COMBINED GEOCHEMICAL AND STABLE ISOTOPE REACTION MODELING: AN IMPORTANT TOOL FOR UNDERSTANDING GEOCHEMICAL TRANSFORMATIONS

Pioneering studies by Plummer (1977) showed that combining mass transfer geochemical modeling with stable isotopes can greatly constrain water-rock reactions in natural systems, sometimes uniquely. Better constrained reaction models mean better understanding of geochemical processes and better decision-making in managing resources and ecosystems.

Similar approaches, but with completely coupled reaction codes, have been successfully applied to seafloor hydrothermal systems (Bowers and Taylor, 1985; Janecky and Shanks, 1988; Bohlke and Shanks, 1994). More recently, Alt and Shanks (2003) have shown that serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated δ³⁴S_sulfide (3.7 to 12.7‰). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400°C alone cannot account for both the high sulfur contents and high δ³⁴S_sulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (~400°C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ~300°C. Subsequent studies of seafloor hydrothermal serpentinization reactions have provided field evidence confirming ubiquitous involvement of gabbros in these hydrothermal systems (Alt and others, 2008)

The development of programs like Geochemist’s Workbench, which includes an algorithm for stable isotope calculations, and increasingly available fractionation data for new stable isotope systems (Cr, Fe, Cu, Zn, Se, Mo, Cd, W, Hg, and Tl) provides an opportunity for better-constrained modeling of geochemical and microbial reactions that affect ecosystems, natural waters, and water-rock reactions.

References:

Alt, J.C., and Shanks, W.C., III, 2003, Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: Sulfur geochemistry and reaction modeling: Geochimica et Cosmochimica Acta, v. 67, p. 641-653.

Alt, J.C., Shanks, W.C., III, Bach, W., Paulick, H., Garrido, C.J., and Beaudoin, G., 2008, Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid-Atlantic Ridge, 15^o20^'N (ODP Leg 209): A sulfur and oxygen isotope study: Geochemistry, Geophysics, Geosystems, v. 8, p. doi:10.1029/2007GC001617.

Bohlke, J.K., and Shanks, W.C., III, 1994, Stable isotope study of hydrothermal vents at Escanaba Trough; observed and calculated effects of sediment-seawater interaction, in Morton, J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, Hydrothermal, and Biologic Studies at Escanaba Trough, Gorda Ridge, Offshore Northern California, U. S. Geological Survey Bulletin 2022, p. 223-239.

Bowers, T.S., and Taylor, H.P., Jr., 1985, An integrated chemical and stable-isotope model of the origin of midocean ridge hot spring systems: Jour. Geophys. Res., v. 90, p. 12583-12606.

Janecky, D.R., and Shanks, W.C., III, 1988, Computational modelling and sulfur isotope reaction processes in seafloor hydrothermal systems: chimneys, massive sulfides, and subjacent alteration zones: Can. Mineral., v. 26, p. 805-827.

Plummer, L.N., 1977, Defining reactions and mass transfer in part of the Floridan Aquifer: Water Resour. Res., v. 13, p. 801-812.