Diffusion-induced nickel isotope fractionation in pyroxenitic xenoliths

Diffusion-induced nickel isotope fractionation in pyroxenitic xenoliths

Xiao-Yang Hu, Shui-Jiong Wang

State Key Laboratory of Geological Processes and Mineral Resources, China

University of Geosciences (Beijing), Beijing 100083, China.

The bulk silicate earth has a homogenous nickel (Ni) isotopic value of +0.11±0.06‰^[1][2]. However, sizable Ni isotope fractionation could occur during mantle metasomatism and melt-rock interaction^{[1][2][3][4][5]}. Here, we analyzed the Ni isotopic composition of a pyroxenitic xenolith (~10cm in length) within Cenozoic intraplate basalts from the Hannuoba region, North China Craton. The host basalts have homogenous δ⁶⁰Ni value of -0.15±0.09‰, whereas the pyroxenitic xenolith has highly variable δ⁶⁰Ni value ranging from -0.05‰ to +0.95‰. In detail, the δ⁶⁰Ni of the pyroxenite exhibit extremely high value at one side of the basalt-pyroxenite boundary, and gradually transitioned to mantle-like δ⁶⁰Ni towards the other side of the basalt-pyroxenite boundary, leading to an stairs-like pattern instead of a U-pattern. Therefore, interaction of the pyroxenitic xenolith with the host basaltic magma after entrainment cannot account for the large Ni isotopic variation. It is likely that the ancient mantle metasomatism, during which, extensive elemental and isotopic exchange between the metasomatic agent and lithospheric mantle, has produced the diffusion-induced Ni isotope fractionation, and later ascending of the Cenozoic intraplate magmas has captured this metasomatized mantle materials, and erupted to the surface. 

 

References:

[1] Wang et al. (2021), Nat Comms, 12, 294; [2] Klaver et al. (2020), GCA, 268, 405-421; [3] Saunders et al. (2020), GCA, 268, 405-421; [4] Gall et al. (2017), GCA, 199, 196-209;  [5] Sheng et al. (2022), JGR-Solid Earth, 127, e2022JB02455.