Abstract
Dense, glassy pyroclasts found in products of explosive eruptions are commonly employed to investigate volcanic conduit processes through measurement of their volatile inventories. This approach rests upon the tacit assumption that the obsidian clasts are juvenile, that is, genetically related to the erupting magma. Pyroclastic deposits within the Yellowstone-Snake River Plain province almost without exception contain dense, glassy clasts, previously interpreted as hyaloclastite, while other lithologies, including crystallised rhyolite, are extremely rare. We investigate the origin of these dense, glassy clasts from a coupled geochemical and textural perspective combining literature data and case studies from Cougar Point Tuff XIII, Wolverine Creek Tuff, and Mesa Falls Tuff spanning 10 My of silicic volcanism. These results indicate that the trace elemental compositions of the dense glasses mostly overlap with the vesiculated component of each deposit, while being distinct from nearby units, thus indicating that dense glasses are juvenile. Textural complexity of the dense clasts varies across our examples. Cougar Point Tuff XIII contains a remarkable diversity of clast appearances with the same glass composition including obsidian-within-obsidian clasts. Mesa Falls Tuff contains clasts with the same glass compositions but with stark variations in phenocryst content (0 to 45%). Cumulatively, our results support a model where most dense, glassy clasts reflect conduit material that passed through multiple cycles of fracturing and sintering with concurrent mixing of glass and various crystal components. This is in contrast to previous interpretations of these clasts as entrained hyaloclastite and relaxes the requirement for water-magma interaction within the eruptive centres of the Yellowstone-Snake River Plain province.
Highlights
IntroductionVolatile elements (predominantly H, C, S, and Cl) are a primary control on many magmatic and volcanic processes
Volatile elements are a primary control on many magmatic and volcanic processes
Trace element analyses of individual glass fragments from the three case studies were performed via laser ablation inductively coupled plasma mass spectrometry (LAICPMS) at the Institute of Geochemistry and Petrology, ETH Zürich
Summary
Volatile elements (predominantly H, C, S, and Cl) are a primary control on many magmatic and volcanic processes. Of particular relevance is the potential for variations in these volatile components of magmas to drive changes in eruptive style. These changes from hazardous explosivity to more benign effusive activity may occur over the history of a volcano, with successive eruptions having different style, or may occur within a single eruption Estimating the pre-eruptive volatile budget of a magma may be carried out through the use of mineral-melt hygrometry (Waters and Lange 2015). While providing important information about the pre-eruptive state of the magma, the hygrometry approach is limited by the fact that it only informs about the water content and that it requires an accurate and precise estimate of the temperature of the magma prior to eruption. The volatile inventories returned by analysis of melt inclusions are known to be potentially compromised by diffusion of elements through the host crystal (Portnyagin et al 2008; Lloyd et al 2013), formation of bubbles within the inclusion (e.g. Moore et al 2015; Wallace et al 2015; Rasmussen et al 2020; amongst others), and post-entrapment crystallisation
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