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Olivine- and pyroxene-rich chondrules. (a) Barred olivine chondrule, PPL, olivine light grey, glass medium grey. (b) Type IIA PO chondrule, XPL, euhedral olivine phenocrysts in glass (black). (c) Metal-rich type IA MPO chondrule, XPL, olivine white and grey, metal black. (d) Type IA MPO chondrule, XPL. (e) Type IA POP, XPL, olivine white and grey in center, pyroxene grey near margin. (f) Type I(A)B P(O)P, XPL, olivine grey and white, pyroxene showing twinning. PPL = plane polarized light; XPL = cross polarized light. Average diameter ~700 P m. (c) – (f) courtesy of B. Zanda. 

Olivine- and pyroxene-rich chondrules. (a) Barred olivine chondrule, PPL, olivine light grey, glass medium grey. (b) Type IIA PO chondrule, XPL, euhedral olivine phenocrysts in glass (black). (c) Metal-rich type IA MPO chondrule, XPL, olivine white and grey, metal black. (d) Type IA MPO chondrule, XPL. (e) Type IA POP, XPL, olivine white and grey in center, pyroxene grey near margin. (f) Type I(A)B P(O)P, XPL, olivine grey and white, pyroxene showing twinning. PPL = plane polarized light; XPL = cross polarized light. Average diameter ~700 P m. (c) – (f) courtesy of B. Zanda. 

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Article
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Chondrule textures depend on the extent of melting of the chondrule precursor material when cooling starts. If viable nuclei remain in the melt, crystallization begins immediately, producing crystals with shapes that approach equilibrium. If not, crystallization does not occur until the melt is supersaturated, resulting in more rapid growth rates a...

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... cooling. There are many experiments in which such textures and overgrowths of the type described by Wasson (2004) have been formed. Lofgren & Le (2000) partially melted crushed ordinary chondrite, and by cooling at a rate of 1000°C/hr produced rather heterogeneous charges containing large magnesian relict grains and small ferroan phenocrysts (Fig. 10), similar to the texture illustrated by Wasson & Rubin (2003). The small grains and the ferroan rims on relicts grew less than 10 Pm thick, while cooling at 1000°C/hr. Hewins & Fox (2004) made overgrowths on relict grains during cooling at about 800°C/hr, with thicknesses ranging from sub-micron on the finest starting materials to tens ...
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... less than 10 Pm thick, while cooling at 1000°C/hr. Hewins & Fox (2004) made overgrowths on relict grains during cooling at about 800°C/hr, with thicknesses ranging from sub-micron on the finest starting materials to tens of mi- crons on the coarsest. For powders with <20 Pm San Carlos olivine, overgrowths were about 2-6 Pm thick for 1470°C (e.g., Fig. 11a) and much thicker for 1550°C, when their abundance was considerably reduced (Fig. 11b). The thickness of olivine rims cannot be used as a cooling rate indicator, as it depends on too many factors (Fox & Hewins 2005). These include surface area of the substrate, i.e. number den- sity or size of crystals, the degree to which the relicts ...
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... on relict grains during cooling at about 800°C/hr, with thicknesses ranging from sub-micron on the finest starting materials to tens of mi- crons on the coarsest. For powders with <20 Pm San Carlos olivine, overgrowths were about 2-6 Pm thick for 1470°C (e.g., Fig. 11a) and much thicker for 1550°C, when their abundance was considerably reduced (Fig. 11b). The thickness of olivine rims cannot be used as a cooling rate indicator, as it depends on too many factors (Fox & Hewins 2005). These include surface area of the substrate, i.e. number den- sity or size of crystals, the degree to which the relicts dissolve, the melt composition, and ability of subsequent phases to nucleate, as well ...
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... that neither can be uniquely defined from textures. The effect of very rapid heating and cooling on chondrule textures has been studied pri- marily as a means of conserving alkalis in chondrules. Samples heated by a plasma gun with 10 ms pulses ( Hewins et al. 2002) had bubble-rich glass over essentially unaltered unsintered starting material (Fig. 12). The association glass + crystal frag- ments is reminiscent of the agglutinates formed by micro-impacts on the lunar sur- face ( Basu & Meinschein 1975), but not of chondrules. However, when charges are inserted into a furnace at superliquidus temperature, which is immediately cooled as fast as possible to reach the liquidus, i.e. 1-10 ...
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... closer matches to chondrule textures were produced in crystallization experiments with a time above the liquidus of at least a minute, for any grain size of starting material, than with heating for less than a second. Figure 12. Very short duration heating produces a layer of glass (top) over starting mate- rial (bottom) insufficiently heated to be well sintered ). ...
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... Yu et al. (2003) reinvestigated isothermal loss of alkalis from chondrule composition melts in a vacuum furnace. With nebular pressures, effec- tively all Na and K are lost in about 40 minutes in residual air (Fig. 13) or 20 minutes in H 2 , though Na is lost faster than K, whereas FeO is lost much more slowly ( ). Based on these results, any chondrule that experienced evaporation of FeO within a canonical nebula should be Na-free. Silicate melts heated at 10 -5 bars in residual air show the predicted Rayleigh fractionation of K isotopes. However, K ...
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... of K isotopes. However, K iso- topic fractionation showed a negative correlation with pressure, and is minimal at 1 atm, because of exchange with the ambient gas. showed that the extent of isotopic fractionation of K from a silicate melt at a P H2 of 1.3×10 -7 bars could be reduced by allowing the K gas to accumulate in a covered crucible (Fig. 14). Figure 13. Fraction of Na retained in K-enriched type IIAB analog chondrule melts at 1450ºC, as a function of pressure (of CO-CO 2 IW-0.5 at 1 atm, or residual air at 10 -5 atm) and time ( Yu et al. 2003). Yu et al. (1996) investigated S loss from chondrule-like melts at 1 atm total pressure and no ambient S species in the vapor. S ...
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... that the extent of isotopic fractionation of K from a silicate melt at a P H2 of 1.3×10 -7 bars could be reduced by allowing the K gas to accumulate in a covered crucible (Fig. 14). Figure 13. Fraction of Na retained in K-enriched type IIAB analog chondrule melts at 1450ºC, as a function of pressure (of CO-CO 2 IW-0.5 at 1 atm, or residual air at 10 -5 atm) and time ( Yu et al. 2003). ...
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... of such data to natural chondrules is difficult, because the total pressure of the ambient gas and the partial pressures of the S species are not known. Figure 14. Fraction of K retained and isotopic composition for charges heated at a P H2 of 1.3×10 -7 bars in covered capsules, as a function of cooling rate between 5000°C/hr and 10°C/hr. ...
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... experiments were conducted by on an anhy- drous CI-composition starting material (initial liquidus temperature ~1633ºC) to see whether the chemical compositions of the various chondrule types could be derived (Fig. 15a). Porphyritic olivine spherules were produced at 1580ºC and P H2 of 1.3u10 -5 atm and then quenched. The oxygen fugacity was controlled by the gas escaping from the melt. The olivine attempting to maintain equilibrium with the evaporating melt is initially similar to olivine in type IIA chondrules, but with FeO loss over 12- 18 hours it ...
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... by the gas escaping from the melt. The olivine attempting to maintain equilibrium with the evaporating melt is initially similar to olivine in type IIA chondrules, but with FeO loss over 12- 18 hours it became ~Fo 99 , similar to that in type IA chondrules. The CaO-MgO and CaO-FeO trends in the olivine match the trends for natural chondrules (Fig. 15b). This correlation shows that evaporation of CI precursors could generate some chon- drules but, though it is not apparent from this figure, silica-rich ones (IIB and IB) would require a more pyroxene-rich precursor. Mass fractionation of Si isotopes is observed in the charges, but not in natural chondrules. The absence of mass ...
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... loss of iron from silicate melt scaled for chondrule size would take about 3 hr with canonical condi- tions but longer in a gas with the high P Fe needed to suppress mass fractionation. Figure 15. Comparison of (MgO+SiO 2 ) in the bulk (a) and CaO in olivine (b) as a func- tion of Fe content of olivine (Fa), for chondrules (Jones 1996, and references therein) and charges evaporated at 1580ºC for 1 to 18 hr (numbers beside runs) in hydrogen at 1.3×10 - 5 bars ( ). ...
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... Na with exposure to this gas for 16 hours at 1330°C, due to condensation. Georges et al. (2000) performed similar 1 atm ex- periments on K condensation into a CMAS liquid, deriving the K by reduction of K 2 CO 3 by graphite. The equilibrium concentrations of K for partial pressures of K close to 10 -3 atm were achieved in 1-2 hours at 1410°C (Fig. 16a), while a K-rich melt evaporated to reach the same composition in the same time. The K isotopic composition of the charge also reaches an equilibrium value when saturation in K is achieved during condensation (Fig. 16b). This was taken to be the composition of the gas when saturation occurs, and subsequent isotopic exchange between the ...
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... graphite. The equilibrium concentrations of K for partial pressures of K close to 10 -3 atm were achieved in 1-2 hours at 1410°C (Fig. 16a), while a K-rich melt evaporated to reach the same composition in the same time. The K isotopic composition of the charge also reaches an equilibrium value when saturation in K is achieved during condensation (Fig. 16b). This was taken to be the composition of the gas when saturation occurs, and subsequent isotopic exchange between the gas, whose isotopic composition should continue to evolve, and the melt appears to be inefficient (Georges et al. 2000). From these two results, we see that alkalis entering chondrules from the nebular gas is a viable ...
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... and melt changed their characteristics as a result of con- densation of silica. Condensation for 2 to 5 minutes at 1350-1450°C and 10 -1 atm CO produced a chemical and mineralogical zonation in the charges. Olivine was replaced by pyroxene as the major phase towards the margin of the charges, and in some cases the rim contained a silica polymorph (Fig. 17). Charges which grew pyroxene around the rim as the result of the influx of Si resemble type IAB chondrules (Scott & Taylor 1983). By contrast, charges cooled without SiO in the ambient gas did not nucleate pyroxene until the temperature was lowered, and then throughout the charge, not just at the margin. The multiple zonation in ...

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... Dynamic crystallization (cooling rates) experiments have successfully reproduced chondrule textures and therefore, until recently, provided the main constraints on chondrule thermal history, including peak temperatures and cooling rates of chondrule melts (Hewins et al. 2005;Desch and Connolly 2002;Desch et al. 2012;Jones et al. 2018). Nucleation and growth theory states that spontaneous crystallization from a melt will not start at saturation but only when the solution becomes supersaturated. ...
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Chondrules in undifferentiated meteorites are former silicate melt droplets of variable texture and composition. Although widely studied, the chondrule formation mechanisms and conditions that explain all properties of chondrules are yet to be identified. To further constrain the processes that affected chondrules in the solar nebula and on the meteorite parent body, we determined in situ Si isotope ratios and major and trace element compositions of minerals in chondrules of variable types and sizes from the Allende CV3 chondrite. The δ³⁰Si in chondrule minerals ranges from −1.28 ± 0.19 to 0.55 ± 0.20‰ (2SE). The δ³⁰Si in chondrules shows no direct relationship with chondrule sizes or with distance between core and rim. Barred olivine-rich chondrules record the highest δ³⁰Si, likely because of faster cooling and less interaction with isotopically light nebular gas. Type I non-porphyritic and some porphyritic chondrules show overall higher δ³⁰Si compared to type II porphyritic chondrules. Furthermore, Mg-rich olivine and Mg-rich pyroxene have systematically higher δ³⁰Si compared to Fe-rich olivine and Fe-rich pyroxene. The variable δ³⁰Si of type I chondrule silicates (Mg-rich) compared to type II chondrule silicates (Fe-rich) may be explained by variable interaction of chondrule silicates with the nebular gas in the solar nebula. We envision a combination of equilibrium and kinetic isotope fractionation of Si between nebular gas and Fe-poor silicates (such as forsterite, anorthite, enstatite and mesostasis) and Fe-rich olivine and orthopyroxene. Petrographic evidence suggests that the enrichment of Fe in some highly altered porphyritic chondrules and at chondrule rims was likely caused by hydrothermal alteration on the parent body. Therefore, the correlation of Fe and δ³⁰Si of the chondrule minerals might serve as an indicator for the extent of further secondary processing of some chondrule minerals. The sum of these observations suggests that the formation and alteration of type II chondrules occurred by oxidation of originally reduced, metal-rich type I chondrules, both in the solar nebula and later on the meteorite parent body. Remaining ³⁰Si depleted gas contributed to the isotopic composition of matrix silicates. The evidence favours the formation of chondrules and matrix of the Allende meteorite in nebular settings rather than by asteroid impacts.
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... Wasson (1996)), the latter ignores other heat sources (by radiation or molecular collisions) which would slow it down, depending on the chondrule formation mechanism, as well as empirical evidence of longer heating (e.g. Hewins and Connolly, 2005;Marrocchi et al., 2018). It should also be noted that even the advocates of such short flash-heatings invoke repetitions of such events, perhaps a dozen of times (Baecker et al., 2017;Wasson and Rubin, 2003), which would in effect allow more time for relaxation to sphericity. ...
Preprint
Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a "nebular" setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .
... Wasson (1996)), the latter ignores other heat sources (by radiation or molecular collisions) which would slow it down, depending on the chondrule formation mechanism, as well as empirical evidence of longer heating (e.g. Hewins and Connolly, 2005;Marrocchi et al., 2018). It should also be noted that even the advocates of such short flash-heatings invoke repetitions of such events, perhaps a dozen of times (Baecker et al., 2017;Wasson and Rubin, 2003), which would in effect allow more time for relaxation to sphericity. ...
Article
Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a “nebular” setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .
... The igneous texture of coarse-grained CAIs could have been formed essentially in the same processes that resulted in chondrule formation: lightning (e.g., Desch and Cuzzi, 2000); nebular shocks (e.g., Desch and Connolly, 2002;Desch et al., 2010;Morris and Desch, 2010;Morris et al., 2012), current sheets (Hubbard et al., 2012;McNally et al., 2014), collision of planetesimals (e.g., Sanders and Scott, 2012;Johnson et al., 2014), among the others. The shock wave models are probably the most developed ones for chondrule formation and provide sufficiently detailed thermal evolution of reheated silicate grains to compare (favorably) to the results of laboratory experiments on chondrule formation (e.g., Hewins et al., 2005). According to the ''canonical" shock model of Desch and Connolly (2002), chondrule precursor materials 0.5 mm in size in such a shock (7 km s À1 , preshock temperature 25°C and pressure 10 À5 bar) could be heated by the hot shock front to $1650°C in a few hours, followed by very fast (minutes) cooling to $1450°C and slower cooling (35-50°C h À1 ) to $950°C. ...
Article
It is widely believed that the precursors of coarse-grained CAIs in chondrites are solar nebula condensates that were later reheated and melted to a high degree. Such melting under low-pressure conditions is expected to result in evaporation of moderately volatile magnesium and silicon and their mass-dependent isotopic fractionation. The evaporation of silicate melts has been extensively studied in vacuum laboratory experiments and a large experimental database on chemical and isotopic fractionations now exists. Nevertheless, it remains unclear if vacuum evaporation of CAI-like melts adequately describes the evaporation in the hydrogen-rich gas of the solar nebula. Here we report the results of a detailed experimental study on evaporation of a such melt at 1600 °C in both vacuum and low-pressure hydrogen gas, using 1.5- and 2.5-mm diameter samples. The experiments show that although at 2 × 10⁻⁴ bar H2 magnesium and silicon evaporate ∼2.8 times faster than at 2 × 10⁻⁵ bar H2 and ∼45 times faster than in vacuum, their relative evaporation rates and isotopic fractionation factors remain the same. This means that the chemical and isotopic evolutions of all evaporation residues plot along a single evaporation trajectory regardless of experimental conditions (vacuum or low-PH2) and sample size. The independence of chemical and isotopic evaporation trajectories on PH2 of the surrounding gas imply that the existing extensive experimental database on vacuum evaporation of CAI-like materials can be safely used to model the evaporation under solar nebula conditions, taking into account the dependence of evaporation kinetics on PH2. The experimental data suggest that it would take less than 25 min at 1600 °C to evaporate 15–50% of magnesium and 5–20% of silicon from a 2.5-mm diameter sample in a solar nebula with PH2 ∼2 × 10⁻⁴ bar and to enrich the residual melt in heavy magnesium and silicon isotopes up to δ²⁵Mg ∼5–10‰ and δ²⁹Si ∼2–4‰. The expected chemical and isotopic features are compatible to those typically observed in coarse-grained Type A and B CAIs. Evaporation for ∼1 h will produce δ²⁵Mg ∼30–35‰ and δ²⁹Si ∼10–15‰, close to the values in highly fractionated Type F and FUN CAIs. These very short timescales suggest melting and evaporation of CAI precursors in very short dynamic heating events. The experimental results reported here provide a stringent test of proposed astrophysical models for the origin and evolution of CAIs.
... Reduction of FeO during partial remelting could have increased the Mg# of the melt from which the pyroxene gen-2 subsequently crystallized. As the Mg# in pyroxene gen-2 is only 3 mol% higher, reduction of FeO in the melt would not have had to be extensive, which is consistent with the observation that reduction of olivine (Jones and Danielson, 1997;Hewins et al., 2005;Ushikubo et al., 2013) is not observed in this sample. However, a single large metal bleb and additionally numerous tiny grains (confirmed to be metal by means of EDX) are disseminated in the mesostasis and within low-Ca pyroxene in the chondrule. ...
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Chondrules from unequilibrated ordinary chondrites are among the oldest Solar system materials and preserve mineralogical, chemical and isotopic signatures that link them to their primary formation mechanisms and environments in the early Solar System. Some chondrules record features indicating modifications by high- to low-temperature processes throughout their residence time in the protoplanetary disk. Chondrules that were partially modified after their primary formation record chemical, isotopic and textural information on their initial formation conditions and subsequent reprocessing that are essential to reconstruct their formation environments and interpret the ages recorded by individual chondrules correctly. The detailed textural and major, minor and trace element analyses of two type-I chondrules from the low petrologic type ordinary chondrites MET 00526 and MET 00452 (L/LL3.05) reveal complex chemical and textural systematics bearing testimony of their multi- stage high temperature evolution, including reheating and partial remelting, in the evolving protoplanetary disk prior to accretion into their parent bodies. During primary crystallization of chondrule MET00526_Ch43, mineral growth, including incipient formation of feldspar in the outer parts of the chondrule, led to the fractionation of melt, eventually resulting in a chemical gradient in the mesostasis. During a later punctuated reheating that ultimately led to partial remelting of the outer parts of the chondrule, mesostasis and low-Ca pyroxene remelted partially. This partial remelting enhanced the chemical differences within the mesostasis and led to the formation of two chemically distinct mesostases in the inner and the outer zone of the chondrule with almost complementary abundances of Rb, Na, K, Ba, Sr and Eu. The calculated bulk mesostasis composition reveals chondritic relative abundances of these elements in the bulk chondrule with a slight depletion of the most volatile elements. Chemical and textural observations further indicate that this disequilibrium remelting occurred under more reducing conditions than the primary melting event preserved in the chondrule centre, allowing for the crystallization of a second generation of low-Ca pyroxene in the outer parts of the chondrule. Very similar processes are also recorded in chondrule MET00452_Ch22 with the degree of remelting being more extensive. A previously determined young 26Al-26Mg age of ~3 Ma after CAIs determined for chondrule MET00452_Ch22 dates the time of the chondrule remelting rather than its primary formation. This is evidence for a late thermal event in the protoplanetary disk and generally indicates that multiple, distinct thermal pulses occurred in the chondrule forming region of the protoplanetary disk throughout the time of chondrule formation. The nonconcentric secondary outer zone around a spherical inner zone may indicate a directed heat source as the cause of partial remelting and reprocessing of primary chondrules.