HR: 11:20h
AN: V12A-05    [Abstracts]
TI: Experimental Evidence for High-Pressure Phase Separation in the H2O-CO2-CaCl2 System: Implications for Rock Rheology
AU: * Selverstone, J
EM: selver@unm.edu
AF: University of New Mexico, Dept Earth Plan. Sciences MSC03 2040, Albuquerque, NM 87131, United States
AU: Chernak, L
EM: Linda-Chernak@brown.edu
AF: Brown University, Dept Geological Sciences, Providence, RI 02912, United States
AU: Tullis, J
EM: Jan-Tullis@brown.edu
AF: Brown University, Dept Geological Sciences, Providence, RI 02912, United States
AU: Cooper, R
EM: Reid-Cooper@brown.edu
AF: Brown University, Dept Geological Sciences, Providence, RI 02912, United States
AB: As part of a study to examine the effect of CO₂ on deformation mechanisms in quartz, axial compression experiments were carried out at 900°C and 1500 MPa on cores of Black Hills quartzite (BHQ) with a layer of dolomite powder (± 0.05 wt% H₂O) in the center of each charge (some runs included buffer assemblages at sample ends). All runs released CO₂ via the reaction dol + qtz = diop + CO₂ during run-up to experimental conditions. BHQ starting material contains three types of naturally occurring fluid inclusions (FIs): pure H₂O, H₂O + 6-18 wt% CaCl₂, and pure CO₂. Deformation experiments on as-is BHQ (no dol powder) result in destruction of most optical FIs. In contrast, experiments with wet dol powder produced visible FIs in nearly all samples, though most were too small to analyze by microthermometry. One hydrostatic experiment with dolomite generated FIs up to 15 microns across near the reaction zone. FIs within this sample fall into two types: (1) superdense CO₂ (homogenization to liquid below -50°C), and (2) H₂O-CO₂-CaCl₂ solutions with variable X(CO₂) and bulk density and up to 40 wt% CaCl₂ (referenced to aqueous phase only). Both inclusion types occur within the same clusters, and likely result from interaction of CO₂ released by dol breakdown with H₂O and FI fluids released from the starting material. Isochores from the Type 1 CO₂ FIs record pressures of 1200- 1400 MPa at 900°C. Estimation of bulk density for Type 2 FIs is hampered by complex microthermometric behavior and incomplete equation of state data for this fluid system, but model isochores overlap with those of Type 1 FIs at 900°C. Entrapment of the two types of FIs and variable phase proportions in Type 2 inclusions are consistent with fluid phase separation at experimental conditions. Deformation experiments run at f(O₂)<graph-CO₂ show significantly less strain close to the dol reaction zone, where CO₂ inclusions are most abundant, relative to sample ends where CO₂ was reduced to graphite. CO₂ reduction may move rocks out of the two-fluid field and result in an increase in both a(H₂O) and f(H₂O), which in turn will facilitate strain accommodation by dislocation and/or diffusion creep. Shmulovich & Graham (2004 CMP) documented a large two-fluid field in the H₂O-CO₂-CaCl₂ system at 800°C and 900 MPa. Our study shows that the region of immiscibility extends to higher pressures and temperatures, and can be anticipated in both lower crustal and upper mantle rocks as well as in subducting slabs. Our data also demonstrate that variations in f(O₂) may be generated and preserved over short distances, and that strain accommodation mechanisms can co-vary with fluid composition, f(O₂) and f(H₂O) in COH-salt systems. We thus anticipate that significant localized differences in rock strength will result from metamorphic reactions that move rocks into and out of the two-fluid field at high pressure.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8045 Role of fluids
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting