The pyroxene-garnet transformation in the earth's mantle

Abstract

An investigation of the solid solubility of pyroxenes in garnet structures in the pressure range 0–250 kb and at about 900°C, is described. At about 100 kb, the solid solubility of enstatite in pyrope increases suddenly, resulting in formation of a garnet of composition Mg 3(AlMg 0.5Si 0.5)Si 3O 12. Analogous relationships are found to exist between calcium-rich pyroxenes and garnets. The results imply the occurrence of a type of pyroxene-garnet transformation, in which the garnets are characterized by partial 6-fold coordination of silicon. The transformation is accompanied by a density increase of about 10%. A variety of evidence strongly indicates that these new garnets are thermodynamically stable. The transformation has also been demonstrated in a typical eclogite, which, at a pressure of 127 kb, crystallizes to an assemblage of 95% garnet and 5% pyroxene, with a mean density of 3.75 g/cm 3. The pyroxene-garnet transformation may be important in the earth's mantle where it would occur at a depth of about 350 km, thus marking the beginning of the transition zone. A speculative model is also presented whereby the occurrence of the garnet-pyroxene transformation in blocks of eclogite which are sinking through the mantle provides a mechanism for deep-focus earthquakes.