Andrew Hajash

Hydrothermal processes along mid-ocean ridges: An experimental investigation

An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200 ° C and 500 ° C at 500–800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na+, Mg++, SO4=, Cl− solution to a reducing, acidic, Na+, Ca++, Cl−, solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid reach concentrations of 200–600 ppm; however, Al abundances remain very low (∼0.5 ppm). Gray and green smectites, anhydrite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed.These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and could be important in the genesis of certain Fe-Cu sulfide ore deposists. The process could also affect the geochemical budgets of certain elements and exert substantial control of the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si, and H to the oceans.

Rate laws for water-assisted compaction and stress-induced water-rock interaction in sandstones

Mineral-water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones, phase relations in metamorphic rocks, mass redistribution and replacement reactions during diagenesis, and loss of porosity in deep sedimentary basins. As a step toward understanding the fundamental geochemical processes involved, using naturally rounded St. Peter sand, we have investigated the kinetics of pore volume loss and quartz-water reactions under nonhydrostatic, hydrothermal conditions in flow-through reactors. Rate laws for creep and mineral-water reaction are derived from the time rate of change of pore volume, sand-water dissolution kinetics, and (flow rate independent) steady state silica concentrations, and reveal functional dependencies of rates on grain size, volume strain, temperature, effective pressure (confining minus pore pressure), and specific surface areas. Together the mechanical and chemical rate laws form a self-consistent model for coupled deformation and water-rock interaction of porous sands under nonhydrostatic conditions. Microstructural evidence shows a progressive widening of nominally circular and nominally flat grain-grain contacts with increasing strain or, equivalently, porosity loss, and small quartz overgrowths occurring at grain contact peripheries. The mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts (termed pressure solution or solution transfer), with a lesser component of time-dependent crack growth and healing. The magnitude of a stress-dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy-induced supersaturations, favoring instead the normal stress dependence of molar Gibbs free energy associated with grain-grain interfaces.

An experimental investigation of high-temperature interactions between seawater and rhyolite, andesite, basalt and peridotite

Natural seawater was allowed to react with rhyolite, andesite, basalt, and peridotite at 200°–500° C, and 1,000 bars at water/rock mass ratios of 5 and 50 in order to investigate the effects of rock type, water/rock ratio, and temperature on solution chemistry and alteration mineralogy. The results indicate that interactions of seawater with various igneous rocks are similar in the production of a hydrous Mg-silicate and anhydrite as major alteration products. Fluids involved in the interactions lose Mg to alteration phases while leaching Fe, Mn, and Si from the rocks. The pH of the solutions is primarily controlled by Mg-OH-silicate formation and therefore varies with Mg and Si concentration of the system. Other reactions which involve Mg (such as Mg-Ca exchange) or which produce free H+, cause major differences in fluid chemistry between different seawater/ rock systems. High water/rock ratio systems (50/1) are generally more acidic and more efficient in leaching than low ratio systems (5/1), due to relatively more seawater Mg available for Mgsilicate production. The experiments show that large-scale seawater/rock interaction could exert considerable control on the chemistry of seawater, as well as producing large bodies of altered rock with associated ore-deposits.Active plate margins of convergence or divergence are suitable environments for hydrothermal systems due to the concurrence of igneous activity, tectonism, and a nearby water reservoir (seawater or connate water). The experimental data indicate that seawater interactions with igneous host rocks could generate many of the features of ore-deposits such as the Kuroko deposits of Japan, the Raul Mine of Peru, the Bleida deposit of Morocco, and deposits associated with ophiolites. Serpentinization of peridotite and alteration of igneous complexes associated with plate margins can also be explained by seawater interaction with the cooling rock. Geothermal energy production could benefit from experimental investigations of hot water/rock systems by development of chemical, temperature, and pressure control systems to maximize the lifetime of hydrothermal flow.

Changes in quartz solubility and porosity due to effective stress: An experimental investigation of pressure solution

Experimental compaction of quartz sand was conducted in semistatic flow-through systems at 150C to examine the chemical and physical processes associated with pressure solution. Pore-fluid chemistry and porosity were monitored through time to investigate the role of effective stress on silica solubility and compaction rate. The concentration of silica in the pore fluid varied directly with effective stress at constant pore-fluid pressure, but not in a linear fashion. Increases in silica were not transient and could be partially reversed by removal of the effective stress. Porosity decreased steadily under constant nonzero effective stress at 150C, but remained essentially constant under identical loading conditions at room temperature. Initial compaction rates increased linearly with effective stress. Deformed samples show indented contacts with fractures and dissolution features, healed microfractures, and a decrease in average grain size. Multiple mechanisms (pressure solution, subcritical crack growth, crack healing) appear to operate at grain contacts during compaction. The authors interpret long-term, time-dependent compaction accompanied by stress-induced changes in fluid chemistry as evidence for pressure solution.

Laboratory deformation of granular quartz sand: Implications for the burial of clastic rocks

We explore the influence of mechanical deformation in natural sands through experiments on water-saturated samples of quartz sand. Stresses, volumetric strain, and microseismicity (or acoustic emission, AE) rates were monitored throughout each test. Deformation of quartz sand at low stresses is accommodated by granular flow without significant grain breakage, whereas at high stresses, granulation and cataclastic flow are dominant. Sands deformed under isotropic conditions show compactive strains with an inverse power-law dependence of macroscopic crushing strength on mean grain size. Triaxial compression at high effective pressures produces compactive strain and a high AE rate associated with considerable particle-size reduction. Triaxial compression at low effective pressure produces dilatant granular flow accommodated by grain boundary frictional sliding and particle rotation. On the basis of experiment results, we predict the evolution of porosity and macroscopic yield strength as a function of depth for extensional and contractional basins. Sand strength increases linearly with depth for shallow burial, whereas for deep burial, strength decreases nonlinearly with depth. At subyield stresses, porosity evolves as a function of applied mean stress and is independent of distortional stress. Our predictions are in qualitative agreement with observations of microfracture density obtained from laboratory creep-compaction experiments and with natural sandstones of the Gulf of Mexico basin. Mechanical deformation contributes as much as a 30% increase to fluid pressure evolution, which has particular application to sedimentary systems that display zones of fluid overpressure. Furthermore, deformational strains cannot be fully recovered during uplift, erosion, and unloading of a sedimentary basin.

The role of carboxylic acids in albite and quartz dissolution: An experimental study under diagenetic conditions

Abstract Simple water soluble organic acids may promote secondary porosity development in sandstones during diagenesis by increasing feldspar solubility and dissolution rate. To test this hypothesis, Amelia albite and Brazilian hydrothermal quartz were reacted with 0.07 m acetate and 0.07 m acetate-0.005 m oxalate solutions at selected pH values, and distilled water. Pore fluid chemistry was monitored through time at various flow rates to obtain both solubility and dissolution rate data. The experiments were conducted in large volume, semi-static, flow-through systems at 100°C and 347 bars. These systems simulate subsurface flow rates, low mass water/rock, and high surface area/fluid mass. Acetate and acetate + oxalate solutions significantly increase albite solubility at temperatures, pressures, and pH values typical of diagenetic environments. Albite solubilities increased in acetate and acetate + oxalate solutions by factors of 2 and 3.4, respectively, compared to distilled water. In these same solutions, Al concentrations were ≈ 140 and ≈480 times higher than that calculated for kaolinite solubility at the same conditions without organic species. These enhanced solubilities occur at pH values (4.6–4.8) that may overlap with formation waters. In contrast to albite, quartz solubility was essentially identical in all solutions investigated. Dissolution rates in the acid region decreased with increasing pH in the acetate and acetate + oxalate solutions. Slopes of log rate vs. pH curves were ≈0.6 for acetate and ≈0.3 for acetate + oxalate. Although the effects of acetate on the dissolution rate are small, the effects of oxalate are significant. A rate law valid for albite dissolution at 100°C, oxalate concentrations to 0.01 m, and pH values ranging from 3.4 to 5.5 is given below (assuming activity coefficients = 1 and acetate rate ≈ the proton-promoted rate): R total = 5.88 × 10 −11 + 5.01 × 10 −8 m 0.56 H+ + 6.7 ×10 −10 2.3 × 10 −4 m O x /(1.0+2.3 × 10 −4 m O x ), where m O x and m H + are oxalate and H + molal concentrations, respectively. Reacted albite grains examined by SEM show extensive dissolution concentrated along cleavage planes and structural imperfections such as twin boundaries and fluid inclusions, consistent with surface-controlled reaction kinetics. No authigenic aluminosilicate minerals were observed. The lack of authigenic clays indicates the efficiency of oxalate and acetate in mobilizing and transporting Al. The combination of enhanced solubility and increased dissolution rates indicates that carboxylic acids may play major roles in feldspar dissolution and secondary-porosity development during diagenesis of feldspathic sandstones.

Experimental seawater/basalt interactions: Effects of cooling

Oceanic tholeiite glass has been reacted with natural seawater at 25°–500° C, 1 kbar, with both low (5) and high (50) water/rock mass ratios. Initial experiments were conducted at constant temperatures between 100° C and 500° C (100° intervals) in order to characterize the mineralogy and chemical exchange trends for both water/rock ratios. However, the primary purpose of this investigation was to study the chemical and mineralogical changes that may take place as reacted seawater cools as it traverses a temperature gradient before exiting onto the seafloor, as may happen in some submarine hydrothermal systems. Consequently, a series of cooling or “temperature gradient” experiments were performed in which seawater that had reacted with basalt at 500° C was cooled to 25° C in a step-wise fashion; mineralogy and fluid chemistry were determined at 100 degree intervals during cooling.For all of the experiments, the elemental exchange trends were the same. With respect to the initial sea-water, Fe, Mn, Ca, Si and H+ increased while Na and Mg decreased. However, the extent of the exchange depended heavily on the temperature and water/rock ratio. During cooling, fluid compositions in the temperature gradient runs generally approached those of the constant temperature experiments. Even though fluid compositions were very similar at 500° C for both water/rock ratios, the high water/rock ratio systems were more efficient in leaching transition metals from the rock and maintained substantial concentrations in solution during cooling, even to temperatures as low as 25° C. The Fe/Mn ratio in the fluid, however, was quite different for the two water/rock ratios; consequently, the effective water/rock ratio appears to be one parameter that can control the Fe/Mn ratio in exiting hydrothermal fluids and may influence the Fe/Mn ratio in metal-rich sediments.Alteration minerals produced in these seawater/ basalt experiments are very similar to those found at submarine springs on the East Pacific Rise, 21° N. Iron sulfides, pyrite and pyrrhotite, precipitated during cooling for both water/rock ratios, demonstrating the ore-forming potential of submarine hydrothermal systems.

Paleomagnetic and radiometric evidence for the age of the Comores Islands, west central Indian Ocean

Abstract Paleomagnetic measurements on 56 lavas and six K Ar age determinations indicate that the Comores Islands, west central Indian Ocean, are no older than late Tertiary. The mean virtual geomagnetic pole for each island coincides with the present spin axis of the earth at the 95% confidence level. Because of the young age, the paleomagnetic data cannot resolve any north-south movement that may have occurred during the time interval recorded by the lavas sampled. The K Ar ages from three of the islands range from 0.01 ± 0.01 to 3.65 ± 0.10 my. The radiometric results indicate that the islands increase in age to the southeast, a sequence consistent with the geomorphology. A magmatic center migrating northwest at about 7 cm/yr is consistent with the between-island age relationships, assuming that the K Ar dates are reasonable estimates of the ages of the islands.

A model for porosity evolution during creep compaction of sandstones

Abstract A coupled creep-compaction and chemical-reaction model is developed to predict the porosity evolution for quartzose sandstones as a function of strain. The model also demonstrates the relative importance of grain-contact dissolution and cementation for both uniaxial and isotropic compaction. Theoretical analysis indicates that porosity reduction during compaction of sandstones is nonlinearly related to strain. In open systems, porosity loss is also related to grain packing, stress state, and pore-fluid saturation state. Grain-contact dissolution is the dominant mechanism for porosity loss in a closed system and, with increasing compaction, cementation becomes increasingly important. Compared to uniaxial compaction, isotropic compaction leads to more porosity loss due to grain-contact dissolution, but less porosity loss due to cementation. With compaction, pore-fluid saturation state has an increasing effect on porosity loss. Higher saturation state enhances porosity loss due to cementation.

Marine diagenesis of feldspathic sand: A flow-through experimental study at 200°C, 1 kbar

Abstract Seawater was reacted with feldspathic sand at 200°C, 1 kbar to investigate the chemical and mineralogical changes that take place under conditions similar to those of burial diagenesis. A non-reactive flow-through system was used that allows pore-fluid chemistry to be monitored as reactions take place at low water/rock mass ratios while controlling temperature, confining pressure, pore-fluid pressure and average volumetric flow rate. Large and rapid changes in fluid chemistry occurred; Mg and Na were removed from solution while Ca, K, SiO2, Fe and Mn were leached from the solids. Smectite was the dominant alteration product although kaolinite, anhydrite, pyrite(?) and secondary albitic plagioclase were also produced. Mg removal from the fluid and smectite formation dominated the reactions. Mg removal generated acidity which attacked the rock and released K, Ca, SiO2, Fe and Mn. Na was also removed from the fluid by the formation of albitic plagioclase and smectite. The extent of chemical exchange was greatest early in the experiments when the ratio of surface area (A) to fluid mass (M) was very high. Preferential dissolution of “fines” lowered A M which decreased the apparent rate of chemical exchange. Although apparent reaction kinetics decreased through time, the chemical exchange trends were the same throughout the experiments and the fluid composition was ultimately related to A M and the flow rate. Pore-fluid evolutionary trends observed early in the experiments may reflect some of those that occur in the subsurface under rock-dominated conditions where A M is high, reaction kinetics are fast, and/or fluid flow rates are slow. Later trends may reflect more water-dominated conditions where apparent reaction kinetics are slowed due to decreased A M and/or rapid fluid flow.