TRACE-ELEMENT PARTITIONING BETWEEN SILICATE MINERALS AND CARBONATITE AT 25 KBAR AND APPLICATION TO MANTLE METASOMATISM

Experiments at 25 kbar and 1000-degrees-C, on a model trace element-enriched carbonatite-peridotite mix, produced augite + pargasite +/- garnet +/- dolomite coexisting with a carbonatite melt. Proton microprobe analysis of the phases showed that key trace elements (Rb, Ba, Sr, Nb, Ta, Zr, Y and REE) all partitioned strongly into the melt (with the exception of Y, Ho and Lu in garnet), verifying that carbonatite is potentially a highly effective metasomatizing agent. The data also indicate that carbonatitic metasomatism will impart higher Ba//Rb, Ba/Nb, Nb/Ta, Sr/Ta, La/Ta, and lower Zr/Y, with little change to Sr/Nb, in affected mantle.