Chromium isotope compositions have been determined for 42 mantle-derived xenoliths from Cameroon, France, Mongolia, Russia and the USA. The samples are mainly spinel and garnet lherzolites, or harzburgites, and yield δCr from −0.33 to 0.43‰, within the range of previously reported compositions, −0.51 to 0.74‰ (Xia et al., 2017). Partial melting has been proposed as a process that alters the δCr of the mantle residue. New non-modal melting models show that the compatible nature of Cr is inconsistent with large changes in the residue’s isotopic composition. The fractionation during melting is constrained by calculated ΔCr-values <0.09 and the measured ΔCr mantle-melt ≈ 0.07‰, which is too small to generate the observed range in δCr at low to medium degrees of melting. Fertile to melt-depleted, unmetasomatised peridotite xenoliths show no correlations between δCr and indicators of partial melting. Almost all LREE-enriched samples studied here define a correlation between δCr and (La/Yb). This is consistent with alteration of Cr isotopic compositions by some process that is related to interactions with low degree partial melts. The low Cr content of small degree partial melts renders metasomatism via such liquids a difficult explanation for the isotopic effects found. A possible explanation is some form of disequilibrium loss of isotopically light Cr into such liquids during their percolation through the mantle. This is similar to a previously proposed mechanism of kinetic isotopic fractionation of Cr associated with interactions between silicate melts and peridotitic mantle (Xia et al., 2017).

中文翻译：

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橄榄岩地幔捕虏体中稳定铬同位素的变化：交代作用与部分熔融

已测定了来自喀麦隆、法国、蒙古、俄罗斯和美国的 42 个地幔来源的捕虏体的铬同位素组成。样品主要是尖晶石和石榴石二辉橄榄石或方辉橄榄石，δCr 值为-0.33 至0.43 ‰，在之前报道的成分范围-0.51 至0.74 ‰之内（Xia 等，2017）。部分熔融被认为是改变地幔残渣 δCr 的过程。新的非模态熔化模型表明，Cr 的相容性质与残留物同位素组成的巨大变化不一致。熔化过程中的分馏受到计算的 ΔCr 值 <0.09 和测量的 ΔCr 地幔熔体 ≈ 0.07‰ 的限制，该值太小，无法在低至中等熔化度下产生 δCr 的观测范围。从肥沃到熔体耗尽、未交代的橄榄岩捕虏体显示 δCr 和部分熔融指标之间没有相关性。这里研究的几乎所有富含轻稀土元素的样品都定义了 δCr 和 (La/Yb) 之间的相关性。这与某些与低度部分熔融相互作用相关的过程对铬同位素组成的改变是一致的。小程度部分熔化的低铬含量使得通过此类液体的交代作用难以解释所发现的同位素效应。一种可能的解释是轻同位素铬在渗过地幔的过程中以某种形式不平衡地损失到这些液体中。这类似于之前提出的与硅酸盐熔体和橄榄岩地幔之间相互作用相关的 Cr 动力学同位素分馏机制（Xia 等，2017）。