Origin of crystal-poor, differentiated magmas: insights from thermal gradient experiments

Abstract

Crystal-poor, differentiated magmas are com- monly erupted from shallow, thermally zoned magma chambers. In order to constrain the origin of these magmas, we have experimentally investigated crystallization, differentiation and crystal-melt separation in presence of a thermal gradient. Experiments have been designed taking advantage of the innate temperature gradient of the piston cylinder apparatus and carried out on a phonolitic system at 0.3 GPa and temperature ranging from 1,050 to 800 C. Crystallization degree and melt composition in experi- mental products vary as a function of the temperature gradient. In particular, melt composition differentiates from tephri-phonolite (starting material) to phonolite moving from the hotter, glassy zone (T B 1,050 C) towards the cooler, heterogeneously crystallized zone (T B 900 C) of the charge. The heterogeneously crystal- lized zone is made up of: (1) a crystal-rich, mushy region (crystallinity [30 vol%), (2) a rigid crystal framework (crystallinity B80 vol%) and (3) glassy belts of phonolitic glass at the top. Thermal gradient experiments picture crystallization, differentiation and crystal-melt separation processes occurring in a thermally zoned environment and reveal that relatively large volumes of crystal-poor melt (glassy belts) can originate as a consequence of the instability and collapse of the rigid crystal framework. Analogously, in thermally zoned magma chambers, the development and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal-poor, differentiated magmas.