Liquid immiscibility in the CaF2-granite system and trace element partitioning between the immiscible liquids

Abstract

The recent discoveries of Ca-fluoride melt in magmatic systems, especially in peralkaline granites with rare earth element enrichment, has raised questions about the presence and nature of liquid immiscibility in CaF2-granite systems, as well as the partitioning behavior of trace elements between such immiscible liquids. We have experimentally explored the immiscibility in this system using natural granite-fluorite starting materials at fast-and slow-cooling rates, temperatures from 500 °C to 1200 °C and 1 atmosphere pressure. Our experiments confirm the presence of a miscibility gap in the CaF2-granite system over a wide range of temperatures. The two immiscible liquids are a mafic fluorosilicate melt (fm) with high CaO and F and a felsic oxysilicate melt (sm) with high SiO2 and alkalis. Immiscibility is encountered when the bulk F-content exceeds approximately 4.4 wt% at 1200 °C and then rapidly decreases to only 0.75 wt% at low temperature (below 900 °C). This indicates that calcic magmas are able to evolve to the liquid immiscibility domain by the residual enrichment of fluorine resulting from fractional crystallization, and that liquid immiscibility should be common. The peritectic reaction fm = sm + fluorite removes the fluorosilicate liquid around 600 °C and could explain the absence of fluorosilicate observations in natural granites. The compositions of the liquids in fast-cooling experiments show a significantly larger miscibility gap which is characterized by, compared to slow-cooling runs, similar sm compositions but higher Ca and F content in fm. This difference is caused by suppression of fluorite nucleation and is interpreted as a metastable extension of the high temperature liquid immiscibility. Immiscibility results in strong fractionation of trace elements which is primarily governed by melt structure and the ionic potential (Z/r) of the elements. The behavior of trace elements can be categorized into three groups: 1. Network formers (Si, Al, Ga, Pb, B) partition into the oxysilicate melt; 2. The partitioning of the alkalis and alkali earths is strongly dependent on the ionic potential, with the partitioning coefficients (D fm/sm) in the order DCs < DRb < DK < DNa < DLi < DBa < DSr < DCa ≤ DMg; 3. High field strength elements (Ti, Zr, Hf, Nb, Ta, Th, U) and rare earth elements (REE) strongly partition into the fluorosilicate melt. The D values vary with the size of the miscibility gap, which is in turn controlled by temperature with more pronounced fractionation as the miscibility gap widens. The fluorosilicate melt effectively sequesters the REE especially at the lower temperatures relevant for granite magmatism and, given its higher density and lower viscosity compared to the coexisting oxysilicate melt, fractionates the REE from the host silicate melt, suggesting a potential REE-mineralising process.