Mechanisms of oxygen isotopic exchange and isotopic evolution of 18O/16O-depleted periclase zone marbles in the Alta aureole, Utah: insights from ion microprobe analysis of calcite

Abstract

Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing (δ18O = 11.8‰) and periclase-free (δ18O = 17.2‰) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in δ18O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.27‰, 2SD). More poorly polished (pitted), texturally retrograde ‘turbid’-looking calcite has lower and more variable δ18O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the δ18O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in δ18O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000–62,000 years at 575–600°C, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the δ18O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization.