Asynchronous infiltration-driven growth of forsterite and periclase during metamorphism in marbles of the inner Alta aureole, Utah: δ18O and textural records of oxygen isotope disequilibrium, rapid forsterite growth and reaction history

Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) delta O-18 analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in delta O-18 at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1 parts per thousand on the thin-section to hand sample-scales. In contrast, within similar to 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale delta O-18 heterogeneities in forsterite are as large as 3.0 parts per thousand, but forsterite crystals do not exhibit systematic center-to-edge decreases in delta O-18 due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from O-18/O-16 depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS delta O-18 profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the delta O-18 profile recorded by forsterite formed earlier than the delta O-18 profile recorded by calcite, but both profiles are prograde features-that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale delta O-18 profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized.