The dynamics of post-collapse magmatism at rhyolitic calderas: Analogue experiments and geochemistry of Yellowstone lavas.

摘要

The dynamics of magma replenishment in silicic magma reservoirs are not well understood. This is an important issue, since replenishment may lead to the rapid accumulation of large volumes of crystal-poor magma, a condition potentially leading to cataclysmic caldera-forming eruptions. To shed light on this process, I have undertaken a combined experimental---geochemical approach designed to provide an integrated and synthetic view of magma replenishment processes. Scaled analogue experiments show that in silicic systems, buoyant injections rise through a magma reservoir without significant mixing with resident material, forming a layered reservoir. When a crystal mush is present at the base of the reservoir, its presence does not hinder buoyant injections to ascend. Indeed, such injections entrain crystals from the mush and transport them to the uppermost layers of the reservoir, resulting in the accumulation of a mix of replenishing magma and mush crystals. This process may have played a significant role in the generation of early post-collapse rhyolites of Yellowstone caldera, the Upper Basin Member rhyolites. In these lavas, I observe the coexistence of large, isolated, sieved plagioclase crystals with small, fresh, more calcic plagioclase crystals occurring as aggregates with pyroxenes and oxides. This unusual mineral assemblage suggests that a higher-temperature, more primitive silicic replenishing magma mixed with crystals from a mush shortly before eruption. In this scenario, the aggregates crystallized from the replenishing magma while the sieved crystals were extracted from the mush. Unlike the Upper Basin Member rhyolites, the younger voluminous Central Plateau Member rhyolites define a cogenetic series in which younger lavas exhibit more evolved mineralogy and trace element signatures, while crystal geochemistry suggests crystallization from progressively cooler melts. At the same time, the crystals also exhibit dissolution textures, suggesting a reheating episode shortly before eruption. Anomalous trace element signatures in some of these rhyolites suggest mixing with a silicic replenishing magma. The Central Plateau Member rhyolites may thus reflect growth of a large reservoir of mushy magma. Within this largely crystalline chamber, certain parts experienced partial melting events, due to localized inputs of heat and/or replenishing magma. These multiple, liquid-rich magma bodies subsequently erupted to form the Central Plateau Member rhyolites.