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

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TaylorEAR-0106696We are attempting to show that equilibrium iron-isotope fractionations among simple iron-bearing complexes and Fe metal can be accurately calculated using a combination of force-field modeling together with available measurements of vibrational frequencies from infrared and Raman spectroscopy. This is important because of its pertinence in interpreting the exciting new results by other workers of high-precision iron isotope measurements on natural materials and between species in aqueous solution and those bound to ion-exchange resins. We are also finishing up an oxygen and radiogenic isotope study of granitic plutons in Nevada and Utah for which preliminary results have shown a systematic involvement of different source rocks during each of three stages of magmatism in the Jurassic, Cretaceous, and Cenozoic. A third goal is to complete our understanding of the remarkable oxygen isotope patterns in phenocrysts and groundmass found in the upper parts of welded ash-flow tuffs, exemplified in our studies of the Bishop Tuff (Holt and Taylor, 1998), where we showed that the low-18O zones exhibited a one-to-one correspondence with mapped fossil fumarolic structures exposed in the uppermost 70-80 meters of the tuff We are in the process of examining the exposed upper 15 meters of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS), Alaska (where the fumaroles are known from direct observation to have terminated about 15 years after eruption), thereby linking the scientific observations made on recently active fumaroles in the VTTS with the fossil fumaroles in the Bishop Tuff.