HR: 11:35h
AN: V12A-06    [Abstracts]
TI: CO2 Strengthening of Quartz in the Dislocation Creep Regime?
AU: * Chernak, L J
EM: Linda_Chernak@brown.edu
AF: Brown University, Dept. of Geological Sciences 324 Brook St., Providence, RI 02912,
AU: Tullis, J
EM: Jan_Tullis@brown.edu
AF: Brown University, Dept. of Geological Sciences 324 Brook St., Providence, RI 02912,
AU: Selverstone, J
EM: selver@unm.edu
AF: Univ. of New Mexico, Dept. of Earth & Planetary Sciences, Albuquerque, NM 87131-0001,
AB: Trace amounts of water have long been known to significantly reduce the strength of quartz deforming by dislocation creep, but the effect of carbonic fluids has not been documented experimentally. In naturally deformed rocks, however, there are indications that carbonic fluids act to strengthen quartz-rich rocks. In schists from the eastern Alps, Selverstone (2005, JMG) observed that quartz in graphitic layers with carbonic fluid inclusions (FIs) deformed in a brittle manner whereas quartz in closely adjacent non-graphitic layers with aqueous FIs deformed by dislocation creep. We have conducted an experimental study to investigate the effect of carbonic fluids on quartz deforming by dislocation creep. Constant displacement rate, axial compression experiments were conducted at 900°C, 1.5 GPa, and 10^-5/s in a modified Griggs apparatus using cylinders of as-is Black Hills quartzite (BHQ) encapsulated in Pt (inner) and Ni (outer) jackets; all samples were shortened 50%. BHQ is a pure (<1% feldspars, clays, and Fe oxides), quartzite with equant grains (average diameter ~100 μm); it contains three types of FIs (carbonic, H₂O + NaCl, and H₂O + CaCl₂) and has an average bulk water content of 0.09 wt % as measured by FTIR. During deformation and recrystallization, most of the original optical-scale FIs appear to be expelled to grain boundaries. In order to test the effect of carbonic fluids, CO₂ was generated in some experiments at P and T by the reaction of quartz + dolomite goes to diopside + CO₂. In these experiments a ~140 μm thick layer of dried, mixed, fine-grained (~5-20 μm diameter) quartz and dolomite powder was placed between two short cores of BHQ. Some samples were held at P and T for 15 hours and some for 85 hours, before starting the deformation. The amount of CO₂ produced by complete reaction is ~0.005 g, or 0.5 wt %. The formation of the diopside layer did not appear to affect the sample deformation. The bulk strength of samples with added CO₂ was similar to or greater than that of as-is samples of BHQ deformed at the same conditions. However, the distribution of deformation was strikingly different. As-is samples are very homogeneously strained, consistent with our very low T gradients. In contrast, the samples with CO₂ produced from the central reaction layer have an hourglass shape; the region close to the reaction layer has abundant CO₂ FIS and is almost unstrained, whereas the regions further away, toward the ends of the sample, have very few CO₂ FIs and are highly strained and recrystallized. Thus it appears that the areas with higher CO₂ were stronger. It is not yet clear whether the CO₂ FIs prevent grain boundary migration recrystallization and/or whether the CO₂ decreases the water fugacity.
DE: 3902 Creep and deformation
DE: 8030 Microstructures
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8045 Role of fluids
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting