Summary and background:

1. qdiffuse qconductive qfocussed qseawater Upflow Zone * * qsource A modeling approach for evaluating stability of subsurface habitats Scott Veirs and Russ McDuff, School of Oceanography, University of Washington scottv@ocean.washington.edu www2.ocean.washington.edu/~scottv/ How shall we model the extent of the subsurface biosphere? Physical circulation models Reactive transport models Summary and background: Thorough characterization of microbial habitats within submarine volcanoes demands that fluid flow models include the chemical reactions that occur between hot rock and seawater. The subsurface region where reduced chemicals in hydrothermal fluids mix with oxygenated seawater may enable high microbial productivity (McCollom and Shock, 1997). This poster explores the nature of such mixing zones and suggests approaches to developing accurate reactive transport models of specific subsurface environments. Extant physical circulation models (e.g. Steefel and Lasaga, 1994) map the temperature and velocity fields in the upper crust, further constraining which portion of the subsurface is inhabitable. Microbes might be expected to thrive anywhere within the volume bounded by the 50oC and 150oC isotherms. The models, however, make important (possibly inaccurate) assumptions about the permeability structure of the upper crust and its modification by chemical precipitation and dissolution. At most spreading centers, geophysical data constrain the depth of circulation by locating the axial heat source (hot rock or magma). This figure juxtaposes hydrothermal vent and micro-earthquake locations at the Endeavour segment of the volcanic ridge in the Northeast Pacific by projecting them parallel to the axis onto a cross-section of the ridge. Since hyperthermophiles can’t live in the hot rock or cold seawater, these data establish the first-order extent of the axial subsurface biosphere: approximately the top 4 km of the oceanic crust. Hydrothermal vents UPFLOW Earthquakes HEAT SOURCE Improved computing power and thermodynamic databases make possible the addition of chemistry to physical circulation models. Depicted above are the results for some minerals that may affect the availability of pore space for microbes (images from www.cas.usf.edu/geology/faculty/steefel/steefel.html). Ridge Axis Mixing and sulfate loss occurs at the Galapagos Vent Field Even more subsurface mixing may occur at the Main Endeavour Field Why doesn’t clogging occur? Or does it? Evidence of near surface processes: Despite ~30 years of hydrothermal vent system study, we still know very little about the processes which generate diffuse flow. Extant large-scale models do not yet characterize the details of mixing, conduction, and precipitation in the shallow subsurface, yet these processes are clearly evident in the growing suite of vent fluid chemistry, mineralogy, and flux observations from different hydrothermal fields. Identifying where and how each process occurs and how they interact should lead to more accurate maps (physical and chemical) of potential microbial habitat. As an example, we focus on the sulfate ion (SO4) here because it can alter permeability in hydrothermal systems (through anhydrite (Ca SO4 precipitation). Separate regressions of Si and Mg data against sample temperature in Galapagos vent fluids (Edmond, et al., 1979; McDuff and Edmond, 1982) both indicate that a high temperature (~345oC) hydrothermal end-member (*) in equilibrium with subsurface rocks was mixed conservatively with >~15 parts seawater. The sulfate data shown here, however, extrapolate to a lower end-member temperature (~270oC, *), suggesting that anhydrite precipitated in the subsurface. Schultz (1992) measured diffuse and focussed vent temperatures and volume fluxes (F) to calculate heat fluxes (q) from a sulfide structure: qdiffuse ~ 10qfocussed Assuming that only mixing is involved in generating the low temperature effluent (qconductive=0) a balance of the volume and heat fluxes dictates that Fseawater ~ 30Fsource Sulfate solubility diagram (after McDuff and Edmond, 1982) shows that when a high-temperature end-member (Ca-rich, Mg- and SO4-poor) mixes conservatively with seawater (rich in Mg and SO4) anhydrite precipitation is expected at intermediate temperatures. K is the solubility product; Q is the reaction quotient (actual concentration ratio in the hydrothermal mixture). F=Volume Flux Fseawater > ~15Fsource References: Edmond, J., Measures, C., McDuff, R., Chan, L., Collier, R., Grant, B., Gordon, L., Corliss, J. (1979) Ridge crest hydrothermal activity and the balances of the major and minor elements in the ocean: The Galapagos data. Earth a and Planetary Science Letters, 46:1-18. McCollom, T. and Shock, E. (1997) Geochemical constraints on chemolithoautotrophic metabolism by microorgansims in seafloor hydrothermal systems. Geochimica et Cosmochimica Acta, 61, 20:4375-4391. McDuff, R. and Edmond, J. (1982) On the fate of sulfate during hydrothermal circulation at mid-ocean ridges. Earth and Planetary Science Letters, 57:117-132. Schultz, A., Delaney, J., and McDuff, R. (1992) On the partitioning of heat flux between diffuse and point source seafloor venting. Journal of Geophysical Research, 97, B9:12299-12314. Steefel. C. and Lasaga, A., (1994) A coupled model for transport of multiple chemical species and kinetic precipitation/dissolution eactions with application to reactive flow in single phase hydrothermal systems. American Journal of Science, 294:529-592. Tivey, M., Humphris, S., Thompson, G., Hannington, M., and Rona, P. (1995) Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data, Journal of Geophysical Research, 100, B7:12527-12555. …determine the rate of porosity modification At the TAG hydrothermal site anhydrite is precipitated through less mixing Combinations of mixing, conduction, and reaction... A new approach to modeling: Both mixing and conduction can affect chemical reactions between rock and water in the subsurface. Construction of a reactive transport model of the hydrothermal upflow zone may help to disentangle the two processes. At the Main Endeavour Field such a construction is aided by estimates of key geophysical parameters and a thorough assessment of end-member chemistry. Future measurements of hydrothermal heat flux partitioning and of fluid chemistry over a full range of venting temperatures will provide opportunities for numerical model verification. A Similarly, the trajectory of a fluid will determine how it modifies porosity in the upflow zone. The solubility diagram (B) represents a situation in which pure mixing may result in anhydrite precipitation. Diagram (A) depicts schematically how conductive heating and mixing together result in more precipitation. Conversely, less precipitation is expected when a hydrothermal mixture experiences conductive cooling (diagram C). Thus, we expect that upflow zones that are efficient at radiating heat will constitute more stable physical and chemical environments for microbes, while “hot” systems will tend to quickly clog potential habitat. A wide variety of mineral assemblages are expected to precipitate from different mixtures of hydrothermal fluid and seawater at different temperatures (Tivey, et al., 1995). The mineralogy observed in massive anhydrite (ANH) samples from the TAG hydrothermal mound are outlined. B C An accurate reactive transport model of a hydrothermal upflow zone may need to encompass any combination of mixing, reaction, and conductive heat transfer. Many different trajectories through the T-mixing fraction-solubility space (depicted above) can generate diffuse fluids! AmSi=amorphous silica Anh=anhydrite Bn=bornite Cp=chalcopyrite Chrys=chrysotile Hm=hematite Py=pyrite Tc=talc