HR: 08:30h
AN: T41G-03    [Abstracts]
TI: Oxygen Isotopic Constraints on the Origin of Syntectonic Veins: Significance for Fluid-Rock Interaction in Convergent Margins
AU: * Gray, D R
EM: drgray@unimelb.edu.au
AF: School of Earth Sciences, University of Melbourne, Melbourne, 3010 Australia
AU: Gregory, R T
EM: bgregory@smu.edu
AF: Geological Sciences, Southern Methodist University PO Box 750395, Dallas, TX 75275 United States
AU: Richards, I J
EM: irichard@smu.edu
AF: Geological Sciences, Southern Methodist University PO Box 750395, Dallas, TX 75275 United States
AB: The oxygen isotope ratios of veins and their coexisting host rocks are sensitive to the conditions of vein formation during crustal shortening that occurs at convergent margins. Vein-host rock measurements from the Lachlan Fold Belt (Australia), the Otago Schist (New Zealand), the Ouachita and Appalachian Orogens (United States) and the Oman Mountains demonstrate that the ratio of the fluid flux rate (u) to the isotopic exchange rate (k) is the parameter that determines the form of the oxygen isotope distribution. For many geologic environments, the isotopic composition of the fluid reaches a quasi-steady state that can be approximated by mixing of a rock-buffered fluid (u/k~0) and an external fluid (u/k>>1); in a fluid + rock binary system, any natural system generates a spectrum of fluid compositions ranging from fluid-buffered to rock-buffered. The primary controls on this critical ratio are permeability (for u) and temperature (for k). Mechanical limits on permeability at depth allow the preservation of surface-derived isotopic heterogeneities to great depths and for geologically long time scales. As pressure and temperature increase, the ratio of u/k is driven toward 0 (declining numerator and increasing denominator) so that the fluid isotopic composition is buffered by the local host rocks (10's of meter scales or less) on time scales of 1000's of years or less while isotopic heterogeneities in the host rock minerals will persist for millions of years. Secondary phases grown in the presence of this rock-buffered fluid will reflect isotopic equilibrium with the local fluid environment and exhibit non-equilibrium fractionations with the remaining partially-exchanged phases. The time scales for this type of fluid-rock interaction are comparable to time scales for metamorphism and deformation in convergent margin settings.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 8108 Continental tectonics: compressional
DE: 8109 Continental tectonics: extensional (0905)
DE: 8110 Continental tectonics: general (0905)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005