Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)

J.M. Donnelly-Nolan

doi:10.1007/s004450050207

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Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)

J.M. Donnelly-Nolan

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Abstract

Oxygen-isotope analyses of lavas from Medicine Lake volcano (MLV), in the southern Cascade Range, indicate a significant change in δ 18 O in Holocene time. In the Pleistocene, basaltic lavas with <52% SiO 2 averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0% SiO 2 ) averaged +5.6‰. No analyzed Pleistocene rhyolites or dacites have values greater than +6.3‰. In post-glacial time, basalts were similar at +5.7‰ to those erupted in the Pleistocene, but intermediate lavas average +6.8‰ and silicic lavas +7.4‰ with some values as high as +8.5‰. The results indicate a change in the magmatic system supplying the volcano. During the Pleistocene, silicic lavas resulted either from melting of low- 18 O crust or from fractionation combined with assimilation of very-low- 18 O crustal material such as hydrothermally altered rocks similar to those found in drill holes under the center of the volcano. By contrast, Holocene silicic lavas were produced by assimilation and/or wholesale melting of high- 18 O crustal material such as that represented by inclusions of granite in lavas on the upper flanks of MLV. This sudden shift in assimilant indicates a fundamental change in the magmatic system. Magmas are apparently ponding in the crust at a very different level than in Pleistocene time.

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Bulletin of Volcanology Article

Source-reported bounding extent: 41.53044724524247° to 41.61144377098603° latitude; -121.65583787658159° to -121.55475257378487° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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J.M. Donnelly-Nolan. 1998. Abrupt shift in δ18O values at Medicine Lake volcano (California, USA). https://doi.org/10.1007/s004450050207

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