¹Noël Chaumard,¹Céline Defouilloy, ¹Noriko T. Kita
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.02.040]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706-1692, USA
Copyright Elsevier

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 29 chondrules in the Murchison CM2 chondrite by secondary ion mass spectrometry (SIMS). The oxygen isotope ratios of analyzed chondrules all plot very close to the primitive chondrule minerals (PCM) line. In each of 24 chondrules, the olivine and/or pyroxene grains analyzed show indistinguishable oxygen isotope ratios. Exceptions are minor occurrences of isotopically distinguished relict olivine grains, which were found in nine chondrules. The isotope homogeneity of these phenocrysts is consistent with a co-magmatic crystallization of olivine and pyroxene from the final chondrule melts and a significant oxygen isotope exchange between the ambient gas and the melts. Homogeneous type I chondrules with Mg#’s of 98.9–99.5 have host chondrule Δ17O values ranging from –6.0‰ to –4.1‰, with one exception (Δ17O: –1.2‰; Mg#: 99.6). Homogeneous chondrules with Mg#’s <96, including four type II chondrules (Mg# ∼65–70), have Δ17O values of around –2.5‰. Five type I chondrules (Mg# ≥99) have internally heterogeneous oxygen isotope ratios with Δ17O values ranging from –6.5‰ to –4.0‰, similar to those of host chondrule values. These heterogeneous chondrules have granular or porphyritic textures, convoluted outlines, and contain numerous metal grains dispersed within fine-grained silicates. This is consistent with a low degree of melting of the chondrule precursors, possibly because of a low temperature of the melting event and/or a shorter duration of melting. The Δ17O values of relict olivine grains in nine chondrules range from –17.9‰ to –3.4‰, while most of them overlap the range of the host chondrule values.

Similar to those reported from multiple carbonaceous chondrites (Acfer 094, Y-82094, CO, CR, and CV), the Δ17O ∼–5‰ and high Mg# (≥99) chondrules, which might derive from a reduced reservoir with limited dust enrichments (∼50× Solar System), dominate the population of chondrules in Murchison. Other chondrules in Murchison formed in more oxidizing environment (Mg#<96) with higher Δ17O values of –2.5‰, in agreement with the low Mg# chondrules in Acfer 094 and CO chondrites and some chondrules in CV and CR chondrites. They might form in environments containing the same anhydrous precursors as for the Δ17O ∼–5‰ and Mg# ∼99 chondrules, but enriched in 16O-poor H2O ice (∼0.3–0.4× the CI dust; Δ17O>0‰) and at dust enrichments of ∼300–2000×.

Regarding the Mg# and oxygen isotope ratios, the chondrule populations sampled by CM and CO chondrites are similar and indistinguishable. The similarity of these 16O-rich components in CO and CM chondrites is also supported by the common Fe/Mn ratio of olivine in type II chondrules. Although they accreted similar high-temperature silicates, CO chondrites are anhydrous compared to CM chondrites, suggesting they derived from different parent bodies formed inside and outside the snow line, respectively. If chondrules in CO and CM chondrites formed at the same disk locations but the CM parent body accreted later than the CO parent body, the snow line might have crossed the the common chondrule-forming region towards the Sun between the time of the CO and CM parent bodies accretion.

¹Andrea Patzer, and ¹Harry Y. McSween Jr.
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13064]
¹Department of Applied Geology, University of Goettingen, Goldschmidtstr. 3, 37077 Goettingen, Germany
²Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee37996–1410, USA
Published by arrangement with John Wiley & Sons

We investigated several olivine-bearing, medium-grained, ophitic to subophitic eucritic clasts from three different Antarctic howardites. Based on grain size (0.5–2 mm), these clasts could represent intrusive igneous units. Based on mineral composition (pyroxene and plagioclase), they are similar to basaltic eucrites. Elemental concentrations of the major silicates and bulk mg#, however, range from those known for basaltic eucrites to those found in cumulate eucrites. Recognizable cumulus phases are absent. Conservatively speaking, the clasts examined may simply classify as relatively coarse-grained unequilibrated basaltic eucrites. Alternatively, at least one of the clasts showing intermediate grain size and a relatively primitive chemical composition (mg# 50) may sample a rock type that could be genetically placed between the basaltic and cumulate eucrite lines of origin. A minor, yet genetically meaningful common feature of the clasts studied is the occurrence of fayalitic olivine. Two distinct categories exist. They are (1) fine veinlets exclusively percolating through pyroxene and (2) more substantial (up to 100 μm wide) veins and/or interstitial deposits. Only the fine veinlets also contain variable amounts of anorthite, ilmenite, and troilite. Although both types of olivine are ferroan, textural aspects suggest distinct paths of generation. The fine veinlets are best explained by decomposition of relatively FeO-rich, heterogeneous, and locally metastable pyroxene, caused in situ by impact heating and subsequent fast cooling. The wider, often very ragged-looking monomineralic olivine fillings, on the other hand, may represent the iron-enriched portion of a highly fractionated magma.