2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 8:00 AM-12:00 PM


POLLINGTON, Anthony D., Dept. of Geology and Geophysics, University of Utah, 135S 1460E, Salt Lake City, UT 84112, BOWMAN, John R., Department of Geology and Geophysics, University of Utah, 135 S 1460 E, Rm 719 WBB, Salt Lake City, UT 84112 and KASZUBA, John P., Isotope and Nuclear Chemistry, Los Alamos National Lab, MS J514, Los Alamos, NM 87545, a.d.pollington@utah.edu

In the outermost talc (Tc) zone of the Alta stock aureole, the Tc reaction front is a quite irregular surface, is in part stratigraphically controlled, and is not parallel to the inner tremolite (Tr), forsterite (Fo), and periclase (Pe) isograds. In contrast to the inner aureole (Pe and Fo zones), the progress of the talc-forming reaction is incomplete; significant quartz (Qtz) + dolomite (Dol) always persists, regardless of whether Qtz is disseminated within dolostone or in the form of chert nodules. Talc reaction progress is very heterogeneous even within restricted stratigraphic intervals. Quantitative estimates of reaction progress require detailed mapping of the distribution of Tc-bearing and Tc-absent nodules and thickness of Tc + calcite (Cc) reaction rims on outcrop scale. Initial results indicate that reaction progress is heterogeneous (1 to 50 moles Tc/m3 rock; ave. 11), can be correlated to stratigraphy, and is generally much less (<10%) than previously measured in the Tr (200 moles Tr/m3 rock), Fo (460 moles Fo/m3 rock) and Pe (5000 moles Pe/m3 rock) zones (Cook and Bowman, 2000). Calcite (Cc) samples from Tc reaction rims surrounding chert nodules have δ18OSMOW (13.4 to 19.8 permil) and δ13CPDB (-3.3 to -0.4 permil) values too low to be in isotope exchange equilibrium with the adjacent matrix dolomite (δ18O = 27.3 to 28.9; δ13C = 0.13 to 2.25) at the estimated T = 350o to 400oC for Tc formation. Measured ΔO (Dol-Cc) fractionations vary significantly between sites, ranging from +7.2 to +14.1 permil, much larger than equilibrium values of approx. 0.6 permil at 400oC. These disequilibrium values may reflect variable kinetic impacts during CO2 evolution and transport accompanying Tc reaction, or variable inheritance of 18O/16O-depleted oxygen from chert nodules (δ18O = +8 to +18), or both.

These field, petrologic, and isotopic characteristics of the Tc zone suggest that metamorphic processes in the outer Alta aureole are characterized by heterogeneous pore fluid composition, variable fluid flux and rock permeability, isotopic disequilibrium, and are influenced by reaction kinetics. Mechanisms that may be complicating fluid flow patterns and mineral reaction in the outer aureole include H2O-CO2 fluid immiscibility or the development of multiple hydrothermal convection cells during evolution of the aureole.