Implications of geochemical fronts in the Notch Peak contact-metamorphic aureole, Utah, USA

Abstract

The geochemistry of the Notch Peak contact-metamorphic aureole was examined to determine the dominant source of fluid attendant on metamorphism and to estimate fluid fluxes. Progressive metamorphism of calc-silicate rocks is defined by phlogopite, diopside and wollastonite isograds. Mineral assemblages at wollastonite and diopside grades indicate water-rich conditions during decarbonation reactions. Near the intrusion, the rocks are highly fractured, in both the horizontal and the vertical direction. Theδ 18O values of most unmetamorphosed through diopside grade rocks range from 16.3 to 20.2‰, whereas in the wollastonite zone and near a pre-intrusion fault the values approach the 9.5‰ value of the granitic stock. Coincident with the zone of 18O depletion are zones of Ba and Zn enrichment. Consideration of solubilities of these elements in magmatic fluids and of published mineral/water distribution coefficients indicates that these elements were advected from the magma. Modelδ 18O profiles for up-temperature flow of formation water, assuming variable Damko¨hler numbers and both static and variable thermal gradients, fit poorly most data and do not reproduce the broadened front in the observed profile. In contrast, model profiles for radial, one-dimensional flow of magmatic water with variable Damko¨hler number under static or variable thermal gradients fit the data well, although the model isotopic front remains less distended that the one observed. The integrated radial flux of magmatic water indicated by the position of the isotopic front is∼ 1.8 × 10 7 mol m −2. Additional broadening of the observed front and the scatter of the data suggest diffusion or hydrodynamic dispersion of oxygen. Regression of the data indicates an effective porosity of ∼ 12% in the inner aureole during fluid flow. The porosity was largely fracture controlled and resulted from decarbonation, sustained exsolution of water from the magma, and up to 30% reaction-produced volume loss in the high-grade rocks. The coincidence of the Ba, Zn andδ 18O fronts suggests that bulk of the magmatic water flow was confined to the wollastontie zone. The exception is the proximity of the fault, where fluids emanating from an underlying part of the intrusion may also have flowed upward through lower grade rocks.