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

Bulletin of Volcanology

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DOI: 10.1007/s004450050207

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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 δ18O in Holocene time. In the Pleistocene, basaltic lavas with <52% SiO2 averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0%SiO2) 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-18O crust or from fractionation combined with assimilation of very-low-18O 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-18O 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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)
Series title:
Bulletin of Volcanology
DOI:
10.1007/s004450050207
Volume
59
Issue:
8
Year Published:
1998
Language:
English
Publisher:
Springer
Description:
8 p.
First page:
529
Last page:
536
Online Only (Y/N):
N
Additional Online Files(Y/N):
N