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Geology; November 1996; v. 24; no. 11; p. 1021-1024; DOI: 10.1130/0091-7613(1996)024<1021:CAHIEF>2.3.CO;2
© 1996 Geological Society of America
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Chemical and hydrogen isotope evidence for in situ dehydrogenation of biotite in silicic magma chambers

T. C. Feeley1 and Z. D. Sharp2

1 Département de Minéralogie, Université de Genève, 13 rue des Maraîchers, 1211 Genève 4, Switzerland
2 Institut de Minéralogie et Pétrologie, Université de Lausanne, BFSH-2, Lausanne CH-1015, Switzerland

To examine the potential for volatile fluxing of magma chambers by in situ degassing of hydrous minerals, we obtained complete chemical analyses for biotite separates from silicic lavas. The separates exhibit unusually low H2O contents that inversely correlate with host lava temperatures, high Fe3+/Fe2+ ratios that inversely correlate with host lava oxygen fugacities, and the highest {delta} D values yet reported for biotite from any silicic igneous rock (up to –19{per thousand}). These results are direct evidence for selective loss of protium (1H) from biotite during dehydrogenation in magma chambers heated from below by intrusion of mafic magma. The maximum P{Delta}V energy generated from dehydrogenation alone can approach 2 x 103 joules per kilogram of magma. This finding provides support for the concept that injection of mafic magma coupled with sudden degassing of hydrous minerals in a volatile-rich magma chamber can increase pressure, and thus enhance the possibility of initiating a volcanic eruption.




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