bollard et al., 2017;Marrocchi et al., 2019)). ...
... While many CAIs are reported to have 26 Mg excesses that provide evidence that they once contained live 26 Al, those CAIs lacking evidence of 26 Al are interpreted as evidence for a heterogeneous distribution in time and/or space of 26 Al in the early solar system (Liu et al., 2009(Liu et al., , 2012Sahijpal and Goswami, 1998;Kita et al., 2013;Mishra and Chaussidon, 2014;Kö ö p et al., 2016a). ...
A ∼175 µm refractory inclusion, A-COR-01 from one of the least altered carbonaceous chondrites, ALHA 77307 (CO3.0), has been found to bear unique characteristics that indicate that it is one of the first solids to have formed at the very birth of the solar system while isotopic reservoirs were still evolving rapidly. Its core is composed mainly of hibonite and corundum, the two phases predicted to condense first from a gas of solar composition, and like many common types of Calcium-, Aluminium-rich Inclusions (CAIs) is surrounded by a rim of diopside.
Core minerals in A-COR-01 are very ¹⁶O-rich (Δ¹⁷OCore = -32.5 ± 3.3 (2SD) ‰) while those in the rim display an O isotopic composition (Δ¹⁷ORim = -24.8 ± 0.5 (2SD) ‰) indistinguishable from that found in the vast majority of the least altered CAIs. These observations indicate that this CAI formed in a very ¹⁶O-rich reservoir and either recorded the subsequent evolution of this reservoir or the transit to another reservoir. The origin of A-COR-01in a primitive reservoir is consistent with the very low content of excess of radiogenic ²⁶Mg in its core minerals corresponding to the inferred initial ²⁶Al/²⁷Al ratio ((²⁶Al/²⁷Al)0 = (1.67 ± 0.31) × 10⁻⁷), supporting a very early formation before injection and/or homogenisation of ²⁶Al in the protoplanetary disk. Possible reservoir evolution and short-lived radionuclide (SLRs) injection scenarios are discussed and it is suggested that the observed isotope composition resulted from mixing of a previously un-observed early reservoir with the rest of the disk.
... While 26 Al may be produced in different stellar environments such as supernovae or AGB stars (Lugaro et al. 2018), a Wolf-Rayet type star is gaining more consensus as the main source of this isotope in the Solar System (Dwarkadas et al. 2017). The variable 26 Al/ 27 Al ratios observed in CAIs and chondrules have been widely used to infer time differences between the formation events of these chondrites components (Mishra & Chaussidon 2014;Bollard et al. 2017). However, it is not clear whether different 26 Al/ 27 Al ratios actually reflect time differences between components or heterogeneity in the distribution of 26 Al in the disk Mishra & Chaussidon 2014). ...
... The variable 26 Al/ 27 Al ratios observed in CAIs and chondrules have been widely used to infer time differences between the formation events of these chondrites components (Mishra & Chaussidon 2014;Bollard et al. 2017). However, it is not clear whether different 26 Al/ 27 Al ratios actually reflect time differences between components or heterogeneity in the distribution of 26 Al in the disk Mishra & Chaussidon 2014). Heterogeneity in the 26 Al within the forming Solar System would, in fact, erase the possibility to use 26 Al/ 27 Al as a chronometer. ...
... Absolute Pb-Pb ages show that CAIs in carbonaceous chondrites were formed over a brief interval (time, t < 160 kyr) at the very beginning of the Solar System (Connelly et al. 2012) from refractory precursors that, according to shortlived 26 Al chronologies, could have condensed over an even shorter period of 10-20 kyr (Jacobsen et al. 2008;Larsen et al. 2011;Mishra & Chaussidon 2014). Such a timescale is shorter than the assembling time of a protoplanetary disk from the infalling molecular cloud (Williams & Cieza 2011). ...
... In Fig. 4(b) we show the time evolution of the temperature in a given location of the disk and the time evolving location of the CAIs condensation front. Massive production of CAIs happens, in the simulation, from t ∼ 40 kyr to t ∼ 70 kyr, in very good agreement with Pb-Pb and 26 Al-26 Mg chronologies of CAIs (Connelly et al. 2012;Mishra & Chaussidon 2014) (Fig. 4(a)). This corresponds to a short time window, when interstellar solids are injected in the disk at temperatures high enough for them to be fully evaporated (i.e. when R c (t) is inward the condensation radius of CAIs, see Fig.1), so that upon spreading outward, the gas can condense into CAIs. ...
Chondritic meteorites, the building blocks of terrestrial planets, are made of an out-of-equilibrium assemblage of solids formed at high and low temperatures, either in our Solar system or previous generations of stars. For decades this was considered to result from large-scale transport processes in the Sun's isolated accretion disk. However, mounting evidence suggests that refractory inclusions in chondrites formed contemporaneously with the disk building. Here we numerically investigate, using a 1D model and several physical and chemical processes, the formation and transport of rocky materials during the collapse of the Sun's parent cloud and the consequent assembling of the Solar Nebula. The interplay between the cloud collapse, the dynamics of gas and dust, vaporization, recondensation, and thermal processing of different species in the disk results in a local mixing of solids with different thermal histories. Moreover, our results also explain the overabundance of refractory materials far from the Sun and their short-formation timescales, during the first tens of kyr of the Sun, corresponding to class 0-I, opening new windows into the origin of the compositional diversity of chondrites. © 2018. The American Astronomical Society. All rights reserved.
... Measurements of Al-Mg formation ages of the least altered chondrules in CV3 chondrites have yielded initial 26 Al/ 27 Al ratios of ($2-6) Â 10 À6 corresponding to $2.0-3.4 Ma after CV3 CAI formation (Mishra and Chaussidon, 2014;Nagashima et al., 2015). In order to be consistent with the radial diffusion model, CV3 chondrite parent body should have accreted at about <3.0-4.4 ...
... Ma after CAI formation from radial diffusion model (Cuzzi et al., 2010) requiring <1 Ma accretion of the CV3 body after CV3 chondrule formation of about 2.0-3.4 Ma after CAI formation (dashed line, Mishra and Chaussidon, 2014;Nagashima et al., 2015). The accretion timing of the CV3 body which fulfills (i-iii) conditions is about 3.2-3.3 ...
Chondritic planetesimals are among the first planetary bodies that accreted inside and outside water snow line in the protoplanetary disk. CV3 carbonaceous chondrite parent body accreted relatively small amount of water ice, probably near the snow line, and experienced water-assisted metasomatic alteration that resulted in formation of diverse secondary minerals, including fayalite (Fa80–100). Chemical compositions of the CV fayalite and its Mn-Cr isotope systematics indicate that it formed at different temperature (10–300°C) and fluid pressure (3–300 bars) but within a relatively short period of time. Thermal modeling of the CV parent body suggests that it accreted ∼3.2–3.3 Ma after CV CAIs formation and had a radius of >110–150 km. The inferred formation age of the CV parent body is similar to that of the CM chondrite parent body that probably accreted beyond the snow line, but appears to have postdated accretion of the CO and ordinary chondrite parent bodies that most likely formed inside the snow line. The inferred differences in the accretion ages of chondrite parent bodies that formed inside and outside snow line are consistent with planetesimal formation by gravitational/streaming instability.
... This period has been estimated by various studies to vary between possibly as little as 4000 years (Larsen et al., 2011) and at most 50,000 years (Thrane et al., 2006) depending on the analytical uncertainties of the measurements. Averaging data from several sources, a duration of 24,400 years is taken by Mishra and Chaussidon (2014). The extent of this underestimation strongly depends on the relative efficiencies to produce small dust particles by condensation of new CAI precursors and fragmentation of previous CAI generations. ...
... This possible bias on bulk rock ages is a possible explanation for the observed difference between the short period for CAIs precursor formation and the longer period of CAI processing determined from mineral isochrons on individual CAIs (see e.g. Kita et al., 2013;Mishra and Chaussidon, 2014). The quantification of this effect is complex and will be addressed in a future paper. ...
Whereas it is generally accepted that calcium-aluminum-rich inclusions (CAIs) from chondritic meteorites formed in a hot environment in the solar protoplanetary disk, the conditions of their formation remain debated. Recent laboratory studies of CAIs have provided new kind of data: their size distributions. We report that size distributions of CAIs measured in laboratory from sections of carbonaceous chondrites have a power law size distribution with cumulative size exponent between -1.7 and -1.9, which translates into cumulative size exponent between -2.5 and -2.8 after correction for sectioning. To explain these observations, numerical simulations were run to explore the growth of CAIs from micrometer to centimeter sizes, in a hot and turbulent protoplanetary disk through the competition of coagulation and fragmentation. We show that the size distributions obtained in growth simulations are in agreement with CAIs size distributions in meteorites. We explain the CAI sharp cut-off of their size distribution at centimeter sizes as the direct result from the famous fragmentation barrier, provided that CAI fragment for impact velocities larger than 10 m/s. The growth/destruction timescales of millimeter- and centimeter-sized CAIs is inversely proportional to the local dust/gas ratio and is about 10 years at 1300 K and up to 104 years at 1670K. This implies that the most refractory CAIs are expected to be smaller in size owing to their long growth timescale compared to less refractory CAIs. Conversely, the least refractory CAIs could have been recycled many times during the CAI production era which may have profound consequences for their radiometric age.
... While many papers have identified this clear heterogeneity of 26 Al, there is no consensus in the field for its origin. The term "heterogeneity" is agnostic about cause, but often it is implied that the heterogeneities are erased by physical mixing between materials in different regions of the disk (e.g., Krot et al. 2012;Mishra & Chaussidon 2014), by processes such as those described by Boss (2013). Such spatial heterogeneities might be expected if 26 Al were produced by irradiation by solar energetic particles in the solar nebula (e.g., Gounelle et al. 2001;Jacquet 2019;Gaches et al. 2020), but there are severe limitations to production of 26 Al in H 2 gas (Clayton & Jin 1995), or in material devoid of H 2 . ...
Most meteoritic calcium-rich, aluminum-rich inclusions formed from a reservoir with ²⁶ Al/ ²⁷ Al ≈ 5 × 10 ⁻⁵ , but some record lower ( 26 Al / 27 Al ) 0 , demanding they sampled a reservoir without live ²⁶ Al. This has been interpreted as evidence for “late injection” of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them “low- ²⁶ Al/ ²⁷ Al corundum/hibonite inclusions” (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel, or hibonite grains 0.5–2 μ m in size with no live ²⁶ Al; live ²⁶ Al is carried on smaller (<50 nm) presolar chromium spinel grains from recent nearby Wolf–Rayet stars or supernovae. In hot (≈1350–1425 K) regions of the disk, these grains and perovskite grains would be the only survivors. These negatively charged grains would grow to sizes 1–10 ³ μ m, even incorporating positively charged perovskite grains, but not the small, negatively charged ²⁶ Al-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ⁴⁸ Ca and ⁵⁰ Ti anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ²⁶ Al in a solar nebula with uniform, canonical ²⁶ Al/ ²⁷ Al. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.
... These external isochrons are interpreted to represent the time at which the CAIs or their precursors were isolated from the bulk solar nebula. Internal 26 Mg isochron data for individual CAIs are more complex (e.g., Wasserburg et al., 2012;Makide et al., 2013;Mishra and Chaussidon, 2014;Kööp et al., 2016b;Ushikubo et al., 2017;Krot, 2019; ; M.C. Liu et al., 2019;Simon et al., 2019;Kawasaki et al., 2018Kawasaki et al., , 2019Kawasaki et al., , 2020Han et al., 2020;Wada et al., 2020;MacPherson et al., 2010MacPherson et al., , 2012MacPherson et al., , 2017aMacPherson et al., , 2018MacPherson et al., , 2020MacPherson et al., , 2022Dunham et al., 2022;Mane et al., 2022;Fig. 1). ...
... L'analyse des échantillons de la mission Genesis a permis de montrer que le soleil était riche en 16 O et, par conséquent, que les CAIs se sont très probablement formés près du Soleil (McKeegan et al., 2011). De plus, ces objets ce sont formés pendant une période très courtes au début du système solaire (MacPherson et al., 2012;Mishra et Chaussidon, 2014). ...
Les éléments volatils comme l’hydrogène et l’azote contrôlent l'évolution des corps planétaires et de leurs atmosphères, et sont des éléments essentiels au développement de la vie sur Terre. Néanmoins, l'origine des éléments volatils et la chronologie de leur accrétion par les planètes telluriques formées au sein du système solaire interne restent un sujet de débat et de controverse en sciences planétaires. Pour répondre à ces questions, les rapports isotopiques de l'hydrogène (D/H) et de l'azote (15N/14N) sont des outils puissants pour tracer l'origine (solaire, chondritique ou cométaire) des éléments volatils piégés par les planètes telluriques. Pour contraindre l’origine(s) des éléments volatils piégés par les planètes rocheuses, nous avons donc mesuré les teneurs et les compositions isotopiques de l’hydrogène et de l’azote par microsonde ionique (LGSIMS) dans des achondrites (angrites, météorites maritennes et aubrites) qui proviennent d’astéroïdes différenciés ou de planètes qui sont considérés s’être formés dans le système solaire interne. Ces météorites conservent un enregistrement des étapes initiales de la formation de leurs corps parents et peuvent imposer des contraintes quant à l’évolution précoce des éléments volatils planétaires. L'analyse in-situ par SIMS est une technique quasi-non-destructive, qui permet de mesurer la teneur et la composition isotopique des éléments volatils de différentes phases dans des échantillons terrestres, extraterrestres et synthétiques. Le développement récent du protocole d'analyse de l'azote dans les échantillons silicatés par sonde ionique nous permet de caractériser des objets de la taille d’une dizaine de microns, tels que des inclusions vitreuses. Au cours de cette thèse, les éléments volatils ont été mesurés dans des inclusions magmatiques piégées dans des minéraux et dans les verres interstitiels. Bien que l’analyse de l’azote dans des aubrites n’a pas pu aboutir, les analyses réalisées sur des météorites martiennes et des angrites ont permis de mettre en évidence la présence de quantité importante d’eau et d’azote au sein de ces météorites et de leurs corps parent. En particulier, l’étude des angrites et plus précisément de la météorite d’Orbigny nous a permis de mettre en évidence la présence d’eau et d’azote ayant des compositions isotopiques similaires à celles des météorites primitives formées dans le système solaire externe (i.e., chondrites carbonées de type CM). Ces résultats impliquent que ces éléments volatils étaient présents ~4 millions d’années après la formation des CAIs (i.e., premiers solides à se former dans le système solaire) dans le système solaire interne et ont pu être piégés par les planètes telluriques lors de leur formation. De plus, l’analyses des météorites martiennes et plus particulièrement de Chassigny a révélé la présence d’azote ayant une composition isotopique enrichie en 15N comparée aux chondrites à enstatite et aux diamants terr estres qui sont supposés représenter la valeur la plus primitive de l’azote sur Terre.
... For exam-ple, spinel-hibonite spherules and 'normal' calcium and aluminum rich inclusions (CAIs) do not show nucleosynthetic variability, while ultrarefractory platy hybonite crystals and CAIs with fractionation and unidentified nuclear effects (also known as FUN CAIs) do. This is interpreted as a record of progressive homogenisation of dust and gas in the inner regions of the protoplanetary disk via turbulent mixing and thermal heat-ing during the T-Tauri phase of the Sun (Mishra & Chaussidon 2014;Pignatale et al. 2018Pignatale et al. , 2019Jacquet et al. 2019). Nucleosynthetic isotope variations are also observed among bulk compositions of meteorites and planetary objects which implies that large scale isotopic heterogeneities, inherited from the proto-solar nebula and/or formed during the evolution of the protoplanetary disk, have been preserved. ...
Isotope variations of nucleosynthetic origin among solar system solid samples are well documented, yet the origin of these variations is still uncertain. The observed variability of ⁵⁴ Cr among materials formed in different regions of the protoplanetary disk has been attributed to variable amounts of presolar, chromium-rich oxide (chromite) grains, which exist within the meteoritic stardust inventory and most likely originated from some type of supernova explosion. To investigate if core-collapse supernovae (CCSNe) could be the site of origin of these grains, we analyze yields of CCSN models of stars with initial masses 15, 20, and 25 M ⊙ , and solar metallicity. We present an extensive abundance data set of the Cr, Mg, and Al isotopes as a function of enclosed mass. We find cases in which the explosive C ashes produce a composition in good agreement with the observed ⁵⁴ Cr/ ⁵² Cr and ⁵³ Cr/ ⁵² Cr ratios as well as the ⁵⁰ Cr/ ⁵² Cr ratios. Taking into account that the signal at atomic mass 50 could also originate from ⁵⁰ Ti, the ashes of explosive He burning also match the observed ratios. Addition of material from the He ashes (enriched in Al and Cr relative to Mg to simulate the make-up of chromite grains) to the solar system’s composition may reproduce the observed correlation between Mg and Cr anomalies, while material from the C ashes does not present significant Mg anomalies together with Cr isotopic variations. In all cases, nonradiogenic, stable Mg isotope variations dominate over the variations expected from ²⁶ Al.
... We note that recent studies do not provide a consensus on the precise location and duration of CAI formation and high-temperature processing. CAI relative chronometry suggests that these objects were formed and were thermally processed over timespans ranging from $40,000 years to $200,000 years (Connelly et al., 2012;MacPherson et al., 2012;Kita et al., 2013;Mishra and Chaussidon, 2015;Liu et al., 2019;Kawasaki et al., 2019). It is theoretically possible that high-temperature processing of CAIs could have lasted as long as $1 Ma because temperatures in the disk are predicted to exceed $1300 K for this extended time interval (Yang and Ciesla, 2012). ...
Short-lived radionuclides (SLRs) once present in the solar nebula can be used as probes of the formation environment of our Solar System within the Milky Way Galaxy. The first-formed solids in the Solar System, calcium-, aluminum-rich inclusions (CAIs) in meteorites, record the one-time existence of SLRs such as ¹⁰Be and ²⁶Al in the solar nebula. We measured the ¹⁰Be–¹⁰B isotope systematics in 29 CAIs from several CV3, CO3, CR2, and CH/CB chondrites and show that all except for a FUN CAI record a homogeneous initial ¹⁰Be/⁹Be with a single probability density peak at ¹⁰Be/⁹Be = 7.4 × 10–4. Integrating these data with those of previous studies, we find that most CAIs (81%) for which ¹⁰Be–¹⁰B isotope systematics have been determined, record a homogeneous initial ¹⁰Be/⁹Be ratio in the early Solar System with a weighted mean ¹⁰Be/⁹Be = (7.1 ± 0.2) × 10–4. This uniform distribution provides evidence that ¹⁰Be was predominantly formed in the parent molecular cloud and inherited by the solar nebula. Possible explanations for why unusual CAIs (FUNs, PLACs, those from CH/CBs, and those irradiated on the parent body) recorded a ¹⁰Be/⁹Be ratio outside of 7.1 × 10⁻⁴ include the following: 1) They incorporated a component of ¹⁰Be that was produced in the nebula by irradiation; 2) they formed after normal CAIs; and 3) they were processed (post-formation) in a way that affected their original ¹⁰Be signatures. Given the rarity of these examples, the overall uniformity of initial ¹⁰Be/⁹Be suggests that Solar System ¹⁰Be was predominantly inherited from the molecular cloud.
... For exam-ple, spinel-hibonite spherules and 'normal' calcium and aluminum rich inclusions (CAIs) do not show nucleosynthetic variability, while ultrarefractory platy hybonite crystals and CAIs with fractionation and unidentified nuclear effects (also known as FUN CAIs) do. This is interpreted as a record of progressive homogenisation of dust and gas in the inner regions of the protoplanetary disk via turbulent mixing and thermal heat-ing during the T-Tauri phase of the Sun (Mishra & Chaussidon 2014;Pignatale et al. 2018Pignatale et al. , 2019Jacquet et al. 2019). Nucleosynthetic isotope variations are also observed among bulk compositions of meteorites and planetary objects which implies that large scale isotopic heterogeneities, inherited from the proto-solar nebula and/or formed during the evolution of the protoplanetary disk, have been preserved. ...
Isotope variations of nucleosynthetic origin among Solar System's solid samples are well documented, yet the origin of these variations is still uncertain. The observed variability of \iso{54}Cr among materials formed in different regions of the proto-planetary disk has been attributed to variable amounts of presolar chromium-rich oxide (chromite) grains, which exist within the meteoritic stardust inventory and most likely originated from some type of supernova explosions. To investigate if core-collapse supernovae (CCSNe) could be the site of origin of these grains, we analyse yields of CCSN models of stars with initial mass 15, 20 and 25 M$_{\odot}$, and solar metallicity. We present an extensive abundance data set of the Cr, Mg, and Al isotopes as a function of enclosed mass. We find cases in which the explosive C-ashes produce a composition in good agreement with the observed \iso{54}Cr/\iso{52}Cr and \iso{53}Cr/\iso{52}Cr ratios as well as the \iso{50}Cr/\iso{52}Cr ratios. Taking into account that the signal at atomic mass 50 could also originate from \iso{50}Ti, the ashes of explosive He-burning also match the observed ratios. Addition of material from the He ashes (enriched in Al and Cr relative to Mg to simulate the make-up of chromite grains) to the Solar System composition may reproduce the observed correlation between Mg and Cr anomalies, while material from the C-ashes does not present significant Mg anomalies together with Cr isotopic variations. In all cases, non-radiogenic, stable Mg isotope variations dominate over the variations expected from \iso{26}Al.
... A homogeneous distribution of 26 Al has also been proposed by several studies (e.g., Villeneuve et al., 2009;Kruijer et al., 2014;Mishra and Chaussidon, 2014;Budde et al., 2018;Luu et al., 2019;Gregory et al., 2020). In particular, the agreement of Al-Mg and Hf-W ages of angrites and chondrules from CV and CR chondrites supports a homogeneous distribution of 26 Al in the early Solar System (Budde et al., 2016(Budde et al., , 2018Nagashima et al., 2017;Tenner et al., 2019). ...
Deciphering the spatial and temporal evolution of chondrules allows for a better understanding of how asteroidal seeds formed, migrated, and eventually accreted into parent asteroids. Here we report high precision Al-Mg ages and oxygen three-isotope ratios of fifteen FeO-poor chondrules from the least metamorphosed Mighei-like (CM) and Ornans-like (CO) carbonaceous chondrites, Asuka 12236 (CM2.9), Dominion Range 08006 (CO3.01), and Yamato-81020 (CO3.05). This is the first report of Al-Mg ages of chondrules from the CM chondrite group. All but one of the fifteen chondrules exhibit a restricted range of inferred initial ²⁶Al/²⁷Al ratios, and all ratios are ≤ 6.0 × 10⁻⁶, which is systematically lower than those of the majority of ordinary chondrite (OC) chondrules. These observations indicate that the majority of chondrules in the outer Solar System were produced ≥ 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which postdates OC chondrule formation in the inner Solar System (≤ 2.2 Ma after CAI formation). We propose that the discrete chondrule-forming events in different disk regions reflect a time difference in growth and orbital evolution of planetesimals within the first 4 Ma of the Solar System.
One chondrule from Asuka 12236 has an age of 1.9 Ma after CAI formation and is therefore significantly older than the other fourteen chondrules, meaning this chondrule formed contemporaneously with the majority of OC chondrules. This old chondrule also exhibits ¹⁶O-depleted oxygen isotope characteristics compared to the other chondrules, suggesting a distinct formation region, probably inside the disk region relative to where the majority of CM and CO chondrules formed. Our results indicate that this old chondrule has migrated from the inner to the outer part of the protoplanetary disk within ∼1 Ma and then accreted into the CM parent asteroid >3 Ma after CAI formation, although its formation exterior to the accretion region of the CM parent asteroid and subsequent inward migration cannot be ruled out completely.
... (2) rare earth elements (REE) 'type II' abundance patterns in hibonite and surrounding pyroxene/glass (3) hibonites and pyroxene/glass within all HIXs (except for pyroxene in Y-791717) are devoid of 'typical' explicit (>2σ) radiogenic excess in Δ 26 Mg (4) most show well resolved deficits in δ 25 Mg (Kurat, 1975;Grossman et al., 1988;Ireland, 1988;Ireland et al., 1990Ireland et al., , 1991Tomeoka et al., 1992;Russell et al., 1998;Simon et al., 1998;Guan et al., 2000). The petrographic, elemental, and isotopic evidences taken together suggest that HIXs formation record one of the earliest, high temperature events of the rapidly evolving cosmochemical environment prior to the end of complete homogenization recorded by Type B CAIs (Mishra and Chaussidon 2014a;Chaussidon and Liu 2015). Table 1 lists some features of these ten hibonite-pyroxene/glass spherules reported so far (Fig. S1). ...
10 Hibonite-pyroxene/glass spherules discovered hitherto are a rare suite of refractory inclusions that show the largest range of exotic isotopic properties (anomalies in neutron rich isotopes (e.g., $^{48}$Ca, $^{50}$Ti), abundance of $^{26}$Al) despite their defining simple spherical morphology and mineralogy consisting predominantly of few hibonites nestled within/with glassy or crystallised calcium, aluminium-rich pyroxene. $^{26}$Al-$^{26}$Mg chronological studies along with petrography and mineralogy of a relatively large (~120 micron diameter), found in Allan Hills 77307 (CO3.03) has been performed. Uniquely, both hibonite and pyroxene show discordant abundance of short-lived now-extinct radionuclide $^{26}$Al that suggest disparate and distinct regions of origin of hibonite and pyroxene. The pristine petrography and mineralogy of this inclusion allow discernment of their genesis and trend of alteration in hibonite-pyroxene/glass spherules.
... Mere thermodynamics offer little insight into the "prehistory" of the solids and igneous cooling rates estimates refer to melting/crystallization events (8), not original gas-solid condensation. Al-Mg dating provides only an upper bound of a few 10s or 100s of millennia for the condensation epoch (9)(10)(11)(12), which says little on the formation timescales of individual condensates. From an astrophysical viewpoint, evolution of isotherms should be fairly slow (on 10 4 to 10 6 -y timescales; ref. 13) and random motions between significant condensation fronts should take similarly long (14), so it is often assumed that condensates formed over prolonged timescales (4,15,16). ...
Chondritic meteorites are composed of primitive components formed during the evolution of the Solar protoplanetary disk. The oldest of these components formed by condensation, yet little is known about their formation mechanism because of secondary heating processes that erased their primordial signature. Amoeboid Olivine Aggregates (AOAs) have never been melted and underwent minimal thermal annealing, implying they might have retained the conditions under which they condensed. We performed a multiisotope (O, Si, Mg) characterization of AOAs to constrain the conditions under which they condensed and the information they bear on the structure and evolution of the Solar protoplanetary disk. High-precision silicon isotopic measurements of 7 AOAs from weakly metamorphosed carbonaceous chondrites show large, mass-dependent, light Si isotope enrichments (–9‰ < δ ³⁰ Si < –1‰). Based on physical modeling of condensation within the protoplanetary disk, we attribute these isotopic compositions to the rapid condensation of AOAs over timescales of days to weeks. The same AOAs show slightly positive δ ²⁵ Mg that suggest that Mg isotopic homogenization occurred during thermal annealing without affecting Si isotopes. Such short condensation times for AOAs are inconsistent with disk transport timescales, indicating that AOAs, and likely other high-temperature condensates, formed during brief localized high-temperature events.
... High-precision in situ internal 26 Al isochrons in CAIs suggest multiple epochs of CAI formation, including a later one at ~0.5 Ma (ref. 29 ), which supports the observed isotopic records. The late epoch of intense irradiation inferred in the present study is in general agreement with the observed isotopic records of 36 Cl in CAIs and chondrules 30,31 . ...
Fossil meteoritic records of short-lived, now-extinct radionuclides provide crucial high-resolution temporal information about the events, processes and activity of the Sun during the early phases of Solar System formation1. The proposed genesis of one such radionuclide,10Be, by spallation reactions of carbon and oxygen2–5 led to the hypothesis of enhanced irradiation in the early Solar System6–8. An alternative scenario of production of 10Be (half-life t1/2 = 1.386 ± 0.016 million years9) by a neutrino process in a supernova arising from the core collapse of a low-mass star (11.8 solar masses, M⊙) has recently been suggested10 and can explain the observed abundance of 10Be in the early Solar System. Here, we report well-resolved excesses in 7Li/6Li of up to ~21.5% in a calcium- and aluminium-rich inclusion (CAI) from the Efremovka meteorite that correlate with 9Be/6Li, suggestive of in situ decay of 7Be. The in situ decay of 7Be to 7Li, with a characteristic half-life of 53.12 ± 0.07 days11, entails multiple episodes of enhanced irradiation in the early Solar System, which have been observed recently in other Sun-like stars12,13. The short half-life of 7Be limits its production by interaction of solar energetic particles (SEPs) with the nebular gas and solids, and provides constraints on the genealogy and chronology of CAIs. Irradiation of the solid and gaseous precursors of CAIs of solar composition by a superflare (X-ray luminosity approximately 1032 erg s−1) during the terminal phase of class I or II of the pre-main-sequence stages of the Sun explains the isotopic properties, distinctive petrographic features and diffusivity constraints in the CAI. Li and Be isotopic measurements of calcium–aluminium inclusions (CAIs) in the Efremovka meteorite suggest that the solid and gaseous precursors of CAIs were irradiated by a superflare during the last stages of the pre-main-sequence evolution of the Sun.
... In detail, we require that reworking of chondrules involves not just remelting but interaction with a vapour atmosphere, to cause resetting of Mg isotope compositions to a chondrule reservoir value. For this to be consistent with the Pb-Pb measurements, Pb exchange (Bizzarro et al., 2004;Galy et al., 2000), other literature bulk chondrule analyses (open diamonds, see references in Fig. 4) and CAI (green filled squares (Bizzarro et al., 2004;Galy et al., 2000;Jacobsen et al., 2008;Larsen et al., 2011;Mishra and Chaussidon, 2014;Thrane et al., 2006;Wasserburg et al., 2012)). Dashed lines indicate mixing between a bulk chondritic composition (as defined by 'chondrule reservoir') and extreme CAI compositions, as can be seen more completely in (b). ...
We have analysed the petrography, major element abundances and bulk Al-Mg isotope systematics of 19 ferromagnesian chondrules from the CV3 chondrites Allende, Mokoia, and Vigarano, together with an Al-rich chondrule and refractory olivine from Mokoia. Co-variations of Al/Mg with Na/Mg and Ti/Mg in our bulk chondrules suggest their compositions are dominantly controlled by reworking of different proportions of chondrule components (e.g. mafic minerals and mesostatis); their precursors are thus fragments from prior generations of chondrules. Our samples show a range in fractionation corrected ²⁶Mg/²⁴Mg (Δ′²⁶Mg) ∼ 60 ppm, relative to precisions <±5 ppm (2se) and these values broadly covary with ²⁷Al/²⁴Mg. The data can be used to calculate model initial ²⁶Al/²⁷Al, or (²⁶Al/²⁷Al)0, of the chondrule precursors. Our resolvably radiogenic chondrules yield model (²⁶Al/²⁷Al)0 ∼ 1–2 × 10⁻⁵, equivalent to model “ages” of precursor formation ≦1 Ma post CAI. However, many of our chondrules show near solar Δ′²⁶Mg and no variability despite a range in ²⁷Al/²⁴Mg. This suggests their derivation either from younger precursor chondrules or open system behaviour once ²⁶Al was effectively extinct ((²⁶Al/²⁷Al)0 < 0.8 × 10⁻⁵, given the resolution here). Evidence for the latter explanation is provided by marked rims of orthopyroxene replacing olivine, indicating reaction of chondrules with a surrounding silicate vapour. Concurrent isotopic exchange of Mg with a near chondritic vapour during late reworking could explain their isotopic systematics. One ferromagnesian object is dominated by a high Mg# olivine with elevated Ti and Ca abundances. This refractory olivine has a markedly negative Δ′²⁶Mg = −16 ± 3 ppm (2se), reflecting its early removal (model age of <0.5 Ma post CAI), from a reservoir with evolving Δ′²⁶Mg. If representative of the chondrule forming region, this grain defines a minimum interval of radiogenic ingrowth for CV chondrites commensurate with (²⁶Al/²⁷Al)0 > 3.4 ± 0.6 × 10⁻⁵. Overall, our samples record a sequence of events from the formation of ferromagnesian objects within 0.5 Ma of CAI to re-equilibration of chondrules and silicate vapour >2 Ma post CAI, assuming an initially homogeneous ²⁶Al/²⁷Al. Metamorphism on the asteroid parent body may have played a subsequent role in affecting Mg isotope composition, but we argue this had a minor influence on the observations here.
... Hence, despite their phylogentic and mineralogical similarities and relatively small size, these objects form a very rare suite of objects that presented the extreme range of exotic isotopic, mineralogical properties that could help understand the early solar system event processes by forming a cog in the continuum from platy hibonite crystals (PLACs), spinel-hibonites (SHIBs), fractionation and unexplained nuclear effects (F/UN) CAIs to normal CAIs. These rare and exotic objects therefore provide records of unique situations/conditions in the early Solar system prior to the end of complete homogenization recorded by Type B CAIs (Mishra and Chaussidon 2014a;Chaussidon and Liu 2015). Table 1 S1). ...
Hibonite-pyroxene spherules are an extremely rare kind of refractory inclusion that show a wide range of exotic isotopic properties despite their defining similarity and simplicity in morphology and mineralogy. One such, relatively large (about 120 micron diameter), inclusion has been found in one of the most pristine meteorites, Allan Hills 77307 (a carbonaceous chondrite of the Ornans group; Petrologic type 3.03). The inclusion consists of two central hibonite laths of about 30x15 micron surrounded by Al, Ca-rich pyroxene. The hibonite laths have uniform composition. The composition of pyroxene surrounding the hibonite is radially homogenously Al,-Ca rich up to about 50-60 microns which transitions to Mg, -Ti rich at the outer boundary. Hibonite-pyroxene spherule found in ALHA 77307 shares many similarities with the other previously found hibonite-pyroxene spherules. A distinguishing feature of the inclusion in ALHA77307 is the presence of two slivers/ wedges at the opposite outer edge of the hibonite- pyroxene spherule that consist of rapidly, poorly crystalized, sub-micron minerals with pristine textures. The pristine petrography and mineralogy of this inclusion allow discernment of the expected general trend of formation and alteration amongst hibonite-pyroxene spherules.
... The uniformity of the initial 26 Al/ 27 Al ratios in those CAIs with canonical 26 Al abundance implies a very short formation duration. Estimates of this duration range from the very short, ∼ 2 × 10 4 years (Thrane et al. 2006;Mishra & Chaussidon 2014), to ∼ 0.3 Myr (MacPherson et al. 2012;Kita et al. 2013;MacPherson et al. 2017;Ushikubo et al. 2017). The time of formation of CAIs with canonical 26 Al abundance is taken as t = 0 for the solar system, and in the simulations we present here. ...
We present a comprehensive evolutionary model of the Sun's protoplanetary disk. The model predicts from first principles the gas densities and temperatures, and abundances of calcium-rich, aluminum-rich inclusions (CAIs) and refractory lithophile elements (Ca, Al, Ti, etc.). A central assumption is that Jupiter's core formed early ($< 1$ Myr) at 3 AU, opening a gap and creating a pressure maximum beyond it in which CAIs were trapped, thereby resolving the "CAI Storage Problem" of meteoritics. Carbonaceous chondrites formed in this pressure trap, while ordinary and enstatite chondrites formed from material inside Jupiter depleted in CAIs by aerodynamic drag. We match the model outputs at different times and locations to each of 11 chondrites, 5 achondrites, and the embryos of Earth and Mars, finding excellent agreement with known meteoritic constraints, and making specific, testable predictions where constraints are lacking. We predict the embryos of the terrestrial planets formed rapidly, in $< 2$ Myr, and that dynamical scattering of asteroids was limited. We predict CI chondrites are depleted in refractory elements relative to the Sun, by about 9\% (0.04 dex). The model demands low levels of turbulence ($\alpha \sim 10^{-4}$) inside 1 AU, falling to lower levels ($\sim 10^{-5}$) beyond 10 AU, suggesting angular momentum transport was dominated by hydrodynamical instabilities augmented by magnetic disk winds, and not by the magnetorotational instability. By 4 Myr, gas had vanished interior to Jupiter, but persisted beyond Jupiter, so that the solar nebula was a transition disk. The model demonstrates the power of meteoritic data to constrain astrophysical disk processes.
... Using these conditions and the appropriate cross sections (Supplementary Table 6), the coproduction of other short-lived radionuclides ( 26 Al, 41 Ca, and 53 Mn) is calculated (Supplementary Table 7). We confirm that 26 Al cannot be produced by irradiation to any meaningful extent 33 , with calculated 26 Al/ 27 Al ratios an order of magnitude lower than the canonical ratio of (5.23 ± 0.13) × 10 −5 (ref. 34 ). ...
X-ray emission in young stellar objects (YSOs) is orders of magnitude more intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of surrounding accretion disks. Protoplanetary disk irradiation has been detected around YSOs by the Herschel Space Observatory3. In our Solar System, short-lived 10Be (with a half-life of 1.39 Myr)4, which cannot be produced by stellar nucleosynthesis, was discovered in the oldest Solar System solids, the calcium–aluminium-rich inclusions (CAIs)5. The high 10Be abundance, as well as the detection of other tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused by solar flares8, 9, 10. Nevertheless, the nature of these flares (gradual or impulsive), the target (gas or dust), and the duration and location of irradiation remain unknown. Here we use the vanadium isotopic composition, together with the initial 10Be abundance to quantify irradiation conditions in the early Solar System11. For the initial 10Be abundances recorded in most CAIs, 50V excesses of a few per mil (‰) relative to chondrites have been predicted8,9. We report 50V excesses in CAIs up to 4.4‰ that co-vary with 10Be abundance. Their co-variation dictates that excess 50V and 10Be were synthesized through irradiation of refractory dust. Modelling of the production rate of 50V and 10Be demonstrates that the dust was exposed to solar cosmic rays produced by gradual flares for less than 300 years at ≈0.1 au from the protosun.
... Whole-rock 26 AlÀ 26 Mg isochron studies using multi-collector inductively coupled plasma source mass spectrometry (MC-ICPMS) indicate the initial 26 Al/ 27 Al ratio (( 26 Al/ 27 Al) 0 ) value of (5.23 ± 0.13) Â 10 À5 , called the canonical (Jacobsen et al., 2008;Larsen et al., 2011), which may reflect the ( 26 Al/ 27 Al) 0 value at the time of Al-Mg fractionation of the CAIs or their precursors by evaporation and condensation processes (MacPherson et al., 2010(MacPherson et al., , 2012. On the other hand, internal 26 AlÀ 26 Mg mineral isochrons for CAIs, estimated using secondary ion mass spectrometry (SIMS), show a range of ( 26 Al/ 27 Al) 0 values MacPherson et al., 2012;Mishra and Chaussidon, 2014;Kawasaki et al., 2015), possibly representing a range of formation ages. The data of MacPherson et al. (2012) show that melted CAIs tend to be younger than unmelted CAIs, suggesting the CAI melting events continued for at least 0.2 Myr after the formation of CAI with the canonical ratio. ...
Fluffy Type A Ca-Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be direct condensates from solar nebular gas. We conducted an investigation of 26Al−26Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals; we also conducted oxygen isotope measurements of coexisting minerals. Two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition, with δ25Mg becoming small along the inferred direction of crystal growth, which supports the idea that they originated through condensation. Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark-Lovering rim. The spinel enclosed by melilite has 16O-rich compositions (Δ17O ∼ −24‰) and an initial value of (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ17O ∼ −12‰ and −17‰) and plots on an isochron with (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ17O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16O-poor (Δ17O values larger than −10‰) to 16O-rich (Δ17O ∼ −25‰). The six reversely zoned melilite crystals show indistinguishable initial 26Al/27Al values with an average (26Al/27Al)0 of (4.7 ± 0.3) × 10−5, which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark-Lovering rim shows 16O-rich compositions (Δ17O ∼ −23‰) with (26Al/27Al)0 = (4.5 ± 0.4) × 10−5. The values of (26Al/27Al)0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18 ± 0.07 Myr from the difference in initial 26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16O-rich to 16O-poor and back to 16O-rich during the first ∼0.2 Myr of Solar System formation.
... A heterogeneous distribution of 60 Fe, and/or nickel isotopes in the early Solar system limits using 60 Fe abundance in terms of chronology. Several isotopic studies and theoretical models suggest a homogeneous early Solar system certainly well before formation of chondrules and as early as by the time of major episode of CAI formation (Boss et al., 2010;Dauphas and Chaussidon, 2011;Mishra and Chaussidon, 2014b;Villeneuve et al., 2009). Hence, a heterogeneous distribution of 60 Fe in the early Solar system cannot be ascribed as a reason for the observed discrepancy. ...
... Whole-rock 26 Mg isochron studies have determined the canonical initial ( 26 Al/ 27 Al) 0 value of $5 Â 10 À5 (Jacobsen et al., 2008;Larsen et al., 2011), which may reflect an initial ( 26 Al/ 27 Al) 0 value at the timing of separation of CAI solids from the solar nebular gas (MacPherson et al., 2012). On the other hand, internal 26 Al- 26 Mg mineral isochrons for CAIs estimated using SIMS show variable initial ( 26 Al/ 27 Al) 0 values for each CAI MacPherson et al., 2012;Mishra and Chaussidon, 2014), which may correspond to differences among each CAI formation age. Although the 26 Al- 26 Mg systematics of CAIs that experienced the multiple incomplete melting should have been reset by each melting and crystallization process, there are very few studies focused on the internal 26 Al- 26 Mg mineral isochrons of each process (Podosek et al., 1991;Hsu et al., 2000;MacPherson et al., 2012). ...
Disequilibrium oxygen isotopic distributions of Ca-Al-rich inclusions (CAIs) correspond to multiple melting events in the solar nebula. 26Al-26Mg systematics may be applicable for age differences among such melting events. We have carried out a coordinated study of detailed petrographic observations and in-situ oxygen and magnesium isotope measurements for a Type C CAI, EK1-04-2, from the Allende CV3 meteorite to determine the melting events and their ages. The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequence of the core minerals is from spinel, anorthite, olivine, and to pyroxene. The mantle has the same mineral assemblage as the core, and shows incomplete melting and solidification textures. Oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral (CCAM) line (δ18O = -44‰ to +9‰), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows a 16O-rich signature (δ18O ∼ -43‰), while anorthite is 16O-poor (δ18O ∼ +8‰). Olivine and pyroxene in the core have the same oxygen isotopic composition (δ18O ∼ -15‰), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16O (δ18O = -13‰ to -4‰) compared with the same minerals in the core. The 26Al-26Mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (26Al/27Al)0 = (3.5 ± 0.2) × 10-5, anorthite is plotted on a line of (-1 ± 5) × 10-7, and olivine and pyroxene in the core are plotted on a line of (-1 ± 7) × 10-6. Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core. This study indicates that the EK1-04-2 Type C CAI underwent multiple heating events after the formation of its CAI precursor. The precursor CAI was formed ∼0.4 Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6 Myr after the precursor CAI formation, the CAI was partially melted and the melt exchanged oxygen isotopes with surrounding 16O-poor nebular gas. 16O-poor olivine and pyroxene in the core crystallized from the melt. Subsequently, Al-rich chondrules accreted onto the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6 Myr and underwent multiple melting events in the nebula, and oxygen and 26Al-26Mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.
... These studies established a "canonical" value of $5 Â 10 À5 for the solar system (see review by MacPherson et al., 1995). In the last decade, multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) and multicollector secondary ion mass spectrometry (MC-SIMS) have made possible much higher precision magnesium isotopic analyses that provide chronologically useful data on low-Al/Mg phases in CAIs and in chondrules, and now allow study of the evolution of initial 26 Mg/ 24 Mg ratios (e.g., Villeneuve et al., 2009;MacPherson et al., 2012;Mishra and Chaussidon, 2014; see Davis and McKeegan, 2014, for a review). ...
... This isochron is, within errors, indistinguishable from the Solar System Initial proposed from 26 Al studies of various types of CAIs and chondrules (( 26 Al/ 27 Al) SSI = (5.62 ± 0.42) Â 10 À5 and d 26 Mg * SSI = À0.052 ± 0.013&; Mishra and Chaussidon, 2014) and from the isochron for bulk Allende CAIs (( 26 Al/ 27 Al) 0 = (5.25 ± 0.12) Â 10 À5 and d 26 Mg * 0 = À0.034 ± 0.032&) calculated from the data of Jacobsen et al. (2008) and Larsen et al. (2011). ...
Nitrogen and noble gas (Ne-Ar) abundances and isotope ratios, determined by CO2 laser extraction static mass spectrometry analysis, as well as Al-Mg and O isotope data from secondary ion mass spectrometry (SIMS) analyses, are reported for a type B calcium-aluminum-rich inclusion (CAI) from the CV3 chondrite NWA 8616. The high (26Al/27Al)i ratio of (5.06 ± 0.50) × 10-5 dates the last melting event of the CAI at ka after “time zero”, limiting the period during which high-temperature exchanges between the CAI and the nebular gas could have occurred to a very short time interval. Partial isotopic exchange with a 16O-poor reservoir resulted in Δ17O > -5‰ for melilite and anorthite, whereas spinel and Al-Ti-pyroxene retain the inferred original 16O-rich signature of the solar nebula (Δ17O ⩽ -20 ‰). The low 20Ne/22Ne (⩽0.83) and 36Ar/38Ar (⩽0.75) ratios of the CAI rule out the presence of any trapped planetary or solar noble gases. Cosmogenic 21Ne and 38Ar abundances are consistent with a cosmic ray exposure (CRE) age of ∼14 to 20 Ma, assuming CR fluxes similar to modern ones, without any evidence for pre-irradiation of the CAI before incorporation into the meteorite parent body. Strikingly, the CAI contains 1.4 to 3.4 ppm N with a δ15N value of +8 to +30 ‰. Even after correcting the measured δ15N values for cosmogenic 15N produced in situ, the CAI is highly enriched in 15N compared to the protosolar nebula (δ15NPSN = -383 ± 8 ‰; Marty et al., 2011), implying that the CAI-forming region was contaminated by 15N-rich material within the first 0.15 Ma of Solar System history, or, alternatively, that the CAI was ejected into the outer Solar System where it interacted with a 15N-rich reservoir.
... This is consistent with the evidence for nucleosynthetic Mg isotope anomalies in CAI characterized by canonical 26 Al/ 27 Al (Wasserburg et al., 2012). Thus, it appears that variations in initial 26 Al/ 27 Al ratios of bulk meteorites and most meteorite components have chronological significance (e.g., Bouvier et al., 2011b;Mishra and Chaussidon, 2014;Villeneuve et al., 2009;Wasserburg et al., 2012), whereas small variations in 26 Mg/ 24 Mg among bulk meteorites may have not. ...
Most meteoritic calcium-rich, aluminum-rich inclusions (CAIs) formed from a reservoir with ${}^{26}{\rm Al}/{}^{27}{\rm Al} \approx 5 \times 10^{-5}$, but some record lower $({}^{26}{\rm Al}/{}^{27}{\rm Al})_0$, demanding they sampled a reservoir without live ${}^{26}{\rm Al}$. This has been interpreted as evidence for "late injection" of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them "Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions" (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel or hibonite grains $0.5 - 2 \, \mu{\rm m}$ in size with no live ${}^{26}{\rm Al}$; live ${}^{26}{\rm Al}$ is carried on smaller ($<$50 nm) presolar chromium spinel grains from recent nearby Wolf-Rayet stars or supernovae. In hot ($\approx$ 1350-1425 K) regions of the disk these grains, and perovskite grains, would be the only survivors. These negatively charged grains would grow to sizes $1 - 10^3 \, \mu{\rm m}$, even incorporating positively charged perovskite grains, but not the small, negatively charged ${}^{26}{\rm Al}$-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ${}^{48}{\rm Ca}$ and ${}^{50}{\rm Ti}$ anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ${}^{26}{\rm Al}$ in a solar nebula with uniform, canonical ${}^{26}{\rm Al}/{}^{27}{\rm Al}$. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.
Igneous processes were quite widespread in the small bodies of the Solar System (SBSS) and were initially fueled by short-lived radioisotopes, the proto-Sun, impact heating, and differentiation heating. Once they finished, long-lived radioisotopes continued to warm the active bodies of the Earth, (possibly) Venus, and the cryovolcanism of Enceladus.The widespread presence of olivine and pyroxenes in planets and also in SBSS suggests that they were not necessarily the product of igneous processes and they might have been recycled from previous nebular processes or entrained in comets from interstellar space. The difference in temperature between the inner and the outer Solar System has clearly favored thermal annealing of the olivine close to the proto-Sun. Transport of olivine within the Solar System probably occurred also due to protostellar jets and winds but the entrainment in SBSS from interstellar space would overcome the requirement of initial turbulent regime in the protoplanetary nebula.
Several short-lived radionuclides (SLRs) were present in the first few million years of Solar System history. Their abundances have profound impact on the timing of stellar nucleosynthesis events prior to Solar System formation, chronology of events in the early Solar System, early solar activity, heating of early-formed planetesimals, and chronology of planet formation. Isotopic analytical techniques have undergone dramatic improvements in the past decade, leading to tighter constraints on the levels of SLRs in the early Solar System and on the use of these nuclides for detailed chronological studies. This review emphasizes the abundances of SLRs when the Solar System formed and how we know them, and briefly discusses the origins of these nuclides and applications in planetary science.
Expected final online publication date for the Annual Review of Nuclear and Particle Science, Volume 72 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic ²⁶Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal ²⁶Al-²⁶Mg isochrons provide initial ²⁶Al/²⁷Al ratios from 5.46 (± 0.38) × 10⁻⁵ to 6.14 (± 0.92) × 10⁻⁵ for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (± 0.08) × 10⁻⁵ to 1.87 (± 0.92) × 10⁻⁵ for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the ²⁵Mg/²⁴Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial ²⁶Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.
We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial 26Al/27Al ratios, (26Al/27Al)0, of these CO CAIs are found to be (4.8 ± 0.5) × 10^–5 and (4.9 ± 0.3) × 10^–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in 10B, ranging from ~370 to ~4300‰ relative to the chondritic B isotopic composition, which are correlated well with 9Be/11B ratios. The correlation indicates in situ decay of 10Be in the present CO CAIs and yields initial 10Be/9Be ratios, (10Be/9Be)0, for the individual CAIs of (2.9 ± 0.6)�× 10^–3 and (2.2 ± 1.0)�× 10^–3 (95% confidence), which are significantly greater than the average (10Be/9Be)0 = ~0.7 ×�10^–3 recorded in CAIs from CV chondrites. The apparent variation in (10Be/9Be)0 between the CO and CV CAIs, despite having indistinguishable (26Al/27Al)0 of 5 ×�10^–5, provides evidence for heterogeneous distribution of 10Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (10Be/9Be)0 and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where 10Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and 26Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (~0.2 million years) inferred from the Al-Mg chronology.
Increasing evidences suggest that the building blocks of Ca-Al-rich inclusions (CAIs) could have formed with the Sun, during the collapse of the parent cloud. However, determination of the relative age of CAIs relies on the homogeneous distribution of their short-lived radionuclide ²⁶ Al that is used as a chronometer. Some CAIs show evidence of ²⁶ Al/ ²⁷ Al variation that is independent of decay.
We investigate the dynamical and chemical evolution of refractories from the collapsing cloud to their transport in the protoplanetary disk focusing to the predicted isotopic anomalies resulting from ²⁶ Al heterogeneities.
The interplay between the thermal properties of the dust, the isotopic zoning in the cloud and disk dynamics produce aggregates that resemble chondrites. An abrupt raise of ²⁶ Al close the center of the cloud followed by a plateau throughout the cloud best matches the observations. As a consequence, the ²⁶ Al -chronometer retains validity from the formation of canonical CAIs onward.
Chondrites are made of a mixture of solids formed at high and low temperatures. This heterogeneity was thought to be produced by large scale transport processes in the Sun’s isolated accretion disk. However, mounting evidences suggest that refractory inclusions in chondrites were produced together with the disk formation.
We present numerical simulations of the formation and transport of rocky materials during the collapse of the Solar Nebula’s parent cloud and the consequent disk assembling.
We find that the interplay between the cloud collapse, the dynamics of gas and dust and thermal processing of different species in the disk, results in a local mixing of solids with different thermal histories. Our simulations return an heterogeneous distribution of refractory material with higher concentration in the outer disk. This refractory material has a short formation timescales, during the first tens of kyr of the Sun (class 0-I). Our results open new frontiers into the origin of the compositional diversity of chondrites.
Chondrules from unequilibrated ordinary and carbonaceous chondrites belong to the oldest and most primitive materials from the early solar system and record chemical and isotopic signatures relating to their formation and evolution. These signatures allow tracing protoplanetary disk processes that eventually led to the formation of planetary building blocks and rocky planets. 26Al-26Mg ages based on mineral-mesostasis isochrons of 31 porphyritic ferromagnesian chondrules, that belong mainly to type-II, constrain the time of chondrule melting prior to incorporation into the respective chondrite parent bodies. For this study chondrules from the unequilibrated L, L(LL) and LL ordinary chondrites (UOCs) NWA 5206, NWA 8276, MET 96503, MET 00452, MET 00526, NWA 7936 and QUE 97008 were selected, which are of petrologic types 3.00-3.15 and were thus least metamorphosed after formation. Magnesium and Al isotopes were measured in-situ by Secondary Ion Mass Spectrometry (SIMS) using a CAMECA 1280 ims. 26Mg excess from in-situ decay of 26Al correlating with 27Al/24Mg has been detected in the mesostasis of all but one chondrule. The initial Al isotopic compositions (26Al/27Al)0 and 26Mg/24Mg ratios (d26Mg*0) deduced from internal mineral isochron regressions range from (9.5 ± 2.8) × 10-6 to (3.1 ± 1.2) × 10-6 and -0.020 ± 0.028‰ to 0.011 ± 0.039‰, respectively. The corresponding chondrule ages (∆tCAI), calculated relative to calcium-aluminum-rich inclusions (CAIs) using the canonical 26Al/27Al = (5.23 ± 0.13) × 10-5, are between 1.76_(-0.27)^(+0.36) and 2.92_(-0.34)^(+0.51) Ma and date the melt formation and thus primary chondrule formation from dust-like precursors or reprocessing of older chondrules. The age range agrees with those acquired with different short-lived chronometers and with published 26Al-26Mg ages, the majority of which were obtained for chondrules from the Bishunpur and Semarkona meteorites, although no chondrule with (26Al/27Al)0 > 10-5 was found.
Chondrules in single chondrite samples or between different chondrite groups show no distinct age distributions. The initial 26Al/27Al of the oldest chondrules in the L(LL)/LL and L chondrite samples are identical within their 1σ uncertainties and yield a mean age of 1.99_(-0.08)^(+0.08) Ma and 1.81_(-0.10)^(+0.11) Ma, respectively. The oldest chondrules from six of the seven studied samples record a mean age of 1.94_(-0.06)^(+0.07) Ma. Since heating events in the protoplanetary disk could have partially reset the Al-Mg systematics in pre-existing chondrules and this would have shifted recorded 26Al-26Mg ages toward younger dates, the oldest mean age of 1.81_(-0.10)^(+0.11) Ma recorded in L chondrite chondrules is interpreted to date the rapid and punctuated onset of chondrule formation. The density distribution of chondrule ages from this study, which comprises the largest single dataset of OC chondrule ages, combined with published ages for chondrules from ordinary and carbonaceous chondrites reveals major age peaks for OC chondrules at 2.0 and 2.3 Ma. Chondrules in ordinary and carbonaceous chondrites formed almost contemporaneously (with a possible distinction between CC groups) in two chemically distinct reservoirs, probably in density-enriched regions at the edges of Jupiter’s orbit. The young formation ages of chondrules suggest that they do not represent precursors but rather by-products of planetesimal accretion.
Oxygen, magnesium, and silicon isotopic compositions in the mineralogically complex, ultrarefractory (UR) calcium-aluminum-rich inclusion (CAI) E101.1 from the reduced CV3 chondrite Efremovka confirm that E101.1 is a compound CAI composed of several lithological units that were once individual CAIs, free-floating in the solar protoplanetary disk. Each precursor unit was found to have had its own thermal history prior to being captured and incorporated into the partially molten host CAI.
Four major lithological units can be distinguished on the basis of their isotopic compositions. (1) Al-diopside-rich sinuous fragments, hereafter sinuous pyroxene, are ¹⁶O-rich (Δ¹⁷O ≤ -20‰) and have light Mg and Si isotopic compositions with mass fractionation down to -3.5‰/amu for both isotopic systems. We attribute these peculiar isotopic compositions to kinetic effects during condensation out of thermal equilibrium. (2) Spinel clusters are ¹⁶O-rich (Δ¹⁷O ∼ -22‰) and have Mg isotope systematics consistent with extensive equilibration with the host melt. This includes (i) δ²⁵Mg values varying between +2.6 ‰ and +6.5 ‰ close to the typical value of host melilite at ∼+5‰, and (ii) evidence for exchange of radiogenic ²⁶Mg with adjacent melilite as indicated by Al/Mg systematics. The spinel clusters may represent fine-grained spinel-rich proto-CAIs captured, partially melted, and recrystallized in the host melt. Al/Mg systematics indicate that both the sinuous pyroxene fragments and spinel clusters probably had canonical or near-canonical ²⁶Al contents before partial equilibration. (3) The main CAI host (Δ¹⁷O ≤ -2‰) had a complex thermal history partially obscured by subsequent capture and assimilation events. Its formation, referred to as the “cryptic” stage, could have resulted from the partial melting and crystallization of a ¹⁶O-rich precursor that underwent ¹⁶O-depletion and a massive evaporation event characteristic of F and FUN CAIs (Fractionated with Unknown Nuclear effects). Alternatively, a ¹⁶O-rich UR precursor may have coagulated with a ¹⁶O-poor FUN CAI having ⁴⁸Ca anomalies, as indicated by perovskite, before subsequent extensive melting. The Al/Mg systematics (2.4 × 10⁻⁵ ≤ (26Al/27Al)0′ ≤ 5.4 × 10⁻⁵, where (26Al/27Al)0′ is a model initial ²⁶Al/²⁷Al ratio per analysis spot) are best understood if the FUN component was ²⁶Al-poor, as are many FUN CAIs. (4) A complete Wark-Lovering rim (WLR) surrounds E101.1. Its Mg and Si isotopic compositions indicate that it formed by interaction of the evaporated interior CAI with an unfractionated ¹⁶O-rich condensate component. Heterogeneities in ²⁶Al content in WLR spinels (3.7 × 10⁻⁵ ≤ (26Al/27Al)0′ ≤ 5.7 × 10⁻⁵) suggest that the previously reported age difference of as much as 300,000 years between interior CAIs and their WLRs may be an artifact resulting from Mg isotopic perturbations, possibly by solid state diffusion or mixing between the interior and condensate components.
The isotopic systematics of E101.1 imply that ¹⁶O-rich and ¹⁶O-poor reservoirs co-existed in the earliest solar protoplanetary disk and that igneous CAIs experienced a ¹⁶O-depletion in an early high temperature stage. The coagulation of various lithological units in E101.1 and their partial assimilation supports models of CAI growth by competing fragmentation and coagulation in a partially molten state. Our results suggest that chemical and isotopic heterogeneities of unclear origin in regular CAIs may result from such a complex aggregation history masked by subsequent melting and recrystallization.
Calcium-aluminum-rich inclusions with isotopic mass fractionation effects and unidentified nuclear isotopic anomalies (FUN CAIs) have been studied for more than 40 years, but their origins remain enigmatic. Here we report in situ high precision measurements of aluminum-magnesium isotope systematics of FUN CAIs by secondary ion mass spectrometry (SIMS). Individual minerals were analyzed in six FUN CAIs from the oxidized CV3 carbonaceous chondrites Axtell (compact Type A CAI Axtell 2271) and Allende (Type B CAIs C1 and EK1-4-1, and forsterite-bearing Type B CAIs BG82DH8, CG-14, and TE). Most of these CAIs show evidence for excess ²⁶Mg due to the decay of ²⁶Al. The inferred initial ²⁶Al/²⁷Al ratios [(²⁶Al/²⁷Al)0] and the initial magnesium isotopic compositions (δ²⁶Mg0) calculated using an exponential law with an exponent β of 0.5128 are (3.1±1.6)×10⁻⁶ and 0.60±0.10‰ (Axtell 2271), (3.7±1.5)×10⁻⁶ and −0.20±0.05‰ (BG82DH8), (2.2±1.1)×10⁻⁶ and −0.18±0.05‰ (C1), (2.3±2.4)×10⁻⁵ and −2.23±0.37‰ (EK1-4-1), (1.5±1.1)×10⁻⁵ and −0.42±0.18‰ (CG-14), and (5.3±0.9)×10⁻⁵ and −0.05±0.08‰ (TE) with 2σ uncertainties. We infer that FUN CAIs recorded ²⁶Al and magnesium isotopic heterogeneity in the CAI-forming region(s). Comparison of ²⁶Al-²⁶Mg systematics, stable isotope (oxygen, magnesium, calcium, and titanium) and trace element studies of FUN and non-FUN igneous CAIs indicates that there is a continuum among these CAI types. Based on these observations and evaporation experiments on CAI-like melts, we propose a generic scenario for the origin of igneous (FUN and non-FUN) CAIs: (i) condensation of isotopically normal solids in an ¹⁶O-rich gas of approximately solar composition; (ii) formation of CAI precursors by aggregation of these solids together with variable abundances of isotopically anomalous grains—possible carriers of unidentified nuclear (UN) effects; and (iii) melt evaporation of these precursors accompanied by crystallization under different temperatures and gas pressures, leading to the observed variations in mass-dependent isotopic fractionation (F) effects.
Two key questions in Solar System formation concern the timescales of high-temperature processing (e.g., formations of solids, temperature fluctuations in the disk, etc.) in the early evolutionary stages, and the astrophysical environment in which the solar protoplanetary disk resided. Astrophysical theories of stellar evolution and astronomical observations of young stellar objects, analog to the forming Sun, provide some constraints on the lifetimes of their different evolutionary stages. Finer scale chronologies of the formation of the first solids and planetary objects in the solar accretion disk can be established from analyses of daughter isotopes from now-extinct, short-lived radionuclides in meteorites and in their components. In this review, we describe the high-temperature components of primitive meteorites, namely Ca-Al-rich Inclusions (CAI) and chondrules, we summarize the current knowledge on the origin of short-lived radionuclides, and we compare the two types of chronologies, the "astrophysical one" (derived from observations of young stellar objects and their disks) and the "meteoritical one" (derived from isotopic analyses of meteorites). Within the first few millions years, most of the mass of the solids, which will be at the origin of the terrestrial planets, were formed.
Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26Al decay to 26Mg and mass dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26Mg/24Mg due to 26Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26Mg/24Mg and 25Mg/24Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25Mg/24Mg ratio can be used to correct the 26Mg/24Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25Mg/24Mg vs. log 26Mg/24Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26Al-26Mg chronology.
A key test of the supernova triggering and injection hypothesis for the
origin of the solar system's short-lived radioisotopes is to reproduce the
inferred initial abundances of these isotopes. We present here the most
detailed models to date of the shock wave triggering and injection process,
where shock waves with varied properties strike fully three dimensional,
rotating, dense cloud cores. The models are calculated with the FLASH adaptive
mesh hydrodynamics code. Three different outcomes can result: triggered
collapse leading to fragmentation into a multiple protostar system; triggered
collapse leading to a single protostar embedded in a protostellar disk; or
failure to undergo dynamic collapse. Shock wave material is injected into the
collapsing clouds through Rayleigh-Taylor fingers, resulting in initially
inhomogeneous distributions in the protostars and protostellar disks. Cloud
rotation about an axis aligned with the shock propagation direction does not
increase the injection efficiency appreciably, as the shock parameters were
chosen to be optimal for injection even in the absence of rotation. For a shock
wave from a core-collapse supernova, the dilution factors for supernova
material are in the range of $\sim 10^{-4}$ to $\sim 3 \times 10^{-4}$, in
agreement with recent laboratory estimates of the required amount of dilution
for $^{60}$Fe and $^{26}$Al. We conclude that a type II supernova remains as a
promising candidate for synthesizing the solar system's short-lived
radioisotopes shortly before their injection into the presolar cloud core by
the supernova's remnant shock wave.
The short-lived now-extinct nuclide 60 Fe, present in the early Solar System, is a unique product of stellar nucleosynthesis. Even though the first hint for its presence in the early Solar System was obtained more than two decades back, a robust value for Solar System initial (SSI) 60 Fe/ 56 Fe is yet to be established. A combined study of 26 Al– 26 Mg and 60 Fe– 60 Ni isotope systematics in chondrules from unequilibrated ordinary chondrites of low petrologic type, Semarkona (LL3.0), LEW 86134 (L3.0), and Y 791324 (L3.1), has been conducted to infer the value of SSI 60 Fe/ 56 Fe. Seven of the analysed chondrules host resolved radiogenic excess in both 60 Ni and 26 Mg resulting from in situ decay of the short-lived nuclides 60 Fe and 26 Al, respectively. The initial 26 Al/ 27 Al values for these chondrules range from (6.9 ± 5.8) Â 10 À6 to (3.01 ± 1.78) Â 10 À5 that suggest their formation between 2.1 and 0.6 Ma after CAIs. The initial 60 Fe/ 56 Fe at the time of formation of these chondrules ranges from (3.2 ± 1.3) Â 10 À7 to (1.12 ± 0.39) Â 10 À6 and show a good correlation with their initial 26 Al/ 27 Al values suggesting co-injection of the two short-lived nuclides, 60 Fe and 26 Al, into the protosolar cloud from the same stellar source. Considering 26 Al as a reliable early Solar System chronometer, this data set yield a SSI 60 Fe/ 56 Fe value of (7.0 ± 1.2) Â 10 À7 , if we adopt a half-life value of 2.6 Ma for 60 Fe reported in a recent study. Model stellar nucleosynthesis yields suggest that both a high mass (5–6.5 M) Asymptotic Giant Branch (AGB) star or a supernova (SN) could be the source of 60 Fe and 26 Al present in the early Solar System. A high mass ($25 M) SN appears more plausible because of the much higher probability of its close association with the protosolar molecular cloud than a high mass AGB star. Such a SN can also account for SSI abundance of 26 Al and its correlated presence with 60 Fe in chondrules.
The precision of the 26Al–26Mg system—one of the most widely used chronometers for constraining the relative timing of events in the early solar system—is presently limited by methods for the determination of 27Al/24Mg ratios, which have seen little improvement in the last decade. We present a novel method for the measurement of 27Al/24Mg ratios in unpurified sample solutions by multiple-collector inductively coupled plasma mass spectrometry. Because Al is monoisotopic we use a modified isotope dilution approach that employs a mixed spike containing isotopically enriched 25Mg and natural 27Al in an accurately known ratio. In order to determine the spike to sample ratio for Al, measurements of spiked aliquots are bracketed by unspiked aliquots, which negates the impact of elemental bias. Unlike conventional isotope dilution, samples do not require chromatographic separation prior to analysis, which both saves time and minimises the risk of contamination of other samples with spike (which is added immediately prior to analysis). Repeat measurements of the BHVO-2, BCR-2, and BIR-1 international rock standards, as well as a gravimetrically prepared Al–Mg reference solution, indicate that our method is both accurate and reproducible to 0.2%. This 4- to 10-fold improvement over previous methods translates directly to an equal gain in the resolution of the 26Al–26Mg chronometer. The approach presented here could, in principle, be applied to other monoisotopic elements such as the Mn–Cr system. Based on multiple measurements of a 2.7 gram piece of the Ivuna CI chondrite, we present a new estimate for the 27Al/24Mg ratio of this meteorite of 0.09781 ± 0.00029.
We report novel methods for the chemical purification of Mg from silicate rocks by ion-exchange chromatography, and high-precision analysis of Mg-isotopes by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS). Using these methods, we have measured the relative and absolute Mg-isotope composition of a number of terrestrial and extraterrestrial materials, including international reference rock standards as well as pure Mg standards, olivine crystals separated from a mantle-derived spinel lherzolite (J12 olivine), one enstatite chondrite, a martian shergottite and sea water samples. Repeated analyses of terrestrial and extraterrestrial samples demonstrate that it is possible to routinely measure the relative Mg-isotope composition of silicate materials with an external reproducibility of 2.5 and 20 ppm for the μ26Mg* and μ25Mg values, respectively (μ notation is the per 106 deviation from a reference material). Analyses of bulk mantle-derived rocks as well as a martian shergottite and an enstatite chondrite define a restricted range in μ25Mg of −120 ± 28 ppm (2sd) relative to the DSM-3 reference standard (μ25,26Mg = 0), suggesting that the Mg-isotope composition of inner solar system bulk planetary materials is uniform within the resolution of our analyses. We have determined the absolute Mg-isotope composition of the J12 olivine, two CI chondrites as well as the DSM-3 and Cambridge-1 reference standards using a mixed 26Mg-24Mg double-spike. The differences between the absolute 25Mg/24Mg ratios of the various materials analyzed relative to the DSM-3 standard are in excellent agreement with results obtained by the sample-standard bracketing method. Based on the averages obtained for the J12 olivine separates, we estimate the absolute Mg-isotope composition for Earth's mantle – and hence that of the bulk silicate Earth – to be 25Mg/24Mg = 0.126896 ± 0.000025 and 26Mg/24Mg = 0.139652 ± 0.000033. Given the restricted range of μ25Mg obtained for bulk planetary material by the sample-standard bracketing technique and the excellent agreement between the data obtained by the relative and absolute methods, we propose that these new values represent the absolute Mg-isotope composition of the bulk inner solar system. Using the absolute Mg-isotope composition of the J12 olivine, we calculate the isotopic abundances of Mg as 24Mg = 0.789548 ± 0.000026, 25Mg = 0.100190 ± 0.000018, and 26Mg = 0.110261 ± 0.000023. Based on this result, we have calculated an atomic weight for Mg of 24.305565 ± 0.000045, which is marginally heavier than previous estimates but a factor of 10 more precise.
A variety of mass fractionation laws have been used to correct Mg isotopic data for natural mass fractionation effects. Using evaporation experiments, we have determined the proper law to use and we explore effects on 26Al-26Mg systematics.
Refractory inclusions [calcium-aluminum-rich inclusions, (CAIs)] represent the oldest Solar System solids and provide information regarding the formation of the Sun and its protoplanetary disk. CAIs contain evidence of now extinct short-lived radioisotopes (e.g., (26)Al, (41)Ca, and (182)Hf) synthesized in one or multiple stars and added to the protosolar molecular cloud before or during its collapse. Understanding how and when short-lived radioisotopes were added to the Solar System is necessary to assess their validity as chronometers and constrain the birthplace of the Sun. Whereas most CAIs formed with the canonical abundance of (26)Al corresponding to (26)Al/(27)Al of ∼5 × 10(-5), rare CAIs with fractionation and unidentified nuclear isotope effects (FUN CAIs) record nucleosynthetic isotopic heterogeneity and (26)Al/(27)Al of <5 × 10(-6), possibly reflecting their formation before canonical CAIs. Thus, FUN CAIs may provide a unique window into the earliest Solar System, including the origin of short-lived radioisotopes. However, their chronology is unknown. Using the (182)Hf-(182)W chronometer, we show that a FUN CAI recording a condensation origin from a solar gas formed coevally with canonical CAIs, but with (26)Al/(27)Al of ∼3 × 10(-6). The decoupling between (182)Hf and (26)Al requires distinct stellar origins: steady-state galactic stellar nucleosynthesis for (182)Hf and late-stage contamination of the protosolar molecular cloud by a massive star(s) for (26)Al. Admixing of stellar-derived (26)Al to the protoplanetary disk occurred during the epoch of CAI formation and, therefore, the (26)Al-(26)Mg systematics of CAIs cannot be used to define their formation interval. In contrast, our results support (182)Hf homogeneity and chronological significance of the (182)Hf-(182)W clock.
The possibility of establishing an accurate relative chronology of the early solar system events based on the
decay of short-lived 26Al to 26Mg (half-life of 0.72 Myr) depends on the level of homogeneity (or
heterogeneity) of 26Al and Mg isotopes. However, this level is difficult to constrain precisely because of
the very high precision needed for the determination of isotopic ratios, typically of �5 ppm. In this
study, we report for the first time a detailed analytical protocol developed for high precision in situ Mg
isotopic measurements (25Mg/24Mg and 26Mg/24Mg ratios, as well as 26Mg excess) by MC-SIMS. As the
data reduction process is critical for both accuracy and precision of the final isotopic results, factors such
as the Faraday cup (FC) background drift and matrix effects on instrumental fractionation have been
investigated. Indeed these instrumental effects impacting the measured Mg-isotope ratios can be as
large or larger than the variations we are looking for to constrain the initial distribution of 26Al and Mg
isotopes in the early solar system. Our results show that they definitely are limiting factors regarding the
precision of Mg isotopic compositions, and that an under- or over-correction of both FC background
instabilities and instrumental isotopic fractionation leads to important bias on d25Mg, d26Mg and D26Mg
values (for example, olivines not corrected for FC background drifts display D26Mg values that can differ
by as much as 10 ppm from the truly corrected value). The new data reduction process described here
can then be applied to meteoritic samples (components of chondritic meteorites for instance) to
accurately establish their relative chronology of formation.
Protostars emit more x-rays, hard and soft, than young sunlike stars in more advanced stages of formation. The x-ray emission
becomes harder and stronger during flares. The excess x-rays may arise as a result of the time-dependent interaction of an
accretion disk with the magnetosphere of the central star. Flares produced by such fluctuations have important implications
for the x-wind model of protostellar jets, for the flash-heating of the chondrules found in chondritic meteorites, and for
the production of short-lived radioactivities through the bombardment of primitive rocks by solar cosmic rays.
Transient heating events that formed calcium-aluminum–rich inclusions (CAIs) and chondrules are fundamental processes in the
evolution of the solar protoplanetary disk, but their chronology is not understood. Using U-corrected Pb-Pb dating, we determined
absolute ages of individual CAIs and chondrules from primitive meteorites. CAIs define a brief formation interval corresponding
to an age of 4567.30 ± 0.16 million years (My), whereas chondrule ages range from 4567.32 ± 0.42 to 4564.71 ± 0.30 My. These
data refute the long-held view of an age gap between CAIs and chondrules and, instead, indicate that chondrule formation started
contemporaneously with CAIs and lasted ~3 My. This time scale is similar to disk lifetimes inferred from astronomical observations,
suggesting that the formation of CAIs and chondrules reflects a process intrinsically linked to the secular evolution of accretionary
disks.
A large number of Ca-Al-rich refractory inclusions in primitive meteorites incorporated the short-lived nuclide 26Al at the time of their formation with an initial 26Al/27Al ratio of 5×10−5. However, there exist inclusions and refractory phases like corundum and hibonite in these meteorites that have no evidence of 26Al. A suite of refractory inclusions from these meteorites also have well-defined initial 26Al/27Al ratios that are lower by factors of 5-1000 compared to the canonical value of 5×10−5. Refractory phases free of 26Al are also devoid of the short-lived nuclide,41Ca. Heterogeneous distribution of the short-lived nuclides in the early solar system and possible formation of refractory inclusions from large clumps of interstellar matter devoid of 26Al are among the suggestions made to explain these observations. In this Letter, we suggest that the refractory phases devoid of 26Al and 41Ca represent some of the first solar system solids that formed in the inner region of the solar nebula prior to the injection of these freshly synthesized nuclides from a stellar source into this region.
It is believed that 26Al, a short-lived (t 1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent solar system (SS) with the initial 26Al/27Al ratio of ~5.2 × 10–5, suggesting an external, stellar origin rather than local, solar source. However, the stellar source of 26Al and the manner in which it was injected into the SS remain controversial: the 26Al could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud, protosolar cloud core, or protoplanetary disk. Corundum (Al2O3) is predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from 16O-rich (Δ17O ~ –25‰) gas of solar composition recorded heterogeneous distribution of 26Al at the birth of the SS: the inferred initial 26Al/27Al ratio ranges from ~6.5×10–5 to <2×10–6; 52% of corundum grains measured are 26Al-poor. Abundant 26Al-poor, 16O-rich refractory objects include grossite- and hibonite-rich calcium-aluminum-rich inclusions (CAIs) in CH (high metal abundance and high iron concentration) chondrites, platy hibonite crystals in CM (Mighei-like) chondrites, and CAIs with fractionation and unidentified nuclear effects CAIs chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory 16O-rich solids in the SS, we infer that 26Al was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between 26Al abundance and O-isotope composition of corundum grains constrains the stellar source of 26Al in the SS.
The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star-formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M ☉ of stars per Myr. The star-formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M ☉ pc–3. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M ☉, given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T bol, the values for L bol are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.
This paper reviews our current understanding of terrestrial planets
formation. The focus is on computer simulations of the dynamical aspects of the
accretion process. Throughout the chapter, we combine the results of these
theoretical models with geochemical, cosmochemical and chronological
constraints, in order to outline a comprehensive scenario of the early
evolution of our Solar System. Given that the giant planets formed first in the
protoplanetary disk, we stress the sensitive dependence of the terrestrial
planet accretion process on the orbital architecture of the giant planets and
on their evolution. This suggests a great diversity among the terrestrial
planets populations in extrasolar systems. Issues such as the cause for the
different masses and accretion timescales between Mars and the Earth and the
origin of water (and other volatiles) on our planet are discussed at depth.
With a half-life of 0.73 Myr, the 26Al-to-26Mg decay system is the most widely used short-lived chronometer for understanding the formation and earliest evolution of the solar protoplanetary disk. However, the validity of 26Al-26Mg ages of meteorites and their components relies on the critical assumption that the canonical 26Al/27Al ratio of ~5 × 10–5 recorded by the oldest dated solids, calcium-aluminium-rich inclusions (CAIs), represents the initial abundance of 26Al for the solar system as a whole. Here, we report high-precision Mg-isotope measurements of inner solar system solids, asteroids, and planets demonstrating the existence of widespread heterogeneity in the mass-independent 26Mg composition (μ26Mg*) of bulk solar system reservoirs with solar or near-solar Al/Mg ratios. This variability may represent heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk at the time of CAI formation and/or Mg-isotope heterogeneity. By comparing the U-Pb and 26Al-26Mg ages of pristine solar system materials, we infer that the bulk of the μ26Mg* variability reflects heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk. We conclude that the canonical value of ~5 × 10–5 represents the average initial abundance of 26Al only in the CAI-forming region, and that large-scale heterogeneity—perhaps up to 80% of the canonical value—may have existed throughout the inner solar system. If correct, our interpretation of the Mg-isotope composition of inner solar system objects precludes the use of the 26Al-26Mg system as an accurate early solar system chronometer.
Meteorites, which are remnants of solar system formation, provide a direct
glimpse into the dynamics and evolution of a young stellar object (YSO), namely
our Sun. Much of our knowledge about the astrophysical context of the birth of
the Sun, the chronology of planetary growth from micrometer-sized dust to
terrestrial planets, and the activity of the young Sun comes from the study of
extinct radionuclides such as 26Al (t1/2 = 0.717 Myr). Here we review how the
signatures of extinct radionuclides (short-lived isotopes that were present
when the solar system formed and that have now decayed below detection level)
in planetary materials influence the current paradigm of solar system
formation. Particular attention is given to tying meteorite measurements to
remote astronomical observations of YSOs and modeling efforts. Some extinct
radionuclides were inherited from the long-term chemical evolution of the
Galaxy, others were injected into the solar system by a nearby supernova, and
some were produced by particle irradiation from the T-Tauri Sun. The chronology
inferred from extinct radionuclides reveals that dust agglomeration to form
centimeter-sized particles in the inner part of the disk was very rapid (<50
kyr), planetesimal formation started early and spanned several million years,
planetary embryos (possibly like Mars) were formed in a few million years, and
terrestrial planets (like Earth) completed their growths several tens of
million years after the birth of the Sun.
All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, (16)O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA's Genesis mission. Our results demonstrate that the Sun is highly enriched in (16)O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in (17)O and (18)O, relative to (16)O, by ~7%, probably via non-mass-dependent chemistry before accretion of the first planetesimals.
The age of the Solar System can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto-Sun. This age is estimated by dating calcium-aluminium-rich inclusions in meteorites. These inclusions are considered as the earliest formed solids in the solar nebula. Their formation marks the beginning for several long- and short-lived radiogenic clocks that are used to precisely define the timescales of Solar System events, such as the formation and evolution of planetary bodies. Here we present the 207Pb-206Pb isotope systematics in a calcium-aluminium-rich inclusion from the Northwest Africa 2364 CV3-group chondritic meteorite, which indicate that the inclusion formed 4,568.2million years ago. This age is between 0.3 (refs 4, 5) and 1.9 (refs 1, 6) million years older than previous estimates and is the oldest age obtained for any Solar System object so far. We also determined the 26Al-26Mg model age of this inclusion, and find that it is identical to its absolute Pb-Pb age, implying that the short-lived radionuclide 26Al was homogeneously distributed in the nebular disc surrounding the proto-Sun. From the consistently old ages in the studied inclusion, we conclude that the proto-Sun and the nebular disc formed earlier than previously thought.
The timing of the formation of the first solids in the solar system remains poorly constrained. Micrometer-scale, high-precision
magnesium (Mg) isotopic analyses demonstrate that Earth, refractory inclusions, and chondrules from primitive meteorites formed
from a reservoir in which short-lived aluminum-26 (26Al) and Mg isotopes were homogeneously distributed at ±10%. This level of homogeneity validates the use of 26Al as a precise chronometer for early solar system events. High-precision chondrule 26Al isochrons show that several distinct chondrule melting events took place from ~1.2 million years (My) to ~4 My after the
first solids condensed from the solar nebula, with peaks between ~1.5 and ~3 My, and that chondrule precursors formed as early
as My after.
Stable-isotope variations exist among inner solar system solids, planets, and asteroids, but their importance is not understood. We report correlated, mass-independent variations of titanium-46 and titanium-50 in bulk analyses of these materials. Because titanium-46 and titanium-50 have different nucleosynthetic origins, this correlation suggests that the presolar dust inherited from the protosolar molecular cloud was well mixed when the oldest solar system solids formed, but requires a subsequent process imparting isotopic variability at the planetary scale. We infer that thermal processing of molecular cloud material, probably associated with volatile-element depletions in the inner solar system, resulted in selective destruction of thermally unstable, isotopically anomalous presolar components, producing residual isotopic heterogeneity. This implies that terrestrial planets accreted from thermally processed solids with nonsolar isotopic compositions.
This letter reports the discovery of an unusual Allende inclusion that is rich in hibonite, Ca(Al, Ti, Mg)12O19, the most refractory and possibly the most primitive major oxide mineral from the solar nebula. The Mg and Ca isotopic compositions of this hibonite-rich inclusion are studied in order to investigate the distribution of Al-26 in the solar system and to extend the search for isotopic anomalies. The Mg results indicate that no Mg isotopic anomalies are present, that the initial Al-26/Al-27 ratio for the inclusion when it crystallized was less than 200 billionths, and that the Mg mass-fractionation effect in the inclusion must be less than about 20 per mil/amu for the hibonite and 10 per mil/amu for other phases. The Ca studies reveal that large Ca mass-fractionation effects of about 7.5 per mil/amu are present and that additional small 'nonlinear' effects of presumably nuclear origin at a level of about 1 to 2 per mil are present in at least Ca-42. A plausible model for the evolution of the hibonite-rich inclusion is outlined.
Enrichments in boron-10/boron-11 in a calcium-aluminum-rich inclusion from the Allende carbonaceous chondrite are correlated with beryllium/boron in a manner indicative of the in situ decay of short-lived beryllium-10. Because this radionuclide is produced only by nuclear spallation reactions, its existence in early solar system materials attests to intense irradiation processes in the solar nebula. The particle fluence inferred from the initial beryllium-10/beryllium-9 is sufficient to produce other short-lived nuclides, calcium-41 and manganese-53, found in meteorites, but the high canonical abundance of aluminum-26 may still require seeding of the solar system by radioactive stellar debris.
We have studied the Mg and Ca isotopic compositions of an unusual Allende inclusion dominated by hibonite, which is the most refractory and possibly the most primitive major oxide mineral. No ^(26)Mg excess was found in spite of the high ^(27)Al/^(24)Mg (1 ≳ 10^3) of some samples, indicating an initial (^(26)Al/^(27)Al)_0 < 2 X 10^(-7), a factor of 250 less than found in some other Allende inclusions.
The upper limit for Mg isotopic fractionation is 20%o per amu. Anomalous but uniform Ca isotopic compositions were found for bulk samples of coexisting phases and microscopic grains. The Ca anomaly is a superposition of a large mass-dependent fractionation effect of 7.5‰ per amu favoring
the heavy isotopes and small (1‰-2‰) "nonlinear" effects of presumably nuclear origin. If the lack of ^(26)Al is due to a time delay of 6 X 10^6 yr for the formation of the hibonite inclusion, then condensation models require modification. The Ca effects suggest the alternative that ^(26)Al was not uniformly distributed in the solar system. These results accentuate the curious and unexplained
association between large mass fractionation and nuclear effects. They also reinforce the scenario which envisages an early solar system consisting of isotopically and chemically distinct reservoirs resulting from the incomplete mixing of several nucleosynthetic components. It is not evident whether these components originated within the solar system or from another star.
The paper describes how the properties of oxygen make it uniquely important in cosmochemistry. It is expected that the interstellar medium should contain molecules and grains with anomalous abundances of the isotopes of oxygen and of the elements with which it is combined, and that these molecules might be preserved in primitive meteorites. Recognition and characterization of such 'presolar carriers" would provide direct experimental information on the processes of nucleosynthesis and on the nature of interstellar materials. Although specific presolar carrier phases have not yet been identified, much can be learned about chemical processes in the early solar system by using the isotopic anomalies as natural tracers for different parts of an originally heterogeneous solar nebula. The paper first examines the origin of isotopic anomalies in oxygen, followed by sections on parent body processes, nebular processes, chondrites, achondrites, iron meteorites, isotopic reservoirs and future research. -after Author
accepted for publication in Nature, 24 pages including Online Supplements, 11 figures, 1 table
This book is a comprehensive treatment of star formation, one of the most active fields of modern astronomy. The reader is guided through the subject in a logically compelling manner. Starting from a general description of stars and interstellar clouds, the authors delineate the earliest phases of stellar evolution. They discuss formation activity not only in the Milky Way, but also in other galaxies, both now and in the remote past. Theory and observation are thoroughly integrated, with the aid of numerous figures and images. In summary, this volume is an invaluable resource, both as a text for physics and astronomy graduate students, and as a reference for professional scientists. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. All rights reserved.
Recent studies of oxygen isotopes in meteorites are reviewed. Findings
by many researchers who have investigated the origin of the isotopic
anomalies in oxygen, parent-body processes, nebular processes,
chondrites, achondrites, iron meteorites, and isotopic reservoirs are
discussed.
High precision SIMS 26Al-26Mg isotope analyses were performed for a pristine Type B1 CAI Leoville 3535-1 on multiple mineral phases that include aluminum-rich zoned melilite mantle (Åk20-Åk60), magnesium-rich melilite (Åk70), fassaite, spinel and anorthite in the core. The data yield a well-defined internal isochron with an inferred initial 26Al/27Al ratio of (5.002 ± 0.065) × 10−5, which is lower than those of bulk CAIs and pristine fine-grained CAIs. Assuming homogeneous distribution of 26Al in the early solar system, Leoville 3535-1 formed ∼50 ka after the time corresponding to the bulk CAI isochron. One anorthite analysis near the grain boundary adjacent to melilite shows sub-μm-scale heterogeneous magnesium distribution, though the 26Al-26Mg data plot on the isochron regression. Thus, the internal 26Al-26Mg system of the CAI remained closed since the last melting event that crystallized anorthite.
Recent whole-rock magnesium-isotopic data for calcium-aluminum-rich inclusions (CAIs) in chondrite meteorites indicate that the primary nebular fractionation of aluminum from magnesium, probably by condensation, occurred within < 20,000 years at 4.567 Ga. However, high-precision multicollector ion microprobe data for diverse CAIs from Vigarano (CV3) yield internal isochrons with a clearly resolved spread in initial 26Al/27Al, meaning that CAIs formed and were reprocessed over a much longer time span. Primitive (unmelted) CAIs have a consistent value of 5.2 ± 0.1 × 10− 5, melted CAIs range from 5.17 × 10− 5 to 4.24 × 10− 5, and one single object with a complex multistage history has an internal range of 26Al/27Al = (4.77 − 2.77) × 10− 5. The entire range corresponds to an age span of ~ 0.7 Ma. Thus not all CAIs formed at “time zero”, and only the most primitive CAIs should be used as benchmarks for earliest Solar System chronology.
– There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium-aluminum-rich inclusion (CAI) gave a good correlation of 26Mg/24Mg with 27Al/24Mg, yielding an isochron corresponding to an initial (26Al/27Al)o = (5.27 ± 0.18) × 10−5 and an initial (26Mg/24Mg)o = −0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008), but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same 26Al/27Al. No inference about uniformity/heterogeneity of 26Al/27Al on a macro scale can be based on the initial (26Mg/24Mg)o values. Different values of 26Al/27Al for samples representing the same point in time would prove heterogeneity of 26Al/27Al. The important issue is whether the bulk solar inventory of 26Al/27Al was approximately 5 × 10−5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with 26Al/27Al from approximately 5 × 10−5 to below 0.2 × 10−5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of 16O-poor material; and the relationship to earlier formed 16O-rich material in the disk. 26Al-free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.
Plots such as ε54Cr vs. ε50Ti and ε54Cr vs. Δ17O reveal a fundamental dichotomy among planetary materials. The “carbonaceous” chondrites, by virtue of high ε50Ti and high ε62Ni, as well as, especially for any given Δ17O, high ε54Cr, are separated by a wide margin from all other materials. The significance of the bimodality is further manifested by several types of meteorites with petrological-geochemical characteristics that suggest membership in the opposite category from the true pedigree as revealed by the stable isotopes. Ureilites, for example, despite having diversely low Δ17O and about the same average carbon content as the most C-rich carbonaceous chondrite, have clear stable-isotopic signatures of noncarbonaceous pedigree. The striking bimodality on the ε54Cr vs. ε50Ti and ε54Cr vs. Δ17O diagrams suggests that the highest taxonomic division in meteorite/planetary classification should be between carbonaceous and noncarbonaceous materials. The bimodality may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula. However, an alternative, admittedly speculative, explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system (carbonaceous) and materials that accreted in the inner solar system (noncarbonaceous). In any event, both the Earth and Mars plot squarely within the noncarbonaceous composition-space. Applying the lever rule to putative mixing lines on the ε50Ti vs. ε54Cr and Δ17O vs. ε54Cr diagrams, the carbonaceous/(carbonaceous+noncarbonaceous) mixing ratio C/(C+NC) is most likely close to (very roughly) 24% for Earth and 9% for Mars. Estimated upper limits for C/(C+NC) are 32% for Earth and 18% for Mars. However, the uncertainties are such that isotopic data do not require or even significantly suggest that Earth has higher C/(C+NC) than Mars. Among known chondrite groups, EH yields a relatively close fit to the stable-isotopic composition of Earth.
