Stable isotope and petrologic evidence for open-system degassing during the climactic and pre-climactic eruptions of Mt. Mazama, Crater Lake, Oregon

Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km(3) of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43 degrees N, long. 122 degrees W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazarna climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing. Sulfur isotope analyses of climactic rhyodacitic whole rocks yield delta S-34 values of 2.8-14.8 parts per thousand with corresponding matrix glass values of 2.4-13.2 parts per thousand. delta S-34 tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in delta S-34 from -0.4 parts per thousand to 5.8 parts per thousand and from -0.1 parts per thousand to 3.5 parts per thousand, respectively. Initial delta S-34 values of rhyodacitic and andesitic magmas were likely near the mantle value of 0 parts per thousand. Hydrogen isotope (delta D) and total H2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values of -103 to -53 parts per thousand and 0.23-1.74 wt%, respectively. Values of delta D and wt% H2O of obsidian decrease towards the top of the fall deposit. Samples with depleted delta D. and mantle delta O-18 values, have elevated delta S-34 values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth's atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14% (to delta S-34 values of 14 parts per thousand or more here) and hydrogen isotopic compositions by 90 parts per thousand (to delta D values of -127 parts per thousand in this case). (C) 2009 Elsevier Ltd. All rights reserved.