D/H ratios and H2O contents record degassing and rehydration history of rhyolitic magma and pyroclasts

Abstract

Volcanic eruptions of rhyolitic magma often show shifts from powerful (Vulcanian to Plinian) explosive episodes to a more gentle effusion of viscous lava forming obsidian flows. Another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra. This dense, juvenile product is similar to the tephra and obsidian flow in composition, but is generally less degassed than its flow counterpart. The formation mechanism(s) of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research. Porous tephra and pyroclastic and flow obsidians from the 1060CE Glass Mountain rhyolitic eruption at Medicine Lake Volcano (California) were analyzed for their porosity, ϕ, water content, H2O, and hydrogen isotopic composition, δD. H2O in porous pyroclasts is correlated negatively with δD and positively with ϕ, indicating that the samples were affected by post-eruptive rehydration. Numerical modeling suggests that this rehydration occurred at an average rate of 10−23.5±0.5 m2s−1 during the ∼960 years since the eruption, causing some pyroclasts to gain up to 1 wt% of meteoric water. Pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity. Comparison between modeled δD-H2O relationships in degassing magma and values measured in the Glass Mountain samples supports the idea that rhyolitic magma degasses in closed-system until its porosity reaches a value of about 65±5%, beyond which degassing occurs in open-system until quench. During the explosive phase, rapidly ascending magma fragments soon after it becomes permeable, creating porous lapilli and ash that continue to degas in open-system within an expanding gas phase. As suggested by recent studies, some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level, partly equilibrating with the continuously fluxing heavier magmatic vapor, explaining the wide range of H2O contents and high variability in δD measured in the pyroclastic obsidians. Using only H2O and δD, it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse, either syn-explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase. During the final effusive phase of the eruption, slowly ascending magma degasses in open-system until it reaches the surface, creating flows with low H2O and δD. This study shows that H2O measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn-eruptive magma degassing pathways, unless careful assessment of post-eruptive rehydration is first carried out. If their mechanism of formation can be better understood, detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases.