Nickel isotopic composition of the mantle

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• 作者：Louise Gall^a ; ^b ; ; Helen M. Williams^c ; Alex N. Halliday^b ; Andrew C. Kerr^d
• 关键词：Nickel ; Stable isotopes ; Mantle geochemistry ; Mineralogy ; Meteorites ; Core formation
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2017
• 出版时间：15 February 2017
• 年：2017
• 卷：199
• 期：Complete
• 页码：196-209
• 全文大小：1412 K
• 卷排序：199

文摘

This paper presents a detailed high-precision study of Ni isotope variations in mantle peridotites and their minerals, komatiites as well as chondritic and iron meteorites. Ultramafic rocks display a relatively large range in δ60Niδ60Ni (permil deviation in 60Ni/58Ni60Ni/58Ni relative to the NIST SRM 986 Ni isotope standard) for this environment, from 0.15 ± 0.07‰ to 0.36 ± 0.08‰, with olivine-rich rocks such as dunite and olivine cumulates showing lighter isotope compositions than komatiite, lherzolite and pyroxenite samples. The data for the mineral separates shed light on the origin of these variations. Olivine and orthopyroxene display light δ60Niδ60Ni whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock δ60Niδ60Ni may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable δ60Niδ60Ni values due to variations in residual mantle and cumulate mineralogy. Based on fertile peridotite xenoliths and Phanerozoic komatiite samples it is concluded that the upper mantle has a relatively homogeneous Ni isotope composition, with the best estimate of δ60Nimantleδ60Nimantle being 0.23 ± 0.06‰ (2 s.d.). Given that >99% of the Ni in the silicate Earth is located in the mantle, this also defines the Ni isotope composition of the Bulk Silicate Earth (BSE). This value is nearly identical to the results obtained for a suite of chondrites and iron meteorites (mean δ60Niδ60Ni 0.26 ± 0.12‰ and 0.29 ± 0.10‰, respectively) showing that the BSE is chondritic with respect to its Ni isotope composition, with little to no Ni mass-dependent isotope fractionation resulting from core formation.