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43498-AC8
Modeling Multi-Phase Fluid-Rock Reactions in the Shallow Crust

John R. Bowman, University of Utah

The principal objectives of this study are to quantify the geological, petrological and geochemical characteristics of the Alta Stock thermal aureole, Alta, Utah. These data will then be used to investigate processes and rates of fluid-rock interaction and mineral growth, the nature of hydrothermal circulation, and the potential role of fluid immiscibility in the aureole.

This year we have focused on the following two activities:

Ion microprobe measurements of oxygen isotope compositions of calcite in the Alta aureole.

Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing (d¹⁸O = 11.8 ä) and periclase-free (d¹⁸O = 17.2 ä) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah.  This work has been done in collaboration with John Valley and Noriko Kita at the WISC-SIMS ion microprobe laboratory at the University of Wisconsin.  These data define a steep, coherent gradient in d¹⁸O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer.  SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.27 ä, 2 SD).  More poorly polished (pitted), texturally retrograde 'turbid'-looking calcite has lower and more variable d¹⁸O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries.  Despite significant lowering of the d¹⁸O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in d¹⁸O.  The results also indicate that grain-to-grain variability in d¹⁸O value is small at any one location along this gradient. Diffusion calculations (spherical geometry, 0.5 to 1.0 mm grain diam.) indicate that conservative time scales required for 90% exchange at calcite grain centers—effective isotopic homogenization—by volume diffusion are 30,000 to 62,000 years at 575-600^oC, the estimated temperature of formation of the periclase zone.  These timescales exceed significantly the timescale (~1250 yrs) estimated for the prograde development of the d¹⁸O gradient at the boundary between these two marble layers.  The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains.  Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization.

Quantitative evaluation of the impact of salinity on phase equilibria in the system CaO-MgO-SiO₂-H₂O-CO₂.

Several equations of state (EOS) have now been developed for the fluid system H₂O-CO₂-NaCl at pressure-temperature-composition conditions appropriate for contact metamorphic environments such as Alta (reviewed in Gottschalk, 2007).  Thus it is now possible to begin to evaluate quantitatively the impact of salinity on fluid immiscibility in this system and on devolatilization phase equilibria relevant to the contact metamorphism of siliceous dolomite carbonate rocks (CaO-MgO-SiO₂-H₂O-CO₂).  Heinrich (2004) has emphasized the potentially significant impact of salinity and fluid immiscibility on both the P-T-X locations of phase equilibria and their roles in driving rock reactions.  We have evaluated several EOS's for H₂O-CO₂-NaCl fluids and a number of thermodynamic data bases for minerals.  We have selected the thermodynamic data base of Gottschalk (1997) and the unpublished EOS of Aranovich and Haefner to calculate phase equilibria in the system CaO-MgO-SiO₂-H₂O-CO₂–NaCl using Perple_X 07 (Connolly, 2005).  The graphical TOC image is an isobaric (P_fl = 150 MPa) T-X(CO₂) diagram showing the impact of salinity on selected phase equilibria relevant to the formation and stability of talc, tremolite, and forsterite in siliceous dolomites.  Of particular focus in our work is the stability of talc.  The phase equilibria show that the stability of talc is constrained to T- X(CO₂) conditions between the reactions 1 and 2 and invariant point I formed by the intersection of these two reactions.  In binary H₂O-CO₂ fluids, these phase equilibria define upper limits to temperature and X(CO₂) of 425^oC and 0.3, respectively.  The impact of increasing salinity is to shift the stability field of talc to higher temperature and lower X(CO₂) (follow the path of the invariant point I to I' and I").  At 150 Mpa pressure, the immiscibility gap does not impinge on the talc stability field in the binary H₂O-CO₂ system.  Increasing salinity also expands the immiscibility gap to lower X(CO₂) and higher T (not shown in the TOC).  A remaining question is whether the low X(CO₂) limb of the solvus intersects the talc stability field with increasing salinity.  The answer has important ramifications for fluid flow and reaction in the outer Alta aureole.

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