Recent investigations of meteorites thought to originate from the asteroid 4 Vesta have suggested an early accretion of water on rocky bodies in the inner Solar System from a carbonaceous chondrite-like source. However, these studies have been based on the hydrogen isotope compositions (δD) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. We have determined the δD and H₂O concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. The H₂O concentrations in clinopyroxenes from eucrites Juvinas, Stannern and Tirhert range from 5 to 18 μg/g, with a weighted average δD of –263 ± 70 ‰. Their apatites and whitlockites exhibit a higher weighted average δD of –165 ± 73 ‰, possibly as a result of H₂ degassing during or after phosphate crystallization. Thermal metamorphism of these eucrites has most probably resulted in the loss of H, and an increase in their original δD values. While the weighted average δD value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 Vesta, the average δD value of Stannern clinopyroxenes is considered to be closest to the initial δD of the source mantle (i.e., –373 ±127 ‰), which is lighter than that of Earth’s depleted upper mantle and most carbonaceous chondrites. We suggest that at least some of the water in 4 Vesta (and possibly other rocky bodies in the inner Solar System) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in Solar System history.

对陨石的最新研究认为，陨石起源于小行星4 Vesta，它暗示了碳质球粒状来源会在太阳系内部的岩石体上早期积聚水。但是，这些研究基于的是在欧亚岩中晚结晶的磷灰石晶粒的氢同位素组成（δD），而该岩石可能没有记录主要的岩浆成分。我们已经确定了几种珠光体中某些最早形成的硅酸盐（斜茂铁）中的δD和H ₂ O浓度，目的是限制其母体源储层的氢同位素组成。H ₂欧亚大陆，朱维纳斯，斯坦恩和蒂尔赫特的斜辉石中的O浓度范围为5至18μg/ g，加权平均δD为–263±70‰。它们的磷灰石和辉晶岩的加权平均δD较高，为–165±73‰，可能是H ₂的结果。在磷酸盐结晶过程中或之后进行脱气。这些珠光体的热变质很可能导致了H的损失，以及其原始δD值的增加。虽然据推断此处报道的真碳质斜辉石的加权平均δD值反映了其母体小行星4 Vesta上的硅酸盐幔储层同位素组成的上限，但认为斯坦坦斜辉石的平均δD值与初始值最接近。源地幔的δD（即–373±127‰），比地球枯竭的上地幔和大多数碳质球粒陨石的δD轻。我们建议至少4 Vesta中的某些水（以及内部太阳系中的其他岩石体）至少来自于原生质星云中一个相对缺乏氘的水库，