Conditions of phlogopite crystallization in ultrapotassic volcanic rocks

ABSTRACT. Phlogopite occurs as an early crystallizing mineral in many ultrapotassic lavas of basaltic affinities. Based on high-pressure experiments in lavas of these compositions, the early crystallization of phlogopite is controlled in large part by the bulk compositions of the liquids from which it crystallizes but also by the total pressure and by the aH20, with early phlogopite forming under a narrow range of aH20, less than that represented by H20-saturated conditions. Variations in fo ~ do not appreciably affect phlogopite crystallization but high ac02 suppresses its crystallization. In ultrapotassic magmas, phlogopite will preferentially incorporate K20, TiO2, MgO, and A1203 relative to the coexisting early silicate minerals, olivine and clinopyroxene, and thus, on fractionation ofthese minerals, phlogopite will be more effective in reducing these oxides in residual liquids. Phenocrysts and microphenocrysts of phlogopite inultra-potassic lavas are directly related with respect to their K/Ti, K/A1, K/(K + Na), and Mg/(Mg + Fe) ratios. Tex-tural relations uggest phlogopite may form by reaction relationships involving liquid with olivine, and/or clino-pyroxene. Such relationships are supported by the experi-mental studies on ultrapotassic rock compositions. THE only common K-bearing minerals stable under upper-mantle conditions are phlogopite and K-richterite; both occurring in mantle derived alkali basalts with K20 ~ Na20 and as nodules in kimberlites. Phlogopite is the more important of these minerals as it has a wider stability field (Kushiro, 1970; Modreski and Boettcher, 1973) and contains more K20. Many authors have discussed variations in phlogopite compositions (particularly TiO2) and their relationship to mantle metasoma