DEGASSING-INDUCED CRYSTALLIZATION IN PERIPHERAL, CRYSTAL-POOR BASALTIC ANDESITE, ANDESITE AND DACITE FLOWS IN WESTERN MEXICO

Crystal-poor basaltic andesite, andesite and dacite flows have erupted peripherally, along the flanks of two andesitic stratovolcanoes, Volcán Sanganguey and Volcán Ceboruco, in western Mexico, which is part of the TransMexican Volcanic Belt. This arc volcanism is a result of subduction of the Rivera plate beneath North America along the Middle America Trench. These crystal-poor flows are in marked contrast to the highly crystalline andesite flows erupted from the central vents of V. Sanganguey and V. Ceboruco. This study focuses on microprobe data obtained from dozens of individual plagioclase crystals within these crystal-poor (< 10 vol %) lava flows ranging in composition from basaltic andesite to dacite (55-68 wt % SiO2). The compositions of individual plagioclase crystals were found to be relatively homogeneous and unzoned, although the plagioclase crystals within a single sample (e.g., XAL-134) vary by up to 35 mol % (e.g., An70-45). The broad range of plagioclase compositions, coupled with the homogenous nature of individual, euhedral grains, suggests that these plagioclase crystals may have formed in response to degassing during rapid ascent to the surface. The plagioclase hygrometer of Lange and Frey (2008) was applied to these crystal-poor andesites and dacites to calculate the range of water concentrations indicated by the variable plagioclase compositions. In XAL-134, the continuum of plagioclase compositions from An70 to An45 leads to a range in calculated water contents from ~5.1 to 1.4 wt % H2O, which is consistent with a scenario of degassing-induced crystallization upon ascent. Analyses of melt inclusions in the euhedral phenocrysts in these crystal-poor samples suggest that caution should be applied before assuming that the inclusion compositions reflect that of the magma at the time of phenocryst growth. We will present evidence that rapid, degassing-induced crystallization led to melt inclusion compositions that were affected by diffusion-controlled boundary layer disequilibria. This may affect interpretation of analyzed H2O concentrations in phenocryst melt inclusions.