Skip to content

Volcanic carbonatites: reconstructing the sodium content through experimentally determined apatite-carbonatite melt partition coefficients

Simone Marioni , Max W. Schmidt

Abstract

Magmatic carbonatites occur in 454 locations, of which 113 have volcanic carbonatites (mostly breccia, agglomerates and lapilli tuffs). Over the last decades, carbonatites have been intensively studied to understand economic-grade REE mineralizations and the role of carbon and other volatiles in the deep Earth cycle. Yet the true composition of carbonatite melts remains challenging to constrain due to the cumulative nature of plutonic occurrences and the rapid alteration of volcanic ones. Oldoinyo Lengai, the only active carbonatite volcano, erupts lavas with 31-35 wt.% Na2O, contrasting the 112 fossil volcanic occurrences, which are alkali-poor (median at 0.22 wt.%), likely due to alteration of Na-rich carbonates to calcite, a process that completes at Oldoinyo Lengai within years (ashes) to hundreds of years (dikes). Further, alkalis plays a key role for the unmixing of carbonatite melts from CO2-bearing alkaline silicate melts and are equally important for transport of REE in alkali-rich fluids. This study determines sodium partition coefficients between apatite and carbonatite melt (DNaapatite/carbonatite) to assess the original magmatic Na-content of carbonatites, experiments being conducted at sub-volcanic conditions (200 MPa, 850-1100°C). Results for REE-free compositions yield that the Na2O-poor (0.2-0.4 wt.%) apatites observed in volcanic carbonatites were in equilibrium with sodic carbonatite melts with 20-28 wt.% Na2O. Yet, REE may enhance the compatibility of sodium in apatite through the coupled substitution REE3+ + Na+ = 2 Ca2+, and hence increase DNaapatite/carbonatite, the above Na2O-range might thus represent maximum values; experiments on the influence of REE are under way.