科研之友
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摘要
We have investigated the quadruple sulfur isotopic composition of inorganic sulfur-bearing phases from 13 carbonaceous chondrites of CM type. Our samples include 4 falls and 9 Antarctic finds. We extracted sulfur from sulfides, sulfates, and elemental sulfur (S-0) from all samples. On average, we recover a bulk sulfur (S) content of 2.11 perpendicular to 0.39 wt.% S (1 sigma). The recovered sulfate, S-0 and sulfide contents represent 25 +/- 12%, 10 +/- 7% and 65 +/- 15% of the bulk S, respectively (all 1 sigma). There is no evidence for differences in the bulk S content between falls and finds, and there is no correlation between the S speciation and the extent of aqueous alteration. We report ranges of Delta S-33 and Delta S-36 values in CMs that are significantly larger than previously observed. The largest variations are exhibited by S-0, with Delta S-33 values ranging between -0.104 perpendicular to 0.012% and +0.256 +/- 0.018% (2 sigma). The Delta S-36/S-33 ratios of S-0 are on average -3.1 +/- 1.0 (2 sigma). Two CMs show distinct Delta S-36/S-33 ratios, of +1.3 + 0.1 and +0.9 + 0.1. We suggest that these mass independent S isotopic compositions record H2S photodissociation in the nebula. The varying Delta S-36/Delta S-33 ratios are interpreted to reflect photodissociation that occurred at different UV wavelengths. The preservation of these isotopic features requires that the S-bearing phases were heterogeneously accreted to the CM parent body. Non-zero Delta S-33 values are also preserved in sulfide and sulfate, and are positively correlated with S-0 values. This indicates a genetic relationship between the S-bearing phases: We argue that sulfates were produced by the direct oxidation of S-0 (not sulfide) in the parent body. We describe two types of models that, although imperfect, can explain the major features of the CM S isotope compositions, and can be tested in future studies. Sulfide and S-0 could both be condensates from the nebula, as the residue and product, respectively, of incomplete H2S photodissociation by UV light (wavelength < 150 nm). This idea requires that FeS formation and the S-0 condensation co-occur. As an alternative, ice accretion to the CM parent body could allow the delivery of S-MIF in CMs. In that case, sulfides would have been the only S-bearing condensate in CM precursors, and S-0 would have been derived from the oxidation of H2S trapped in ices, after its photodissociation at low temperature (< 500 K) in the nebula. In our models, the observations of H2S UV photodissociation is required to occur at the disk surface, and allowed in nebular environments with canonical C/O ratios. Vertical motions in the disk would redistribute phases that condensed at high altitude to the midplane, where they accreted in the phases that make up the chondritic matrix.
- 出版日期2017-1-1
