9316477 Wolff The proposed research project will exploit the measurable evolution of 87Sr/86Sr over the lifetime of a large rhyolitic magma system with very high Rb/Sr, to investigate and constrain the rates and nature of processes causing compositional zonation. The rhyolitic system to be studied is that which erupted at 1.51 Ma to form the Otowi (or Lower) Member of the Bandelier Tuff, Jemez Mountains, New Mexico. The procedure to be followed relies on analyzing very small amounts of Sr (down to 1 ng) in small samples of feldspar, matrix glass, and glass inclusions. The results of this work will enable the contribution of various processes involved in compositional zonation (closed-system differentiation, phenocrysts accumulation, wall-rock contamination, magmatic recharge) of the Otowi Member magma to be quantified. More significantly, the timing, rates, and mechanism of any contribution from closed-system differentiation can be constrained more closely than is possible in a conventional geochemical study.
