Oxygen Isotope Systematics in Polymetamorphic Rocks: The Effects of Multiple Periods of Deformation and Mineral Growth

点击翻译按钮获取中文摘要

9814991MoecherOxygen isotope analyses of metamorphic minerals provide critical constraints on the thermal evolution of, and fluid flow in, the crust. Studies of the oxygen isotope systematics of minerals in regional metamorphic rocks have mainly assessed the extent of isotopic equilibrium with increasing metamorphic grade based on isotopic fractionations among porphyroblasts and matrix phases. However, few studies explicitly consider the deformation history of regional metamorphic rocks in terms of what effect punctuated fabric development (sequential foliation/crenulation development) and discontinuous porphyroblast nucleation and growth may have on the extent of isotopic equilibrium among minerals. The current research will investigate the effects of deformation partitioning and punctuated fabric development on oxygen isotope systematics in minerals from two regional metamorphic terranes. Specifically, the hypothesis that all minerals in all fabric elements are in oxygen isotopic equilibrium will be tested. This will require spatial resolution on the order of millimeters and analysis of milligram quantities of material. Such resolution will be obtained using laser heating fluorination of small volumes of minerals (including refractory porphyroblasts) separated from distinct domains within a multiply deformed rock. In the context of this study, oxygen isotope analysis will serve to link the geochemical (isotopic and major element equilibrium), thermal (based on isotopic fractionations) and physical (mineral reactions and deformation history) evolution of metamorphic rocks. In addition to oxygen isotope equilibrium, stable isotope analysis provides insight into which minerals in a metamorphic assemblage are likely to be in major element equilibrium. This has important implications regarding equilibrium among metamorphic minerals used in conventional geothermobarometry to reconstruct P-T-t paths in ancient orogens.