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Abstract

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions. Science297, 1679–1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene–olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from 26Al and 60Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10–50 km radius accreting within 1–3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.

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... The H-chondrite Ste. Marguerite plots slightly above the line defined by angrites and Allende CAIs (CAI-2) which most likely reflects a slight disturbance of the Hf Amelin et al. (2006), Amelin (2008), Kleine et al. (2004), Kleine et al. (2008), Burkhardt et al. (2007), Bouvier et al. (2007). this sample . ...
... This is inconsistent with the general assumption that chondrites, which comprise the major fraction of the world`s meteorite collections, are the precursor material from which the iron meteorites formed. 4568.5 ± 0.5 Myr based on a combined CAI isochron using Allende and Efremovka CAIs (Bouvier et al., 2007). Their formation interval, based on the short-lived 26 Al-26 Mg chronometer (T 1/2 =0.73 Myr) has recently been found to be extremely short, in the range of 150,000 years (Bizzarro et al., 2004). ...
... Using the angrite D`Orbigny as an anchor they derived an absolute age of 4568.5 ± 0.8 Myr. This age is in remarkable agreement with the Pb-Pb age of 4568.5 ± 0.5 Myr determined for CAIs from Bouvier et al. (2007), but is somewhat older than the CAI age of 4567.2 ± 0.6 Myr determined by Amelin et al. (2002). ...
... When three (Ausson, New Concord, and Marion) or five (plus Alfianello and Barwell) samples with the most obvious terrestrial Pb contamination are excluded, similar ages of 4543 AE 87 Ma and 4535 AE 37 Ma, respectively, are obtained (Table 2). Given the uncertainties, these results agree with published Pb-Pb ages of between 4529 and 4553 Ma for three L5/L6 chondrites , of~4550 Ma for L4/L5/L6 chondrites (Unruh 1982), and of about 4525-4555 Ma for silicates and phosphates of L5/L6 chondrites (G€ opel et al. 1994;Bouvier et al. 2007), which together are thought to reflect the protracted metamorphic history of the parent body. ...
... For G€ opel et al. 1994;Bouvier et al. 2007), which are considered to record metamorphic equilibration and cooling, the agreement is unsurprising given the large uncertainties of our LL chondrite ages. ...
... Overall, the data thus provide credible evidence for the former presence of live 205 Pb on the LL chondrite parent body during cooling. The youngest possible 205 Pb-205 Tl age of 31 Ma after CCs is thereby still younger than the Pb-Pb ages of about 4545-4557 Ma, which were determined for silicates and phosphates of LL5/LL6 chondrites G€ opel et al. 1994;Bouvier et al. 2007), and that are thought to record the cooling history of the parent body. This observation suggests a low closure temperature for the 205 Pb-205 Tl system relative to U-Pb closure in silicates (at~1000-1100 K; Cherniak 2001) and phosphates (~800 K; Cherniak et al. 1991), in accord with the highly mobile character of Tl even during moderate thermal overprinting (Matza and Lipschutz 1977;Ngo and Lipschutz 1980). ...
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New Tl, Pb, and Cd concentration and Tl, Pb isotope data are presented for enstatite as well as L- and LL-type ordinary chondrites, with additional Cd stable isotope results for the former. All three chondrite suites have Tl and Cd contents that vary by more than 1–2 orders of magnitude but Pb concentrations are more uniform, as a result of terrestrial Pb contamination. Model calculations based on Pb isotope compositions indicate that for more than half of the samples, more than 50% of the measured Pb contents are due to addition of modern terrestrial Pb. In part, this is responsible for the relatively young and imprecise Pb-Pb ages determined for EH, L, and LL chondrites, which are hence only of limited chronological utility. In contrast, four particularly pristine EL chondrites define a precise Pb-Pb cooling age of 4559 ± 6 Ma. The enstatite chondrites (ECs) have highly variable ε114/110Cd of between about +3 and +70 due to stable isotope fractionation from thermal and shock metamorphism. Furthermore, nearly all enstatite meteorites display ε205Tl values from −3.3 to +0.8, while a single anomalous sample is highly fractionated in both Tl and Cd isotopes. The majority of the ECs thereby define a correlation of ε205Tl with ε114/110Cd, which suggests that at least some of the Tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205Tl from 205Pb decay. Considering L chondrites, most ε205Tl values range between −4 and +1, while two outliers with ε205Tl ≤ −10 are indicative of stable isotope fractionation. Considering only those L chondrites which are least likely to feature Pb contamination or stable Tl isotope effects, the results are in accord with the former presence of live 205Pb on the parent body, with an initial 205Pb/204Pb = (1.5 ± 1.4) × 10−4, which suggests late equilibration of the Pb-Tl system 26–113 Ma after carbonaceous chondrites (CCs). The LL chondrites display highly variable ε205Tl values from −12.5 to +14.9, also indicative of stable isotope effects. However, the data for three pristine LL3/LL4 chondrites display an excellent correlation between ε205Tl and 204Pb/203Tl. This defines an initial 205Pb/204Pb of (1.4 ± 0.3) × 10−4, equivalent to a 205Pb-205Tl cooling age of 55 + 12/−24 Ma (31–67 Ma) after CCs.
... Some of these bodies, like the eucrite parent body 4-Vesta, for example, also began to differentiate in this time frame due to radioactive heat produced by decay of short-lived nuclides such as 26 Al and 60 Fe (Bizzarro et al., 2005). Chondrite parent bodies did not accrete rapidly enough or become sufficiently large to form differentiated planetesimals, but have had different metamorphic and cooling histories (e.g., Brearley and Jones, 1998;Bouvier et al., 2007). Ordinary and enstatite chondrite parent bodies have been subjected to intense thermal metamorphism and are represented by chondrites of petrographic types 3 to 6, corresponding to temperature peaks of 800 to 1100 K (Huss et al., 2006). ...
... Carbonaceous chondrites are characterized by variable degrees of aqueous alteration (types 1 and 2) or generally low degrees of thermal metamorphism on their parent bodies (b700 K for type 3). Carbonaceous chondrites are, therefore, suggested to be among the last chondritic planetesimals to form (e.g., Kunihiro et al., 2004;Bouvier et al., 2007). ...
... Using our new Sm-Nd and Lu-Hf CHUR parameters, we can calculate the initial composition of the Solar System at 4.5685 Ga (Bouvier et al., 2007). For Sm-Nd, we obtain 143 Nd/ 144 Nd (SSi) = 0.506686 ± 70, which corresponds to an uncertainty of 1.4 ɛ Nd units, Fig. 3. Sm-Nd isotope composition of chondrites. ...
Article
Editor: R.W. Carlson Keywords: Lu–Hf Sm–Nd chondrite CHUR bulk silicate Earth thermal metamorphism The Lutetium–Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu–Hf system parameters need to be more tightly constrained, in particular the Lu–Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu–Hf and Sm–Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5, and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu–Hf isotopic composition of CHUR. Our new determinations of Lu–Hf CHUR parameters are 176 Lu/ 177 Hf = 0.0336 ± 1 and 176 Hf/ 177 Hf = 0.282785 ± 11 (2σ m), which are higher than previous estimates and, together with average Sm–Nd chondrite compositions of unequilibrated chondrites of 147 Sm/ 144 Nd = 0.1960 ± 4 and 143 Nd/ 144 Nd = 0.512630 ± 11 (2σ m), now provide firm constraints on the chondritic parameters for both Lu–Hf and Sm–Nd isotopic systems. A comparison of Lu–Hf and Sm–Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu–Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the 176 Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.
... The utility of the Pb-phosphate system stems from two key characteristics: (i) Its temporal resolution can resolve protracted conductive cooling histories in excess of 100 Ma that short-lived thermochronologic systems (e.g., Al-Mg and Hf-W) cannot and (ii) its capacity to internally verify closed-system behavior with paired 238 U-206 Pb and 235 U-207 Pb systematics (see Results) that is not shared by other long-lived thermochronologic systems (e.g., Ar-Ar and apatite/merrillite fission track). Yet, to date, the Pbphosphate measurements of LL chondrites were limited and apparently inconsistent with onion shell cooling (18,19,23,24). This study resolves these deficits and inconsistencies using Pb-phosphate measurements of five additional LL chondrites coupled with thermal simulations that reveal the thermal history, size, and accretionary time frame of the LL parent planetesimal. ...
... (B) Pb-phosphate cooling dates simulated by coupled planetesimal thermal and Pb production-diffusion in phosphate models at depths corresponding to petrologic types 4 to 7 in (A). (C) Summary of measured LL chondrite Pb-phosphate model cooling dates from this and previous studies (19,23,24). Pb-phosphate dates are calculated using the revised bulk chondritic 238 U/ 235 U of (14). ...
... We used the primordial Pb c composition of Canyon Diablo troilite (43) to correct for initial Pb c . We chose this composition over that suggested more recently by (44) given its long history of use in OC phosphate studies (18,19,23,24) and the fact that for measurements with Pb*/Pb c >2 (the threshold at which we calculate Pb-phosphate dates), the effect on the calculated age is negligible ( fig. S2). ...
Article
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Chondritic meteorites, derived from asteroidal parent bodies and composed of millimeter-sized chondrules, record the early stages of planetary assembly. Yet, the initial planetesimal size distribution and the duration of delay, if any, between chondrule formation and chondrite parent body accretion remain disputed. We use Pb-phosphate thermochronology with planetesimal-scale thermal models to constrain the minimum size of the LL ordinary chondrite parent body and its initial allotment of heat-producing ²⁶ Al. Bulk phosphate ²⁰⁷ Pb/ ²⁰⁶ Pb dates of LL chondrites record a total duration of cooling ≥75 Ma, with an isothermal interior that cools over ≥30 Ma. Since the duration of conductive cooling scales with parent body size, these data require a ≥150-km radius parent body and a range of bulk initial ²⁶ Al/ ²⁷ Al consistent with the initial ²⁶ Al/ ²⁷ Al ratios of constituent LL chondrules. The concordance suggests that rapid accretion of a large LL parent asteroid occurred shortly after a major chondrule-forming episode.
... Many carbonaceous chondrites have cosmic-ray exposure ages <1 million years ago, indicating that they were only recently detached from their parent bodies (12,13). Any fluid flow that occurred during their ejection, subsequent transport to Earth, and/or entry into the atmosphere is therefore potentially detectable using U-series disequilibria (14,15). ...
... There is petro-fabric (petrologic alignment of minerals) evidence for impact-related deformation in these meteorites, and the degree of aqueous alteration correlates with the strength of, and may in some cases postdate, fabric formation (27). Moderate velocity (~1 km/s) impacts are sufficient to melt ice (14). Our observations are consistent with postaccretional surface regolith fluid flow on the parent bodies (28) and indicate that they still contain substantial quantities of ice. ...
... This could potentially affect the radiogenic parent-daughter Rb/Sr ratios-for example, a 10% recent change in a present-day Rb/Sr ratio would be equivalent to a 0.01 difference in a time-integrated 87 Sr/ 86 Sr ratio at 4.56 billion years ago (7). Differences in initial U isotope and U/Pb ratios could change the dating of the age of the Solar System by 1 million to 2 million years (13,14). The continuing presence of ice in carbonaceous chondrites could deliver water (30) and methane (31) to Earth. ...
Article
Recent fluid flow in ancient meteorites Carbonaceous chondritic meteorites are thought to be fragments broken off parent bodies that orbit in the outer Solar System, largely unaltered since their formation. These meteorites contain evidence of reactions with liquid water that was thought to have been lost or completely frozen billions of years ago. Turner et al. examined uranium and thorium isotopes in several carbonaceous chondrites, finding nonequilibrium distributions that imply that uranium ions were transported by fluid flow. Because this signature disappears after several half-lives of the radioactive isotopes, the meteorites must have been exposed to liquid within the past million years. The authors suggest that ice may have melted during the impacts that ejected the meteorites from their parent bodies. Science , this issue p. 164
... Similar timescales have been suggested for the accretion and differentiation of Mars (Dauphas and Pourmand 2011). Unmelted planetesimals such as the chondrite parent bodies accreted within 2-10 Myr after calciumaluminum-rich inclusions (CAIs; e.g., Bouvier et al. 2007;Henke et al. 2013) when the main source of radiogenic heat from the decay of short-lived radionuclides such as 26 Al (with a half-life of 0.705 Ma) was already declining. Generally, igneous differentiation and formation of magma oceans on planets and planetesimals were driven by the release of radioactive heat from the decay of 26 Al and the gravitational energy of early planetary accretion and differentiation (e.g., Ghosh et al., 2003). ...
... The accretion and differentiation of some planetesimals took place very early in solar system history, as early as less than 1 Ma after the formation of CAIs, for the parent bodies of magmatic iron meteorites as deduced from 182 Hf-182 W systematics (Kleine et al. 2005b;Kruijer et al. 2014) and Pb isotopes (Blichert-Toft et al. 2010). Excesses in 26 Mg produced by the short-lived radionuclide 26 Al have been found in bulk samples of basaltic and cumulate eucrites, mesosiderites (e.g., Bizzarro et al. 2005;Schiller et al. 2010), and mineral isochrons of angrites Spivak-Birndorf et al. 2009), and are in agreement with 53 Mn-53 Cr chronology (Lugmair and Shukolyukov 1998), indicating that the magma sources of these rocks formed within 3 Ma of the beginning of the solar system defined here as the age of CAI formation at 4568 Ma (Bouvier et al. 2007;Bouvier and Wadhwa 2010). This was possible because the internal temperature of the planetary bodies of eucrites and angrites increased from the radiogenic heat produced by the decay of 26 Al until reaching metamorphic and melting temperatures in the internal parts of the planetesimals (e.g., Bizzarro et al. 2005). ...
Article
Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.
... This age is identical to that of typical LL5 and LL6 chondrites (4.54-4.56 Ga) 13,14 . Although analytical uncertainty regarding the obtained U-Pb age is large, that 26 Mg-excess was not observed in Itokawa particles 15 suggests that thermal metamorphism continued for several million years (until 26 Al decayed) after the CAI formation. ...
... Tatsumi et al. 31 also suggested a crater retention age of 3-33 Myr based on more realistic collision experiments 31 . Taking the probable size of the parent body of Itokawa (>20 km 7 ) and/or LL chondrites (10-50 km 14 ) into consideration, we can state that our radiometric impact age of 1.36 ± 0.24 Ga is approximately consistent with the timescale of asteroid collision frequency in the main belt and the crater chronology of S-type asteroids in the main belt and slightly longer than the crater age of Itokawa. This outcome may be the result of global resurfacing that resets the 1-10 m deep surface layer, which may have occurred in the main belt long after the possible catastrophic disruption of the rigid parent of Itokawa 31 . ...
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Understanding the origin and evolution of near-Earth asteroids (NEAs) is an issue of scientific interest and practical importance because NEAs are potentially hazardous to the Earth. However, when and how NEAs formed and their evolutionary history remain enigmas. Here, we report the U-Pb systematics of Itokawa particles for the first time. Ion microprobe analyses of seven phosphate grains from a single particle provide an isochron age of 4.64 ± 0.18 billion years (1σ). This ancient phosphate age is thought to represent the thermal metamorphism of Itokawa's parent body, which is identical to that of typical LL chondrites. In addition, the incorporation of other particles suggests that a significant shock event might have occurred 1.51 ± 0.85 billion years ago (1σ), which is significantly different from the shock ages of 4.2 billion years of the majority of shocked LL chondrites and similar to that of the Chelyabinsk meteorite. Combining these data with recent Ar-Ar studies on particles from a different landing site, we conclude that a globally intense impact, possibly a catastrophic event, occurred ca. 1.4 Ga ago. This conclusion enables us to establish constraints on the timescale of asteroid disruption frequency, the validity of the crater chronology and the mean lifetime of small NEAs.
... Similar timescales have been suggested for the accretion and differentiation of Mars (Dauphas and Pourmand 2011). Unmelted planetesimals such as the chondrite parent bodies accreted within 2-10 Myr after calciumaluminum-rich inclusions (CAIs; e.g., Bouvier et al. 2007;Henke et al. 2013) when the main source of radiogenic heat from the decay of short-lived radionuclides such as 26 Al (with a half-life of 0.705 Ma) was already declining. Generally, igneous differentiation and formation of magma oceans on planets and planetesimals were driven by the release of radioactive heat from the decay of 26 Al and the gravitational energy of early planetary accretion and differentiation (e.g., Ghosh et al., 2003). ...
... The accretion and differentiation of some planetesimals took place very early in solar system history, as early as less than 1 Ma after the formation of CAIs, for the parent bodies of magmatic iron meteorites as deduced from 182 Hf-182 W systematics (Kleine et al. 2005b;Kruijer et al. 2014) and Pb isotopes (Blichert-Toft et al. 2010). Excesses in 26 Mg produced by the short-lived radionuclide 26 Al have been found in bulk samples of basaltic and cumulate eucrites, mesosiderites (e.g., Bizzarro et al. 2005;Schiller et al. 2010), and mineral isochrons of angrites Spivak-Birndorf et al. 2009), and are in agreement with 53 Mn-53 Cr chronology (Lugmair and Shukolyukov 1998), indicating that the magma sources of these rocks formed within 3 Ma of the beginning of the solar system defined here as the age of CAI formation at 4568 Ma (Bouvier et al. 2007;Bouvier and Wadhwa 2010). This was possible because the internal temperature of the planetary bodies of eucrites and angrites increased from the radiogenic heat produced by the decay of 26 Al until reaching metamorphic and melting temperatures in the internal parts of the planetesimals (e.g., Bizzarro et al. 2005). ...
... Min et al. (2013) constrained the single grain (U-Th)/He ages from a number of merrillite and chlorapatite grains separated from St. Séverin (SS). They combined those data with the Pb/Pb phosphate age (Bouvier et al., 2007), whole rock (WR) 40 Ar/ 39 Ar ages (Hohenberg et al., 1981, with revision as discussed by Min et al., 2013) and WR (U-Th)/He ages (Wasson and Wang, 1991), along with the commonly accepted values for the closure temperatures (T c ) for each decay system, to construct a temperature-time (T-t) path for cooling of St. Séverin below 500°C. The age vs. T c data for the different decay systems, which are illustrated in Fig. 4 by filled symbols, indicate nonlinear cooling below $500°C, with an average rate of $2.6°C/My, as estimated by Min et al. (2013). ...
... Thermochronological data (T c vs. age: filled red circles) for St. Séverin as summarized byMin et al. (2013). The original sources of the data and uncertainty of the ages (in My) are as follows: phosphate(Bouvier et al., 2007; ±0.1); Ar-Ar ...
Article
We have carried out detailed thermometric and cooling history studies of several LL-, L- and H-chondrites of petrologic types 5 and 6. Among the selected samples, the low-temperature cooling of St. Séverin (LL6) has been constrained in an earlier study by thermochronological data to an average rate of ∼2.6 °C/My below 500 °C. However, numerical simulations of the development of Fe–Mg profiles in Opx–Cpx pairs using this cooling rate grossly misfit the measured compositional profiles. Satisfactory simulation of the latter and low temperature thermochronological constraints requires a two-stage cooling model with a cooling rate of ∼50–200 °C/ky from the peak metamorphic temperature of ∼875 °C down to 450 °C, and then transitioning to very slow cooling with an average rate of ∼2.6 °C/My. Similar rapid high temperature cooling rates (200–600 °C/ky) are also required to successfully model the compositional profiles in the Opx–Cpx pairs in the other samples of L5, L6 chondrites. For the H-chondrite samples, the low temperature cooling rates were determined earlier to be 10–20 °C/My by metallographic method. As in St. Séverin, these cooling rates grossly misfit the compositional profiles in the Opx–Cpx pairs. Modeling of these profiles requires very rapid cooling, ∼200–400 °C/ky, from the peak temperatures (∼810–830 °C), transitioning to the metallographic rates at ∼450–500 °C. We interpret the rapid high temperature cooling rates to the exposure of the samples to surface or near surface conditions as a result of fragmentation of the parent body by asteroidal impacts. Using the thermochronological data, the timing of the presumed impact is constrained to be ∼4555–4560 My before present for St. Séverin. We also deduced similar two stage cooling models in earlier studies of H-chondrites and mesosiderites that could be explained, using the available geochronological data, by impact induced fragmentation at around the same time. Diffusion kinetic analysis shows that if a lower petrological type got transformed by the thermal effect of shock impacts to reflect higher metamorphic temperature, as has been suggested as a possibility, then the peak temperatures would have had to be sustained for at least 10 ky and 80 ky, respectively, for transformation to the petrologic types 6 and 4. Finally, we present a model that reconciles textural data supporting an onion-shell parent body of H-chondrites with rapid cooling rate at high temperature caused by impact induced disturbance, and also discuss alternatives to the onion shell parent body model.
... These isotope systems provide a high-resolution chronology of the few million years that around the same time as accretion of primitive chondrite parent bodies (e.g., Kleine et al., 2005;Kruijer et al., 2014a). In particular, the negative ε 182 W of −3.3 to −3.5 in magmatic iron meteorites (Quitté and Birck, 2004;Lee, 2005;Kleine et al., 2005;Scherstén et al., 2006;Markowski et al., 2006;Qin et al., 2008a;Kruijer et al., 2012) suggests accretion of their parent bodies <1.5 Ma after CAI formation and an early core formation of <2 to 6 Ma after CAI formation (Bouvier et al., 2007;Sahijpal et al., 2007;Qin et al., 2008a;Kruijer et al., 2012Kruijer et al., , 2014a. As a consequence, chondrite parent bodies might not be the remnants of the oldest planetesimals, and possibly accreted at around the same time when the first iron meteorite parent bodies already differentiated. ...
... In order to find which components of chondrites carry the Ba isotopic anomalies, a detailed study on CAIs of the carbonaceous chondrite Allende has been carried out [22]. CAIs are the first solids to condensate during the cooling of the solar nebula [37][38][39]; thus their isotopic composition reflects the one of the early stage of the solar system formation. However an isotopic anomaly has been reported between CAIs and bulk carbonaceous chondrites on the one hand, and terrestrial samples, angrites, and eucrites on the other hand [21][22][23]. ...
Article
While the isotopic variations of barium were reported for the first time fourty years ago, the number of studies on barium isotopes significantly increased only after 2010. Barium isotope anomalies in meteorites have been successfully used to provide constraints about the origin of presolar SiC grains. In carbonaceous chondrites Ba isotope anomalies are indicative of the heterogeneity of the early solar system, possibly resulting from of a later injection of material after the cooling of solar system. Barium isotope fractionation in the same carbonaceous chondrites suggests that a strong magnetic field was present in the innermost part of the early solar system. Barium mass-dependent isotope fractionation has also been detected throughout Earth surface materials. While igneous rocks show limited Ba isotopic variations, relatively large isotopic variations are observed amongst and within soils, rivers, and biological materials. Indeed, plants seem to fractionate Ba isotopes during Ba uptake from soil solutions. Therefore, Ba isotope signatures have the potential to provide clues on the biological cycling of Ba at the Earth surface. In seawater, Ba isotopic variations have been mapped out, and are mainly related to barite precipitation, which is in turn related to organic matter remineralization in the water column. This makes Ba isotopes a potentially powerful tool to reconstruct past ocean productivity, although constraints are still lacking regarding the inputs of dissolved Ba to the oceans by rivers or hydrothermalism.
... The CAIs are the oldest Solar System condensates, with ages of 4.568 Ga (e.g. Bouvier et al., 2007;Connelly et al., 2008Connelly et al., , 2012Jacobsen et al., 2008;Amelin et al., 2010;Bouvier and Wadhwa, 2010). Their origin and evolution involved condensation from the solar nebula followed by various thermal modifications, including high-T melting and shock reheating (e.g. ...
Article
High-precision stable Ba isotope ratios are reported in a variety of terrestrial samples, undifferentiated primitive meteorites, and calcium-aluminum-rich inclusions (CAIs) from the Allende chondrite. All whole-rock terrestrial and meteorite samples are isotopically indistinguishable at a 50 parts per million (ppm) level per atomic mass unit (amu). Three CAIs are isotopically light, with δ138/137Ba (permil deviation of the 138Ba/137Ba ratio from a terrestrial standard) values down to -0.6‰ compared to whole-rock meteorites, whereas the matrix is enriched in heavy isotopes (δ138/137Ba: +0.2‰). Similar light isotope enrichments in CAIs have been previously observed for Eu, Sr, and Ca, while for most other elements CAIs are enriched in the heavier isotopes (e.g. Mg, Fe). Kinetic isotopic fractionation is a possible explanation for the enrichment in the lightest isotopes, either by condensation from a vapor phase enriched in light isotopes by kinetic effects or by kinetic fractionation during non-equilibrium condensation of an undercooled gas as suggested for Ca isotopes. However, the common property of Ba, Eu, and Sr is that they all have a low first ionization potential. We suggest that electromagnetic sorting of ionized species in the early Solar System is a possible alternative mechanism to explain the depletion in heavy isotopes observed in refractory inclusions for those elements.
... Physical and chemical conditions during the earliest stages of Solar System evolution can be studied in chondritic meteorites and Interplanetary Dust Particles (IDPs), believed to be among the most primordial objects left over from the formation of the Solar System some 4.567 billion years ago (Bouvier et al., 2007). In this work we describe the unaltered mineralogy and light element (i.e. ...
Article
Micrometeorites are sub-millimetric extraterrestrial samples, which dominate the flux of extraterrestrial matter entering the Earth atmosphere. Every micrometeorite is altered by the interaction with the atmosphere. However, they can be found embedded in larger meteorites, in which case they are called microxenoliths. Microxenoliths are ancient micrometeorites, and they allow the study of past epochs of the Solar System, not accessible by studying micrometeorites. New microxenoliths have been discovered and studied in H chondrites and in the carbonaceous chondrite Isheyevo. Many different techniques (scanning and transmission electron microscopy, Raman spectroscopy, mass spectrometry) have been applied to characterize them. Also, numerical simulations have been performed to investigate their origin and the effects they suffer during the passage through the Earth atmosphere.
... The diffusion of Ti in olivine has been taken as a proxy for Hf (Cherniak and Liang, 2014) and the diffusion of Hf in plagioclase has been taken as in diopside (Bloch and Ganguly, 2014). The diffusion history of this system is divided in the following steps (Fig. 1b): T 0 = 4.5685 Gyr (Bouvier et al., 2007) T 1 = 4.5664 Gyr; accretion of the chondrite parent body. This time has to be delayed relative to T 0 because of the presence of 26 Al that could melt the asteroid parent body if accreted earlier. ...
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The ¹⁷⁶Lu–¹⁷⁶Hf isotopic system is widely used for dating and tracing cosmochemical and geological processes, but still suffers from two uncertainties. First, Lu–Hf isochrons for some early Solar System materials have excess slope of unknown origin that should not be expected for meteorites with ages precisely determined with other isotopic chronometers. This observation translates to an apparent Lu decay constant higher than the one calculated by comparing ages obtained with various dating methods on terrestrial samples. Second, unlike the well constrained Sm/Nd value (to within 2%) for the chondritic uniform reservoir (CHUR), the Lu/Hf ratios in chondrites vary up to 18% when considering all chondrites, adding uncertainty to the Lu/Hf CHUR value. In order to better understand the Lu–Hf systematics of chondrites, we analyzed mineral fractions from the Richardton H5 chondrite to construct an internal Lu–Hf isochron, and set up a numerical model to investigate the effect of preferential diffusion of Lu compared to Hf from phosphate, the phase with the highest Lu–Hf ratio in chondrites, to other minerals. The isochron yields an age of 4647±210 million years (Myr) using the accepted ¹⁷⁶Lu decay constant of 1.867±0.008×10−11yr−1. Combining this study with the phosphate fractions measured in a previous study yields a slope of 0.08855±0.00072, translating to a ¹⁷⁶Lu decay constant of 1.862±0.016×10−11yr−1 using the Pb–Pb age previously obtained, in agreement with the accepted value. The large variation of the Lu/Hf phosphates combined with observations in the present study identify phosphates as the key in perturbing Lu–Hf dating and generating the isochron slope discrepancy. This is critical as apatite has substantially higher diffusion rates of rare earth elements than most silicate minerals that comprise stony meteorites. Results of numerical modeling depending of temperature peak, size of the grains and duration of the metamorphic event, show that diffusion processes in phosphate can produce an apparently older Lu–Hf isochron, while this effect will remain negligible in perturbing the Sm–Nd chronology. Our results suggest that only type 3 chondrites with the lowest metamorphic grade and large minerals with minimal diffusive effects are suitable for determination of the Lu–Hf CHUR values and the Lu decay constant respectively.
... This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium. core formation | core composition | mineral physics | experimental petrology | earth's accretion E arth formed ∼4.56 billion years ago (1)(2)(3), over a period of several tens of millions of years, through the accretion of planetary embryos and planetesimals (4). The energy delivered by progressively larger impactors maintained Earth's outer layer as an extensively molten (4) magma ocean. ...
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The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.
... Temperature-time evolution paths for chondritic meteorites of different thermal metamorphic grade from the same parent body can potentially be used to constrain the thermal structure, and possibly the source of heat present in that parent body, yet the thermal structures and cooling histories of chondritic parent bodies are still debated (e.g., Rubin, 2004;Amelin et al., 2005;Harrison and Grimm, 2010;Henke et al., 2012;Ganguly et al., 2013;Scott et al., 2014). For instance, the H chondrites experienced a large range of thermal-metamorphic conditions with isotopic closure ages obtained from different chronologic systems (e.g., Rb-Sr, K-Ar, U-Pb) recording diverse temperature versus time paths for individual meteorites (Wasserburg et al., 1969;Göpel et al., 1994;Trieloff et al., 2003;Bouvier et al., 2007). An "onion-shell" structure of increasing temperature with depth (heated by 26 Al decay) has been proposed by some to account for variable metamorphic grades and cooling rates of ordinary chondrites (e.g., Pellas and Storzer, 1981). ...
Article
The siderophile, or iron-loving elements have many applications in the Earth and planetary sciences. In primitive meteorites, differences in the relative abundances of these elements are likely due to both nebular and parent body processes. In addition, some siderophile elements are also characterised by isotopically distinctive nucleosynthetic signatures. Thus, the relative abundances and isotopic compositions of these elements can be used to trace the genetics of primary planetary building blocks. Although these elements are largely concentrated in the metallic cores of differentiated planetary bodies, their absolute and relative abundances, as well as their isotopic compositions can also reveal important information regarding conditions of core formation and the chemical evolution of the silicate portions of the planetary bodies. The lithophile-siderophile nature of the radiogenic Hf- W system allow it to be used to place chronologic constraints on planetary core formation. The differing incompatibilities of the two elements in silicate systems further mean that the system can also be used to study early differentiation processes and subsequent efficiency of mixing in the silicate portions of differentiated bodies, including Earth. The abundances of siderophile elements in the terrestrial mantle are used to assess primary and secondary melting processes, and resulting metasomatic interactions. In addition, the Re-Os isotope system can, in some instances, be used to place chronologic constraints on when these processes occurred. The abundances of siderophile elements, and Os/ Os and Os/ Os ratios in the mantle sources of ocean island basalts can be used to place constraints on the age of recycled materials, and in some instances, the types of recycled materials present in these mantle domains.
... These isotope systems provide a high-resolution chronology of the few million years that around the same time as accretion of primitive chondrite parent bodies (e.g., Kleine et al., 2005;Kruijer et al., 2014a). In particular, the negative ε 182 W of −3.3 to −3.5 in magmatic iron meteorites (Quitté and Birck, 2004;Lee, 2005;Kleine et al., 2005;Scherstén et al., 2006;Markowski et al., 2006;Qin et al., 2008a;Kruijer et al., 2012) suggests accretion of their parent bodies <1.5 Ma after CAI formation and an early core formation of <2 to 6 Ma after CAI formation (Bouvier et al., 2007;Sahijpal et al., 2007;Qin et al., 2008a;Kruijer et al., 2012Kruijer et al., , 2014a. As a consequence, chondrite parent bodies might not be the remnants of the oldest planetesimals, and possibly accreted at around the same time when the first iron meteorite parent bodies already differentiated. ...
Conference Paper
The formation of chondrules, matrix and other components of chondrites is not yet fully understood. Knowing their relative formation ages and genetic relationships can provide crucial information on their formation conditions. Short lived radionuclides can provide new insights into the chronology of the formation of early solar system materials. In the past, the 26Al26Mg system has been mainly used for this purpose, but recently the 182Hf182W system emerged as a promising tool to date chondrule formation. Thus, components of chondrites that never underwent high temperature metamorphism, like CV3 chondrites, are well suited candidates for Hf-W investigations. Earlier studies of Allende chondrules with the Al-Mg und PbPb systems indicate that they formed up to 3.2 Ma after CAI formation1-4 . We prepared 23 Allende and 13 Vigarano separates for Hf and W isotope measurements, covering both reduced and oxidized CV chondrites. These include pure handpicked chondrule and matrix fractions as well as magnetic separates and bulk aliquots. The separates all define isochrons which indicates a contemporaneous formation of all components. The ages defined by the isochrons suggest formation of Allende and Vigarano components within 3 Ma after CAI formation. These ages are consistent with chondrule formation ages obtained from other isotopic systems. Notably, W and Hf concentrations in chondrule and matrix fractions from Allende and Vigarano vary considerably. Chondrules from Allende exhibit high Hf/W ratios (2.5 to 3.0), whereas matrix and strong magnetic fractions exhibit low Hf/W ratios (0.5 to 0.9). Unlike in Allende, Vigarano contains chondrules with extremly low Hf/W ratios. As suggested from petrological observations, these low ratios can be attributed to the presence of metal inclusions within the reduced Vigarano chondrules.
... The hit-and-run collisions described by Asphaug et al. (2006) occur during the accretion phase of planetary formation, which lasts until approximately 1.5 Myr post-CAI formation (Yang et al., 2007). Alternatively, the catastrophic impacts that disrupted parent bodies of ordinary chondrites occur at or near-peak temperatures for the bodies (Lucas et al., 2020), which are expected to be reached prior to 10 Myr post-CAI formation (Bouvier et al., 2007). As the cooling timescale required by a pure iron Psyche to retain a bulk porosity of ∼52 vol% extends beyond the hit-and-run collision and catastrophic impact epochs of the early solar system, a collision scenario for the formation of a highly porous, pure iron Psyche is not likely. ...
Article
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Some M‐type asteroids have measured bulk densities much lower than expected based on their metal‐rich surfaces. In particular, the density of the largest M‐type asteroid 16 Psyche would require a bulk porosity of ∼52 vol% if it has a pure iron composition. We determine that a pure iron Psyche must have cooled to and remained below 800 K to maintain sufficient porosity for that porosity to persist until present. Iron bodies smaller than Psyche could preserve long‐lived high porosities (>40%), yet even the smallest M‐type asteroids would require temperatures below 925 K. Our results indicate that all iron asteroids must cool to and remain at low temperatures prior to a porosity‐adding event in order to retain porosity on timescales longer than a few million years. Accordingly, a pure iron Psyche would not have sufficiently cooled to retain high porosities when large porosity adding events are most likely to have occurred in the early solar system. Hence, Psyche is not likely to be a pure iron exposed core. However, this structure may be more viable for M‐type asteroids smaller than Psyche which cool more quickly.
... These meteorites are thought to come from a single undifferentiated parent body. Accreted early in the history of the solar system, i.e. a few million years after CAI formation (Göpel et al., 1994;Amelin et al., 2005;Bouvier et al., 2007), this parent body has been heated by radioac-sistent with the concept of an 'onion-shell' thermal structure for which samples originating closer to the centre of the original parent body have experienced higher degrees of thermal metamorphism than samples from close to the surface (i.e. Miyamoto et al., 1981;Bennett and McSween, 1996;Ghosh et al., 2003;Trieloff et al., 2003). ...
... In both cases, relative I-Xe intervals have been revised assuming a 129 I half-life of 16.1 Myr. I-Xe intervals were then converted to ages relative to the formation of the Solar System using the absolute calibration of the I-Xe system to the Pb-Pb chronometer and the absolute Pb-Pb age of CAI formation 33 ; the origin of the Solar System then predates closure of the I-Xe system in Shallowater enstatite by 6 Myr. Models of the resetting of the I-Xe chronometer were evaluated against this dataset. ...
Article
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Many newly formed Sun-like stars show evidence of debris disks composed of dust generated through destructive collisions among residual planetesimals. These are inferred to survive at a detectable level over the first ~100 Myr of their parent star’s lifetime. We hypothesize that the most primitive meteorites were processed as a result of impacts as our Solar System’s debris disk dissipated, rather than as a result of heat generated by decay of 26Al. We show how the iodine–xenon (I–Xe) record from chondrules in the Chainpur meteorite supports this hypothesis, and use it to constrain the decline in the impact rate. We demonstrate that it is the creation of I–Xe sites during compaction that is recorded by the chondrule dataset. We show that, to account for the broader I–Xe record from primitive material, a consistent picture requires that the dissipation of our Solar System’s debris disk had a timescale of around 40–50 Myr in this period. Against this backdrop, the late addition of siderophiles to the Earth in a single large impact may represent a rare phenomenon in planetary system formation that has been anthropically selected. The iodine–xenon record in meteorites can be used to probe late-time processes within the Solar System, during the dissipation of its debris disk. Most primitive meteorites were processed and heated by impacts during this ~50 Myr period, rather than by the decay of 26Al.
... Blank levels posed a limitation to our pyroxene Pb isotope data for the following reason: because of limited availability of sample material, it was necessary to separate Pb, Sm, Nd, Lu, and Hf from the same sample dissolutions. While this in itself is not a problem, in order to achieve complete sample dissolution and sample-spike equilibration for the Sm-Nd and Lu-Hf parts of the combined analytical work, we were constrained to using steeljacketed Teflon bombs for sample digestion and this step results in higher Pb blanks than if the samples could have simply been dissolved in PFA beakers (see discussion about blank levels of high-pressure bombs in Bouvier et al. (2007)). On closer inspection of literature Pb isotope data, however, augite and pigeonite separates are commonly inconsistent with other mineral phases (e.g., oxides and pyroxene residues of Zagami in Fig. 4 of Borg et al. (2005) and pyroxene residues of Shergotty in Fig. 1 of Chen and Wasserburg (1986a)). ...
... We judged the quality of each random parameter combination by comparing the thermal evolutions at depth throughout the added chondritic material to the measured ages of multiple H chondrites that have been dated using multiple geochronological systems with different closure temperatures (Amelin et al., 2005;Blinova et al., 2007;Bouvier et al., 2007;Kleine et al., 2008;Trieloff et al., 2003). We considered the 182 Hf-182 W, 207 Pb-206 Pb in silicates, 207 Pb-206 Pb in phosphates, 40 Ar-39 Ar and 244 Pu-fission track ages measured from the Richardton, Kernouvé, and Estacado H chondrites (Table S1 in the supporting information), and the 207 Pb-206 Pb in phosphates, 40 Ar-39 Ar, and 244 Pu-fission track ages measured from Ste. Marguerite. ...
Article
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The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo-generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X-ray microscopy to metallic veins in the slowly cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates that this meteorite recorded a magnetic field over a period of tens to hundreds of years at ∼100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo-generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 105 to 106 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict that the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces.
... Meteorites provide direct samples of some of the most primitive solid materials found in the Solar System, yielding insights into early disk processes including chemical partitioning in the protoplanetary disk, the assembly of dust into planets, and subsequent planetary dynamical evolution (e.g., Scott, 2007). We now have a broad understanding that the parent asteroids of chondritic meteorites experienced heating (recorded by thermal metamorphism) predominantly by the decay of short-lived radionuclides for the first 60 million years (Myr) of Solar System history (e.g., Bouvier et al., 2007). A large body of evidence shows that impacts continued to disturb these objects beyond the end of thermal metamorphism, resetting a number of mineral geochronometers in the process (e.g., Wittman et al., 2010). ...
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The geochemistry and textural associations of chondritic phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. Here, we present a quantitative assessment of phosphorus-bearing mineral textures in the three variously-shocked lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. Phase associations, microtextures, and microstructures of phosphates are extremely variable within and between the differently-shocked lithologies investigated in the Chelyabinsk meteorite. We observe continuously strained as well as unstrained phosphate populations. Unstrained (recrystallized and annealed) grains are present only in the more intensely shocked dark lithology, indicating that phosphate growth in Chelyabinsk predates the development of primary shock-metamorphic textures. This disruption event is also recorded by complete melting of portions of the meteorite to produce the shock-melt lithology, which contains a population of phosphorus-rich olivine grains. We interpret the textures and phase associations of Chelyabinsk to have resulted from initial phosphate growth via metasomatic olivine replacement, followed by major deformation during an early shock-melting impact and a subsequent minor event. This minor event appears to have generated a sub-population of phosphates that display patchy CL textures, in both the light and dark lithology. Finally, we propose a new classification scheme to describe various types of Phosphorus-Olivine-Assemblages (Type I-III POAs), which can be used to classify shock metamorphic events and define the associated physicochemical processes.
... This subgroup of ordinary chondrites is of interest as one of the primitive bodies which contain chondrules, the crystallization products of silicate melt droplets, whose formation and metamorphism provide clues about the earlier history of the Solar System. Therefore, earlier studies of the Bjurböle L/LL4 meteorite were mostly related to chondrules, trace elements and isotope composition analyses (see, e.g., [6][7][8][9][10][11]). Studies of the chemical composition of the Bjurböle L/LL4 meteorite showed that olivine (Fe, Mg) 2 SiO 4 , as a solid solution of fayalite Fe 2 SiO 4 (Fa) and forsterite Mg 2 SiO 4 , contains Fa of 26 mol% [12] and 26.2 mol% [13]. ...
Article
Bjurböle L/LL4 ordinary chondrite was studied using scanning electron microscopy with energy dispersive spectroscopy, Raman spectroscopy, X-ray diffraction, magnetization measurements and Mössbauer spectroscopy. The phase composition and the relative iron fractions in the iron-bearing phases were determined. The unit cell parameters for olivine, orthopyroxene and clinopyroxene are similar to those observed in the other ordinary chondrites. The higher contents of forsterite and enstatite were detected by Raman spectroscopy. Magnetization measurements showed that the temperature of the ferrimagnetic-paramagnetic phase transition in chromite is around 57 K and the saturation magnetic moment is ~7 emu/g. The values of the ⁵⁷Fe hyperfine parameters for all components in the Bjurböle Mössbauer spectrum were determined and related to the corresponding iron-bearing phases. The relative iron fractions in Bjurböle and the ⁵⁷Fe hyperfine parameters of olivine, orthopyroxene and troilite were compared with the data obtained for the selected L and LL ordinary chondrites. The Fe²⁺ occupancies of the M1 and M2 sites in silicate crystals were determined using both X-ray diffraction and Mössbauer spectroscopy. Then, the temperatures of equilibrium cation distribution were determined, using two independent techniques, for olivine as 666 K and 850 K, respectively, and for orthopyroxene as 958 K and 1136 K, respectively. Implications of X-ray diffraction, magnetization measurements and Mössbauer spectroscopy data for the classification of the studied Bjurböle material indicate its composition being close to the LL group of ordinary chondrites.
... AE 9.5 Ma (95% confidence level, MSWD = 1.07; Fig. 12b) with precision on 207 Pb/ 206 Pb ratios ranging from 0.6% to 5.0% (Table S4) (G€ opel et al. 1994 (G€ opel et al. 1994;Bouvier et al. 2007). Hamburg has an estimated S2 shock level. ...
Article
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The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic ³He, ²¹Ne, and ³⁸Ar are ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic ¹⁰Be concentration point to a small 40–60 cm diameter meteoroid. An ⁴⁰Ar‐³⁹Ar age of 4532 ± 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA‐ICPMS give an average Pb‐Pb age of 4549 ± 36 Ma. This is in good agreement with the average SIMS Pb‐Pb phosphate age of 4535.3 ± 9.5 Ma and U‐Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.
... Meteorites provide direct samples of some of the most primitive solid materials found in the solar system, yielding insights into early disk processes including chemical partitioning in the protoplanetary disk, the assembly of dust into planets, and subsequent dynamical evolution (e.g., Scott 2007). We now have a broad understanding that the parent asteroids of chondritic meteorites experienced heating (recorded by 1 thermal metamorphism) predominantly by the decay of short-lived radionuclides for the first 60 million years (Myr) of solar system history (e.g., Bouvier et al. 2007). ...
Article
Full-text available
The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids.
... The Pb sample cuts were brought into solution in 200-800 lL of 3% HNO 3 and spiked with a Tl standard solution to obtain a final concentration of 1 ppb Tl for correction of the instrumental mass bias. The total procedural blanks (chemistry and mass spectrometry) were 0.8 and 1.4 pg for leachates and residues, respectively, and their contribution to the sample isotopic compositions was corrected following the method described in Bouvier et al. (2007). The amount of Pb in the residue of the pyroxene separate (PX-R) was too small for high-precision isotopic analysis and Pb isotope data for only the last leachate (L 8 ) are reported for this sample. ...
Article
Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca-phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The 26Al-26Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of 26Al, but a slight excess in the radiogenic 26Mg in a bulk sample allows the determination of a model 26Al-26Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The 207Pb-206Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole-rock isochron provides a re-equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.
... Equation 7 allows for the construction of a Pb-Pb isochron diagram in 206 Pb/ 204 Pb-207 Pb/ 204 Pb space known as Holmes-Houtermans diagram (Holmes, 1946;Houtermans, 1946 (Figure 1.13). This approach in combination with modern analytical techniques, allows very precise calculation of 207 Pb/ 206 Pb*, which is weighted toward samples that are the least sensitive to Pb c (nonradiogenic (or common) Pb introduced during laboratory work or naturally prior to sampling) (e.g., Amelin et al., 2002;Baker et al., 2005;Bouvier et al., 2007;Connelly et al., 2008;Wadhwa et al., 2009;Amelin et al., 2010;Connelly et al., 2012). (Amelin et al., 2002). ...
Thesis
In this thesis I report the petrographic, elemental and isotopic (oxygen, Al-Mg, Rb-Sr and uranium) compositions as well as U-Pb ages of five coarse-grained igneous inclusions (Type A CAIs 1 and 4a, Type B CAIs 5, 6 and 7) from the CV chondrite North West Africa (NWA) 4502 the second largest CV chondrite so far after Allende. At a mineralogical level, NWA 4502 is rather pristine. In comparison with CAIs from Allende and other CV chondrites, CAIs from the NWA 4502 show many similarities with CAIs from other CV3 chondrites. However, all studied NWA 4502 CAI types have low levels of secondary hydrothermal alteration, thermal and shock metamorphism that affected CAIs in other CV chondrites. Even though NWA 4502 shows pristine parent-body characteristics, complexities discovered in some of the isotope systems especially Rb-Sr indicate significant imprint of terrestrial weathering. Acid leached residues of NWA 4502 CAI fractions yield a Pb-Pb age (4567.40 ± 0.27 Ma) and initial 87Sr/86Sr (0.698886 ± 0.000026) and an average initial 26Al/27Al (4.90 ± 1.51 x 10-5) ratio consistent with values reported for Allende and Efremovka CAIs inspite of the differences in secondary processes between these CV chondrites. This leads to the conclusion that secondary processing did not bias the U-Pb isotopic ages of these CV CAIs. The evaluation of three acid leaching procedures aimed at removing weathering products and terrestrial contamination has shown that U-Pb dating of CAIs using the most radiogenic material can be achieved using mild acid leaching methods on coarse grained pyroxene and melilite-rich fractions. This study enabled us to expand the population of well characterized CV CAIs and provided an insight to better understanding the influence of secondary processes on isotopic systems and the chronology of CAIs.
Article
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Los meteoritos son rocas que se originaron en las etapas tempranas del Sistema Solar y representan fragmentos de asteroides que no llegaron a formar un planeta completo, o bien, restos de algún planeta diferenciado. La importancia de estudiarlos radica en que constituyen una de las evidencias principales para conocer su composición geoquímica y la historia de formación y evolución de los planetas, particularmente de los terrestres. Básicamente hay dos clases de meteoritos, los pétreos y los metálicos; cada uno de ellos representa una región específica de un cuerpo planetario. La historia de un meteorito puede ser muy azarosa pues, a lo largo de sus más de 4,500 millones de años, ha sido transformado por varios procesos que van desde su acreción, hasta el metamorfismo, intemperismo, colisiones, fusiones, caídas en la superficie terrestre y el hallazgo de éste. Además, los meteoritos muestran que en otros planetas también hubo procesos magmáticos similares a los terrestres. En este trabajo se muestra cómo se puede estudiar la historia de cada meteorito tomando como ejemplo algunos estudios mexicanos, y se observa que los elementos químicos, sus isótopos y las proporciones entre ellos son elementos clave para alcanzar este fin.
Chapter
DefinitionA field of research constraining thermal histories of meteorites (rocks produced in extraterrestrial environment and delivered to Earth) and interpreting the results in the context of Earth and planetary science.IntroductionSince the formation of our solar system at 4,568.2 Ma (Bouvier and Wadhwa 2010), individual solid objects in the solar system experienced a range of planetary processes associated with temperature changes. Accretion, impact, breakup, metamorphism, and alteration are some of these planetary processes, which provide valuable information into the early evolution of the solar system. In addition, peak temperature conditions and cooling paths are crucial in understanding various physical properties (e.g., dimensions, internal structures) of solid bodies. The field of research known as “thermochronology” provides a means of investigating the thermal histories of meteorites (or their parental rock bodies) and interpreting the resulting data.Methods Thermal histori ...
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Meteorites are samples of small parent bodies accreted in the first five million years of the solar system. The diversity of meteorites can be related to different thermal evolutions of their parent bodies, from thermal metamorphism to complete mantle/core differentiation. This thesis focuses on the silicate-metal relative mobility during heating, in particular for solid systems which approach or exceed the onset of silicate melting. Two approaches are considered. Firstly, the textural evolutions in natural meteorites have been quantified. Secondly, laboratory experiments have been performed in order to identify and understand the processes which govern these textural changes. A textural study of metal and sulphide grains in H-chondrites shows that as metamorphism grade increases, phases separate, change in shape and grow. This evolution occurs progressively, making it possible to define textural criteria that vary continuously across petrographic boundaries. This evolution is consistent with independent geochemical data and thermal model. We propose a new scale of metamorphism allowing the subdivision of types 4 to 6. Grain growth experiments have been performed in synthetic analogues of meteorites: the system forsterite+nickel±melt silicate (Fo:Ni±M). The synthesis of starting materials required special care. A new sintering technique, seldom used in geosciences, has been developed: Spark Plasma Sintering. Experimental results show that mechanisms of grain growth of Fo and Ni are largely dependent of proportion and composition of each phase. Finally, results are in good agreement with natural observations and can be used to precise thermal history of planetesimals.
Article
Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any 26Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.
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Natural objects evolve from initial chaotic motions to order through a fascinating internal self-organization, which is embedded in their structure. The dynamics of this process is the focus of this work. They can be subjected to temporal and spatial variations or retain their stability for a long time. Ordered structures surround us ubiquitously on Earth; numerous examples of self-organization are observed in space. Turbulent flows characterized by a great variety of dynamical processes are widespread in the surrounding world. We mainly focus on the problems of macroscopic modeling these natural flows.
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Over the past two decades, large strides have been made in the field of planet formation. Yet fundamental questions remain. Here we review our state of understanding of five fundamental bottlenecks in planet formation. These are: 1) the structure and evolution of protoplanetary disks; 2) the growth of the first planetesimals; 3) orbital migration driven by interactions between proto-planets and gaseous disk; 4) the origin of the Solar System's orbital architecture; and 5) the relationship between observed super-Earths and our own terrestrial planets. Given our lack of understanding of these issues, even the most successful formation models remain on shaky ground.
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This chapter discusses the energy source and the matter content from which the initial Earth’s mantle was formed. The PT conditions by which the mantle accumulation is provided are defined. Attention is drawn to the role of chondrites of different composition in the frame of the Earth’s heterogeneous accumulation model. The conditions are formulated by which a melted layer at the bottom of the mantle is formed. At the bottom boundary of the layer fraction crystallization occurs. The crystallization of Mg-pyroxene and magnesio-wüstite will lead to the formation at the bottom of the mantle of a layer comprising a mixture of these minerals. We note that, based on seismic data, it can be concluded that, namely by that mineral association, there exists a transition layer “D” on the modern core–mantle boundary. The boundaries between layers shift following the body’s growing surface.
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The last thirty years of cosmochemistry and planetary science have shown that one major Solar System reservoir is vastly undersampled in the available suite of extra-terrestrial materials, namely small bodies that formed in the outer Solar System (>10 AU). Because various dynamical evolutionary processes have modified their initial orbits (e.g., giant planet migration, resonances), these objects can be found today across the entire Solar System as P/D near-Earth and main-belt asteroids, Jupiter and Neptune Trojans, comets, Centaurs, and small (diameter < 200 km) trans-Neptunian objects. This reservoir is of tremendous interest, as it is recognized as the least processed since the dawn of the Solar System and thus the closest to the starting materials from which the Solar System formed. Some of the next major breakthroughs in planetary science will come from studying outer Solar System samples (volatiles and refractory constituents) in the laboratory. Yet, this can only be achieved by an L-class mission that directly collects and returns to Earth materials from this reservoir. It is thus not surprising that two White Papers advocating a sample return mission of a primitive Solar System small body (ideally a comet) were submitted to ESA in response to its Voyage 2050 call for ideas for future L-class missions in the 2035-2050 time frame. One of these two White Papers is presented in this article.
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Tridimensional model with large level of detail and reliability of the Bacubirito meteorite is determined by laser scanner measurements. By means of this model and densities published in the literature, we estimate the mass, main geometrical quantities, and regmaglypts distribution on the meteorite. A Monte Carlo method is proposed for uncertainty estimations of the derived geometrical magnitudes. The Bacubirito meteorite mass is m = 19.43 ± 0.51 tons with a maximum length of 4.130 ± 0.005 m; Bacubirito is set as the world’s fifth largest and the longest reported to date.
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The most abundant components of primitive meteorites (chondrites) are millimeter-sized glassy spherical chondrules formed by transient melting events in the solar protoplanetary disk. Using Pb-Pb dates of 22 individual chondrules, we show that primary production of chondrules in the early solar system was restricted to the first million years after formation of the Sun and that these existing chondrules were recycled for the remaining lifetime of the protoplanetary disk. This is consistent with a primary chondrule formation episode during the early high-mass accretion phase of the protoplanetary disk that transitions into a longer period of chondrule reworking. An abundance of chondrules at early times provides the precursor material required to drive the efficient and rapid formation of planetary objects via chondrule accretion.
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We review the abundances, distributions and isotopic compositions of halogens in chondritic meteorites and discuss formation of the mineral and other carriers of these elements. Halogens provide tracers of processes that occurred in circumstellar and interstellar environments, within the solar nebula, and after accretion within asteroidal meteorite parent bodies. Knowledge of the halogen abundances in chondrites is fundamental to understanding the halogen contents of the Earth and other terrestrial planets. However, the full potential of halogens to constrain processes in the solar nebula and the chondrite parent bodies has not yet been realized due to both analytical challenges and uncertainties in halogen condensation temperatures. Analytical challenges make robust determinations of absolute halogen abundances extremely difficult, and some large uncertainties remain. Halogens in chondritic meteorites are present in both soluble (water extractable) and insoluble fractions, and they are therefore highly susceptible to alteration by weathering. Potential contamination from terrestrial sources is a perennial problem, even for meteorite falls. However, although there are significant variations in the analytical data, even within individual chondrite groups, it is clear that there are distinct, but complex differences between the three main classes of chondrites, carbonaceous, ordinary, and enstatite. In most chondrites, the abundances of the halogens are controlled by their cosmochemical volatility, and the halogens are depleted in most of the chondrite groups relative to CI chondrites. Measurements of halogen isotope compositions in chondrites are somewhat limited. In general, bulk δ³⁷Cl values show little deviation from the terrestrial standard (−0.3 ± 0.3‰ relative to SMOW), although there are some notable exceptions with more negative values down to –4‰ that are possibly related to either the initial condensation of ices, or to parent body fluid interactions. In addition, ¹²⁹Xe and ³⁶S excesses in individual components of chondritic meteorites indicate that the short-lived radioisotopes, ¹²⁹I and ³⁶Cl, were present in the early solar system. The ¹²⁹I-¹²⁹Xe system is well established as an important chronometer for constraining the early chronology of solar system materials, but the chlorine-S system has not yet been used widely as a chronometer because of uncertainties in the initial ³⁶Cl/³⁵Cl ratio. Distinct halogen-bearing phases are generally rare in chondritic meteorites, but include silicates, silicate glasses, aluminates, sulfides, halides, phosphates, and oxides. With the exception of the enstatite chondrites, where chlorine (and possibly other halogens) are present in djerfisherite and chlorine-enriched chondrule glass, the primary (nebular) mineralogical carriers of the halogens in the most pristine (low petrologic type 3) carbonaceous and ordinary chondrites are poorly constrained. In type 1 and 2 carbonaceous chondrites that have been affected by interaction with aqueous fluids at low temperatures, such as the CI, CM, and CR chondrites, the mineralogical carriers of the halogens are also highly uncertain. In contrast, the redistribution and concentration of halogens into discrete minerals, particularly chlorine-rich phases, has occurred in essentially all carbonaceous and ordinary chondrites that have been affected by even moderate degrees of metamorphism, sometimes in the presence of aqueous fluids. Fluorine-bearing chlorapatite occurs typically as the major halogen-bearing phase in type 4–6 ordinary chondrites, as well as CK and other metamorphosed carbonaceous chondrites. Extensive metasomatic effects observed in the Allende subgroup of the oxidized CV chondrites (CVOxA) have resulted in the formation of sodalite, wadalite, and the rare aluminates chlormayenite and adrianite, most typically in CAIs, although sodalite also occurs in chondrules and matrices. Evidence for limited development of sodalite and rare scapolite is also present in chondrules in some petrologic type 3 ordinary chondrites and in CAIs in CO3 chondrites. Rare occurrences of fluor-richerite and fluorphlogopite have been reported as crystallization products of impact melts in enstatite chondrites, and fluorine- and chlorine-bearing biotite also occurs in some high petrologic type R chondrites.
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Here we report in situ secondary ionization mass spectrometry Ca‐phosphate U‐Pb ages for an L‐impact melt breccia (NWA 7251), which are integrated with petrological and mineral chemical studies of this meteorite. NWA 7251 is a heavily shocked rock that is composed mainly of the chondrite host, impact melt portion, and melt veins (crosscutting and pervasive type). The host is an L4 chondrite that has been shocked to S4. The impact melt portion has a fine‐grained igneous texture, and is composed mainly of olivine, low‐Ca pyroxene, high‐Ca pyroxene, and albitic glass. The impact melt was generated at pressure of >30–35 GPa and temperature of >1300–1500 °C during an impact event. The Ca‐phosphate grains in the host were affected by a shock heating event. Most of the Ca‐phosphate grains in the melt were neocrystallized, but relatively large grains enclosed by or adjacent to metal veins or melt globules are likely inherited. The U‐Pb isotopic systematics of Ca‐phosphates in NWA 7251 yield an upper intercept age of 4457 ± 56 Ma and a lower intercept age of 574 ± 82 Ma on the normal U‐Pb concordia diagram. The age of 4457 ± 56 Ma is interpreted to be related to an early shocking event rather than the thermal metamorphism of the parent body. The impact melt and veins in NWA 7251 were generated at 574 ± 82 Ma, resulting from disruption of the L chondrite parent body.
Thesis
La présence de magmas en profondeur permet de contraindre des processus géologiques passés et actuels. Ces magmas (i.e. liquides silicatés) participent aux cycles géochimiques des éléments volatils comme vecteur de matière.Nous étudions deux éléments volatils complémentaires : l'iode (I), un halogène, et le xénon (Xe), un gaz rare. Leur système radioactif éteint 129I/129Xe (T1/2 = 15.7Ma) est utilisé pour dater les processus hadéens et la formation de l'atmosphère, issu de l'évolution d'un océan magmatique. Or on connait peu le comportement de l'iode et du xénon dans les magmas en profondeur à haute pression et température.Notre protocole expérimental vise l'étude de l'incorporation de l'iode et du xénon et de leur solubilité dans les magmas. Pour étudier l'incorporation, la structure des silicates liquides a été caractérisée par diffraction de rayons X avec des expériences in situ réalisées dans des cellules à enclumes de diamant et dans des presses Paris-Édimbourg. Les teneurs de solubilité de l'iode et du xénon ainsi que l'eau ont été mesurés par les méthodes PIXE et ERDA.À hautes pressions, l'iode possède une forte solubilité (quelques %pds) dans les magmas. Les résultats préliminaires sur son incorporation dans du basalte montrent que l'iode ne formerait pas des liaisons covalentes. À haute pression et température (T>300°C - P>1GPa), le xénon forme une liaison covalente Xe-O avec les oxygènes des anneaux de 6 tétraèdres SiO44-. Le xénon a une solubilité élevée dans les magmas (4pds% - 1600°C - 3.5GPa).Les modèles de datation et des cycles géochimiques de l'iode et du xénon doivent être revus en tenant compte de leur comportement différentiel dans les magmas.
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Context. The cooling history of individual meteorites can be reconstructed if closure temperatures and closure ages of different radioisotopic chronometers are available for a couple of meteorites. If a close similarity in chemical and isotopic composition suggests a common origin from the same parent body, some basic properties of this body can be derived. Aims. The radius of the L chondrite parent body, its formation time, and its evolution history are determined by fitting theoretical models to empirical data of radioisotopic chronometers for L chondrites. Methods. A simplified evolution model for the L chondrite parent body was constructed considering sintering of the initially porous material, temperature dependent heat conductivity, and an insulating regolith layer. Such models were fitted to thermochronological data of five meteorites for which precise data for the Hf-W and U-Pb-Pb thermochronometers have been published. Results. A set of parameters for the L chondrite parent body is found that yields excellent agreement (within error bounds) between a thermal evolution model and thermochonological data of five examined L chondrites. Empirical cooling rate data also agree with the model results within error bounds such that there is no conflict between cooling rate data and the onion-shell model. Two models are found to be compatible with the presently available empirical data: one model with a radius of 115 km and a formation time of 1.89 Ma after CAI formation, and another model with 160 km radius and formation time of 1.835 Ma. The central temperature of the smaller body remains well below the Ni,Fe-FeS eutectic melting temperature and is consistent with the apparent non-existence of primitive achondrites related to the L chondrites. For the bigger model, incipient melting in the central core region is predicted, which opens the possibility that primitive achondrites related to L chondrites could be found.
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Lead isotope compositions were measured on both single and combined chondrules from the CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by the radiogenic 208Pb*/206Pb* values. All samples were aggressively acid step-leached to separate radiogenic from primordial lead. It is found that apparent Th/U varies both between individual chondrules and between the different leaching fractions of each chondrule or group of chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the existence of internal heterogeneities, while inter-chondrule variations reveal the presence of external heterogeneities. Three main explanations for the observed Th-U fractionation that are not mutually exclusive prevail: (1) uranium species, notably UO and UO2, coexisted in the nebular gas at high temperature, whereas Th existed exclusively as ThO2; (2) chondrules interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the measured apparent Th/U variability is due to each of these particular processes is difficult to assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear arrays, or mixing lines, are assigned isochronous significance.
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Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3–6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with cooling rate in the parent body. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (�500–200 �C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Ko¨ hler [1990]) and Ca-in-olivine thermometry (TCa-Ol; Kohler and Brey [1990]), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected within an onion shell structure, we obtain fast cooling at rates J0.3 �C/y from peak temperatures of �900 �C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (�700 to 800 �C) at �10�2 to 10�1 �C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (�500 �C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.
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The 53Mn-53Cr isotope systematics in ordinary chondrites constrains the accretion and thermal history of their parent bodies. Mineralogical observations and olivine-spinel geothermometry suggest that chromite in ordinary chondrites formed during prograde thermal metamorphism with the amount of chromite increasing with petrologic grades in type 3 to type 6 ordinary chondrites. Assuming a chondritic evolution of the respective parent bodies, 53Cr/52Cr model ages for chromite range from 3.99 +0.93/-0.79 to 11.1 +6.0/-2.8 Ma after the formation of calcium-aluminium-rich inclusions (CAIs). Chromite and silicate-metal-sulphide isochrons define an age range from 3.49 +0.55/-0.50 to 16.1 +2.4/-1.6 Ma. Both chromite model ages and isochron ages correlate with the petrological grade of the samples, which is consistent with an onion-shell structure of the chondrite parent bodies. The study shows that unlike the isochron ages, which are prone to impact-related disturbances or partial re-equilibration during cooling from high temperatures, the chromite model ages are not easily affected by thermal metamorphism or later events and yield robust mineral growth ages. The results are consistent with a homogenous distribution of 53Mn and an initial canonical 53Mn/55Mn = 6.28 x 10-6. The estimated closure temperatures for the Mn-Cr system in chromites range from ~760 °C for type 6 to ~540-620 °C for type 3 ordinary chondrites. The high closure temperatures estimated for type 3 and type 6 ordinary chondrites imply that the chromite ages correspond to the peak metamorphic temperature reached during the thermal history of the chondrite parent bodies. The oldest chromite model age obtained for type 3 samples along with the established Al-Mg chondrule formation ages constrain the accretion of the parent bodies to > 2.1 Ma after CAI formation, implying that planetesimal accretion immediately followed chondrule formation.
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L’accrétion des éléments volatils sur Terre continue d’être débattue malgré de nombreux travaux de recherche sur le sujet. Les observables géochimiques et cosmochimiques ont été expliquées par une large gamme de scénarios allant de (1) l’accrétion des éléments volatils durant les phases principales de l’accrétion ter- restre, jusqu’à (2) l’accrétion de matériaux pauvres en volatils, avec une addition tardive de matériel riche en éléments volatils après la fin de la différenciation, aussi appelée vernis tardif. Les signatures mantelliques des éléments volatils et sidérophiles sont des enregistrements des processus liés à la volatilité et des processus de différenciation. L’étude du comportement de ces éléments dans un équilibre métal-silicate à haute pression et haute température permet de séparer l’effet de la différenciation de l’effet des processus volatils sur les abondances de ces éléments, et donc de discriminer les scénarios capables d’expliquer les abondances observées. Dans ce travail de thèse, nous cherchons à déterminer le ou les scénarios d’accrétion de la Terre permettant d’expliquer les abondances en éléments volatils et sidérophiles observées dans le manteau. Des expériences menées dans des presses piston-cylindre et multi-enclume de 2 à 20 GPa et de 1700 à 2600 K ont permis d’observer les comportements élémentaires et isotopiques d’une sélection d’éléments volatils et sidérophiles. L’étude du coefficient de partage de Sn, Cd, Bi, Sb et Tl a permis de déterminer l’effet de la température, la pression, la fO2, et la concentration en S dans le métal sur l’affinité de ces éléments pour le métal relativement au silicate. Le coefficient de partage de ces éléments est reconstruit au cours de l’accrétion terrestre, aux conditions de pression et température appropriées. Différents scénarios d’accrétion sont testés dans le but de reproduire les mêmes abondances finales que celles observées dans le manteau. Les éléments étudiés présentent une sidérophilité élevée, qui ne permet pas d’expliquer les abondances élevées observées dans le manteau. Un scénario impliquant un équilibre partiel entre noyau et manteau ainsi qu’une arrivée des éléments volatils dans les 10-20 derniers pourcents des phases principales d’accrétion de la Terre, sous la forme d’un impacteur de grande taille, permettent d’expliquer les abondances de ces éléments, en accord avec d’autres études basées sur d’autres observables. Environ 3 wt.% de S dans le noyau sont également nécessaires afin d’expliquer l’abondance en Sn. Un vernis tardif représentant 0,5 % de la masse terrestre est également requis afin d’expliquer l’abondance en Bi dans le manteau. L’étude du fractionnement isotopique de l’Sn entre métal et silicate a permis de mesurer un facteur significatif de ~0.3 ‰ à 2 GPa et 2000 K (∆¹²²/¹¹⁸Snmetal−silicate), impliquant qu’un fractionnement isotopique de l’Sn pourrait avoir lieu aux conditions de la différenciation terrestre, ayant pour effet d’enrichir le manteau en isotopes légers. Les chondrites carbonées présentent une composition similaire à la Terre silicatée ce qui en fait les meilleures candidates comme source des volatils sur Terre, d’après les résultats de cette étude. Ces résultats sont compatibles avec le scénario d’accrétion favorisé au vu des données élémentaires. Les fractionnements élémentaires et isotopiques des éléments entre métal et silicate étant fortement dépendant de la composition du métal, l’effet de la teneur en Ni dans le métal sur le fractionnement isotopique du Fe est testé. Les expériences réalisées (22 expériences, de 0 à 70 % Ni) ne permettent pas de détecter un effet du Ni sur le fractionnement du Fe. Cette étude apporte donc de nouveaux éléments de réponse concernant le timing et le mécanisme d’accrétion, et l’origine des éléments volatils sur Terre...
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The solar system started to form about 4.56 Gyr ago and despite the long intervening time span, there still exist several clues about its formation. The three major sources for this information are meteorites, the present solar system structure and the planet-forming systems around young stars. In this introduction we give an overview of the current understanding of the solar system formation from all these different research fields. This includes the question of the lifetime of the solar protoplanetary disc, the different stages of planet formation, their duration, and their relative importance. We consider whether meteorite evidence and observations of protoplanetary discs point in the same direction. This will tell us whether our solar system had a typical formation history or an exceptional one. There are also many indications that the solar system formed as part of a star cluster. Here we examine the types of cluster the Sun could have formed in, especially whether its stellar density was at any stage high enough to influence the properties of today's solar system. The likelihood of identifying siblings of the Sun is discussed. Finally, the possible dynamical evolution of the solar system since its formation and its future are considered.
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The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the ~130°C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggest very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type three chondrites. -from Authors
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In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 yr of COBE Differential Microwave Radiometer (DMR) observations, with additional details in a set of companion Letters. The anisotropy is consistent with a scale-invariant power-law model and Gaussian statistics. With full use of the multifrequency 4 yr DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of n = 1.2 +/- 0.3 and a quadrupole normalization Qrms-PS = 15.3^{+3.8}_{-2.8} mu K. For n = 1 the best-fit normalization is Qrms-PS|n=1 = 18 +/- 1.6 mu K. These values are consistent with both our previous 1 yr and 2 yr results. The results include use of the l = 2 quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The final DMR 4 yr sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is ~2 per 10 deg sky map patch. The improved signal-to-noise ratio of the 4 yr maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.
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Estimates of the abundances of volatile, alkali elements (K, Rb, and Cs) in the bulk Silicate Earth vary considerably. The K and Rb abundances are constrained by the K/U (-1.3 X 104), K/Rb (-380), Rb/Sr (-0.03), and Ba/Rb (-11) ratios of the bulk Earth and by Sr, Nd, and Hf isotope systematics. The Cs abundance of the Silicate Earth is constrained by estimates of the Rb/Cs ratios of the continental crust and mantle. The continental crust has a Rb/Cs ratio of about 25, whereas the depleted MORB source and OIB plume source regions have a Rb/Cs ratio of about 80. There is evidence suggestive of a secular change in the Rb/Cs ratios of the depleted mantle, which may have been caused by continental crust fo~ation and cast-mantle recycling processes. The RbfCs ratio in the Silicate Earth is estimated to be about 28, based upon studies of the continental crust, MORB source, and OIB (plume) source. The continental crust contains about 37% of the total K present in the Silicate Earth, 50% of its Rb, and 55% of its Cs, whereas the residual mantle (the MORB and OIB source) contains about 20% of the K, 10% of the Rb, and only 4% of the Cs. Together, these reservoirs account for only about 60% of the total inventory of K, Rb, and Cs in the Earth today, indicating the existence of a less depleted reservoir in the mantle that contains the remainder of these elements, The average Rb/Cs ratio for all lunar samples is about 22 and this is believed to represent the bulk RbfCs ratio of the Moon. Within the limits of uncertainty which apply to both estimates, this ratio is similar to that of the Silicate Earth. Hence, we conclude that the existence of significant difference in the Rb/Cs ratios of the Earth and Moon cannot be inferred from the presently available data base. Thus, we disagree with the claim of KREUTZBERGER et Rb/Cs ratio than the Earth.
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The isotopic composition of Mg was measured in different phases of a Ca-Al rich inclusion in the Allende meteorite. Large excesses of ²⁶Mg of up to 10% were found. These excesses correlate strictly with the ²⁷Al/²⁴Mg for four coexisting phases with distinctive chemical compositions. Models of in situ decay of ²⁶Al within the solar system and of mixing of interstellar dust grains containing fossil ²⁶Al with normal solar system material are presented. The observed correlation provides definitive evidence for the presence of ²⁶Al in the early solar system. This requires either injection of freshly synthesized nucleosynthetic material into the solar system immediately before condensation and planet formation, or local production within the solar system by intense activity of the early Sun. Planets promptly produced from material with the inferred ²⁶Al/²⁷Al would melt within approx.3 x 10⁵ yr.
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Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°-1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: D Sm = 2.3 × 10 -6 exp (-52,200/ RT ) D Sr = 412 exp (-100,000/ RT ). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: D Pb = 0.035 exp (-70,000/ RT ). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C. When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events.
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1. Historical introduction 2. Potential meteorite parent bodies 3. Chondrites and their main properties 4. Chondrules and their main properties 5. Theories for the origin of chondrules 6. Discussion of theories for the origin of chondrules 7. Making the chondrites: chondrule sorting and metal-silicate fractionation 8. So how far have we come and where do we go next?
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The first observation of radiogenic Mg-26 in nonrefractory meteoritic material, a plagioclase-bearing, olivine-pyroxene clast chondrule in the Semarkona ordinary chondrite, is reported. The inferred initial abundance of Al-26 is sufficient to produce incipient melting in well-insulated bodies of chondritic composition. It is concluded that planetary accretion and diffentiation must have begun on a timescale comparable to the half-life of Al-26 and that, even if widespread melting did not occur, Al-26 heating played a significant role in thermal metamorphism on small planets.
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ABSTRACT Jarosewich, Eugene, Roy S. Clarke, Jr., and Julie N. Barrows, editors. The Allende Meteorite Reference Sample. Smithsonian Contributions to the Earth Sciences, number 27, 49 pages, 32 tables, 1986.—A reference material for comparative analytical studies and standardization was prepared from fresh, clean specimen material from the Allende, Mexico, Type CV3 carbonaceous chondrite fall of 8 February 1969. Fragments weighing 4 kg were powdered, homogenized, and split into 1 g and 5 g subsamples. Analytical results for a total of 74 elements were provided by 24 analysts or groups of analysts. A variety of techniques were used, and many elements were determined by more than one technique. Reports from contributors of data outline their procedures and give their results in detail. Sample homogeneity,has been evaluated in terms of this body of data, and "recommended values" are suggested for 43 elements. OFFICIAL PUBLICATION DATE is handstamped,in a limited number,of initial copies and is recorded in the
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U-Pb age of three CAIs from Allende and two CAIs from Efremovka is determined at 4568+/-0.4 Ma.
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Chemical diffusion of Pb under anhydrous, pO2-buffered conditions has been measured in natural pyroxenes of three different compositions: a chromian diopside, an augitic clinopyroxene, and a near-end member enstatite. Sources of diffusant consisted of PbS powder and ground pyroxene mixed together, pre-reacted in evacuated silica capsules at 1050°C, and re-ground. Prepared sample capsules were annealed for times ranging from several hours to a few months, at temperatures from 850°C to 1050°C. The Pb distributions in the pyroxene specimens were profiled by Rutherford Backscattering Spectrometry (RBS).
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Carbonaceous chondrites (CCs) are derived from undifferentiated icy planetesimals and are the most primitive meteorites. The information that we can derive from CCs depends largely on our understanding the effects of water in carbonaceous chondrite parent bodies (CCPBs). The way water influenced the parent bodies' evolution depends partly on the flow rates and patterns of the water circulation. The first quantitative models for the thermal evolution of CCPBs were based on parameterized hydrothermal convection and homogeneous alteration. Recent work has presented full models of hydrothermal convection in an internally heated, self-gravitating porous sphere. These results illustrate that the convective patterns in CCPBs are not uniform. Some regions of the body experience little to no pore water flow while other regions experience hundreds of pore volumes. It has long been held that CC meteorites of different chemical groups come from distinct parent bodies. Simulations showing heterogeneous patterns of fluid flow in CCPBs have led to the suggestion that parent bodies could be heterogeneously altered and, consequently, one parent body could be a source for multiple groups of CC meteorites. Previously, no numerical convection simulations of CCPBs have included water-rock reactions. We have coupled the computer code MAGHNUM with the reaction package PHREEQC. We use MAGHNUM to simulate the dynamic freezing, thawing and flow of water in a radiogenically-heated, self-gravitating body. The accompanying water-rock interactions are modeled with PHREEQC. Flow and chemistry are coupled through, for example, reaction rates and temperature. This work was supported by a grant from the Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory.
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New techniques of isotopic measurements by a new generation of mass spectrometers equipped with an inductively-coupled-plasma source, a magnetic mass filter, and multiple collection (MC-ICPMS) are quickly developing. These techniques are valuable because of (1) the ability of ICP sources to ionize virtually every element in the periodic table, and (2) the large sample throughout. However, because of the complex trajectories of multiple ion beams produced in the plasma source whether from the same or different elements, the acquisition of precise and accurate isotopic data with this type of instrument still requires a good understanding of instrumental fractionation processes, both mass-dependent and mass-independent. Although physical processes responsible for the instrumental mass bias are still to be understood more fully, we here present a theoretical framework that allows for most of the analytical limitations to high precision and accuracy to be overcome. After a presentation of unifying phenomenological theory for mass-dependent fractionation in mass spectrometers, we show how this theory accounts for the techniques of standard bracketing and of isotopic normalization by a ratio of either the same or a different element, such as the use of Tl to correct mass bias on Pb. Accuracy is discussed with reference to the concept of cup efficiencies. Although these can be simply calibrated by analyzing standards, we derive a straightforward, very general method to calculate accurate isotopic ratios from dynamic measurements. In this study, we successfully applied the dynamic method to Nd and Pb as examples. We confirm that the assumption of identical mass bias for neighboring elements (notably Pb and Tl, and Yb and Lu) is both unnecessary and incorrect. We further discuss the dangers of straightforward standard-sample bracketing when chemical purification of the element to be analyzed is imperfect. Pooling runs to improve precision is acceptable provided the pooled measurements are shown to be part of a single population. Second-order corrections seem to be able to improve the precision on 143Nd/144Nd measurements. Finally, after discussing a number of potential pitfalls, such as the consequence of peak shape, correlations introduced by counting statistics, and the effect of memory on double-spike methods, we describe an optimal strategy for high-precision and accurate measurements by MC-ICPMS, which involves the repetitive calibration of cup efficiencies and rigorous assessment of mass bias combined with standard-sample bracketing. We suggest that, when these simple guidelines are followed, MC-ICPMS is capable of producing isotopic data precise and accurate to better than 15 ppm.
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The Tuxtuac meteorite fell in Zacatecas state, Mexico, on October 16, 1975, at 1820 hours. Two partly crusted masses, weighing 1924 g and 2340 g, were recovered. The stone is an ordinary chondrite, LL5, with olivine Fa30 and 19.22 wt pct total iron. The silicates contain numerous voids and a froth-like mesostasis is present within some chondrules. Metal phases present are kamacite (5.7-6.4 pct Ni, 6-7 pct Co) and high nickel metal (taenite 37-41 pct Ni, 1.7 + or - 0.3 pct Co; tetrataenite 47-52 pct Ni, 0.8-1.4 pct Co). The stone is unusual for an LL-group chondrite in that it exhibits neither large-scale brecciation features nor dark veins.
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A compilation of over 1500 Mg-isotopic analyses of Al-rich material from primitive solar system matter (meteorites) shows clearly that 26Al existed live in the early Solar System. Excesses of 26Mg observed in refractory inclusions are not the result of mixing of "fossil" interstellar 26Mg with normal solar system Mg. Some material was present that contained little or no 26Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CMs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether 26Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.
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Mass transport was responsible for establishing the main stratification of the Earth (core, mantle, and crust) and also was the dominant means of heat transport through most of the Earth's history. During early accretion, impacts supplied the heat input and also enhanced heat loss by stirring the near-surface. Initial core segregation probably began catastrophically when the Earth was about half of its present radius and homogenized the temperature near the melting temperature of the mantle. Thereafter, core segregation would have been rate-limited by accretion. Subsequent thermal evolution would have depended on whether the surface was insulated by a thick atmosphere or a buoyant crust, either of which might have permitted a magma ocean to accumulate. As the Earth cooled, there may have been one or more transitions in tectonic style, but how these might relate to the accumulation, chemistry, and structural style of the preserved continental crust is still not clear. The lack of clear evidence for any compositional stratification in the present mantle cautions against models involving substantial early stratification.
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Abstract— The radiogenic 207Pb/206Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi-stage evolution of U-Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is ±0.5–1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U-Pb system in meteorites.
Article
Abstract— Widespread evidence exists for heating that caused melting, thermal metamorphism, and aqueous alteration in meteorite parent bodies. Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high-grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories.