Deuterium to hydrogen isotope ratios in unequilibrated ordinary chondrites (UOCs) which have undergone little-to-no thermal metamorphism pose an interesting problem when looking at water in the early Solar System. Bulk chondrite studies have shown that UOCs of the lowest subtypes have D/H ratios as high as comets from the outer Solar System, which, along with bulk UOC water abundances, decrease with thermal metamorphism. Since bulk UOC analyses represent a complex mixture of organic and hydrated phases, it is not clear what phase(s) is responsible for the high bulk D/H values. In this study, we report secondary ion mass spectrometry (SIMS) measurements of the H isotope composition of the fine-grained matrix of UOCs with petrological subtypes ranging from 3.00 to 3.9. We find that for matrix areas in UOCs of petrologic subtype ≥3.2, correlations between D-rich organic material and D-poor phyllosilicates give relatively D-poor intrinsic water isotopic compositions, with δD values between −320 ± 91 ‰ and −71 ± 71 ‰, which are inherited from parent body accretion. Therefore, we conclude that OC parent bodies accreted D-poor water ice that had an H isotopic composition similar to that of CM and CV chondrite parent bodies. We find that matrix in UOCs of the lowest subtypes (Semarkona, Bishunpur, and Ngawi) show similar water and organic H isotope compositions to higher type UOCs. Our analyses also show that matrix areas in these pristine UOCs contain a third, thus far unidentified, component that carries the high D/H signature, with δD values up to ∼6000 ‰. We propose that this component is pristine amorphous silicates preserved from the molecular cloud or early protoplanetary disc that is extremely sensitive to thermal and aqueous alteration on asteroidal parent bodies.

中文翻译：

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未平衡普通球粒陨石基质中原始高 D/H 成分的鉴定


在观察早期太阳系中的水时，几乎没有经历过热变质作用的不平衡普通球粒陨石（UOC）中的氘与氢同位素比率提出了一个有趣的问题。块状球粒陨石研究表明，最低亚型的 UOC 的 D/H 比与来自太阳系外的彗星一样高，而块状 UOC 的水丰度随着热变质作用而减少。由于本体 UOC 分析代表有机相和水合相的复杂混合物，因此尚不清楚是什么相导致本体 D/H 值较高。在这项研究中，我们报告了岩石学亚型范围为 3.00 至 3.9 的 UOC 细粒基质的 H 同位素组成的二次离子质谱 (SIMS) 测量结果。我们发现，对于岩石学亚型≥3.2的UOC中的基质区域，富含D的有机物质和贫D的层状硅酸盐之间的相关性给出了相对贫D的内在水同位素组成，δD值在-320±91‰和-71±71之间‰，这是从母体增生遗传而来的。因此，我们得出结论，OC 母体积聚了贫 D 水冰，其 H 同位素组成与 CM 和 CV 球粒陨石母体相似。我们发现最低亚型（Semarkona、Bishunpur 和 Ngawi）UOC 中的基质显示出与高级类型 UOC 相似的水和有机 H 同位素组成。我们的分析还表明，这些原始 UOC 中的基质区域包含第三种迄今为止尚未识别的成分，该成分带有高 D/H 特征，δD 值高达~6000 ‰。我们认为该成分是从分子云或早期原行星盘中保存下来的原始无定形硅酸盐，对小行星母体的热和水蚀变化极其敏感。