Nickel isotopic composition of the upper continental crust

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-gran odiorite (TTG), loess, river sediments and glacial diamictites, to constrain the Ni isotopic composition of the UCC. Significant variations in delta Ni-60 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 delta Ni-60 values of fine-grained clastic sediments (including loess, river sediments and glacial diamictites) range from-0.01% to 0.23%. Such delta Ni-60 variations cannot be explained by Ni isotopic fractionation during chemical weathering because there are no clear correlations between delta Ni-60 and Ni/Al2O3, or the chemical index of alteration (CIA). Instead, the delta Ni-60 variations in fine-grained clastic sediments are likely inherited from source rocks. The delta Ni-60 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 delta Ni-60 in the felsic igneous UCC since 3.5 Ga. Similarly, the delta Ni-60 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 delta Ni-60 value of 0.12 +/- 0.15% (2SD) for the UCC (ranging from-0.07% to 0.36%). And the weighted average delta Ni-60 is estimated to be 0.07 +/- 0.10% (2SD) or 0.11 +/- 0.09% (2SD) depending on the assumed delta Ni-60 of the metamorphic rocks. Thus, a lithology-weighted average delta 60Ni needs to be further determined by future studies when the delta 60Ni values of metamorphosed sedimentary rocks in the UCC are constrained. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study establishes the Ni isotopic composition of the upper continental crust (UCC) and reveals significant variations in different rock types and sediments. The variations are attributed to magmatic differentiation and source heterogeneity. The results also suggest limited variation in the Ni isotopes of the UCC since 3.5 Ga.