Denis Norton

Theoretical prediction of phase relations among aqueous solutions and minerals: Salton Sea geothermal system

Abstract Thermodynamic calculations of compositional relations among aqueous solutions and minerals in the system Na2O-K2O-CaO-MgO-Fe2O3 Al2O3-SiO2-H2O-CO2 HCl at pressures and temperatures corresponding to liquid-vapor equilibrium of H2O permit quantitative description and interpretation of phase relations among rock forming minerals and aqueous solutions in magma-hydrothermal systems. The extensive data base on the Salton Sea geothermal system provides an exemplary case for predicting the chemical characteristics of geothermal fluids associated with metasomatic mineral zones observed in deep drillhole samples. Near the Elmore No. 1 well aqueous species activity ratios of a Na + a H + and a K + a H + vary several tenths of a log unit with increasing depth and temperature from ∼ 0.6 km and ∼250°C to ∼ 2.2 km and ∼350°C, whereas a Ca 2+ a 2 H + decreases ∼ 2 orders of magnitude for a comparable range in depth and temperature. The fugacity of CO2 gas is ∼1.5–6 bars at ≲ 310°C. Calculated values of a Sio 2( aq ) , a Na + a K + , a Ca 2+ a Mg 2+ and fCO2(g), in the fluid phase coexisting with observed mineralogic phase relations are in remarkably close agreement with measured solute concentrations in geothermal fluids produced from deep drillholes near the Salton Sea. Hydrolysis reactions representing observed phase relations and written with alkali and alkaline earth cations as products have negative standard molal enthalpies (ΔH0P,T,r) and volumes (ΔV0P,T,r) of reactions; consequently, a Na + a H + , a K + a H + , a Ca 2+ a 2 H + , and a Mg 2+ a 2 H + decrease with increasing temperature at constant pressure, but increase with increasing pressure at constant temperature. An approximate linear relationship exists among these activity ratios and the reciprocal of absolute temperatures because ΔH0P,T,r varies only slightly with increasing temperature at ≲ 250 to 300°C. However, at ≳300°C, ΔH0P,T,r and ΔV0P,T,R decrease dramatically as a consequence of extrema in the thermodynamic and electrostatic properties of the solvent near the critical region of H2O. Hence, empirical solute geothermometers based on linear regressions of geothermal fluid compositions at ≲250–300°C cannot be confidently extrapolated to higher temperature systems.

Influence of magma chamber geometry on hydrothermal activity at mid-ocean ridges

Two-dimensional models of the ridge environment are utilized in this study to examine the behavior of ridge hydrothermal systems, with particular emphasis on the influence of magma chamber geometry. Seismological evidence supports a dike-like 2 km half-width chamber, and models of this chamber indicate that: (1) complete crystallization of the magma requires 30,000 years, (2) hydrothermal upflow and hot springs are concentrated in a narrow band within 1.5 km of the ridge axis for the lifetime of the system, (3) a large hydrothermal cell forms and remains centered 2 km from the axis for the lifetime of the system, (4) hydrothermal activity ends by 70,000 years. Petrological evidence supports a wide sill-like chamber up to 15 km in half-width, and models of this chamber indicate that: (1) complete crystallization of the magma requires 120,000 years, (2) hydrothermal vents are present at the ridge axis, but most of the vents are located 5–10 km away from the axis, (3) one large hydrothermal cell develops 15 km from the axis, while a series of small vigorous cells form directly above the intrusion, all of these features migrate toward the axis as the magma solidifies, (4) hydrothermal activity ends by 170,000 years. Substantially different hydrothermal systems develop around these two chamber geometries because they generate different distributions of near-criticalP-T conditions for H2O. These zones of near-critical conditions are the primary control on driving forces for ridge hydrothermal systems, and provide the primary link between magmatic and hydrothermal events. Narrow magma chambers produce a very narrow near critical zone, and therefore produce hydrothermal and hot-spring activity that is strong focused at the ridge axis. Wide chambers produce broad near-critical zones, and widely distributed hot-spring activity that sweeps towards theaxis with time.

Chemical mass transfer in the Rio Tanama system, west-central Puerto Rico

Abstract Equilibrium relationships are defined between stream waters and weathering products, kaolinite and calcium montmorillonite, for the Rio Tanama system, west-central Puerto Rico. The major element composition of 46 water samples of springs and streams define a reaction path in the system CaO-Na 2 O-MgO-Al 2 O 3 -SiO 2 -H 2 O between acid waters containing low concentrations of alkali cations and detrital reactant minerals. The principal reactant phases appear to be chlorite, plagioclase and orthoclase and occasionally anhydrite or calcite. Headward erosion by the Rio Tanama supplies the reactant phases to the stream silt load. The chemical denudation rate calculated for the Rio Tanama system is about 30 m/million yr. The chemical stream load appears to be buffered by the product phases in the main river over the 15–20 km river length sampled in this study. The silt and soil mineralogy and water compositions are used to define a log K for the hydrolysis of Ca-montmorillonite at 25°C of 35.0 ± 0.8. This value is in reasonable agreement with the value of 37.1 ± 1.0 defined by Garrels and Mackenzie (1967) in a similar manner for spring waters in the Sierra Nevada.

Silicate interactions with ammonia-water fluids on early Titan

Plausible models of the early history of Titan suggest that ammonia and water were present in liquid form at the surface. We show here by thermodynamic modeling that such an ocean could have reacted with silicates to put substantial quantities of sodium and potassium into solution. Following the formation of an ice crust by cooling, mantle ammonia-water fluids enriched in potassium would have been brought to the surface through the cryogenic equivalent of volcanism. Later impacts would have released the 40Ar produced by decay of the 40K into the atmosphere. The abundance of atmospheric 40Ar, measurable by the Huygens probe gas chromatograph mass spectrometer, may be dominated by this source and hence gives a proxy indication of the volume of ammonia-water resurfacing on Titan over geologic time.

Complex behavior of magma- hydrothermal processes: Role of supercritical fluid

Abstract Magmas emplaced into the upper portions of the earth’s crust are accompanied by extensive hydrothermal activity. Hydrothermal activity is represented as a system of coupled processes that dissipate thermal, mechanical, and chemical energy into the magma’s lithocap, primarily by convection of H2O-rich fluids. To investigate dynamical behavior of the system, a serial experiment was undertaken in which T(t) and P(t) values are computed for a pluton location during the time the region was subjected to near-critical hydrothermal convective flow. The consequent evolution of fluid buoyancy, ∇xρf, ion stability, ΔḠ°, and fracture extension, δL/L0 during this time indicates that variations in density gradients increase smoothly until 70,000 yr then burst into frequent, ≈100-yr oscillations. Oscillations first increase in magnitude then decrease. Oscillatory behavior of state conditions derived from numerical experiments illustrate resonant effects in chemical equilibrium and fracture extension processes and show the sensitivity of the stable mineral assemblage to either of the competing chemical and mechanical transport processes. An oscillatory zoned tourmaline that formed at near-critical conditions of H2O from the Geysers Geothermal deposit appears to provide evidence of nonlinear systematics in hydrothermal activity. Mathematical analogs to this system demonstrate that processes in this system record their dynamical behavior in the supercritical region and suggest that alteration events are generated by the complex, “chaotic” behavior of these processes. This type of behavior appears to be further augmented by strong divergence of H2O-fluid properties toward ± infinity at commonly encountered state conditions in the shallow reaches of magma-hydrothermal activity. System behavior elucidated here arises from affording for connectivity of processes by numerical experiments of hydrothermal activity for a region near the contact of a magma and its lithocap. The cumulative data from numerical experiments, equation-of-state (EOS) relationships, geologic and geochemical observations support the proposition that magma- hydrothermal processes should be thought of as complex dynamical systems whose behavior at state conditions near the supercritical region of the fluid is likely chaotic.

Scientific Rationale for Establishing Long-Term Ocean Bottom Observatory/Laboratory Systems

The oceanographic community is in a position scientifically and technologically to initiate programs leading to the installation of one or more permanently instrumented observatory/laboratory complexes on submarine spreading centers. The dynamic nature of these systems is well established. Yet, there has been no long term, inter-disciplinary effort focused on specific sites to document rates of change in system components, nor the interactions linking the physical, chemical, and biological processes involved. The ultimate goal of this natural laboratory approach would be to establish, then model, the temporal, and the spatial, co-variation among the active processes involved in generation and aging of 60 percent of the planetary surface. The technological and intellectual stimulation involved in successful implementation of natural seafloor laboratories will provide a new generation of dynamically-based, quantitatively testable models of ocean lithosphere genesis and of the biological and chemical consequences of its formation.

Determination of the sources and circulation paths of thermal fluids: the Abano region, northern Italy

Abstract Water samples from natural springs and artesian wells in the Abano region of northern Italy are characterized by anomalous temperatures and compositions. Concentrations of major components and oxygen isotopes in samples of these fluids have been interpreted in the context of the regional geologic environment, circulation of groundwaters, and reactions between rocks and circulating groundwaters. These considerations define sourceregions and pathlines of the groundwaters. Circulation of fluids in the Abano region is interpreted to be the result of meteoric water infiltration into outcrops of Permian and Mesozoic aquifers in the pre-Alps, which lie north of the Abano region. These aquifers have southerly dips and, therefore, extend under the Abano region at depths between 0.5 and 2.5 km. This aquifer geometry is conducive to forced convection both of aqueous ions derived from the evaporate, limestone and dolomite bearing formations, and of thermal energy along flow paths which extend from the outcrops in the sourceregions to depths of 2 km below the Abano region and upward along high angle faults to thermal springs. Local variation in compositions of water samples is consistent with the mixing of local meteoric waters and formation fluids that were ultimately derived from Alpine sources.