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

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.