Oxygen and Hydrogen Isotope Studies of Igneous Rocks and Associated Hydrothermal Alteration

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9725811 Taylor This proposal is part of a continuing effort to understand water-rock interactions in the Earth's crust and the mechanisms of 18O/16O exchange between minerals and fluids. Recently, the PI and coworkers have discovered a new type of 18O/16O signature was discovered in the welded ash-flow tuffs, namely the coexistence of 18O-depleted volcanic glass with feldspar and quartz phenocrysts that have preserved their primary magmatic 18O/16O values. This implied that large amounts of low-18O meteoric groundwater had tocirculate through the tuff at high temperatures (500(-650(C), on a short enough time scale ((25 years) that the feldspar (18O values were undisturbed, suggesting an analog to the spectacular fumarolic activity that occurred in 1912-1924 at the Valley of Ten Thousand Smokes, Mt. Katmai National Park, Alaska. The present proposal aims to test these concepts on an outflow-tuff sheet; such a study would be more directly analogous to the activity seen on the surface at Katmai. The 760,000 year old, 200-m thick Bishop Tuff in California appears to be an ideal field area, because of good zones of shallow-dipping columnar joints. The amounts of magmatic H2O in these systems and the mechanisms of hydrologic recharge for these fossil fumaroles will be investigated by making detailed 18O/16O studies, as well as making a regional 18O/16O map and comparing it to what is known about the pre-eruptive surface topography of the BishopTuff. This will allow a test as to whether these fossil fumaroles were fed by groundwater flow from the newly-formed Long Valley Lake down through the upper zone of permeable, partially welded ash-flow tuff. The role of columnar jointing in fumarolic meteoric-hydrothermal circulation will also be studied.