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Nickel isotopic composition of the mantle

Gall, Louise, Williams, Helen M, Halliday, Alex N and Kerr, Andrew C. 2017. Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199 , pp. 196-209. 10.1016/j.gca.2016.11.016

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Abstract

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 δ60Ni (permil deviation in 60Ni/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 δ60Ni whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock δ60Ni may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable δ60Ni 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 δ60Nimantle 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 δ60Ni 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Nickel; Stable isotopes; Mantle geochemistry; Mineralogy; Meteorites; Core formation
Additional Information: PDF uploaded in accordance with publisher's polices at http://www.sherpa.ac.uk/romeo/issn/0016-7037/ (accessed 25.11.16) This journal has an embargo period of 24 months.
Publisher: Elsevier
ISSN: 0016-7037
Date of First Compliant Deposit: 25 November 2016
Date of Acceptance: 9 November 2016
Last Modified: 24 Nov 2020 19:44
URI: http://orca.cf.ac.uk/id/eprint/96056

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