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

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

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..539n&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 4, p. 539-559

Computer Science

13

Contacts (Geology), Fluid Flow, Geochemistry, Limestone, Magma, Metamorphism (Geology), Oxygen 18, Trace Elements, Utah, Advection, Barium, Composition (Property), Decarbonation, Intrusion, Lithology, Minerals, Zinc

Scientific paper

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. The delta O-18 values of most unmetamorphosed through diopside grade rocks range from 16.3 to 20.2 parts per thousand, whereas in the wollastonite zone and near a pre-intrusion fault the values approach the 9.5 parts per thousand value of the granitic stock. Coincident with the zone of O-18 depletion are zones of Ba and Zn enrichment. Consideration of solubilitties of these elements in magmatic fluids and of published mineral/water distribution coefficients indicates that these elements were advected from the magma. Model delta O-18 profiles for up-temperature flow of formation water, assuming variable Damkoehler 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 Damkoehler number understatic or variable thermal gradients fit the data well, although the model isotopic front remains less distended than the one observed. The integrated radial flux of magmatic water indicated by the position of the isotopic front is approximately 1.8 X 10(exp 7) mol/sq m. 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 effect of porosity of approximately 12% in the inner aureole during fluid flow. The porosity was largely fracture controlled and resulted from the 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 and delta O-18 fronts suggests that bulk of the magmatic water flow was confined to the wollastonite 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.

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