HR: 17:15h
AN: V34A-06    [Abstracts]
TI: The Role of Brines in low Temperature, Fault-related Deformation of Quartz Arenites
AU: * O'Kane, A
EM: aokane@bgnet.bgsu.edu
AF: Bowling Green State University, Department of Geology 190 Overman Hall, Bowling Green, OH 43403 United States
AU: Onasch, C M
EM: conasch@bgnet.bgsu.edu
AF: Bowling Green State University, Department of Geology 190 Overman Hall, Bowling Green, OH 43403 United States
AU: Farver, J
EM: jfarver@bgnet.bgsu.edu
AF: Bowling Green State University, Department of Geology 190 Overman Hall, Bowling Green, OH 43403 United States
AB: Fluids play an integral role in deformation within the Earth\'{}s crust over a wide range of physical conditions. At low temperatures ($<$$300\deg$C) the effect is dominantly mechanical, largely through the effects of pore fluid pressure. At higher temperatures ($>$$300\deg$C), chemical processes, such as diffusive mass transfer, advective mass transfer, and hydrolytic weakening dominate. Brines, because of their greater reactivity, enhance certain chemical processes during deformation. In the transition between high and low temperature regimes, both mechanical and chemical processes operate and interact in complex ways. This study investigates the role of brines in the deformation of quartz arenite in a map scale fault zone deformed under conditions transitional between low and high temperature regimes. The fault zone is also known to have been a conduit for fluids thought to be largely basinal brines. The Cove Mountain fault zone in south central Pennsylvania contains several map-scale blocks of quartz arenite which display a wide range of brittle and ductile microstructures. Abundant evidence of fluids is present in the form of quartz veins, microveins, fluid inclusion planes, cataclastic bands, and stylolites. Three different fluids are recognized based on cathodoluminescence color of quartz, and homogenization (T$_{h}$) and melting temperatures (T$_{m}$) from fluid inclusions. Blue-green luminescing quartz has a T$_{h}$ of 185 to $215\deg$C and a T$_{m}$ of -15 to -$17\deg$C; red luminescing quartz has a T$_{h}$ of 165 to $200\deg$C and a T$_{m}$ of -16 to -$20\deg$C; and zoned quartz with both red and blue-green luminescence has a T$_{h}$ of 180 to $220\deg$C and a T$_{m}$ of -18 to -$23\deg$C. The eutectic temperature of all three fluids is approximately -$50\deg$C suggesting that CaCl$_{2}$ is the dominant salt species. Grains adjacent to fluid conduits (microfractures, cataclastic bands, and stylolites) display more crystal plastic microstructures than those farther away. Compared to quartz in veins and in undeformed portions of the rock, which have water contents of ~1,000 and ~12,000 H/10$^{6}$ Si, respectively (determined by FTIR), grains adjacent to fluid conduits have water contents as high as 26,000 H/10$^{6}$ Si indicating that water was able to penetrate these grains and thus promote ductile deformation in a dominantly brittle regime. Access of water to grain centers was provided by both microfracturing and diffusion. Evidence for diffusion is shown by the presence of quartz grains adjacent to fluid conduits which have blue luminescing centers and red rims. Water contents in these grains ranges from 2,000-22,000 H/10$^{6}$ Si with some correlation between water content and luminescence color. The large amount of pressure solution relative to similar rocks in the region is believed to be related to the salinity of the brines. Also, the thick accumulation of goethite in stylolites and fractures precipitated by these fluids is unusual in similar rocks in the region suggesting that the fluid chemistry was atypical. The microstructures in this fault zone indicate that water is important in controlling the operative deformation mechanisms in the transition from low to high temperature deformation. Furthermore, the chemistry of these brines may account for greater solubility of quartz and faster diffusion rates than would be the case with low salinity fluids.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting