Non-alkali metals titrate silica out of carbonatite melts into insoluble silicate minerals

Abstract

Silica solubility in crustal carbonatite melts is a controversial topic with suggested solubility limits varying wildly from nearly 0 % up to 15 % SiO2. Here, we present experiments utilising alumina crucibles and CO2 atmospheres. We tested low pressure phase relations and compositions in simplified Na-K-Ca-Fe3+-Al-Si-F systems. We found that substantial mutual solubility of CaO and SiO2 was not possible. Instead, they reacted to form combeite and other Na-Ca silicates. Kalsilite formed on crucible walls, and occasionally within the liquid pools. In Fe2O3-rich systems, SiO2 was likewise poorly soluble in the carbonatite melt with both components partitioning to separate silicate phases. Silica is substantially soluble in carbonatite melts only if they contain Na2O or K2O and no other non-alkali cations that can react with SiO2 to form refractory silicates. In these alkali-dominated systems, immiscible silicate liquids form in equilibrium with Si-bearing, Ca-poor carbonatite melts. Our findings agree with experimentally-derived carbonatite compositions obtained over the past several decades, particularly in decreasing temperatures and increasing alkali regimes. The Ca-poor character of both experimental immiscible liquids may explain the unusual alkali-rich composition of carbonatites and nephelinites in Oldoinyo Lengai, formed at near-atmospheric pressures. Since virtually all natural carbonatite melts contain much more Ca (and other non-alkali metals such as Mg or Fe) than Si, actual natural melt compositions will be SiO2-free for all practical purposes at temperatures below approximately 1000 degrees C, resulting from SiO2 being titrated out to refractory silicates. We propose that low-temperature silicocarbonatites are likely to be antiskarns, igneous silicate mineral assemblages within carbonatites where SiO2 was externally supplied by adjacent silicate rocks. Likewise, we suggest that many carbonatite-associated silicate rocks in ring complexes did not entirely form as cumulates from SiO2-bearing carbonatite melts. Instead, some formation by antiskarn metasomatism is a plausible mechanism. We suggest that currently-observed quartz in carbonatites forms post-magmatically. Therefore, previously hypothesised silica-dependent ore-forming processes may not occur in natural carbonatites, because SiO2 cannot attain sufficient concentrations in primary carbonatitic melts and mineralising fluids.