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 types (e.g. Nixon, 1987; O'Reilly & Griffin, 1996). of: (1) protogranular, Cr-diopside-bearing harzburgite; (2) poikilitic, Studies of upper-mantle xenoliths in alkali basalts, Mg-augite-bearing harzburgite and cpx-poor lherzolite; (3) dunite kimberlites, lamproites and carbonatites have improved that contains clinopyroxene, spinel phlogopite, and rarely amphibole. our understanding of materials and processes involved Trace element data for rocks and minerals identify distinctive in the geochemical evolution of the mantle (e.g. Downes signatures for the different rock types and record upper-mantle & Dupuy, 1987; The harzburgites reflect an initial partial melting event & Boullier, 1997). The variation and magnitude of followed by metasomatism by mafic alkaline to carbonatitic melts. geochemical heterogeneities in the lithospheric mantle The dunites were first formed by reaction of a harzburgite protolith reflect the composition of mantle melts and fluids and with tholeiitic to transitional basaltic melts, and subsequently the efficiency of heat and mass transfer. Mantle plumes developed metasomatic assemblages of clinopyroxene + phlogopite are important for initiating such transfer processes. amphibole by reaction with lamprophyric or carbonatitic melts. The Kerguelen plume is remarkable because of its We measured two-mineral partition coefficients and calculated volume, the persistence of volcanic activity for at least mineral-melt partition coefficients for 27 trace elements. In most 115 My, and its migration across diverse geotectonic samples, calculated budgets indicate that trace elements reside in environments through time as a result of spreading of the constituent minerals. Clinopyroxene is the major host for REE, the Indian Ocean Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxene