ELEMENTAL AND ISOTOPIC INDICATORS OF INFILTRATION-DRIVEN METAMORPHISM IN FORSTERITE FROM THE ALTA, UTAH CONTACT AUREOLE

Reaction progress calculations and measured ¹⁸O/¹⁶O depletion patterns in carbonate host rocks indicate that at least the periclase (Per) and forsterite (Fo) zones of the Alta aureole formed by infiltration of lower δ¹⁸O, H₂O-rich fluids outward from the Alta stock. Trace element and oxygen isotope compositions of Fo also record this infiltration-driven metamorphism. In the Per zone and most of the Fo zone, boron is significantly enriched in Fo (50 to 1000 ppm) and whole rock (6 to 50 ppm) compared to the dolostone protolith for these marbles (< 2 ppm). These enrichments record extensive boron metasomatism locally from the Alta stock into the aureole. In addition, samples of Fo from < 250 m from the igneous contact have significantly higher FeO concentrations (ave. = 0.61 wt. %; max of 1.59 wt. %) than Fo from the outer Fo zone (ave. = 0.26 wt. %; max of 0.53 wt. %). Fo samples from > 250 m from the igneous contact are homogeneous with respect to δ¹⁸O, with δ¹⁸O values of ≥ 20.3‰. In contrast, Fo grains in lower δ¹⁸O calcite (Cal) host marble (≤20‰) of the Per and innermost Fo zones have δ¹⁸O values as low as 14.6‰. Ion microprobe (SIMS) analyses indicate that some of these Fo grains are heterogeneous in δ¹⁸O by as much as 3‰. Multi-spot SIMS analyses show that these heterogeneous Fo grains are complexly zoned, with no systematic increase or decrease in δ¹⁸O from core to rim. This complex zonation suggests that multiple processes [variation in reaction overstepping, dissolution/re-precipitation during infiltration-driven change in δ¹⁸O of the Cal matrix, change in temperature (T) of reaction] are operating during growth of these isotopically zoned Fo grains.

Except for samples from the inner Per zone, measured oxygen isotope fractionation factors [Δ¹⁸O_(Cal-Fo)] are systematically less than expected equilibrium fractionation factors by ~1.5-3‰ based on measured Cal-dolomite solvus T’s. These measured fractionations are increasingly smaller than expected equilibrium values with increased metamorphic grade (toward the igneous contact). These measured values of Δ¹⁸O_(Cal-Fo) cannot be explained by higher T and suggest either that nucleation and growth of Fo occurred under non-equilibrium conditions, and/or that preferential δ¹⁸O depletion of Cal occurred as a result of continued fluid infiltration during retrograde cooling.