Trace element residence and partitioning in mantle xenoliths metasomatized by highly alkaline, silicate- and carbonate-rich melts (Kerguelen Islands, Indian Ocean)

Abstract

Mantle xenoliths in alkaline lavas of the Kerguelen Islands consist of: (1) protogranular, Cr-diopside-bearing harzburgite; (2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite; (3) dunite that contains clinopyroxene, spinel phlogopite, and rarely amphibole. Trace element data for rocks and minerals identify distinctive signatures for the different rock types and record upper-mantle processes. The harzburgites reflect an initial partial melting event followed by metasomatism by mafic alkaline to carbonatitic melts. The dunites were first formed by reaction of a harzburgite protolith with tholeiitic to transitional basaltic melts, and subsequently developed metasomatic assemblages of clinopyroxene + phlogopite ± amphibole by reaction with lamprophyric or carbonatitic melts. We measured two-mineral partition coefficients and calculated mineral-melt partition coefficients for 27 trace elements. In most samples, calculated budgets indicate that trace elements reside in the constituent minerals. Clinopyroxene is the major host for REE, Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxene for Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc.