Salton Sea Geothermal System Research Articles

Pyrobaculum igneiluti sp. nov., a novel anaerobic hyperthermophilic archaeon that reduces thiosulfate and ferric iron.

A novel anaerobic, hyperthermophilic archaeon was isolated from a mud volcano in the Salton Sea geothermal system in southern California, USA. The isolate, named strain 521T, grew optimally at 90 °C, at pH 5.5-7.3 and with 0-2.0 % (w/v) NaCl, with a generation time of 10 h under optimal conditions. Cells were rod-shaped and non-motile, ranging from 2 to 7 µm in length. Strain 521T grew only in the presence of thiosulfate and/or Fe(III) (ferrihydrite) as terminal electron acceptors under strictly anaerobic conditions, and preferred protein-rich compounds as energy sources, although the isolate was capable of chemolithoautotrophic growth. 16S rRNA gene sequence analysis places this isolate within the crenarchaeal genus Pyrobaculum. To our knowledge, this is the first Pyrobaculum strain to be isolated from an anaerobic mud volcano and to reduce only either thiosulfate or ferric iron. An in silico genome-to-genome distance calculator reported <25 % DNA-DNA hybridization between strain 521T and eight other Pyrobaculum species. Due to its genotypic and phenotypic differences, we conclude that strain 521T represents a novel species, for which the name Pyrobaculum igneiluti sp. nov. is proposed. The type strain is 521T (=DSM 103086T=ATCC TSD-56T).

Characterizing the Microbial Sulfur Metabolizing Community of the Salton Sea Geothermal System

Reduction of sulfur compounds has been proposed to be the earliest form of respiration, appearing 3.5 billion years ago as a mechanism for oxidation-reduction cycling in a nutrient-deficient environment. Today, sulfur-based respiration is common in environments where the availability of electron acceptors is limited, such as deep subsurface environments. The anoxic, high-temperature fluids ejected by the Salton Sea mud volcanoes is one such environment. Located in the Imperial and Riverside counties in Southern California, USA, the Salton Sea Geothermal System is a diverse ecosystem consisting of the endorheic Salton Sea, salt pools, petroleum seeps, and mud volcanoes. Water and various gases are collected in the subsurface of these volcanoes, heated, and bubbled out in a continuous cycle. Here, microbial life is diverse, ranging from thermophiles, halophiles, and obligate and facultative anaerobes. Characterization of the community has been performed via a wide-reaching 16S rRNA community analysis and by an isolate-specific analysis. This dual method allows for both breadth and depth in the characterization of sulfur metabolism in the mud volcanoes. Community analysis has been approached in two ways. In the first method, DNA is extracted from raw mud samples, followed by PCR amplification targeted at the highly-conserved 16S RNA region of the genome, isolation of specific amplicons by cloning into an E. coli vector, and amplicon purification and sequencing. In the second method, the 16S amplicons are sequenced directly using high throughput sequencing techniques. Sequences are checked for quality and high matches are used for alignment and phylogeny building. By using both methods we are able to gain a more complete picture of the entire community present in this deep subsurface environment. In addition to using community analysis, we have isolated sulfur and thiosulfate reducing microbes from this system. One of the isolates was characterized in detail and determined to be a novel anaerobic, hyperthermophilic archaeon that grows only in the presence of thiosulfate and ferric iron as electron acceptors. Genome annotation identified several candidate genes proposed to be involved in hydrocarbon oxidation and sulfur respiration. The isolate is neutrophilic, growing optimally at 90°C and 0–3% w/v NaCl. 16S rRNA sequence analysis places this isolate within the genus Pyrobaculum, a group of hyperthermophilic Crenarchaeota, and a neighbor joining phylogenetic tree from 16S RRNA sequence alignments was constructed. Based on genotypic and phenotypic differences between this isolate and the established members of the Pyrobaculum genus, the isolate is concluded to be a new species, for which the name Pyrobaculum igneolutum is proposed. Characterization of the metabolic schemes of the microbial life responsible for sulfur reduction is of great importance to the elucidation of complex biogeochemical cycling and is of practical relevance to cleaner hydrocarbon oxidation in the oil industry. Support or Funding Information This project is supported by NSF award MCB 1518306. Figure 1Open in figure viewerPowerPoint The 16S neighbor joining phylogenetic tree of the Pyrobaculum genus and related species, with Pyrobaculum igneolutum included. GenBank accession numbers are provided in parentheses. Introns were removed prior to alignment. Each number indicates the bootstrap value from 1000 trials. Bar, 1 substitution per 100 bases. Figure 2Open in figure viewerPowerPoint The effect of (A) pH, temperature, and (C) NaCl on the growth of the isolate.

Fluid origin, gas fluxes and plumbing system in the sediment-hosted Salton Sea Geothermal System (California, USA)

The Salton Sea Geothermal System (California) is an easily accessible setting for investigating the interactions of biotic and abiogenic geochemical processes in sediment-hosted hydrothermal systems. We present new temperature data and the molecular and isotopic composition of fluids seeping at the Davis-Schrimpf seep field during 2003–2008. Additionally, we show the first flux data for CO 2 and CH 4 released throughout the field from focused vents and diffuse soil degassing. The emitted gases are dominated by CO 2 (~ 98%) and CH 4 (~ 1.5%). By combining δ 13C CO2 (as low as − 5.4‰) and δ 13C CH4 (− 32‰ to − 17.6‰) with 3He/ 4He (R/Ra > 6) and δD CH4 values (− 216‰ to − 150‰), we suggest, in contrast to previous studies, that CO 2 may have a significant Sub-Continental Mantle source, with minimal crustal contamination, and CH 4 seems to be a mixture of high temperature pyrolitic (thermogenic) and abiogenic gas. Water seeps show that δD and δ 18O increase proportionally with salinity (Total Dissolved Solids in g/L) ranging from 1–3 g/L (gryphons) to 145 g/L (hypersaline pools). In agreement with elemental analyses, the isotopic composition of the waters indicate a meteoric origin, modified by surface evaporation, with little or no evidence of deep fossil or magmatic components. Very high Cl/Br (> 3,000) measured at many seeping waters suggests that increased salinities result from dissolution of halite crusts near the seep sites. Gas flux measurements from 91 vents (pools and gryphons) give a conservative estimate of ~ 2,100 kg of CO 2 and 11.5 kg of CH 4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over 51,000 m 2) revealed that 7,310 kg/d CO 2 and 33 kg/d CH 4 are pervasively released to the atmosphere. These results emphasise that diffuse gas emission from soil can be dominant (~ 75%) even in hydrothermal systems with large and vigorous gas venting. Sediment-hosted hydrothermal systems may represent an intermediate class of geologic methane sources for the atmosphere, with emission factors lower than those of sedimentary seepage in petroleum basins but higher than those of traditional geothermal-volcanic systems; on a global scale they may significantly contribute to the atmospheric methane budget.

Dynamics of hydrothermal seeps from the Salton Sea geothermal system (California, USA) constrained by temperature monitoring and time series analysis

Water‐, mud‐, gas‐, and petroleum‐bearing seeps are part of the Salton Sea geothermal system (SSGS) in southern California. Carbon dioxide is the main component behind the seeps in the Davis‐Schrimpf seep field (∼20,000 m2). In order to understand the mechanisms driving the system, we have investigated the seep dynamics of the field by monitoring the temperature of two pools and two gryphons for 2180 h (90.8 days) in the period from December 2006 to March 2007, with a total of 32,700 measurements per station. The time series have been analyzed by statistical methods using cross correlation, autocorrelation and spectral analysis, and autoregressive modeling. The water‐rich pools never exceed 34.0°C and are characterized by low‐amplitude temperature variations controlled by the diurnal cycles in air temperature. The long‐term validity of these results is evident from a second period of temperature monitoring of one of the pools from December 2007 to April 2008 (120 days). In contrast to the pools, the mud‐rich gryphons have a strikingly different behavior. The gryphons are hotter (maximum 69.7°C) and have large amplitude variations (standard deviation of 6.4) that overprint any signal from external diurnal forcing. Autoregressive modeling shows the presence of distinct hot and cold pulses in the gryphon temperature time series, with amplitudes up to 3°C. These pulses likely reflect a combination of hydrothermal flux variations from the SSGS and the local temporal changes in bubbling activity within the gryphons.

Buried rhyolites within the active, high-temperature Salton Sea geothermal system

Previously unrecognized pulses of rhyolite volcanism occurred in the Salton Trough between 420 ± 8 ka and 479 ± 38 ka (2 σ), based on high-spatial resolution U–Pb zircon geochronology. Presently, these rhyolite lavas, tuffs and shallow subvolcanic sills are buried to depths between ~ 1.6 and 2.7 km at ambient temperatures between 200 and 300 °C, and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz, K-feldspar, chlorite, epidote, and minor pyrite. Alteration resistant geochemical indicators (whole-rock Nd-isotopes, zircon oxygen-isotopes) reveal that these rhyolites are derived from remelting of MORB-type crust that was chilled and hydrothermally altered by deep-circulating hydrothermal waters. U–Pb zircon dating confirms the presence of Bishop Tuff in well State 2-14 at ~ 1.7 km depth, approximately 5 km NE of the geothermal wells that penetrated the buried rhyolites. These results indicate accelerated subsidence towards the center of the Salton Trough, increasing from 2.2 mm/a to 3.8 mm/a. Based on these results, the present-day Salton Sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism, intense heat flow and metamorphism that predates present-day geothermal activity and Holocene volcanism by at least ~ 400 ka.

Characteristics and Origin of the Oak Dam East Breccia-Hosted, Iron Oxide Cu-U-(Au) Deposit: Olympic Dam Region, Gawler Craton, South Australia

Oak Dam East is a large (~560 Mt of 41–56 wt % Fe), Cu-Au-U-bearing, massive Fe oxide deposit in the northeastern Gawler craton, South Australia. It is one of several unmined deposits in the same province as the giant Olympic Dam deposit, and it lies beneath more than 500 m of younger cover rocks. The Oak Dam East breccia complex formed in a near-surface environment and was characterized by an early history of skarnlike albite-calc-silicate ± magnetite formation (from hypersaline, 400°–500°C fluids). Large-scale brecciation was an early feature, and the resulting complex was progressively filled in and replaced by (1) hematite and goethite (hematite stage I), (2) magnetite-apatite-quartz ± actinolite (magnetite stage), (3) hematite-goethite-quartz-chalcedony-pyrite-chlorite-monazite (hematite stage II), and (4) chalcopyrite-pitchblende-illite-hydromus-covite-florencite-carrolite (Cu-U-(Au) mineralization stage). Hematite stage II assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent NaCl equiv that were boiling at 170° to 190°C and had δ18O values more negative than −3.7 to +1.7 per mil, interpreted as meteoric water. The upper 200 m of the breccia complex contains spectacular colloform hematite void fillings, interpreted to have replaced hydrothermal goethite. The Cu-U mineralized zones resemble Olympic Dam ore in terms of geometries, being grossly subhorizontal, 10 to 70 m thick, and crosscutting with respect to the host breccias. The mineralization is zoned vertically from pyrite (δ34S = −6 to 0‰) at depth to shallower chalcopyrite (δ34S = −14 to −7‰). High-grade pitchblende occurs centrally within the chalcopyrite zone (e.g., 10 m of 0.46% Cu and 3,700 ppm U3O8), straddling a boundary between K mica + illite alteration and underlying Fe-Mg chlorite alteration. The deposit is assigned to the 1570 to 1595 Ma iron oxide Cu-Au-U event in the eastern Gawler craton on the basis of its similarity to other Cu-Au-U deposits in the province. An LA-ICPMS Pb isotope age for monazite in hematite stage II was calculated to be 1455 ± 20 Ma, and possibly as young as 1370 ± 54 Ma, to explain all isotopic data. This is an age of isotopic resetting. Our preferred model is that Cu-U mineralization developed within a stratified hydrothermal brine complex analogous to the Salton Sea geothermal system. Copper, Fe, and S in the deep brine was sourced from an evolving deep reservoir, with local compositional controls on sulfur isotopes including cooling, boiling, and replacement of previous pyrite. Chalcopyrite (±U) veins and disseminations formed as a result of mixing with overlying, steam-heated, U-rich, saline ground water. Mixing-induced cooling and dilution only occurred episodically, following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex. By contrast, high-grade UO2 formation occurred as deep upflow waned, via reaction of the upper brine with hematite stage II Fe-Mg chlorite along an interface that descended with time. This resulted in an increase of U grades through time caused by leaching and reprecipitation.

Processes controlling water and hydrocarbon composition in seeps from the Salton Sea geothermal system, California, USA

Water-, mud-, gas-, and petroleum-bearing seeps are part of the Salton Sea geothermal system (SSGS) in Southern California. Seeps in the Davis-Schrimpf seep fi eld (~14,000 m 2 ) show considerable variations in water temperature, pH, density, and solute content. Water-rich springs have low densities ( 98 vol%). Halogen geochemistry of the waters indicates that mixing of deep and shallow waters occurs and that near-surface dissolution of halite may overprint the original fl uid compositions. Carbon isotopic analyses suggest that hydrocarbon seep gases have a thermogenic origin. This hypothesis is supported by the presence of petroleum in a water-dominated spring, composed of 53% saturated compounds, 35% aromatics, and 12% polar compounds. The abundance of polyaromatic hydrocarbons and immature biomarkers suggests a hydrothermal formation of the petroleum, making the SSGS a relevant analogue to less accessible hydrothermal seep systems, e.g., the Guaymas Basin in the Gulf of California.

The dynamics of submarine geothermal heat pipes

To better understand natural two‐phase hydrothermal systems, we have constructed one‐dimensional heat‐pipe solutions for NaCl‐H2O fluids and explored the effects of basal heat flux and permeability on their behavior. For seafloor conditions, saline brines form quickly at the base of the heat pipe; and in some cases halite is precipitated. NaCl‐H2O heat pipes may become liquid or vapor dominated but, in contrast to their pure‐water counterparts, often do not achieve steady state. When steady state solutions do exist, they are characterized either by broad, weak counter‐flow or by vigorous counter‐flow across a thin layer. The latter behavior may be analogous to that occurring in the Salton Sea Geothermal System, California.

Layered thermohaline convection in hypersaline geothermal systems

Thermohaline convection occurs in hypersaline geothermal systems due to thermal and salinity effects on liquid density. Because of its importance in oceanography, thermohaline convection in viscous liquids has received more attention than thermohaline convection in porous media. The fingered and layered convection patterns observed in viscous liquid thermohaline convection have been hypothesized to occur also in porous media. However, the extension of convective dynamics from viscous liquid systems to porous media systems is complicated by the presence of the solid matrix in porous media. The solid grains cause thermal retardation, hydrodynamic dispersion, and permeability effects. We present simulations of thermohaline convection in model systems based on the Salton Sea Geothermal System, California, that serve to point out the general dynamics of porous media thermohaline convection in the diffusive regime, and the effects of porosity and permeability, in particular. We use the TOUGH2 simulator with residual formulation and fully coupled solution technique for solving the strongly coupled equations governing thermohaline convection in porous media. We incorporate a model for brine density that takes into account the effects of NaCl and CaCl2. Simulations show that in forced convection, the increased pore velocity and thermal retardation in low-porosity regions enhances brine transport relative to heat transport. In thermohaline convection, the heat and brine transport are strongly coupled and enhanced transport of brine over heat cannot occur because buoyancy caused by heat and brine together drive the flow. Random permeability heterogeneity has a limited effect if the scale of flow is much larger than the scale of permeability heterogeneity. For the system studied here, layered thermohaline convection persists for more than one million years for a variety of initial conditions. Our simulations suggest that layered thermohaline convection is possible in hypersaline geothermal systems provided the vertical permeability is smaller than the horizontal permeability, as is likely in sedimentary basins such as the Salton Trough. Layered thermohaline convection can explain many of the observations made at the Salton Sea Geothermal System over the years.

Fluid density distribution in a high temperature, stratified thermohaline system: implications for saline hydrothermal circulation

Density distribution within the Salton Sea geothermal system, of fluids ranging from 20°C to 325°C, has been computed using chemical and thermal data from geothermal production well tests and curve-fit models of NaCaK chloride solution properties. Density corrections can easily be made to ±0.01 g/cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation. Field data on dissolved CO2 is too sparse to routinely compute a correction, however this moderately, but variably abundant component can be shown to produce only minor (< 0.01 g/cm3) errors in density estimates.Fluid density within the shallow, cool (< 250°C), low salinity portions of the system decreases markedly with increasing depth and temperature, from ≈ 1.0 to ≈ 0.85 g/cm3. A sharp interface separates these relatively dilute fluids from hypersaline brines with TDS > 20 wt%. The density of brine climbs rapidly to near 1.0 g/cm3 as the salinity increase across this interface overwhelms the thermal effect on fluid density. This steep density gradient precludes all but diffusional-conductive or perhaps double diffusive-convective mass and heat transfer in this transitional regime. Measurement uncertainties for reservoir depths, temperatures and salinities commonly exceed errors of our density model, limiting the accuracy of details in our modeled density distribution. Gross scale relationships of this salinity stratified geothermal system are, however apparent, permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems' evolution.

Chemical mass transfer modelling of ore-forming hydrothermal systems: current practise and problems

Chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid-rock reactions in natural hydrothermal systems, based on thermodynamic data for minerals and fluid species. Quantitative simulations of hydrothermal wall-rock alteration and ore-mineral deposition can be used to develop and test geological models for ore formation, which are a basis for mineral exploration and resource assessment. In practice, the application of thermodynamic mass transfer modelling to specific ore-forming systems faces two main problems, even for the limiting assumption of local equilibration between fluids and minerals. The first problem relates to the geometry of fluid-rock interaction, where the scheme of the model must be adjusted to the geological question of interest. One-dimensional reactor models, defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock, do not necessarily provide a more realistic description of chemical mass-transfer than simple ‘titration’ models without temporal and spatial dimension, because in most ore-depositing systems the fluids are partially channelized by structures like veins or faults. The efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand, and wall-rock reaction promoting ore-mineral precipitation on the other. An example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates, how ore-deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two-dimensional ‘multibox’ model. The second problem relates to the extent and quality of thermodynamic data. Explicit consideration of Fe-bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid-silicate-sulphide reactions associated with hydrothermal ore formation. Although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements, significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits: aqueous species in high-temperature aqueous brines; silicate/oxide/carbonate minerals including FeMg silicates; and sulphur-bearing minerals including sulphides and sulphates. Discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the Salton Sea geothermal system suggest that the redox-scale for rock-forming silicates may be inconsistent with that of sulphides and aqueous SCOH species. This analysis is preliminary but illustrates the importance of field-testing of thermodynamic data, particularly for hydrothermal conditions between 150° and 450°C, which lie between the high temperatures of reliable equilibrium experiments among rock-forming minerals (> 500°C) and the low temperatures of many experiments defining the thermodynamics of aqueous species (mostly < 300°C).

Evaluation of Kinetic Models for the Smectite to Illite Transformation

AbstractThree different models have been reported previously to describe the kinetics of the transformation of smectite to illite (Pytte 1982; Velde and Vasseur 1992; Huang et al. 1993). In order to evaluate the general utility of these models to calculate the timing and extent of this transformation, each model was applied to four different geologic settings (Denver Basin, Gulf Coast, the Salton Sea Geothermal System, and Paris Basin) in which the ages, geothermal gradients and potassium ion activities vary markedly. The model results are compared to the measured percentages of illite in illite/smectite (I/S) and the K/Ar ages of I/S (if available) to test the utility of a given model to a particular basin.Although individual models can be applied to study this transformation within a specific setting, none of these models was successful in simulating the transformation for all four basins. The Salton Sea was simulated best using the model by Huang et al. (1993), which incorporated an increased geothermal gradient during the last 20,000 years. These results indicate that a large fraction of illite formed due to this increased geothermal gradient, and underscores that temperature is a dominant kinetic factor in forming illite. The Denver Basin was simulated well by the models of Velde and Vasseur (1992) and Pytte (1982). The Gulf Coast was simulated very well by the model of Huang et al. (1993) using a term that terminates the transformation at 75% illite. For the Paris Basin, the results are mixed. The models can be refined by comparing the calculated and measured ages of illite such as the K/Ar ages of I/S to understand the thermal history of a particular basin. The calculated ages of illitization derived from these refined models can be used to indicate the time at which source rocks became thermally mature to form oil and gas.

Dating and tracing of fluids using 129I and 36Cl: results from geothermal fluids, oil field brines and formation waters

Preliminary results are presented for 129I/I and 36Cl/Cl ratios in formation waters from the KTB project in Germany, geothermal waters from the Salton Sea Geothermal System in California and oilfield brines from the Anadarko Basin in Oklahoma. The results demonstrate the use of these isotopic systems to determine residence times, source formations and pathways of fluids in different geologic situations.

Al-Fe3+ and F-OH substitutions in titanite and constraints on their P-T dependence

Aluminous titanites occur in low-grade metasediments from the Salton Sea geothermal system, U.S.A., and in granitoids and associated fluorite-bearing skarns from southwest Japan. Average (Al+Fe 3+ ) content of the metamorphic titanites in the Salton Sea geothermal system decreases systematically with increasing temperature and depth from 0.21-0.37 per formula unit (pfu,) in the chlorite-calcite zone (T = 180-320°C), through 0,12-0.23 pk. in the biotite zone (320-340°C), to 0,12-0.13 pfu. in the clinopyroxene zone (340-360°C). Fluorine content varies from 0.0 to 0.20 pfu., and average X F [= F/(F+Cl+OH)] is less than 0.5 in all samples. Titanites in the granitoids are divided into magmatic titanites occurring as discrete grains in the matrix or as inclusions in plagioclase, and secondary titanites occurring as tiny granules in chlorite that alters biotite. The (Al+Fe 3+ ) content and F contents are, respectively, 0,18 ± 0.03 and 0.14 ± 0.04 pfu, for the magmatic titanites and 0,45 ± 0.05 and 0,35 ± 0.08 pfu, for the secondary titanites. The (Al+Fe 3+ ) content and F contents of titanites in fluorite-bearing skarns are 0.21 ± 0.05 and 0,16 ± 0.06 pfu., respectively. The titanites in the granitoids and skarns have high X F of 0.77 ± 0.17 and 0.75 ± 0.16, respectively. The low-grade metamorphic and secondary titanites have variable Al content and X F ; their compositions are controlled by both substitutions (Al, Fe 3+ )FTi −1 O −1 and (Al, Fe 3+ )(OH)Ti −1 O −1 . The Ca(Al, Fe 3+ )(OH)SiO 4 component of titanites was preferentially decomposed with increasing temperature and the (Al, Fe 3+ )FTi -1 O −1 substitution mainly controls Al content of titanite under high-temperature conditions. On the other hand, the Ca(Al, Fe 3+ )FSiO 4 component increases with increasing pressure at fixed temperature, and stabilizes titanite in high-temperature and high-pressure conditions such as eclogite-facies metamorphism

Double-diffusive convection in geothermal systems: the salton sea, California, geothermal system as a likely candidate

Much has been published about double-diffusive convection as a mechanism for explaining variations in composition and temperature within all-liquid natural systems. However, relatively little is known about the applicability of this phenomenon within the heterogeneous rocks of currently active geothermal systems where primary porosity may control fluid flow in some places and fractures may control it in others. The main appeal of double-diffusive convection within hydrothermal systems is-that it is a mechanism that may allow efficient transfer of heat mainly by convection, while at the same time maintaining vertical and lateral salinity gradients. The Salton Sea geothermal system exhibits the following reservoir characteristics: (1) decreasing salinity and temperature from bottom to top and center toward the sides, (2) a very high heat flow from the top of the system that seems to require a major component of convective transfer of heat within the chemically stratified main reservoir, and (3) a relatively uniform density of the reservoir fluid throughout the system at all combinations of subsurface temperature, pressure, and salinity. Double-diffusive convection can account for these characteristics very nicely whereas other previously suggested models appear to account either for the thermal structure or for the salinity variations, but not both. Hydrologists, reservoir engineers, and particularly geochemists should consider the possibility and consequences of double-diffusive convection when formulating models of hydrothermal processes, and of the response of reservoirs to testing and production.

A brine interface in the Salton Sea Geothermal System, California: Fluid geochemical and isotopic characteristics

Data from 71 geothermal production intervals in 48 wells from the Salton Sea Geothermal System (SSGS) indicate that fluids in that system cluster into two distinct populations in terms of their salinity and their stable isotopic compositions. The distinctive, hot, hypersaline brine (typically >20 wt% total dissolved solids) for which the SSGS is known is overlain by a cooler (<260°C) fluid with distinctly lower salinity (typically <10 wt% total dissolved solids). Hypersaline brines have high and rather consistent 18O shifts produced by water-rock interaction and have a very narrow range in δD values. Low TDS fluids, on the other hand, show a wide range in both δD and δ 18 O. Production of both types of fluid from closely spaced geothermal wells in many regions of the SSGS indicates that a relatively sharp salinity interface exists over much of the field. The fluid interface typically cross-cuts sedimentary bedding but is consistently found where reservoir temperatures are approximately 260°C. At these temperatures, hypersaline brines have densities of approximately 1.0 gm/cm 3, while the low TDS fluids have densities as low as 0.85 gm/cm 3. This stable, density-stratified interface acts as a barrier to convective heat and mass transfer in the SSGS, isolating the hypersaline reservoir from overlying dilute fluids. A lithologic “cap” implied by previous SSGS models is unnecessary in such a stratified system since heat and mass transfer across the interface must occur by slow conductive, diffusional and interface mixing processes regardless of local permeability.

Sulfur isotopic variations among minerals and aqueous species in the Salton Sea geothermal system; a SHRIMP ion microprobe and conventional study of active ore genesis in a sediment-hosted environment

Sulfur isotopic variations among minerals and aqueous species in the Salton Sea geothermal system; a SHRIMP ion microprobe and conventional study of active ore genesis in a sediment-hosted environment

Sulfur Isotopic Variations Among Minerals and Aqueous Species in the Salton Sea Geothermal System; a SHRIMP Ion Microprobe and Conventional Study of Active Ore Genesis in a Sediment-Hosted Environment

Sulfur Isotopic Variations Among Minerals and Aqueous Species in the Salton Sea Geothermal System; a SHRIMP Ion Microprobe and Conventional Study of Active Ore Genesis in a Sediment-Hosted Environment

Epidote‐Bearing Veins in the State 2–14 Drill Hole: Implications for Hydrothermal Fluid Composition

Epidote‐bearing veins in State 2–14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy, parageneses, texture, and composition of vein minerals. In order of decreasing abundance, minerals in epidote‐bearing veins are pyrite, calcite, K‐feldspar, quartz, anhydrite, hematite, chlorite, Fe‐Cu‐Zn sulfides, actinolite, titanite, and allanite. The downhole distribution of minerals in epidote‐bearing veins (+ pyrite and quartz) varies as a function of depth and includes: (1) calcite above ∼2000 m, (2) K‐feldspar between 1700 and 2745 m, (3) anhydrite between 2195 and 2745 m, (4) hematite ± sulfides above 2773 m, and (5) actinolite below ∼2890 m. Where present, K‐feldspar was the first mineral to precipitate in veins followed by epidote. In all other veins, epidote was the earliest vein mineral to form. Calcite, quartz, anhydrite, hematite, and sulfides were paragenetically later. Compositional zoning, common in most vein epidotes, is typically symmetric with Al‐rich cores and Fe3+ ‐rich rims. The minimum mole fraction of Ca2Fe3Si3O12(OH) (XPs) in vein epidotes decreases systematically with increasing depth from ∼0.33 at 906 m to ∼0.21 at 2900 m, and the maximum XPs at any given depth is greater than 0.33. Thermodynamic analyses of phase relations among vein‐filling minerals and aqueous solutions at depths near 1867 m and 300°C indicate that the modern reservoir fluid in the Salton Sea geothermal system is in equilibrium with calcite + hematite + quartz + epidote (XPs = 0.33) ± anhydrite. The predicted fugacity of CO2 (∼14 bars) for the modern Salton Sea brine is in close agreement with the calculated value of fCO2 for the 1867 m production fluid. Theoretical phase diagrams in the system CaO‐K2O‐Fe2O3‐Al2O3‐SiO2‐H2O‐O2‐S2‐CO2 demonstrate that the mineralogies and mineral parageneses recorded hi epidote‐bearing veins and the observed variations in Al‐Fe3+ content of vein epidotes may result from only minor changes in the fugacity of CO2, O2, and S2 of the geothermal fluid.

Compositional, Order/Disorder, and Stable Isotope Characteristics of Al‐Fe Epidote, State 2–14 Drill Hole, Salton Sea Geothermal System

Epidote (Ca2Fe3Si3O12(OH)‐Ca2Al3Si3O12(OH)) is a common hydrothermal mineral in metasediments and veins at depths &gt;900 m in the State 2–14 drill hole of the Salton Sea geothermal system. The mole fraction of Ca2Fe3Si3O12(OH) in epidotes (Xps) from this drill hole ranges from 0.11 to 0.42, and complex compositional zoning of octahedral Fe3+ and Al is typical within single grains. With increasing depth there is an overall, but irregular, decrease in the Fe3+ content of epidotes in metasandstones. In most samples, vein epidotes are more Fe3+ ‐rich and exhibit a wider compositional range than metasandstone epidotes from the same depth. Octahedral Fe3+ in the M(1) sites in four epidotes, evaluated by 57Fe Mössbauer spectroscopy, ranges from 7.5 ± 1.7% to 11.4 ± 1.5% of the total iron. The most ordered epidote formed in veins at 2618 m in the biotite zone. This epidote has an ordering parameter (σ = 1–2XFe3+,M(1)) of 0.85 ± 0.03 which corresponds to a calculated state of equilibrium order/disorder at ∼390° ± 60°C, in close agreement with the probable downhole temperature of ∼340°C, Two epidote samples from the chlorite + calcite zone (1420 m, ∼265°C; 1867 m, ∼300°C) are more disordered, corresponding to calculated states of equilibrium order/disorder of &gt;450°C, These data suggest that the analyzed epidotes from the chlorite + calcite zone are in a metastable state of substitutional order/disorder. The measured value of δDepidote from 1420 m is −96‰, which is 5–6‰ lighter than epidotes from 1867 m (δDepidote = −90‰), 2227 m (δDepidote = −91‰), and 2618 m (δDepidote = −90 ‰). When compared to the reservoir fluid from the December 1985 flow test, the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is −19‰ at 1867 m and ∼340°C. This value is in accord with published experimental hydrogen isotope fractionations between iron‐rich epidote and aqueous electrolyte solutions. Therefore, considering the complexity of the Salton Sea brine, the isotopic temperature is similar to the inferred downhole temperature.