Primary clinopyroxene spherulites in basaltic lavas from the Pacific–Antarctic Ridge

Abstract

Fresh glassy basaltic andesite samples recovered from the northern part of the Pacific–Antarctic Ridge contain abundant spherulites consisting of arrays of closely packed clinopyroxene fibers. The spherulites frequently enclose elongated vesicles that are tear drop-shaped or tailed. The long axes of the elongated vesicles were found to be always parallel to the orientation of the surrounding crystal fibers. In several cases, elongated vesicles having different orientations are hosted by a single spherulite. The existence of elongated vesicles provides unequivocal evidence that a significant proportion of the clinopyroxene fibers must have crystallized directly from a supercooled liquid at temperatures above the glass transition. Moreover, the absence of fractures within the spherulites and the surrounding glass as well as the observed nucleation of clinopyroxene fibers at vesicle walls are interpreted to be consistent with a primary origin of the clinopyroxene spherulites. Based on these textural observations it has to be concluded that spherulites do not represent a diagnostic texture for the devitrification of volcanic glass that occurs below the glass transition temperature.