ИСТИНА

Nickel is a strongly compatible element in olivine, so fractional crystallization of olivine typically looks as a concave-up trend on a Fo-Ni diagram. We consider Ni-enriched olivine compositions that fall above a crystallization trend. Ni-enriched olivine crystals are abundant in mafic alkaline rocks (Fig. 1a). Unusual olivine compositional trends with Ni-enriched olivine cores were also recently found in subduction-related volcanic rocks from Kamchatka (Fig. 1b). To explain the origin of Ni-enriched olivine phenocrysts, we develop a set of theoretical and computational models to describe how primitive olivine phenocrysts from a parent (high-Mg, high-Ni) basalt re-equilibrate with an evolved (low-Mg, low-Ni) melt through diffusion. These models describe the progressive loss of Fo and Ni in olivine cores during protracted diffusion for various crystal shapes and different ratios of diffusion coefficients for nickel and forsterite, which define the curvature of Fo-Ni diffusion trends. In the case when the diffusivity of Ni is lower than that for Fo then olivine phenocrysts affected by diffusion form a concave-down trend that contrasts with the concave-up crystallization trend. Models for different simple geometries show that this result does not depend on the size of the crystals and only weakly depends on their shape. We also find that diffusion anisotropy has the same effect as effect of phenocryst shape. Thus, both the crystal shape and the diffusion anisotropy 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 Ni and Fo diffusion coefficients. The initial and final points of the diffusion trend are in turn determined by the compositional contrast between mafic and more evolved melts that have mixed to cause disequilibrium between olivine cores and surrounding melt. 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 behaviour. In each case the ratio of Ni and Fe-Mg 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, (see blue and red lines on Fig. 1). This research was supported by DFG grant No. Wo 362/51-1, and RFBR grants No. 16-55-12040 and 20-55-50001.