Hydrothermal Melting Curves in Silicate–Water Systems at Pressures greater than 10 Kilobars

Abstract

Fusion curves, or melting intervals, for many elements, minerals and rocks have now been measured experimentally to high pressures, but most determinations of melting relationships in the presence of water vapour have been limited to pressures below 10 kbar. It is well known that the solubility of water vapour under pressure in silicate liquids produces a marked depression of melting temperatures, as illustrated by the negative slopes, dP/dT, of the univariarit curves for the reactions: albite + vapour = liquid (Fig. 1); albite + nepheline + vapour = liquid (Fig. 2). Barth, Smith and Kadik and Khitarov have predicted that, because of the progressive change in the relative properties of water in vapour and liquid phases with increasing pressure, the slopes, dP/dT, of hydrothermal melting curves in silicate–water systems (solid–liquid–vapour) should change from negative to positive at moderate pressures, probably within the interval 3 kbar–10 kbar, with the curves passing through a temperature minimum where dT/dP = 0. We have recently extended water saturated univariant melting curves in the system NaAlSiO_4–SiO_2–H_2O from 10 to 35 kbar, using piston–cylinder apparatus, and none of the curves exhibited a temperature minimum where dT/dP = 0. Figs. 1 and 2 show some of our results. These are not complete phase diagrams; they show only the reactions encountered by two composition joins through the ternary system. Further reactions required by the existence of invariant points, I, and a singular point, S_5, will be illustrated elsewhere.