Sodic nature of carbonatite melts and the origin of calcite versus dolomite carbonatites

Abstract

Carbonatites are magmatic rocks that host most economic rare earth element (REE) deposits and bear on the deep Earth volatile cycle. Yet, the cumulative character of plutonic carbonatites and the rapid posteruptive alteration of volcanic carbonatites impede a direct determination of the melt compositions crystallizing these rocks. This study focused on sodium in carbonatite melts, a critical component for their genesis, magmatic evolution, and also for REE mobility, given that most carbonatite-hosted REE mineralizations are formed by alkali-rich fluids. Through experiments at subvolcanic conditions (100−200 MPa, 800−1000 °C), we determined the partition coefficient of Na between apatite and carbonatite melt, DNaapatite/carbonatite, which is rather uniformly 0.010 ± 0.003 at temperatures ≥800 °C. Applying our DNaapatite/carbonatite value to natural samples yields 8−22 wt% Na2O for the melts forming calcic carbonatites and 26−32 wt% Na2O for melts forming dolomite carbonatites. The latter are hence more evolved than calcic carbonatite melts, increasing their potential to shed alkali-rich fluids. A model accounting for calcite fractionation from calcite carbonatite melts prior to apatite saturation indicates primary Na2Omelt contents of 16−23 wt% at 900 °C and 9−11 wt% at 1100 °C, consistent with formation by liquid immiscibility from alkaline silicate magmas, but inconsistent with direct mantle derivation, in particular for dolomitic melts.