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Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: indication for a single large formation region across snow line.

ABSTRACT: In-situ oxygen three-isotope analyses of chondrules and isolated olivine grains in the Paris (CM) chondrite were conducted by secondary ion mass spectrometry (SIMS). Multiple analyses of olivine and/or pyroxene in each chondrule show indistinguishable Δ¹⁷O values, except for minor occurrences of relict olivine grains (and one low-Ca pyroxene). A mean Δ¹⁷O value of these homogeneous multiple analyses was obtained for each chondrule, which represent oxygen isotope ratios of the chondrule melt. The Δ¹⁷O values of individual chondrules range from -7‰ to -2‰ and generally increase with decreasing Mg# of olivine and pyroxene in individual chondrules. Most type I (FeO-poor) chondrules have high Mg# (~99) and variable Δ¹⁷O values from -7.0‰ to -3.3‰. Other type I chondrules (Mg# ≤97), type II (FeO-rich) chondrules, and two isolated FeO-rich olivine grains have host Δ¹⁷O values from -3‰ to -2‰. Eight chondrules contain relict grains that are either ¹⁶O-rich or ¹⁶O-poor relative to their host chondrule and show a wide range of Δ¹⁷O values from -13‰ to 0‰. The results from chondrules in the Paris meteorite are similar to those in Murchison (CM). Collectively, the Δ¹⁷O values of chondrules in CM chondrites continuously increase from -7‰ to -2‰ with decreasing Mg# from 99 to 37. The majority of type I chondrules (Mg# >98) show Δ¹⁷O values from -6‰ to -4‰, while the majority of and type II chondrules (Mg# 60-70) show Δ¹⁷O values of -2.5‰. The covariation of Δ¹⁷O versus Mg# observed among chondrules in CM chondrites may suggest that most chondrules in carbonaceous chondrites formed in a single large region across the snow line where the contribution of ¹⁶O-poor ice to chondrule precursors and dust enrichment factors varied significantly.

SUBMITTER: Chaumard N

PROVIDER: S-EPMC8588000 | biostudies-literature | 2021 Apr

REPOSITORIES: biostudies-literature

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Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: indication for a single large formation region across snow line.

Chaumard Noël N Defouilloy Céline C Hertwig Andreas T AT Kita Noriko T NT

Geochimica et cosmochimica acta 20210218

In-situ oxygen three-isotope analyses of chondrules and isolated olivine grains in the Paris (CM) chondrite were conducted by secondary ion mass spectrometry (SIMS). Multiple analyses of olivine and/or pyroxene in each chondrule show indistinguishable Δ<sup>17</sup>O values, except for minor occurrences of relict olivine grains (and one low-Ca pyroxene). A mean Δ<sup>17</sup>O value of these homogeneous multiple analyses was obtained for each chondrule, which represent oxygen isotope ratios of t ...[more]

PMID: 34776527

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Project description: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; Δ170>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.

Project description:Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#'s of 99.0 to 99.2 and Δ17O of -4.2‰ to -5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range from (3.5 ± 1.3) × 10‒6 to (6.0 ± 3.9) × 10‒6. This corresponds to formation times of 2.2 (-0.5/+1.1) Myr to 2.8 (‒0.3/+0.5) Myr after CAIs, using a canonical (26Al/27Al)0 of 5.23 × 10-5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#'s (94.2 to 98.7) and generally higher Δ17O (-0.9‰ to -4.9‰) than chondrules with resolvable excess 26Mg. Their inferred (26Al/27Al)0 upper limits range from 1.3 × 10‒6 to 3.2 × 10‒6, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00-3.05 chondrites. We conclude that the resolvable (26Al/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (26Al/27Al)0, Mg#, and Δ17O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ17O ~-5‰ chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ~100×. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher Δ17O (-5‰ to -1‰), with low (26Al/27Al)0, and hence no resolvable excess 26Mg. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (26Al/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al/27Al)0. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.

Project description:Oxygen three-isotope analysis by secondary ion mass spectrometry of chondrule olivine and pyroxene in combination with electron microprobe analysis were carried out to investigate 24 FeO-poor (type I) and 2 FeO-rich (type II) chondrules from the Kaba (CV) chondrite. The Mg#'s of olivine and pyroxene in individual chondrules are uniform, which confirms that Kaba is one of the least thermally metamorphosed CV3 chondrites. The majority of chondrules in Kaba contain olivine and pyroxene that show indistinguishable Δ17O values (= δ17O - 0.52 × δ18O) within analytical uncertainties, as revealed by multiple spot analyses of individual chondrules. One third of chondrules contain olivine relict grains that are either 16O-rich or 16O-poor relative to other indistinguishable olivine and/or pyroxene analyses in the same chondrules. Excluding those isotopically recognized relicts, the mean oxygen isotope ratios (δ18O, δ17O, and Δ17O) of individual chondrules are calculated, which are interpreted to represent those of the final chondrule melt. Most of these isotope ratios plot on or slightly below the primitive chondrule mineral (PCM) line on the oxygen three-isotope diagram, except for the pyroxene-rich type II chondrule that plots above the PCM and on the terrestrial fractionation line. The Δ17O values of type I chondrules range from ~ -8‰ to ~ -4‰; the pyroxene-rich type II chondrule yields ~0‰, the olivine-rich type II chondrule ~ -2‰. In contrast to the ungrouped carbonaceous chondrite Acfer 094, the Yamato 81020 CO3, and the Allende CV3 chondrite, type I chondrules in Kaba only possess Δ17O values below -3‰ and a pronounced bimodal distribution of Δ17O values, as evident for those other chondrites, was not observed for Kaba. Investigation of the Mg#-Δ17O relationship revealed that Δ17O values tend to increase with decreasing Mg#'s, similar to those observed for CR chondrites though data from Kaba cluster at the high Mg# (>98) and the low Δ17O end (-6‰ and -4‰). A mass balance model involving 16O-rich anhydrous dust (Δ17O = -8‰) and 16O-poor water ice (Δ17O = +2‰) in the chondrule precursors suggests that type I chondrules in Kaba would have formed in a moderately high dust enriched protoplanetary disk at relatively dry conditions (~50-100× dust enrichment compared to Solar abundance gas and less than 0.6× ice enhancement relative to CI chondritic dust). The olivine-rich type II chondrule probably formed in a disk with higher dust enrichment (~2000× Solar).

Project description:The presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early Solar System; however, the process of aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) chondrites are particularly well-suited for the study of aqueous alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots of the alteration process through time. We studied oxygen isotopes in secondary-minerals from six CR chondrites of varying hydration states to determine how aqueous fluid conditions (including composition and temperature) evolved on the parent body. Secondary minerals analyzed included calcite, dolomite, and magnetite. The O-isotope composition of calcites ranged from δ18O ≈ 9 to 35 ‰, dolomites from δ18O ≈ 23 to 27 ‰, and magnetites from δ18O ≈ -18 to 5 ‰. Calcite in less-altered samples showed more evidence of fluid evolution compared to heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted on a single trend, with the exception of grains from the extensively hydrated chondrite MIL 090292. The MIL 090292 magnetite diverges from this trend, possibly indicating an anomalous origin for the meteorite. If magnetite and calcite formed in equilibrium, then the relative 18O fractionation between them can be used to extract the temperature of co-precipitation. Isotopic fractionation in Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (~60°C). Assuming that the CR parent body experienced closed-system alteration, a similar exercise for parallel calcite and magnetite O-isotope arrays yields "global" alteration temperatures of ~55 to 88 °C. These secondary mineral arrays indicate that the O-isotopic composition of the altering fluid evolved upon progressive alteration, beginning near the Al Rais water composition of Δ17O ~ 1 ‰ and δ18O ~ 10 ‰, and becoming increasingly 16O-enriched toward a final fluid composition of Δ17O ~ -1.2 ‰ and δ18O ~ -15 ‰.

Dataset Information

Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: indication for a single large formation region across snow line.

Publications

Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: indication for a single large formation region across snow line.

Similar Datasets

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