Formation and metamorphism of group A5 chondrules in ordinary chondrites

Abstract-We report bulk and olivine compositions in 66 stony cosmic spherules (Na 2 O < 0.76 wt%), 200-800 lm in size, from the Transantarctic Mountains, Antarctica. In porphyritic cosmic spherules, relict olivines that survived atmospheric entry heating are always Ni-poor and similar in composition to the olivines in carbonaceous or unequilibrated ordinary chondrites (18 spherules), and equilibrated ordinary chondrites (one spherule). This is consistent with selective survival of high temperature, Mg-rich olivines during atmospheric entry. Olivines that crystallized from the melts produced during atmospheric entry have NiO contents that increase with increasing NiO in the bulk spherule, and that range from values similar to those observed in chondritic olivines (NiO generally <0.5 wt%) to values characteristic of olivines in meteoritic ablation spheres (NiO > 2 wt%). Thus, NiO content in olivine cannot be used alone to distinguish meteoritic ablation spheres from cosmic spherules, and the volatile element contents have to be considered. We propose that the variation in NiO contents in cosmic spherules and their olivines is the result of variable content of Fe, Ni metal in the precursor. NiO contents in olivines and in cosmic spherules can thus be used to discuss their parent body. Ni-poor spherules can be derived from C-rich and ⁄ or metal-poor precursors, either related to CM, CI, CR chondrites or to chondritic fragments dominated by silicates, regardless of the parent body. Ni-rich spherules (NiO > 0.7 wt%) that represent 55% of the 47 barred-olivine spherules we studied, were derived from the melting of C-poor, metal-rich precursors, compatible with ordinary chondrite or CO, CV, CK carbonaceous chondrite parentages.

“…1996; Jones 1996; Buchanan et al. 1997; Greshake 1997; Huang and Sears 1997; Krot et al. 1997, 2005, 2006; Leshin et al.…”

Section: Resultsmentioning

confidence: 99%

Section: Cosmic Spherule Versus Ablation Spheresmentioning

confidence: 99%

Nickel abundance in stony cosmic spherules: Constraining precursor material and formation mechanisms

Cordier¹,

Ginneken

Folco

2011

“…2) and the generally higher alkali contents of Semarkona matrix-rims compared to Bishunpur matrix-rims is consistent with more extensive exchange between chondrules and matrix-rims in Bishunpur. Also, in the metamorphic sequence Semarkona + Bishunpur + Krymka + Chainpur, there is a general increase in the proportions of normative albite relative to anorthite in chondrule mesostases, as well as an increase in the total normative feldspar (Sears et al, 1995a,b;Huang and Sears, 1997). These data are consistent with both the addition of Na to chondrule mesostases during metamorphism and the fact that it was an important process even for Bishunpur.…”

Section: Isotopic Exchange On the Parent Body?mentioning

confidence: 98%

The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Alexander

Grossman

Wang

et al. 2000

116

Abstract-In a search for evidence of evaporation during chondrule formation, the mesostases of 11Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K-isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had 841K compositions that were normal within error (typically ?3%, 20). The six "anomalous" points are probably all artifacts. Experiments have shown that free evaporation of K leads to large 41K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3%0 enrichment in 841K is produced by -12% loss of K. The range of L-chondrite-normalized K/Al ratios (a measure of the K-elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L-chondrite-like K abundances and the K loss occurred via Rayleigh fractionation, the most K-depleted chondrules would have had compositions of up to 841K = 200%0. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of WA1 fractionation in chondrules does not explain the K-isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K-isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim-matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K-isotopic fractionation in them requires that the K-isotopic exchange took place during chondrule formation, which would probably require gas-chondrule exchange. Potassium evaporated from fine-grained dust and chondrules during chondrule formation may have produced sufficient K-vapor pressure for gas-chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.

“…The causes of the compositional differences among the chondrule types are the subject of debate. Theories to explain the variability can be divided into two categories: 1) dissimilarities in chondrule precursors (Grossman and Wasson 1983;Rubin 1986;Jones and Scott 1989;Alexander 1994;Jones 1994Jones , 1996Rubin, Sailer, and Wasson 1999), and 2) volatile loss and Fe reduction during chondrule formation (Jones 1990;Alexander 1994;Hewins et al 1996Hewins et al , 1997Huang et al 1996;Huang and Sears 1997 Fig. 4b and as a thin rim between the phosphate and chondrule mesostasis in Fig.…”

Section: The Composition and Chemistry Of Chondrule Meltsmentioning

confidence: 99%

Opaque minerals in chondrules and fine‐grained chondrule rims in the Bishunpur (LL3.1) chondrite

Лауретта

Buseck

2003

Abstract-We present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the Bishunpur (LL3.1) ordinary chondrite. There are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary, even though the two types of metal grains can be separated by only a few microns. Metal grains in chondrule interiors exhibit minor alteration in the form of oxidized P-, Cr-, and Si-bearing minerals. Metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite. The results of this study suggest that many metal grains in Bishunpur reacted with a type-I chondrule melt and incorporated significant amounts of P, Cr, and Si. As the system cooled, some metal oxidation occurred in the chondrule interior, producing metal-associated phosphate, chromite, and silica. Metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation. It appears that chondrulederived metal and its corrosion products were incorporated into the fine-grained rims that surround many type-I chondrules, contributing to their Fe-rich compositions. We propose that these finegrained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine-grained mineral fragments and microchondrules.