Carbonatites are silica-poor, carbonate-rich magmatic rocks that have attracted the attention of the scientific community for hosting the majority of the world's rare earth element (REE) deposits and for possibly bearing geochemical signatures linked to mantle processes involving volatiles. Understanding the mineralization mechanisms and disentangling the mantle-related features requires a better knowledge of carbonatite magmatism, hindered by the cumulative nature of plutonic carbonatites and the rapid alteration of volcanic occurrences. Experimental petrology played a key role in constraining the genesis of carbonatite melts, and the conditions that lead to liquid immiscibility from alkaline silicate melts. Alkalis play a pivotal role in the immiscible segregation of the carbonatite melt, as well as in the complexation and transport of REE in and around carbonatites. Immiscibility experiments predicted the primary carbonatite melts to bear >7 wt.% alkalis, contrasting the alkali-poor whole-rock compositions of natural carbonatites (median at 0.28 wt.% Na2O+K2O).

This study explores the apatite-carbonatite melt partitioning of sodium by mean of rapid-quench pressure vessel experiments at sub-volcanic conditions (100-200 MPa, 800-1000°C). Both experimental and natural literature data suggest an influence of REE availability on the sodium partition coefficient (DNaapatite/carbonatite) through the substitution Na+ + REE3+ = 2Ca2+. Using REE-poor compositions, the resulting DNaapatite/carbonatite is 0.010 ± 0.002 over the whole pressure and temperature interval investigated, and for apatite compositions relevant for carbonatite rocks. This DNaapatite/carbonatite suggests that Na-poor (0.05-0.15 wt.% Na2O) apatite commonly present in plutonic carbonatites was in equilibrium with a melt with 5-15 wt.% Na2O, confirming the predictions of immiscibility experiments. The same DNaapatite/carbonatite applied to apatite found in fossil carbonatite lavas yield an original Na2O content in the melt within the 15-30 wt.% range, indicating calcite fractionation prior to eruption, and compositions tending towards the Oldoinyo Lengai lavas (30-35 wt.% Na2O).