EVIDENCE FOR CARBONATITE METASOMATISM IN SPINEL PERIDOTITE XENOLITHS FROM WESTERN VICTORIA, AUSTRALIA

The experimental discovery that sodic dolomitic carbonatite melt can be generated in equilibrium with pargasitic lherzolite [1] at pressures corresponding to depths of 90 km or more, has led to the development of models for mantle metasomatism by ephemeral carbonatite melts [2]. A suite of mantle xenoliths (magnesian wehrlites to lherzolites) from Mt Leura and Mt Shadwell (western Victoria, Australia) display the predicted petrographic and geochemical signatures of such metasomatism, namely replacement of primary orthopyroxene by jadeitic clinopyroxene and forsteritic olivine, the presence of significant accessory apatite, unusually high whole-rock CaO/Al2O3 and Na2O/Al2O3 values, and extreme LILE enrichment without concomitant TiO2 enrichment. Isotopic data suggest a petrogenetic association of the carbonatite metasomatic event with the host alkali olivine basalts. We propose a model whereby carbonated, undersaturated silicate melts crystallizing pargasitic amphibole at depths of greater-than-or-equal-to 90 km, produce a mobile carbonatite melt which can segregate and ascend at very low melt fractions, and undergo decarbonation reactions with refractory lithosphere at 55-65 km depth, producing highly magnesian, apatite-bearing wehrlites and lherzolites.