Instability of garnet from the mantle: Glass as evidence of metasomatic melting

Abstract

Instability of garnets is manifest by incongruent melting in the kimberlite of Fayette County, Pennsylvania. Quenching of this reaction has resulted in the formation of alkali-rich Ne- and Qz-normative glasses, spinel, aluminous Ca-rich and Ca-poor pyroxenes, and olivine. The crystalline products have a range of metastable textures and compositions; these represent quenching of the reaction at various stages of its completion during a progressive increase in cooling-rate consequent upon the final rise of the kimberlite through the upper mantle and lower crust. The melting was not isochemical; addition of alkalis and probably volatiles are required to balance the overall reaction. These metasomatic reactants may have their origin in the host kimberlitic melt or may have infiltrated from the adjacent mantle. Similarities of some of the features described herein with other reported kelyphite occurrences suggest that incongruent melting of garnet may be more commonplace than heretofore appreciated in “kelyphite” formation.