PHASE EQUILIBRIA IN THE SILICA-UNDERSATURATED PART OF THE KAISIO4 - MG2SIO4 - CA2SIO4 - SIO2 - F SYSTEM AT 1 ATM AND THE LARNITE-NORMATIVE TREND OF MELT EVOLUTION

Abstract

The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been studied on two silica-undersaturated joins of the KAlSiO4- Mg2SiO4- Ca2SiO4- SiO2- F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F-=O2-). The first join is the pseudo-ternary Forsterite - Diopside - KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine, spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite - F-phlogopite join that intersects the Forsterite - Diopside - KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo+lc+ l=phl, and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite - F-phlogopite join, but is preserved in a part of the Forsterite - Diopside - KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from ``normal'' melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure.