HETEROGENEOUS FLUID FLOW, REACTION PROGRESS, AND ISOTOPE DISEQUILIBRIUM IN THE TALC ZONE, OUTER ALTA AUREOLE, UTAH

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 δ¹⁸O_SMOW (13.4 to 19.8 permil) and δ¹³C_PDB (-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 = 350^o to 400^oC for Tc formation. Measured Δ^O (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 400^oC. 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.