Experimental constraints on the textures and origin of obsidian pyroclasts
Obsidian pyroclasts are commonly preserved in the fall deposits of explosive silicic eruptions. Recent work has suggested that they form by sintering of ash particles on the conduit walls above the fragmentation depth and are subsequently torn out and transported in the gas-particle dispersion. Although the sintering hypothesis is consistent with the general vesicle textures and dissolved volatiles in obsidian pyroclasts, previous sintering experiments do not capture all of the textural complexities observed in the natural pyroclasts. Here, we design experiments in which unimodal and bimodal distributions of rhyolitic ash are sintered at temperatures and H2O pressures relevant to shallow volcanic conduits and under variable cooling rates. The experiments produce dense, welded obsidian that have a range of textures similar to those observed in natural pyroclasts. We find that using a unimodal distribution of particles produces obsidian with evenly distributed trapped vesicles, while a bimodal initial particle distribution produces obsidian with domains of poorly vesicular glass among domains of more vesicle-rich glass. We also find that slow cooling leads to resorption of trapped vesicles, producing fully dense obsidian. These broad features match those found in obsidian pyroclasts from the North Mono (California, USA) rhyolite eruption, providing strong support to the hypothesis that obsidian can be produced by ash sintering above the fragmentation depth during explosive eruptions.
KeywordsObsidian Ash Sinter Volatile Explosive eruption
JEG and JMW were partially supported by grants from the National Science Foundation (EAR–1725186 and EAR–1725207). EWL and JPC acknowledge support from the UK Natural Environment Research Council via grant NE/N002954/1. FBW acknowledges a fellowship from the Centre for Advanced Study at the Ludwig-Maximilians-Universität, Munich. All data from the study can be obtained from JEG. The authors thank two anonymous reviewers and the Associate Editor for their insights, which have improved the manuscript.
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