Heterogeneous nickel isotope compositions of the terrestrial mantle – Part 2: Mafic lithologies

Abstract

We report stable Ni isotope compositions (δ 60/58 Ni, relative to SRM986) for mafic lavas with a range of -0.16 ‰ to +0.20 ‰ (n=44), similar to that of peridotite samples. Ocean island basalts (OIB) have been analysed from Iceland (n=6), the Azores (n=3), the Galápagos Islands (n=2), and Lōʻihi, Hawaii (n=1). Samples from Iceland (average δ 60/58 Ni = +0.13±0.16‰, 2s, n=7) display the greatest range in Ni isotope compositions from a single OIB location in this work, of +0.01 ‰ to +0.23 ‰. Samples from the Azores (average δ 60/58 Ni = -0.10±0.10 ‰, 2s) and Galápagos (average δ 60/58 Ni = -0.01±0.04 ‰, 2s) are generally isotopically lighter. The single Lōʻihi sample has a δ 60/58 Ni of +0.17 ‰. The lightest analysed bulk rock δ 60/58 Ni in this work, -0.16 ‰, is from the Azores island, Pico. Enriched mid ocean ridge basalts (E-MORB), which have (La/Sm)N>1, are isotopically lighter than normal type MORB (N-MORB), as shown by data from the Mid Atlantic Ridge (n=9) and East Pacific Rise (n=3). All E-MORB average δ 60/58 Ni = Heterogeneous Ni isotope compositions of the terrestrial mantle: Part 2 2 +0.00±0.06 ‰ (2s, n=7), whereas N-MORB average δ 60/58 Ni = +0.14±0.10 ‰ (2s, n=5).A suite of 15 mafic samples from the Cameroon Line, comprising lithologies ranging from nephelinites to hypersthene-normative basalts, have Ni isotope compositions that are identical within analytical uncertainty (average δ 60/58 Ni = +0.08±0.06 ‰, 2s). Similarly, MORB samples display no relationship between δ 60/58 Ni and geochemical indicators of degree of partial melting or fractional crystallisation. Host lavas for two previously analysed ultramafic xenolith suites have δ 60/58 Ni identical to the average δ 60/58 Ni of their respective xenolith suites. This is consistent with previously published evidence from peridotites and komatiites that Ni isotopes are not greatly fractionated by melting. Therefore, mafic rocks may preserve the δ 60/58 Ni of their mantle source. Sampling a greater volume of mantle, their average Ni isotope composition +0.07±0.17 ‰ (2s, n=44) may also be a better representation of the Bulk Silicate Earth (BSE), than estimates based purely on peridotites.The δ 60/58 Ni of MORB co-varies with La/Sm, Rb/Sr, europium anomaly (Eu/Eu*), and K2O/(K2O+Na2O).The relationships between these parameters and δ 60/58 Ni are consistent with mixing between two model endmembers. One could be depleted MORB or depleted MORB mantle (DMM) with a relatively heavy Ni isotope composition; the other a more enriched endmember that has isotopically lighter δ 60/58 Ni. The link between lighter δ 60/58 Ni and enriched lithologies in the mantle is further supported by published evidence of light Ni isotope compositions associated with some pyroxenite xenoliths. However, the curvature of the apparent mixing arrays defined by basalts is hard to reconcile with admixing of geochemically enriched but isotopically fractionated oceanic crustal lithologies. High [Ni] enriched magmas such as kimberlites may be a closer match to the enriched endmember. Ho...

Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth's Core Formation and Differentiation of the Ureilite Parent Body

“…Lack of clear relationships between δ 60/58 Ni values and Ni contents and olivine core Mg# indicate limited Ni isotopic fractionation during igneous processes on the UPB. This is consistent with the fact that igneous processes do not effectively fractionate Ni stable isotopes on Earth, as evidenced by the absence of δ 60/58 Ni variations between peridotites (0.10 ± 0.07‰) and basalts (0.03 ± 0.16‰) (Klaver et al, 2020;Saunders et al, 2020Saunders et al, , 2021Wang et al, 2021). However, the variation of Mg# of ureilites may result from a mixing process, based on C, 54 Cr, and noble gas isotope evidence (Barrat et al, 2017;Broadley et al, 2020;Zhu, Moynier, Schiller, Wielandt, et al, 2020) Note.…”

Sulfide Dissolution on the Nickel Isotopic Composition of Basaltic Rocks

“…Published Ni isotopic data for mafic lithologies show large variations (Cameron et al., 2009; Gall et al., 2017; Gueguen et al., 2013; Saunders et al., 2022; Wang et al., 2019, 2021). In general, the high‐degree mantle melts (e.g., komatiite) have similar Ni isotopic compositions to the peridotitic source; the lower‐degree melting products represented by MORBs and OIBs are isotopically more variable but statistically lighter than the komatiites; the extremely low‐degree melts as represented by the nephelinites have even lighter Ni isotopic compositions (Figure 8).…”

“…Komatiites which represent high‐degree melting products of the mantle (up to ∼50 %, e.g., Arndt, 2003), have similar Ni isotopic compositions to the peridotites. Mafic lithologies such as mid‐ocean ridge basalts (MORBs) and oceanic island basalts (OIBs), generated by <25% partial melting, show even larger variations but generally lighter Ni isotopic compositions (Cameron et al., 2009; Gueguen et al., 2013; Saunders et al., 2022; Wang et al., 2021). Whether this difference is a sign of melting‐induced Ni isotope fractionation or not is unclear.…”

“…They proposed that the mafic lithologies represent their source mantle domains by assuming that there is no Ni isotope fractionation during igneous differentiation. They attributed the Ni isotopic heterogeneity to crustal recycling, with the primitive mantle endmember close to the chondritic Ni isotopic value (Saunders et al, 2020(Saunders et al, , 2022. Their model requires the hypothetical recycled materials to have anomalously high Ni concentrations and low δ 60/58 Ni values in order to modify the mantle Ni isotopic systematics (Saunders et al, 2022).…”

The effect of alkalinity on Ni–O bond length in silicate glasses: Implications for Ni isotope geochemistry

“…For instance, it is argued that the Ni isotope composition of the primitive mantle is distinct from Earth's main building blocks (i.e., chondritic meteorites), but the origin of this dichotomy remains unclear (Klaver et al, 2020;Saunders et al, 2020;Wang et al, 2021). Moreover, with an increasing number of Ni isotope data becoming available, it appears that basalts could be isotopically lighter than their peridotite source (Wang et al, 2021;Saunders et al, 2022), potentially pointing to isotopic fractionation during partial melting. Understanding to what extent the composition and structure of silicate melts play a role in causing Ni isotope fractionation during higher temperature magmatic processes is key to developing Ni isotopes into a mature geochemical proxy.…”