OXYGEN ISOTOPE ZONATION AND TRACE ELEMENT ACTIVATORS OF CATHODOLUMINESCENCE IN FORSTERITE FROM THE ALTA, UTAH CONTACT AUREOLE

Infiltration-driven metamorphism has produced widespread development of forsterite (Fo) in dolomitic marbles of the contact aureole surrounding the Alta, Utah stock. Secondary ion mass spectrometry (SIMS) measurements indicate that metamorphic Fo and calcite (Cal) grains are homogenous with respect to δ¹⁸O (± 0.3‰ 2SD) at the grain-scale throughout most of the Fo zone. However, some Fo grains in lower δ¹⁸O (Cal) host marble (≤20‰) of the periclase (Per) and innermost Fo zones are heterogeneous in δ¹⁸O by as much as 3‰. Isotopically zoned grains record either a systematic increase or decrease in δ¹⁸O from core-to-rim, suggesting that multiple parameters [variation in reaction overstepping, infiltration-driven change in δ¹⁸O of Cal matrix, change in temperature (T) of reaction] are changing during growth of these Fo grains. Measured oxygen isotope fractionation factors [Δ¹⁸O_(Cal-Fo)], using either grain-scale SIMS δ¹⁸O analyses or δ¹⁸O analyses of bulk mineral separates, are systematically less than expected equilibrium fractionation factors for all samples by ~1.5-3‰ based on measured Cal-dolomite solvus T’s. Excluding the inner Per zone, 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, or that preferential δ¹⁸O depletion of calcite occurred as a result of continued fluid infiltration during retrograde cooling.

Many of the Fo grains from the Fo zone exhibit distinct cathodoluminescence (CL) structure, but SIMS analyses indicate these grains are homogeneous with respect to δ¹⁸O. Laser ablation (LA)-ICP-MS measurements show that domains of high CL intensity have significantly higher concentrations of one or more of lithium (Li, 30-300 ppm), phosphorus (P, 50-1050 ppm) and titanium (Ti, 25-200 ppm) compared to domains of low CL intensity where Li, P and Ti values range from 5-25 ppm, 1-50 ppm and 1-20 ppm, respectively. Concentrations of Li and P are well correlated. However, the measured 1:3 to 1:4 atomic proportions of Li to P suggest either multiple coupled substitutions and/or involvement of vacancies (defects) in these substitutions.