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Results from experiments in this work have been combined with existing data to yield tracer diffusivities of divalent cations in natural multicomponent garnets at 10-40 kb, 1100 - 1475°C. For the garnet compositions studied in this work, the activation energy for tracer diffusion of Mn < Fe ≃ Mg, while activation volumes increase in the order Mg < Fe < Mn. These data may be used with theoretical models to calculate the full multicomponent diffusion coefficient matrix as a function of pressure, temperature, composition and oxygen fugacity. An analytical model has been developed to describe the relaxation of compositional zoning in metapelitic garnets during metamorphism. It is found that a significant amount of relaxation occurs during heating and the composition at the core of a garnet crystal is disturbed very early in the process. The extent of relaxation depends on a number of factors which include the grain size, the initial shape of the compositional profile, details of the thermal history and the textural mode of occurrence of the garnet. Complications such as oscillations in a compositional profile may be explained by the mathematical form of the equations describing the process. Graphical representation of the results of this model have been provided that allow quick and easy determination of the extent of relaxation or time scale of metamorphic processes. Calculations using the diffusion data indicate that Fe-Mg exchange geothennometry and Sm-Nd geochronology involving garnets may not yield the peak metamorphic conditions for certain geologically realistic situations. Model calculations on natural assemblages suggest that the diffusion data may be used to yield infonnation on metamorphic processes ranging from time scales to crystal growth rates during metamorphism. Veracity of P-T paths calculated from garnet zoning may also be checked using the diffusion data. Illustrative examples of such calculations have been provided.

The accuracy of the geochemical model PHRQPITZ (Plummer et al., 1988), which uses the ion-interaction method (Pitzer, 1973) for the calculation of activity coefficients in predicting mineral solubilities, is evaluated. Furthermore, a method for estimating computational error in saturation indices for minerals in natural systems is developed. The published experimental solubility data for celestite (SrSO₄), barite (BaSO₄), fluorite (CaF₂), and calcite (CaCO₃) are evaluated and compared to calculated solubility data for the minerals in simple binary (mineral-H₂O) and ternary (mineral-NaCl-H₂O) systems. The solubility modeling error determined for minerals in these simple systems is then extrapolated to several examples of natural systems. The error in magnitude and direction gives a good estimate of the computational uncertainty in the mineral saturation indices calculated by PHRQPITZ for the natural systems. The only adjustment to PHRQPITZ is the addition of polynomial-based log K(sp) equations for the minerals being evaluated.

The surface water composition of Pinal Creek has been affected by input of a low-oxygen ground water plume with high metal concentrations, particularly manganese and copper, and low pH. The pH of the stream water increases from approximately 6.0 to 7.8 downstream of the point of entry, which may affect the rate of manganese oxide precipitation. The purpose of this study was to determine the role of gas exchange processes on the observed downstream increase in pH at Pinal Creek. A finite-difference model was developed to calculate downstream changes in concentration of volatile solutes in small streams. Gas exchange rate constants used in the model were estimated using propane tracer techniques. Modeled simulations of dissolved oxygen and dissolved inorganic carbon transport in a 2.6 km reach at Pinal Creek, Arizona closely matched field observations. Based on the carbon balance and alkalinity balances, it can be concluded that downstream changes in pH can be sufficiently explained by acid/base chemistry of carbonate and CO2 degassing. Laboratory batch studies conducted in the laboratory using Pinal Creek water indicated that biological activity may be an important control on the precipitation of the black manganese oxides that were observed to coat the sediments in Pinal Creek.

Enrichment and depletion of trace elements occurs in host rocks distal to porphyry copper deposits as a result of hydrothermal metasomatism. Subtle geochemical distributions in weakly propylitized host rocks is sufficient to indicate proximity to a mineralized system and may be applied to porphyry copper exploration. Samples collected adjacent to four porphyry copper deposits were analyzed for a multi-element suite, then normalized to the elemental concentrations of the fresh host rocks. The probability that an element has been enriched or depleted is determined by using concentrations in the unaltered host in conjunction with a calculated standard deviation. The probabilities have distinct zoning that is related to alteration around the deposits. Contribution lateral to deposits was observed with: Ag, As, Au, Bi, Br, Ca, Cu, Hg, Mn, Mo, Pb, Sb, Se, V, and Zn. Proximal to the mineralized portion of the systems elemental removal was observed with: Ba, Br, Ca, Mg, Mn, P, Pb, Ti, V, Y, and Zn.