Benoît Dubacq

XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry

XMapTools is a MATLAB^(C)-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure-temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data and includes functions to calculate the oxide weight percent compositions for various minerals. A set of external functions is provided to calculate structural formulae from the standardized analyses as well as to estimate pressure-temperature conditions of crystallization, using empirical and semi-empirical thermobarometers from the literature. Two graphical user interface modules, Chem2D and Triplot3D, are used to plot mineral compositions into binary and ternary diagrams. As an example, the software is used to study a high-pressure Himalayan eclogite sample from the Stak massif in Pakistan. The high-pressure paragenesis consisting of omphacite and garnet has been retrogressed to a symplectitic assemblage of amphibole, plagioclase and clinopyroxene. Mineral compositions corresponding to ~165,000 analyses yield estimates for the eclogitic pressure-temperature retrograde path from 25kbar to 9kbar. Corresponding pressure-temperature maps were plotted and used to interpret the link between the equilibrium conditions of crystallization and the symplectitic microstructures. This example illustrates the usefulness of XMapTools for studying variations of the chemical composition of minerals and for retrieving information on metamorphic conditions on a microscale, towards computation of continuous pressure-temperature-and relative time path in zoned metamorphic minerals not affected by post-crystallization diffusion.

Fluid-Mineral Reactions and Trace Metal Mobilization in an Exhumed Natural CO2 Reservoir, Green River, Utah

Red sandstones near Green River, Utah (United States), have been bleached by diagenetic fluids. Field relationships, modeling, fluid inclusion and isotopic data suggest that the causal fluid was a CO2-charged brine, distinguishing this site from hydrocarbon-related bleaching elsewhere on the Colorado Plateau. Mineralogical and chemical profiles from unbleached to bleached sandstone show that bleaching is related to hematite dissolution and precipitation of a 1–2 cm band of secondary oxide and carbonate at the reaction front. Trace metals are mobilized by the fluid and concentrated near the reaction front. High-flux fluid pathways are more heavily altered with large-scale secondary calcite and iron oxide precipitation. Changes may be modeled by a reaction with stoichiometry 20Fe2O3 + 5CH4 + 64CO2 + 19H2O + 11H+ = 30Fe2+ + 10FeHCO3+ + 59HCO3–. The Fe-rich, reduced fluid precipitates iron-oxides and carbonate at the reaction front between bleached and unbleached sandstone. These findings make the site an analogue for processes occurring over long time scales in geological carbon storage projects. Trace metals moblized by CO2-charged brines are likely to be rapidly re-precipitated at reaction fronts.