T242 Coupling of Deformation and Chemical Processes in Earth

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Hall E

USING MICROSTRUCTURES TO TEST THE ASSUMPTION OF EQUILIBRIUM FRACTIONATION FOR OXYGEN ISOTOPE THERMOMETRY - THE QUARTZITIC KETTLE DETACHMENT (WA, USA)

Paper number: T242-12
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

Meteoric fluid flow along detachments during crustal scale extension is now well established, yet the interaction among fluids, deformation processes, and rock chemistry in this tectonic context remains unclear. Here we document that microstructural evolution along the quartzite mylonite that delineates the Kettle dome metamorphic core complex reflects changes in mineral chemistry, stable isotope signature, and fluid-rock interaction. High-resolution sampling (~5 m) of the Kettle detachment over approximately 100 m of section indicates that microstructures evolve from dislocation creep regimes 1-2 to regimes 2-3 (Hirth and Tullis, 1992) downsection, with increasing recovery and recrystallization, hydration of feldspar, and changes in mica chemistry. EBSD analyses produce very constant pole figures and highlight that the quartzite experienced non-coaxial, simple shear dominated deformation.

Over the 100 m section, delta D isotopic ratios from synkinematic muscovite grains are consistent with a meteoric fluid source (-130 ‰). Results of quartz-muscovite (Qz-Ms) oxygen isotope thermometry (Chacko et al., 1996) indicate that temperatures of fractionation remain stable (~350°C) across the section. However, in the upper part of the section, close to the hanging wall, where quartzite is deformed in regimes 1-2, Qz-Ms oxygen isotopes did not reach isotopic equilibrium, as shown by “unrealistic” temperatures derived from the oxygen isotope thermometer. Geologically “realistic” temperatures associated with high recovery/recrystallization rates and the presence of unzoned micas may indicate pervasive fluid flow. In contrast, the presence of zoned micas and low recovery/recrystallization rates in the upper part of the quartzite mylonite, coupled with the suggested isotopic disequilibrium results, may indicate channelized fluid flow. Therefore, we conclude that microstructural development, and especially the degree of dynamic recrystallization, provides a qualitative measure of oxygen isotope equilibrium.