The development of an in-situ UV ablation GC-IRMS technique for the analysis of oxygen isotopes in metamorphic minerals, and its application to polymetamorphic schists from Western Massachusetts, U.S.A.
This thesis describes an attempt to develop a pioneering method for the analysis of oxygen isotopes in metamorphic rocks. This technique is then applied to a suite of metapelites from Massachusetts, U.S.A. with the aim of investigating metamorphic history. The study of oxygen isotopes is a rapid and efficient way of deciphering the reaction history of a metamorphic rock, and they are particularly useful for quantifying the role of fluids during metamorphism. Technological advances have given the opportunity to develop a new laser fluorination facility capable of in-situ oxygen isotope analysis on the 100μm scale. The use of UV laser ablation coupled with helium carrier flow and isotope ratio mass spectrometry gives the potential for liberation, transfer and analysis of nanomoles of oxygen. This analytical technique is developed herein, and applied to garnets from high alumina metapelites of the Hoosac Schist of Western Massachusetts. These large garnets contain concentric unconformity textures which are attributed to at least two metamorphic events. Core-rim zoning profiles from three Hoosac garnets has been accomplished. Metamorphic modelling in the complex chemical system Na2O-CaO-MnO-K2O-FeO-MgO-Al2O3- SiO2-H2O has yielded P-T estimates for garnet cores of 520°C and 8.5 kbar, and rims at 590°C and 8-10kbar. Within this framework, a new approach enables calculation of oxygen isotope shifts with reaction progress in the presence of a non-equilibrium fluid. Fitted profiles from the Hoosac garnets imply prograde core growth during inflow of external low-δ18O fluid, and calculations suggest a minimum time integrated fluid flux for the first garnet growth event of the order of 0.2 cm3/cm2, some four to five orders of magnitude less than other New England studies.