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Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit.

Gardner, J.E. and Llewellin, E.W. and Watkins, J.M. and Befus, K.S. (2016) 'Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit.', Earth and planetary science letters., 459 (1). pp. 252-263.

Abstract

The ranges in intensity and style of volcanic eruptions, from highly explosive Plinian eruptions to quiescent lava extrusions, depend on the style and efficiency of gas loss from ascending magma. Obsidian pyroclasts – small, glassy pieces of quenched magma found in some volcanic tephra beds – may preserve valuable information about magma degassing in their vesicle textures and volatile contents. Accurate interpretation of their textures and volatiles, however, requires understanding the mechanism of formation of the pyroclasts. Obsidian pyroclasts from the ca. 1325–1350 C.E. North Mono eruption of Mono Craters (CA, USA) were analyzed and found to have H2O and CO2 contents indicating that they were formed at pressures in the approximate range of 3–40 MPa. Many also contain domains with differing vesicle textures, separated by boundaries containing xenocrystic material, indicating that they are composed of smaller fragments that have sutured together. More than half of the pyroclasts analyzed contained small (∼10 μm), highly distorted vesicles, with multi-cuspate morphology, interpreted as the remnants of interstitial gas trapped amongst sintered fragments of melt/glass. Rounded vesicles are also common and are interpreted to result from surface tension-driven relaxation of the distorted vesicles. Calculated timescales of sintering and relaxation are consistent with timescales for pyroclast formation indicated by H2O re-equilibration within the heterogeneous pyroclasts. This sintering model for the origin of obsidian pyroclasts is further supported by the observation that spherical vesicles are found mainly in H2O-rich pyroclasts, and distorted vesicles mainly in H2O-poor pyroclasts. We conclude that obsidian pyroclasts generated during the North Mono eruption were formed by cycles of fragmentation, sintering/suturing, and relaxation, over a very wide range of depths within the conduit; we find no evidence to support pumice (foam) collapse as the formation mechanism. Similar textures, and the occurrence of xenolithic material, in obsidian pyroclasts in other eruptions suggest that sintering may be generally responsible for the origin of obsidian pyroclasts. Our conceptual model indicates that volatile contents in obsidian pyroclasts reflect both degassing of bubbly magma and the composition of gas trapped between sintering particles.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1016/j.epsl.2016.11.037
Publisher statement:This article is available under the terms of the Creative Commons Attribution License (CC BY). You may copy and distribute the article, create extracts, abstracts and new works from the article, alter and revise the article, text or data mine the article and otherwise reuse the article commercially (including reuse and/or resale of the article) without permission from Elsevier. You must give appropriate credit to the original work, together with a link to the formal publication through the relevant DOI and a link to the Creative Commons user license above. You must indicate if any changes are made but not in any way that suggests the licensor endorses you or your use of the work.
Date accepted:20 November 2016
Date deposited:23 November 2016
Date of first online publication:06 December 2016
Date first made open access:No date available

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