The role of fluorine in carbonatite magma evolution

Abstract

CARBONATITE magmas require some agent in solution to maintain them in the liquid state at geologically relevant temperatures and pressures¹. Although the liquidus temperatures of carbonate systems are greatly lowered in the presence of water², an unrealistically large water content (∼95 wt%) is required for maximum lowering of the minimum melting temperature². Here we report experimental results which show that, in several carbonate systems, ∼8 wt% fluorine lowers the minimum melting and liquidus temperatures to a similar extent as do these very large amounts of water. Thus, although water may well be present in most carbonatite magmas, it is neither the only nor necessarily the main agent by which they can remain liquid. Fluorine has the further effect of breaking the 'thermal barrier' imposed by the nyerereite composition in the system Na₂CO₃–K₂CO₃–CaCO₃, thereby allowing a low-alkali calcitic carbonatite magma to differentiate into a highly sodic carbonatite magma of the Oldoinyo Lengai type. Although this neither proves nor disproves a possible origin by liquid immiscibility, it restores the credibility of fractional crystallization as an important process in developing an alkali enrichment trend in carbonatites.