Contrasting plagioclase textures and geochemistry in response to magma dynamics in an intra-caldera rhyolite system, Okataina volcano

摘要

The changing magmatic dynamics of the rhyolite caldera volcano, Okataina Volcanic Centre, New Zealand, is revealed in plagioclase growth histories. Crystals from the -0.7 ka Kaharoa eruption are characterized by resorbed cores displaying a cellular texture of high-An (>40) zones partially replaced by low-An (<30) zones, surrounded by a resorption surface and a prominent normal-zoned rim (An(50-20)). Elevated An, Fe, Mg, Sr, and Ti follow the resorption surface and display rimward depletion trends, accompanied by Ba and REE enrichment. The zonation is consistent with fractional crystallization and cooling. The cores display wide trace element diversity, pointing to crystallization in a variety of melts, before transport and mixing into a common magma where the rims grew. Plagioclase from the -36 ka Hauparu eruption display several regrowth zones separated by resorption surfaces, which surround small resorbed cores with a spongy cellular texture of variable An content (An(40-50)). The crystals display step-wise regrowth of successively higher An, Fe, Mg, and Ti content, consistent with progressive mafic recharge. Two crystal groups are distinguished by trace element chemistry, indicating growth in separate melts and co-occurrence via magma mingling. For plagioclase in both eruption deposits, partition coefficients (D) estimated from crystal rim-groundmass glass analyses, produce melt compositions similar to the array of rock and glass compositions erupted and are consistent with the processes of fractional crystallization and recharge. However, D values estimated from some published formulations based on An content and temperature produce unrealistic melts. The contrasting zoning patterns in plagioclase correspond to the evolutionary history of magmatism at Okataina. Emptying of the magma reservoir following caldera eruption at 46 ka reduced barriers to mafic magma ascent. This is recorded by the frequent resorption and recharge episodes in Hauparu crystals. Subsequent redevelopment of a more silicic reservoir zone (post-26 ka) dampened thermal and mass perturbations, resulting in simpler growth histories of the Kaharoa crystals. The plagioclase lack features associated with rapid decompression events that are common in andesite systems. This reflects the rapid ascent of the rhyolite magmas and lack of precursory eruptions that could decompressed the system. (C) 2015 Elsevier B.V. All rights reserved.