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

Abstract

We report stable Ni isotope compositions (δ60/58Ni, 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 δ60/58Ni = +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 δ60/58Ni = −0.10 ± 0.10‰, 2s) and Galápagos (average δ60/58Ni = −0.01 ± 0.04‰, 2s) are generally isotopically lighter. The single Lōʻihi sample has a δ60/58Ni of +0.17‰. The lightest analysed bulk rock δ60/58Ni 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 δ60/58Ni = +0.00 ± 0.06‰ (2s, n = 7), whereas N-MORB average δ60/58Ni = +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 δ60/58Ni = +0.08 ± 0.06‰, 2s). Similarly, MORB samples display no relationship between δ60/58Ni and geochemical indicators of degree of partial melting or fractional crystallisation. Host lavas for two previously analysed ultramafic xenolith suites have δ60/58Ni identical to the average δ60/58Ni 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 δ60/58Ni 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 δ60/58Ni of MORB co-varies with La/Sm, Rb/Sr, europium anomaly (Eu/Eu*), and K2O/(K2O + Na2O). The relationships between these parameters and δ60/58Ni 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 δ60/58Ni. The link between lighter δ60/58Ni 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. However, this needs further study.