Reversed Experiments on Biotite-Quartz-Feldspar Melting in the System Kmash: Implications for Crustal Anatexis

Abstract

Melting temperatures of biotite-quartz and biotite-quartz-feldspar assemblages in the system $$K_{2}O-MgO-Al_{2}O_{3}-SiO_{2}-H_{2}O$$ have been determined in the pressure range 0.5-18 kbar. The subsolidus dehydration of biotite + quartz also has been investigated from 300 to 500 bars. The melting equilibria studied are phlogopite + quartz + sanidine + vapor = liquid [O], phlogopite + quartz + vapor = enstatite + liquid [K], phlogopite + quartz = enstatite + liquid [K, V], and phlogopite + quartz + sanidine = enstatite + liquid [V]. At 10 kbar, the [O], reaction occurs at 700-720°C, the [K] reaction at 780-790°C, the [K, V] reaction at 815-850°C, and the [V] reaction at ~800-850°C. Vapor-saturated reactions were reversed isobarically in two-stage experiments in which the starting assemblage was held at a temperature ~10° above the temperature of previously observed melting for a duration equivalent to an initial melting experiment, followed by lowering of the temperature below the temperature of initial melting for times greater than 200 hrs. Textural evidence for reversal included abundant mica growth at the expense of enstatite and liquid in the quenched charges. Vapor-absent melting reactions were reversed by a series of two-stage experiments in which the reactant assemblage was first held ~135° above the solidus for approximately 80 hrs, followed by an average of 200 hrs at a lower temperature in the same run. Textural evidence for mica regrowth from liquid defined the reversed solidus. Melts generated by reactions [O], [K], and [K, V] are granitic in composition, and the melt generated by the [V] reaction is quartz-syenitic in composition. Variable Al content of phlogopite causes slight detectable divariance of vapor-absent reactions. Peraluminous phlogopite participates in the vapor-absent melting reactions first, but is replaced by more nearly stoichiometric phlogopite with increasing temperature. The volume percentage of melt produced under vapor-absent conditions is approximately equal to the volume percentage of phlogopite entering the melting reaction. This limits the extent of partial melting at near-solidus temperatures in felsic rocks in the deep crust.