OXYGEN ISOTOPE AND TEXTURAL EVIDENCE FOR FLUID INFILTRATION-DRIVEN REACTION OVERSTEPPING DURING DEVELOPMENT OF FORSTERITE MARBLES IN THE ALTA, UTAH CONTACT AUREOLE

Forsterite (Fo) is developed extensively in dolomitic marbles in the Alta, Utah contact aureole through infiltration-driven metamorphism. Measured oxygen isotope fractionation factors [Δ¹⁸O(Cal-Fo)], based on bulk separates of Fo and calcite (Cal), range from 4.1 to 1.4 ‰. Temperatures of metamorphism defined by calcite-dolomite thermometry range from ~460^oC at the Fo isograd to ~625^oC at the igneous contact. Equilibrium Δ¹⁸O(Cal-Fo) factors should range from 6.5 to 4.2 ‰ for this T range (Chiba et al., 1989). Excluding the inner Per zone, measured Δ¹⁸O(Cal-Fo) fractionation factors generally decrease, and are increasingly smaller than equilibrium values, with increased metamorphic grade (toward the igneous contact). These measured Δ¹⁸O(Cal-Fo) fractionation factors reflect nucleation and growth of Fo under non-equilibrium conditions, zoned Fo, or preferential ¹⁸O/¹⁶O depletion of calcite in response to continued fluid infiltration during retrograde cooling. Trends in textural parameters of the Fo are compatible with the first two alternatives. As the igneous contact is approached, the number of Fo crystals/mol Fo increases from a minimum of 2.4E+04 to as high as 2.7E+07, and the average Fo crystal size decreases from 0.49 to 0.004 mm². These trends in oxygen isotope fractionation factors and Fo textures suggest increased reaction overstepping (increased reaction affinity) with increasing T and/or increased heating rates during development of Fo in the aureole.

However, between different carbonate strata in the same outcrop, the number of Fo crystals/mol Fo and average Fo crystal size can vary by a factor of 10, and Δ¹⁸O(Cal-Fo) factors can vary by as much as 1.2 ‰. As the T-time histories of samples at the same location within the aureole should be the same, these variations cannot be explained by differences in T or heating rates. At most of these sites, the number of Fo crystals/mol Fo and the δ¹⁸O value of the carbonate matrix of the sample are negatively correlated. This negative correlation suggests that reaction overstepping is driven at least in part by the infiltration of low X(CO₂) fluids (Ferry et al., 2011), as lower δ¹⁸O of the marble correlates with higher fluid flux in the inner Alta aureole.

Chiba et al. (1989), Geochim. Cosmochim. Acta, 53, 2985-2995.

Ferry et al. (2011), J. Petrology, 52, 1619-1640.