Steep oxygen-isotope gradients at marble-metagranite contacts in the northwest Adirondack Mountains, New York, USA: products of fluid-hosted diffusion

Steep gradients in oxygen-isotope ratios are recorded from two contacts between Grenville marbles (calcite δ¹⁸O = 21-24%) and metamorphosed granitic intrusives (whole rock δ¹⁸O = 11-12%) in the northwest Adirondacks. Oxygen- and carbon-isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures. The δ¹⁸O profiles are sigmoidal with inflection points at the contacts, and are developed over 5-6 m. Quantitative fluid infiltration-diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid-hosted diffusion. For porosities of 10^-3 to 10^-6, the profiles could have formed in 10³-10⁷ years via an interconnected grain boundary fluid. The granites were probably emplaced at shallow crustal levels prior to the regional Grenville metamorphism, and isotopic exchange is most likely to have occurred at that time. Contact metamorphism at these localities involved devolatilisation of the marbles, and the time estimates may reflect the periods over which fluid-producing reactions took place and an enhanced porosity was maintained. In the marbles, there is a progressive increase in the percentage of diopside and K-feldspar at the expense of phlogopite and tremolite towards the metagranites. These mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of H₂O from the granite. The two localities lie on either side of the regional-metamorphic amphibolite- to granulite-facies transition. The preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism, or at any time thereafter. Steep isotopic gradients are present at orthogneiss-marble contacts elsewhere in the Adirondacks, suggesting that this conclusion may be generally applicable to this terrain.