Nickel isotopic composition of the upper continental crust

Abstract

Establishing the nickel (Ni) isotopic composition of the upper continental crust (UCC) is crucial for using the Ni isotope system to trace biogeochemical processes and understand crust-mantle interactions. This study reports the Ni isotopic composition of eighty-four well-characterized upper crustal samples, including granites, granodiorites, tonalite-trondhjemite-granodiorite (TTG), loess, river sediments and glacial diamictites, to constrain the Ni isotopic composition of the UCC. Significant variations in δ60Ni are revealed for I-type (0.02–0.26‰), A-type (−0.05–0.08‰) and S-type (0.08–0.36‰) granites for the first time. These Ni isotopic variations are attributed to magmatic differentiation for I- and A- type granites and source heterogeneity for S-type granites. The δ60Ni values of fine-grained clastic sediments (including loess, river sediments and glacial diamictites) range from −0.01‰ to 0.23‰. Such δ60Ni variations cannot be explained by Ni isotopic fractionation during chemical weathering because there are no clear correlations between δ60Ni and Ni/Al2O3, or the chemical index of alteration (CIA). Instead, the δ60Ni variations in fine-grained clastic sediments are likely inherited from source rocks. The δ60Ni values of our samples for 3.2–3.5 Ga TTGs (0.00–0.13‰), 2.4–2.5 Ga TTGs (0.04–0.13‰) and < 0.4 Ga granites (excluding S-type granites) are statistically indistinguishable (P < 0.05, student's t-test), implying limited variation of δ60Ni in the felsic igneous UCC since 3.5 Ga. Similarly, the δ60Ni values of glacial diamictites suggest insignificant temporal variation in the weathered UCC since 2.4 Ga. The data gathered in this study combined with literature data yields an arithmetic mean δ60Ni value of 0.12 ± 0.15‰ (2SD) for the UCC (ranging from −0.07‰ to 0.36‰). And the weighted average δ60Ni is estimated to be 0.07 ± 0.10‰ (2SD) or 0.11 ± 0.09‰ (2SD) depending on the assumed δ60Ni of the metamorphic rocks. Thus, a lithology-weighted average δ60Ni needs to be further determined by future studies when the δ60Ni values of metamorphosed sedimentary rocks in the UCC are constrained.