TRACE AND MINOR ELEMENT PARTITIONING BETWEEN GARNET AND AMPHIBOLE AND CARBONATITIC MELT

The very low viscosity of carbonatite melts potentially makes them a highly effective agent of mantle metasomatism and it is important to quantify how the interaction of these melts with a lherzolitic mantle may affect trace element budgets. Our experiments yield measurements of amphibole-carbonatite melt and garnet-carbonatite melt partition coefficients for the appropriate P,T conditions (18-33 kbar) for a suite of trace and minor elements (Ti, Nb, Zr, Ta, Ce, Lu, Y, Sr, Ba, Rb and K). Measured partition coefficients permit prediction of fractionation among trace element ratios in a carbonatite melt relative to garnet and amphibole in the mantle source for this melt. For example, equilibration of a carbonatite melt with garnet will not fractionate Zr/Y or Nb/Ta ratios in the melt but will decrease Zr/Nb, Lu/Y, Zr/LILE and Y/LILE ratios in the melt. A carbonatite melt in equilibrium with amphibole will fractionate Zr/Nb with Nb enrichment in the melt, but to a lesser extent than for garnet. Metasomatism of the mantle wedge beneath island arcs through addition of carbonatite melt may explain the lower Zr/Nb of island-arc basalts relative to mid-ocean ridge basalts. Ti behaved compatibly with garnet and amphibole in all experiments, implying relative depletion of Ti in coexisting carbonatite melt and a decrease in Ti/LILE ratio in such a melt under equilibrium conditions.