A compositional classification scheme for meteoritic chondrules

Sears, D. W. G.; Lü, Junfu; Benoit, P. H.; Dehart, J. M.; Lofgren, G. E.

doi:10.1038/357207a0

Cited by 56 publications

(29 citation statements)

References 20 publications

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“…Sears et al (1992) and Haak and Scott (1993) first suggested that it was type IA chondrules that were preferentially lost from the OC formation region, but as was pointed out by Grossman (1996), the typically low Na/Si ratios of type IA chondrules compared to CI and bulk OC (Fig. 11) means that the loss of type IAs alone cannot account for the volatile element fractionations.…”

Section: What Was Fractionated From What?mentioning

confidence: 99%

“…The Jones type IIA chondrules and the INAA averages are also very similar to the bulk silicate OC compositions. Sears et al (1992) have argued that type I chondrules are underrepresented in the chondrules studied by INAA. However, as pointed out by Grossman (1996), the significantly smaller sizes and masses of the type I chondrules means that even if they are underrepresented, this will only have a modest effect on the bulk chondrule composition.…”

Section: Major Element Compositions Of Chondrites Chondrules and Mamentioning

confidence: 99%

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Re-examining the role of chondrules in producing the elemental fractionations in chondrites

Alexander¹

2005

Meteoritics &Amp; Planetary Science

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Abstract-The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI-like component. The matrix-normalized abundances of the more volatile elements (condensation temperatures <750-800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI-like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe-metal is added to the chondrule composition. Measured matrix compositions are not CI-like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ∼10-30% of refractory-rich, volatile depleted material was added to matrix.If most of the more volatile elements are in a CI-dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg-SiFe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.

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Section: What Was Fractionated From What?mentioning

confidence: 99%

Section: Major Element Compositions Of Chondrites Chondrules and Mamentioning

confidence: 99%

Re-examining the role of chondrules in producing the elemental fractionations in chondrites

Alexander¹

2005

Meteoritics &Amp; Planetary Science

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“…The consequence of this is that the fields in the ternary discriminant diagram of DeHart et al (1992) and Sears et al (1992) delineating A5, B1, B2, B3 are not justified. There probably are real differences between the chondrules that these authors classified as B1 and those that plot in the other fields: these are the chondrules with mesostasis most susceptible to beam damage, meaning that they are chondrules with less crystalline albite.…”

Section: Classification Of Chondrulesmentioning

confidence: 99%

“…This does not mean, however, that shock events elsewhere on the parent body could not have generated the heat or fluids associated with metamorphism in these chondrites. DeHart et al (1992) and Sears et al (1992) developed a classification scheme for chondrules based on three properties: CL of major phases, especially olivine and mesostasis, CaO-FeO relationships in olivine, and the normative composition of mesostasis. This scheme divides chondrules into two major groups, A (bright CL) and B (low CL), which are in turn divided into subgroups A1-A5 and B1-B3 on the basis of mineral compositions.…”

Section: The Role Of Shock Metamorphismmentioning

confidence: 99%

The onset of metamorphism in ordinary and carbonaceous chondrites

Grossman

Brearley

2005

Meteorit & Planetary Scien

342

441

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Received May 18, 2004; revision accepted November 3, 2004) Abstract-Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by Xray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.

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“…Another 5-10 showed uniformly high Na in their mesostasis, but without visible zoning. Among the type II (high-FeO, Sears group B) chondrules, which greatly outnumber type I chondrules (Sears et al, 1992) and which are much richer in alkalis, only a few potentially weakly zoned chondrules were found using the Na maps. Thus, -15% ofSemarkona chondrules are overtly zoned in Na, almost all of which are type I.…”

Section: Abundance Of Zoned Chondrules In Semarkonamentioning

confidence: 99%

Zoned chondrules in Semarkona: Evidence for high‐ and low‐temperature processing

Grossman

Alexander

Wang

et al. 2002

Meteorit & Planetary Scien

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Abstract-At least 15% of the 10w-FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins. We have studied zoned chondrules using electron microprobe methods (x-ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved.Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water). Calcium is depleted in areas ofvolatile enrichment. Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another; Mn follows Na in some chondru1es but not in others; Fe and Mg are unzoned. D/H ratios increase in the water-rich areas of zoned chondrules. Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside. Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification.Systematic variations in zoning patterns among pyroxene-and olivine-rich chondrules may indicate that fractionation oflow-and high-Ca pyroxene played some role in Ti, Cr, Mn, Si, AI, and some Ca zoning. But direct condensation ofelements into hot chondrules, secondary melting oflate condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub-parallel zoning profiles ofmany elements, the presence ofH20, or elemental abundance patterns.Zoning ofmoderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid. This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material. Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, 10w-FeO chondrules appear to have fully altered mesostasis.

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A compositional classification scheme for meteoritic chondrules

Cited by 56 publications

References 20 publications

Re-examining the role of chondrules in producing the elemental fractionations in chondrites

Re-examining the role of chondrules in producing the elemental fractionations in chondrites

The onset of metamorphism in ordinary and carbonaceous chondrites

Zoned chondrules in Semarkona: Evidence for high‐ and low‐temperature processing

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