Mass independent sulfur isotope signatures in CMs: Implications for sulfur chemistry in the early solar system
Mass independent sulfur isotope signatures in CMs: Implications for sulfur chemistry in the early solar system
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DOI：
10.1016/j.gca.2016.09.036
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发表时间：
2017-01-01
影响因子：
5
通讯作者：
Oduro, H.
中科院分区：
文献类型：
--
作者：
Labidi, J.;Farquhar, J.;Oduro, H.
我们研究了 13 颗 CM 型碳质球粒陨石的无机含硫相的四重硫同位素组成。我们的样本包括 4 个瀑布和 9 个南极发现物。我们从所有样品中的硫化物、硫酸盐和元素硫 (S-0) 中提取了硫。平均而言，我们回收的散装硫 (S) 含量为 2.11，垂直于 0.39 wt.% S (1 sigma)。回收的硫酸盐、S-0 和硫化物含量分别占本体 S 的 25 +/- 12%、10 +/- 7% 和 65 +/- 15%（均为 1 sigma）。没有证据表明掉落和发现之间的大量硫含量存在差异，并且硫形态与水体蚀变程度之间没有相关性。我们报告 CM 中 Delta S-33 和 Delta S-36 值的范围明显大于之前观察到的值。 S-0 表现出最大的变化，Delta S-33 值范围在 -0.104 垂直于 0.012% 和 +0.256 +/- 0.018% (2 sigma) 之间。 S-0 的 Delta S-36/S-33 比率平均为 -3.1 +/- 1.0 (2 sigma)。两个 CM 显示出不同的 Delta S-36/S-33 比率，分别为 +1.3 + 0.1 和 +0.9 + 0.1。我们认为这些与质量无关的 S 同位素成分记录了星云中的 H2S 光解离。不同的 Delta S-36/Delta S-33 比率被解释为反映了在不同 UV 波长下发生的光解离。这些同位素特征的保存要求含硫相异质地附着到 CM 母体上。非零 Delta S-33 值也保留在硫化物和硫酸盐中，并且与 S-0 值呈正相关。这表明含硫相之间存在遗传关系：我们认为硫酸盐是由母体中的 S-0（不是硫化物）直接氧化产生的。我们描述了两种类型的模型，虽然不完美，但可以解释 CM S 同位素组成的主要特征，并且可以在未来的研究中进行测试。硫化物和S-0都可能是星云的冷凝物，分别是紫外光（波长<150 nm）不完全H2S光解的残留物和产物。这个想法要求 FeS 形成和 S-0 缩合同时发生。作为替代方案，CM 母体的冰积聚可以允许在 CM 中传递 S-MIF。在这种情况下，硫化物将是 CM 前体中唯一的含 S 凝结物，而 S-0 可能源自冰中捕获的 H2S 在星云中低温（< 500 K）光解离后的氧化。在我们的模型中，H2S 紫外光解离的观测需要发生在盘表面，并且允许在具有规范 C/O 比的星云环境中进行。圆盘中的垂直运动会将在高空凝结的相重新分布到中平面，在那里它们以构成球粒状基质的相吸积。 (C) 2016 Elsevier Ltd. 保留所有权利。
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. (C) 2016 Elsevier Ltd. All rights reserved.