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Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition.

ABSTRACT: Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors.

SUBMITTER: Tartese R

PROVIDER: S-EPMC6112742 | biostudies-literature | 2018 Aug

REPOSITORIES: biostudies-literature

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Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition.

Tartèse Romain R Chaussidon Marc M Gurenko Andrey A Delarue Frédéric F Robert François F

Proceedings of the National Academy of Sciences of the United States of America 20180806 34

Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. T ...[more]

PMID: 30082400

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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 ?17O values ranging from -6.0‰ to -4.1‰, with one exception (?17O: -1.2‰; Mg#: 99.6). Homogeneous chondrules with Mg#'s <96, including four type II chondrules (Mg# ~65-70), have ?17O values of around -2.5%. Five type I chondrules (Mg# ?99) have internally heterogeneous oxygen isotope ratios with ?17O 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 ?17O 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 ?17O ~-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 ?17O 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 ?17O ~-5% and Mg# ~99 chondrules, but enriched in 16O-poor H2O ice (~0.3-0.4× the CI dust; ?170>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.

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Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition.

Publications

Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition.

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