The evolution of chondrules

The range of olivine compositions forming anomalous inclusions in chondrules suggests fractional condensation, as does the sequence of minerals in fine matrix particles. Chondrule bulk compositions vary in a random way as the precursors (condensates) were assembled heterogeneously. Chondrule heating and cooling were moderately rapid, and at high oxygen fugacities, suggesting processing in a localized particle-rich clump in the nebula. Rims deposited on chondrules show that condensation continued after chondrule formation. The hot particle-rich clump slows radiative cooling of chondrules, allows condensation, provides high oxygen fugacity as a result of evaporation and generates the collisions needed to fragment chondrules. The simultaneous evaporation/condensation and melting/crystallization are explained by pressure gradients in the clump. In the thickest parts of the clump, high vapor pressure allows melting, but at the outside evaporation and condensation would proceed. The observation that round chondrules are more magnesian and fragments are more ferroan is similarly consistent with formation in outer low-pressure reduced regions and the thick clump center, respectively. Turbulent migrations of condensates in and out of such clumps would explain the random nature of precursor compositions, while evaporation and gas dissipation would explain changes in oxygen fugacity which chondrules record plus their shape-redox relationship

Evaporation and recondensation of sodium in Semarkona Type II chondrules

We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa8 and Fa13, with 0.0022–0.0069 ± 0.0013 wt.% Na2O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ∼Fa35 and 0.03 wt.% Na2O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent DNa, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent DNa values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium DNa to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases. Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high PT. The high Na concentrations in olivine and mesostases indicate very high PNa while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high PT, PNa and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, "cluster chondrites", compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange

On the possible role of elemental carbon in the formation of reduced chondrules

Recent experiments have been designed to produce chondrule textures via flash melting while simultaneously studying the nature of chondrule precursors. However, these experiments have only been concerned with silicate starting material. This is a preliminary report concerning what effects elemental carbon, when added to the silicate starting material, has on the origin of chondrules produced by flash melting

Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo

Meeting Abstracts: Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo Clearwater Beach, FL, USA. 9-11 June 201

Evaporation and recondensation of sodium in Semarkona type II chondrules. Geochimica Et Cosmochimica Acta 78

Abstract We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa 8 and Fa 13 , with 0.0022-0.0069 ± 0.0013 wt.% Na 2 O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine-liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are $Fa 35 and 0.03 wt.% Na 2 O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent D Na , indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent D Na values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium D Na to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases. Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high P T . The high Na concentrations in olivine and mesostases indicate very high P Na while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high P T , P Na and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, "cluster chondrites", compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange

A petrological and chemical reexamination of Main Group pallasite formation

Abstract The large number of pallasite parent bodies suggests that their formation was a common event in asteroid evolution. New microprobe data has been generated for pallasite olivines and chromites that indicate subsolidus redox processes, while literature data has been collated and correlated to form new results and interpretations. The new data include information on metal compositions; the metal cooling rates; the formation of round and fragmental olivine; the close-packing of olivine; the crystallization of the phosphates, phosphoran olivine and orthopyroxene; and the bulk pallasite P and S contents. A model for Main Group pallasites has been developed using fractional melting of a chondritic precursor to make a multi-layered parent body. One portion of this contains a gradational metal-olivine (pallasite) texture. Concentric downward crystallization of the molten metal is indicated, based on cooling rates and metal compositions. Fractional crystallization of the residual silicate and metallic melts produces the minor phases on cooling. Comparisons to and a brief review of past models, particularly those utilizing mixing, have been made

Refractory inclusions as Type IA chondrule precursors: Constraints frommelting experiments

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