Stability of sanidine + forsterite and its bearing on the genesis of potassic magmas and the distribution of potassium in the upper mantle

Abstract

Experimental determination of the reaction KAlSiO 4 + 4MgSiO 3 ⇌KAlSi 3 O 8 + 2Mg 2 SiO 4 between 17.5 and 30 kbar and 900–1300°C defines an average slope of 33 bar/°C. The reaction delineates the maximum pressure stability of sanidine in a forsterite-bearing assemblage; sanidine and anorthoclase megacrysts in alkalic magmas must have crystallized at lower pressures. The reaction occurs at slightly lower pressures than the transition from silicate liquids (extremely silica-undersaturated) to carbonate-normative liquids as observed in partial melting studies of the systems KAlSiO 4 MgO SiO 2 CO 2 and KAlSiO 4 MgO SiO 2 H 2 O CO 2 . Both the reaction and the change in melt compositions reflect the rapid expansion of the enstatite stability volume with increasing pressure. Suggestions that potassium resides in sanidine or a hollandite structure in the deep upper mantle need to be revised. The stable K Al silicate at high pressures in anhydrous lherzolite compositions is kalsilite.