2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 10
Presentation Time: 10:50 AM

VARIATIONS IN QUARTZITE RHEOLOGY AS A FUNCTION OF METAMORPHIC FLUID COMPOSITION: RESULTS OF COUPLED FLUID INCLUSION AND EXPERIMENTAL STUDIES


SELVERSTONE, Jane, Earth & Planetary Sciences, Univ. of New Mexico, Albuquerque, NM 87131-1116, TULLIS, Jan, Brown Univ, PO Box 1846, Providence, RI 02912-1846 and HOLYOKE III, Caleb W., Department of Geological Sciences, Brown Univ, Box 1846, Providence, RI 02912, selver@unm.edu

Synmetamorphic devolatilization reactions may dramatically affect the rheology of quartz-rich rocks during deformation. Water is well known to weaken qtz deforming by dislocation creep (DC); the effects of other fluid compositions have not been systematically investigated, but observations suggest that carbonic fluids cause embrittlement at conditions where qtz would otherwise behave plastically (Post et al. 1996 JGR; Selverstone 2001 GSA abs). We have analyzed the results of >60 deformation experiments (axial compression & shear) carried out on Black Hills (BH) and Heavitree (HT) quartzite to assess the role of initial fluid composition on quartzite rheology. Fluid inclusions (FIs) are abundant and diverse in BH starting material: H2O-NaCl, H2O-CaCl2, and CO2. Measurable FIs in HT are nearly pure CH4. Experimental runs over a range of conditions (700-900°C, 1000-1500 MPa, 5-60% axial strain, gamma=1.3-6, strain rate=10-5-10-6/s) show consistent differences in behavior between the two quartzites: (1) HT microstructures are systematically about 50°C 'colder' than those in BH deformed at the same conditions. (2) BH samples develop distinctive 'hot' and 'cold' bands perpendicular to s1; cold bands are marked by s1-parallel microcracks and webs of CO2 FIs. TEM analysis of dislocation microstructures in banded BH samples deformed by regime 1 DC (Hirth & Tullis, JSG) shows distinct differences: 'hot' bands have dislocation tangles and strain-free recrystallized grains, but 'cold' bands have linear dislocations and microcracks. HT developed banding only when NaCl was introduced into the sample during deformation. We interpret these differences in behavior as resulting from release of different FI fluids. Low fH2O fluids in HT and in 'cold' BH bands inhibit dislocation glide, climb and grain boundary migration, producing 'cold' microstructures. High fH2O fluids in BH at the same conditions facilitate DC processes. Band development in BH may result from self-organization of immiscible brines and CO2 during deformation. Introduction of salt into HT likely also caused fluid separation at run conditions, leading to localized brittle failure. Our data show that changes in metamorphic fluid composition can lead to dramatic changes in rock rheology, in some cases causing embrittlement even at high P&T.