J.M. Donnelly-Nolan
1998
<p><span> Oxygen-isotope analyses of lavas from Medicine Lake volcano (MLV), in the southern Cascade Range, indicate a significant change in δ</span><span class="a-plus-plus">18</span><span>O in Holocene time. In the Pleistocene, basaltic lavas with <52% </span>SiO<span class="a-plus-plus">2</span><span> averaged +5.9‰, intermediate lavas averaged +5.7‰, and silicic lavas (≥63.0%</span>SiO<span class="a-plus-plus">2</span><span>) 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-</span><span class="a-plus-plus">18</span><span>O crust or from fractionation combined with assimilation of very-low-</span><span class="a-plus-plus">18</span><span>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-</span><span class="a-plus-plus">18</span><span>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.</span></p>
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10.1007/s004450050207
en
Springer
Abrupt shift in δ18O values at Medicine Lake volcano (California, USA)
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