Abstract

Crystal-liquid equilibria have been determined in a portion of the system MgO-Al2O3-SiO2 at 15 kb. The isobaric binary eutectic between enstatite and forsterite has been established at a composition of En85Fo15 (weight percent) at 1710°C. The pressure at which enstatite changes from incongruent to congruent melting, as pressure is increased, is estimated to be approximately 4 kb. Six isobaric invariant points within the ternary system have been established which involve the phase relations between a liquid and the crystalline phases forsterite, spinel, aluminous enstatite, sapphirine, quartz, sillimanite, and corundum. Enstatite was found to contain up to 18.9 wt percent Al2O3 in the assemblage quartz + sapphirine + enstatite at 1420°C. The AlAl/MgSi substitution in enstatite seems to be highly dependent upon temperature. Due to a reduction in the size of the primary phase field of forsterite with increasing pressure, partial fusion of a simplified upper-mantle material consisting of enstatite, forsterite, and spinel would produce an initial liquid richer in MgO and poorer in SiO2 and Al2O3 as pressure is increased to 15 kb. Due to a reaction relation between aluminous enstatite, forsterite, and liquid, silica-enriched liquids can be produced from silica-undersaturated compositions by fractional crystallization at high pressures. Similarly, aluminous enstatite can melt at pressures greater than approximately 4 kb, in an anhydrous environment, to produce silica-enriched liquids. The effect of alumina solid-solution and the effect of water on the melting of enstatite at high pressures are complementary effects which allow the incongruent melting of enstatite at greater depths than previously expected and support the possibility of generating silica-enriched magmas by partial fusion at high pressures as well as the production of such magmas by high-pressure fractional crystallization.

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