Hydrogen isotope analysis of rehydrated silicic lavas: implications for eruption mechanisms

Bulk and step-heating hydrogen isotope measurements of glassy and vesicular samples from three young rhyolite lavas demonstrate for the first time tha t magmatic H2O can be distinguished from meteoric H2O added during rehydrat ion. Step-heat extractions of gases obtained at five temperatures show that most meteoric H2O is released at around 400 degreesC, while magmatic H2O c omes off during the fusion step at 1100-1200 degreesC. Magmatic H2O content s from vertically-arrayed lava samples show a downward increase consistent with the equilibrium pressure-dependence of H2O solubility. These gradients imply that volatile concentrations high enough to generate pyroclastic flo ws during front collapse may be found in the upper portions of silicic dome s produced by relatively high eruption rates. (C) 2001 Elsevier Science B.V . All rights reserved.