Chemical and hydrogen isotope evidence for in situ dehydrogenation of biotite in silicic magma chambers

Abstract

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 H 2 O contents that inversely correlate with host lava temperatures, high Fe 3+ /Fe 2+ ratios that inversely correlate with host lava oxygen fugacities, and the highest δ D values yet reported for biotite from any silicic igneous rock (up to −19‰). These results are direct evidence for selective loss of protium ( 1 H) from biotite during dehydrogenation in magma chambers heated from below by intrusion of mafic magma. The maximum P Δ V energy generated from dehydrogenation alone can approach 2 × 10 3 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.