HR: 0800h
AN: V41A-0391    [Abstracts]
TI: The Iceland Deep Drilling Project (IDDP): (III) Hydrothermal Fluid Geobarometry
AU: * Reed, M H
EM: mhreed@uoregon.edu
AF: Department of Geological Sciences, University of Oregon, Eugene, OR 97403, United States
AU: Palandri, J L
EM: palandri@uoregon.edu
AF: Department of Geological Sciences, University of Oregon, Eugene, OR 97403, United States
AU: Elders, W
EM: wilfred.elders@ucr.edu
AF: Department of Earth Sciences, University of California, Riverside, CA 92521, United States
AU: Fridleifsson, G O
EM: gof@isor.is
AF: Hitaveita Sudurnesja Ltd, Brekkustig 36, Reykjanesbaer, 260, Iceland
AB: The IDDP wells will penetrate high pressure geothermal reservoirs where an understanding of the pressure effects on mineral equilibria is essential. The chemical compositions of fluids from active hydrothermal systems have long been applied to estimating reservoir temperature in subaerial geothermal systems at temperatures less than 300 °C and pressures along the H2O liquid/vapor P-T curve, where the pressures are low and the pressure effects on mineral equilibria are small. At pressures of hundreds of bars beneath mid-ocean ridge black smoker springs, the effect of pressure on mineral solubilities is substantial, and can be exploited to estimate pressure and temperature from fluid composition. In practice we compute mineral saturation indices, log(Q/K), for a given fluid for a wide range of P-T combinations, then plot log(Q/K) for alteration minerals against pressure at a series of temperatures so as to identify a possible "knot" in P-T-log(Q/K) space where a group of probable alteration minerals equilibrated with the fluid. We find that saturation index surfaces distinctly converge to zero in a narrow range of pressure and temperature. As an example, we estimate that for an East Pacific Rise 21 °N NGS fluid with a vent T=273 °C and vent P=260 bar, the reservoir conditions are likely T=370-420 °C and P=480-530 bar. To explore what aspect of the fluid chemistry causes the strong pressure effect on mineral solubilities, we computed the effect of pressure change on the activities of aqueous H+, Na+, K+, Ca2+, and other significant species in the 21 °N NGS fluid. At 420 °C, pH changes from 8 to 5 as pressure changes from 200 to 700 bar, an effect resulting from dissociation of HCl with increasing pressure. Similarly, chloride complex dissociations yield approximately 10-fold increases in Ca2+, Na+, and K+ concentrations with a 200 to 700 bar pressure increase. In another series of calculations, we synthesized a seawater-like fluid that was equilibrated at 400 °C and 500 bar with clinopyroxene, chlorite, epidote, feldspars, and quartz, then treated the fluid as an "unknown" for estimating P-T. Even for small departures from equilibrium P-T (e.g. +/- 25 °C), the mineral saturation surfaces change markedly, thereby supporting the conclusion that pressure effects on fluid composition are large enough to enable meaningful pressure and temperature estimations in deep hydrothermal systems.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1039 Alteration and weathering processes (3617)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting