ИСТИНА

Mg and Ni are both compatible elements in olivine. Fractional crystallization of olivine typically results in a concave up trend on a Fo-Ni diagram. “Ni-enriched” olivine phenocrysts that fall off and above the crystallization trend of are commonly observed in subduction and intraplate settings. To explain such “Ni-enriched” olivine crystals, we develop a set of theoretical and computational models to describe how primitive (high-Fo, high-Ni) olivine phenocrysts from a parent basalt re-equilibrate with an evolved (low-Fo, low-Ni) melt through diffusion. These models describe Fo and Ni decreasing in olivine cores during protracted diffusion for different crystal shapes and variable relative diffusivities for Ni (DNi) and Mg-Fe exchange (DFo). Such dependence is considered as a diffusion trend responsible for the formation of Ni-enriched olivine phenocrysts. Theoretical models show if the diffusivity of Ni is lower than the diffusivity of Fo, then affected by diffusion the olivine phenocrysts form a concave down trend that contrasts with the concave up crystallization trend. These models with different geometries allow showing the diffusion trend does not depend on the size of the crystal and weakly depends on its shape. In addition, it is shown that the anisotropy of the diffusion coefficient affects the diffusion trend in the same way as a change in the crystal shape and both features, anisotropy and shape, do not significantly change the concave down diffusion trend. Three-dimensional numerical diffusion models using a range of realistic olivine morphologies with anisotropy corroborate this conclusion. Thus, the curvature of the concave down diffusion trend is mainly determined by the ratio of the diffusion coefficients DNi/DFo. The initial and final points of the diffusion trend are determined by the compositional contrast between mafic and more evolved melts that have mixed to cause disequilibrium between olivine cores and surrounding melt. These endmember compositions may also vary in nature. We present several examples of measurements on olivine from arc basalts from Kamchatka, and several published olivine datasets from mafic magmas from non-subduction settings (lamproites and kimberlites) that are consistent with diffusion-controlled Fo-Ni behavior. In each case the ratio of the Ni and Fo diffusion coefficients is indicated to be <1. These examples show that crystallization and diffusion can be distinguished by concave up and concave down trends in Fo-Ni diagrams. Research was supported by grants GSF Wo 362/51-1 and RFBR 16-55-12040.