Abstract High precision nickel stable isotopic compositions (δ60/58Ni) are reported for 22 peridotite xenoliths from the USA (Kilbourne Hole, New Mexico), Tanzania, and Cameroon. For a subset of these, δ60/58Ni is also reported for their constituent mineral separates (olivine, orthopyroxene, clinopyroxene, and spinel). Bulk peridotites show significant heterogeneity in Ni isotopic composition, ranging from +0.02‰ to +0.26‰. Unmetasomatised fertile peridotites from three localities, define an average δ60/58Ni of +0.19±0.09‰ (n = 18). This value is comparable to previous estimates for the δ60/58Ni of the bulk silicate earth (BSE), but is unlikely to be representative, given observed heterogeneity, presented here and elsewhere. Samples with reaction rims and interstitial glass (interpreted as petrographic indications of minor metasomatism) were excluded from this average; their Ni isotopic compositions extend to lighter values, spanning nearly the entire range observed in peridotite worldwide. Dunites (n = 2) are lighter in δ60/58Ni than lherzolites and harzburgites from the same location, and pyroxenites (n = 5) range from +0.16‰ to as light as −0.38‰. The δ60/58Ni in the Kilbourne Hole xenoliths correlate negatively with bulk-rock Fe concentration and positively with 143Nd/144Nd, providing evidence that light δ60/58Ni is associated with mantle fertility and enrichment. The trend between δ60/58Ni and Fe concentration in bulk rocks appears to be global, replicated across the peridotites in this work from other localities, and in literature data. The inter-mineral fractionations are small; the maximum difference between heaviest and lightest phase is 0.12‰. This provides evidence that bulk rock δ60/58Ni variation does not result from differences in modal mineralogy, fractional crystallization or degrees of partial melting. The δ60/58Ni fractionation appears to be an equilibrium effect and usually is in the decreasing order spinel > olivine = orthopyroxene > clinopyroxene. However, the fractionation between clinopyroxene and orthopyroxene varies in magnitude and sign, and is correlated with pyroxene Si/Fe positively, and Fe/Mg negatively. The magnitude of inter-pyroxene fractionation also correlates with other pyroxene compositional ratios (e.g. La/Smclinopyroxene); as well as bulk rock δ60/58Ni, and [U]. These data provide evidence that Ni isotopes fractionate at the bulk rock and mineral scale in response to mantle enrichment processes, possibly related to recycling of isotopically light subducted components.

中文翻译：

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地幔中的非均质镍同位素组成——第 1 部分

摘要 报告了来自美国（新墨西哥州基尔伯恩霍尔）、坦桑尼亚和喀麦隆的 22 种橄榄岩捕虏体的高精度镍稳定同位素组成 (δ60/58Ni)。对于其中的一个子集，还报告了 δ60/58Ni 的组成矿物分离物（橄榄石、斜辉石、斜辉石和尖晶石）。块状橄榄岩在 Ni 同位素组成方面表现出显着的异质性，范围从 +0.02‰ 到 +0.26‰。来自三个地区的未交代肥沃橄榄岩的平均 δ60/58Ni 定义为 +0.19±0.09‰ (n = 18)。该值与先前对大块硅酸盐地球 (BSE) 的 δ60/58Ni 的估计相当，但鉴于观察到的异质性，不太可能具有代表性，在此处和其他地方都有介绍。具有反应边缘和间隙玻璃的样品（解释为轻微交代的岩相学迹象）被排除在这个平均值之外；它们的 Ni 同位素组成扩展到更轻的值，几乎涵盖了全球橄榄岩中观察到的整个范围。Dunites (n = 2) 在 δ60/58Ni 中比来自同一位置的锂辉石和斜方辉石轻，辉石 (n = 5) 的范围从 +0.16‰ 到轻至 -0.38‰。Kilbourne Hole 捕虏体中的 δ60/58Ni 与大块岩石 Fe 浓度呈负相关，与 143Nd/144Nd 呈正相关，证明轻 δ60/58Ni 与地幔肥力和富集有关。大块岩石中 δ60/58Ni 和 Fe 浓度之间的趋势似乎是全球性的，在这项工作中从其他地方和文献数据中复制到橄榄岩中。矿物间的分馏很小；最重相和最轻相之间的最大差值为 0.12‰。这提供了证据表明大块岩石 δ60/58Ni 的变化不是由模态矿物学、分步结晶或部分熔融程度的差异引起的。δ60/58Ni 分馏似乎是一种平衡效应，通常是尖晶石 > 橄榄石 = 斜辉石 > 斜辉石的递减顺序。然而，斜辉石和斜辉石之间的分馏在大小和符号上有所不同，并且与辉石 Si/Fe 正相关，Fe/Mg 负相关。辉石间分馏的幅度也与其他辉石组成比（例如 La/Smclinopyroxene）相关；以及大块岩石δ60/58Ni，和[U]。