HR: 16:30h
AN: V34A-03 INVITED     [Abstracts]
TI: Controls on Transition Metal Concentrations in Crustal Brines
AU: * Yardley, B W
EM: bruce@earth.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment - Earth Sciences , Leeds, LS2 9JT United Kingdom
AB: Experimental studies of mineral solubilities have systematically explored the effects of pH and other parameters on metal concentrations over relatively narrow temperature ranges. This study has compiled a data base of brine analyses, ranging from low temperature shield and formation brines to magmatic brines, including geothermal and metamorphic brine analyses. The data includes both analyses of samples from drilling, and fluid inclusion analyses, and there is a span of over an order of magnitude in chloride concentration. Concentrations of Fe, Mn, Zn and Pb vary systematically across the entire data set, and the principal controls on their concentrations are salinity and temperature. In each suite of analyses in the data set, metal concentrations increase linearly with Cl over the entire salinity range, with a slope of between 1 and 1.5 in log mol units. For Fe and Mn in all the data sets, Me/Cl remains nearly constant over a wide range of salinities at constant temperature, but there is almost 6 orders of magnitude variation in Me/Cl between low-T formation brines and magmatic brines. Larger scatter in the Fe data may be attributed to variations in redox, and correlates with Mn/Fe. The slope of the data array on a Zn-Cl plot may be somewhat higher for formation waters than for magmatic fluids, indicating a possible change in complexing with temperature, but at no temperature is there evidence for a change in complexing with Cl concentration. Pb data is sparse but shows similar trends, though with less dependence on temperature. The continuity in crustal brine chemistry from sedimentary to metamorphic and magmatic fluids demonstrates the importance of wall rock buffering for the control of crustal fluid composition, and shows that the variation in pH, {\it f}S$_{2}$ and redox environment between different lithologies is not sufficiently large for variation in these parameters to dominate the variation in metal contents of fluids. In contrast, temperature and salinity emerge as the major controls on transition metal contents of crustal fluids, with little evidence for significant changes in chloride complexing over the range of compositions and conditions encountered in the crust. Preliminary results on low salinity, gas-rich fluid inclusions suggest that their transition metal contents fit on the same trends as those of brines.
DE: 8424 Hydrothermal systems (8135)
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting