Natural and Experimental Phase Relations of Lavas from Serocki Volcano

Abstract

Phase equilibrium experiments on Serocki lava ALV-1690-20 have been carried out at 1 atm and 8 kbar. The results provide a framework for quantifying the effects of fractional crystallization on the abundant parental group MORB near the Kane Fracture Zone (KFZ), Mid-Atlantic Ridge. The least-evolved parental group MORBs, as exemplified by Serocki lava ALV-1690-20, are not in equilibrium with a mantle peridotite residue in the plagioclase stability field. Multiply saturated liquids from 8 kbar experiments have pigeonite as their low-Ca pyroxene, and have low Mg#, high Na20 and high Ti02 compared to the parental group lavas. The variation in composition between glass and whole rock from Serocki volcano can be accounted for by plagioclase accumulation. Elevated pressure fractional crystallization of olivine, plagioclase, and augite at 5-6 kbar can account for compositional variation in spatially related lavas from the vicinity of Serocki volcano. Plagioclase phenocrysts or xenocrysts, which are abundant in Serocki lavas, did not crystallize from a liquid of Serocki lava composition at any pressure under anhydrous conditions. These exotic An-rich plagioclase (An84) may have crystallized at elevated pressure from a precursor magma with higher MgO and CaO and lower Na20 contents, or may record a mixing event with a distinctively low-Na20 magma type.