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

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 Iherzolite; (3) dunite that contains clino pyroxene, spinel phlogopite, and rarely amphibole. Trace element data for r ocks and minerals identify distinctive signatures for the different rock ty pes and record upper-mantle processes. The harzburgites reflect an initial partial melting event followed by metasomatism by mafic alkaline to carbona titic melts. the dunites were first formed by reaction of a harzburgite pro tolith with tholeiitic to transitional basaltic melts, and subsequently dev eloped metasomatic assemblages of clinopyroxene + phlogopite +/- amphibole by reaction with lamprophyric or carbonatitic melts. We measured two-minera l partition coefficients and calculate mineral-melt partition coefficients for 27 trace elements. In most sample, calculated budgets indicate that tra ce 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; orthopyro xene for Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc.