Growth of large sulfide structures on the endeavour segment of the Juan de Fuca ridge

Mapping and sampling with DSRV “Alvin” has established that sulfide blocks 0.5 m across, dredged from the axial valley of the Endeavour Segment at 47°57′N, are samples of unusually large sulfide structures. The steep-sided structures, up to 30 m in length, 20 m in height, and 10–15 m across, are localized by venting along normal faults at the base of the western axial valley wall, and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest. High-temperature fluids (350 to more than 400°C) pass through the massive sulfide structures and enter seawater through small, concentric “nozzle-like” features projecting from the top or the sides of the larger vent structures. Diffuse, low-temperature flow is pervasive in the vicinity of the active sulfide structures, exiting from basalt and sulfide surfaces alike. Evidence of recent volcanic activity is sparse.The two largest samples taken with the dredge would not have been recoverable using the submersible. These samples represent massive, complex portions of the sulfide structures which were not closely associated with rapid high-temperature fluid flow at the time of sampling; they contain textural evidence of sealed hydrothermal fluid exit channels. Mineralogy is dominated by Fe sulfides nnd amorphous silica. Pyrite, marcasite, wurtzite, chalcopyrite, and iss are the most common sulfide phases. Pyrrhotite, galena, and sphalerite are present in trace amounts. Barite, amorphous silica, and chalcedony are the only non-sulfide phases; anhydrite is not observed in any of the dredge samples, although it is common in the chimney-like samples recovered by “Alvin”.Specific mineralogical-textural zones within the dredge samples are anaoogous to individual layers in East Pacific Rise at 21°N and southern Juan de Fuca Ridge samples, with two exceptions: a coarse-grained, highly porous Fe sulfide-rich interior containing sulfidized tubeworm casts, and a 2–5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica. The porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber. The amorphous silica deposited from a seawater/hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber; conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate. These silica-rich zones are the densest, most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features.Observations in these samples are consistent with two distinct phases of development. Phase 1 is analogous to chimney growth and construction at 21°N and ends when flow channels become sealed to rapid flow of through-going fluid. The flow is evidently redirected within the structure. Phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater/hydrothermal fluid mixtures. Evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures.     

Geothermal assessment of the island of ischia (southern Italy) from isotopic and chemical composition of the delivered fluids

The Ischia geothermal system is hosted by silicic rocks of the Quaternary Potassic Roman Province, in southern Italy. Exploration drilling down to 1156 m depth in the mid-1950s provided information on boiling profiles (up to 250°C) and on the depth and permeability of the potential reservoirs. Discharge fluid samples were collected and analyzed to define the inflow of surrounding seawater (C1 ranges from 2.5 to 20 g/kg) into the system.Analyses of samples from surface manifestations and shallow wells collected during 1983 and 1988 point to the existence of three distinct mixing regimes, involving three water components. A dishomogeneous body of diluted water (Cl less than 2.5 g/kg), that occurs at depths > 700 m and reequilibrates at 240°C at least, is overlain by an aquifer of groundwater variably mixed with variably seawater (Cl from 4 to 10 g/kg), which tends to reequilibrate at 160°C. Steam-heated waters locally develop and act as dilutants of the rising geothermal fluids.Dilution, mixing, and evaporation of the ascending chloride fluids are supported by oxygen and hydrogen isotopic data the thermal waters being enriched in ¹⁸O and D with respect to local meteoric water by up to 7 and 30‰, respectively. The relative composition of the major cations in thermal solutions was used to discriminate the two main groups of thermal waters, the reservoir temperatures of which are estimated from the Na/K-gethermometer. K-Mg geothermometer indicates reequilibration in near-surface conditions.The isotopic composition of the fumarolic steam varies from −7 to −12‰ in ∂⁸O and from − 35 to − 70‰ in ∂D, in agreement with a deep mixed fluid that boils adiabatically from 240 to 80°C. The deuterium content of the H₂O-H₂ pair gives enrichment factor of about 830‰, corresponding to equilibrium temperature conditions slightly higher than the surface boiling temperatures. The ∂¹³C of CO₂is almost constant at −4.5‰ (1δ=0.4), suggesting an important magmatic contribution, and the ∂¹⁸O values of CO₂appears to in equilibrium with accompanying steam at the measured temperatures.The CO₂/Ar and H₂/Ar chemical ratios have been used to derive aquifer temperatures, the values obtained being consistent with those of solute geothermometers.     

Geochemistry and isotopes of fluids from sulphur springs, Valles Caldera, New Mexico

Detailed geochemistry supported by geologic mapping has been used to investigate Sulphur Springs, an acid-sulfate hot spring system that issues from the western flank of the resurgent dome inside Valles Caldera. The most intense activity occurs at the intersection of faults offsetting caldera-fill deposits and post-caldera rhyolites. Three geothermal wells in the area have encountered pressures <1 MPa and temperatures of 200°C at depths of 600 to 1000 m. Hot spring and fumarole fluids may discharge at boiling temperatures with pH 1.0 and SO₄ 8000 mg/l. These conditions cause argillic alterations throughout a large area.Non-condensible gases consist of roughly 99% CO₂ with minor amounts of H₂S, H₂, and CH₄. Empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280°C. Comparison of ¹³C and ¹⁸O between CaCO₃ from well cuttings and CO₂ from fumarole steam indicates a fractionation temperature between 200 and 300°C by decarbonation of hydrothermally altered Paleozoic limestone and vein calcite in the reservoir rocks. Tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids (Baca #13, 278°C) are 2.1 and 1.0 T.U. respectively, suggesting the steam originates from a reservoir whose water is mostly >50 yrs old. Deuterium contents of fumarole steam, deep reservoir fluid, and local meteoric water are practically identical even though ¹⁸O contents range through 4‰, thus, precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation. From analysis of D and ¹⁸O values between fumarol steam and deep reservoir fluid, steam reaches the surface either (1) by vaporizing relatively shallow groundwater at 200°C or (2) by means of a two-stage boiling process through an intermediate level reservoir at roughly 200°C.Although many characteristics of known vapor-dominated geothermal systems are found at Sulphur Springs, fundamental differences exist in temperature and pressure of our postulated vapor-zone. We propose that the reservoir beneath Sulphur Springs is too small or too poorly confined to sustain a “true” vapor-dominated system and that the Sulphur Springs system may be a “dying” vapor-dominated system that has practically boiled itself dry.     

S, O and Sr isotope systematics of active vent materials from the Mariana backarc basin spreading axis at 18°N

Stable isotope ratios of S, O and Sr have been measured for active vent materials which were first found and sampled in April 1987 from the Mariana backarc spreading axis at 18°N. Chimneys consisted mostly of barite with a lesser proportion of sulfide minerals such as sphalerite, galena, chalcopyrite and pyrite. Theδ³⁴S values of sphalerite and galena taken from several chimneys and various parts of a chimney showed a narrow range from 2.1 to 3.1‰, suggesting uniform conditions of fluid chemistry during chimney growth. The sulfur isotopic results imply a contribution of hydrogen sulfide reduced from seawater sulfate in the deep hydrothermal reaction zone, considering that fresh glasses of the Mariana Trough basalts haveδ³⁴S= −0.6 ± 0.3‰. Sulfur isotopic compositions of hydrogen sulfide in the high temperature vent fluids (δ³⁴S= 3.6–4.8‰) which are higher than those of the sulfide minerals suggest the secondary addition of hydrogen sulfide partially reduced from entrained seawater SO₄²⁻ at a basal part of the chimneys. This interpretation is consistent with theδ³⁴S values of barite (21–22‰) that are higher than those of seawater sulfate. The residence time of the entrained SO₄²⁻ was an order of an hour on a basis of oxygen isotopic disequilibrium of barite. Strontium isotopic variations of barite and vent waters indicated that Sr in barite was mostly derived from the Mariana Trough basalts with a slight contribution from Sr in circulating sea-water, and that 10–20% mixing of seawater with ascending hydrothermal fluids induced precipitation of barite at the sea-floor.     

High-resolution garnet chronometry and the rates of metamorphic processes

Garnets in an amphibolite-facies metasediment from Sulitjelma, North Norway yield precise and concordant SmNd, UPb and RbSr ages that relate directly to the pressure (P) and temperature (T) conditions of mineral growth. Differential mineral reaction between graphitic and non-graphitic layers within this sample preserves a record of theP-T and time (t) history experienced during Barrovian regional metamorphism. Garnets in graphitic layers grew during prograde metamorphism at462 ± 16°C and5.2 ± 0.5 kbar under conditions of lowaH₂O, and yield indistinguishable¹⁴⁷Sm¹⁴³Nd and²³⁸U²⁰⁶Pb ages of434.1 ± 1.2 Ma and433.9 ± 1.0 Ma, respectively. In contrast, garnet growth in adjacent graphite-free layers did not occur untilP-T conditions of540 ± 18°C and8.0 ± 1.0 kbar were attained, with continued growth in response to minor heating and decompression with final matrix equilibration at544 ± 16°C and7.0 ± 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable¹⁴⁷Sm¹⁴³Nd and²³⁸U²⁰⁶Pb ages of424.6 ± 1.2 Ma and423.4± 1.7 Ma, respectively. These results provide precise estimates for average heating and burial rates during prograde metamorphism of 8.6_−4.4^+7.5°C Ma⁻¹ and 0.8_−0.5^+0.9 km Ma⁻¹, respectively. Rb and Sr exchange between coexisting silicates in the graphite-free assemblage continued for some 37 Ma after the “peak” of metamorphism, and require an average cooling rate of about 4.0°C Ma⁻¹ during uplift. These results illustrate a clear relationship between reaction history and the timing of mineral growth and provide definitive constraints on the rates of thermal and tectonic processes accompanying regional metamorphism.     

Evaluation of approximations in modelling the cooling of magmatic bodies

The cooling of a magmatic intrusion is simulated by a simple model of a non-homogeneous earth, with thermal properties depending on temperature, in which heat transfer is assumed to take place by conduction only. The mathematical problem consists in solving a non-linear partial differential equation with continuity conditions on temperature and heat flux imposed at the contacts between different rocks. This has been done numerically by a finite difference method. The model is then adopted as “reality” against which a number of commonly used approximations are tested. It is found that the effect of latent heat liberation can be reasonably taken into account by attributing an effective initial temperature to the magma (errors within 20°C for t > 10⁵ years, when the temperature of the magma is still as high as 600°C); the effective specific heat approximation does not work as well. The dependence of thermal conductivity and specific heat on temperature may be eliminated by maintaining the errors within 30°C for t < 5 × 10⁵ years. The assumption that magma and country rocks have the same thermal properties allows an estimate of the temperature field in the host rocks with errors of 50°C at most. The assumption that all rocks have the same constant conductivity yields results that are far from “reality” (errors of 100–200°C even at shallow depth).     

Oxygen diffusion rates in quartz exchanged with CO2

Oxygen self diffusion rates were determined in quartz samples exchanged with¹⁸O-enriched CO₂ between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D₀) at P(CO₂) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10⁻⁸ cm²/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O₂ exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with¹⁸O₂. The effect of higher CO₂ pressures is small. At 900°C, the diffusion rate exchanged with CO₂ is = 2.35 × 10⁻¹⁵ cm²/s at 100 bar, 2.24 × 10⁻¹⁵ cm²/s at 3.45 kbar and 8.13 × 10⁻¹⁵ cm²/s at 7.2 kbar.There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO₂ systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H₂O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100°C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.     

Sulfur isotopic effects in the disproportionation reaction of sulfur dioxide in hydrothermal fluids: implications for the δS variations of dissolved bisulfate and elemental sulfur from active crater lakes

Sulfur isotope effects during the SO₂ disproportionation reaction to form elemental sulfur (3SO₂+3H₂O→2HSO₄⁻+S+2H⁺) at 200–330°C and saturated water vapor pressures were experimentally determined. Initially, a large kinetic isotopic fractionation takes place between HSO₄⁻ and S, followed by a slow approach to equilibrium. The equilibrium fractionation factors, estimated from the longest run results, are expressed by 1000 ln α_HSO4⁻–S=6.21×10⁶/T²+3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions.δ³⁴S values of HSO₄⁻ and elemental sulfur were examined for active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Maly Semiachik (Kamchatka), Poás (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). Δ_HSO4⁻–S values are 28‰ for Keli Mutu, 26‰ for Kawah Ijen, 24‰ for Ruapehu, 23‰ for Poás, 22‰ for Maly Semiachik, 21‰ for Yugama, 13‰ for Bannoe, 9‰ for Niseko, 4‰ for Khloridonoe, and 0‰ for Noboribetsu, in the decreasing order. The SO₂ disproportionation reaction in the magmatic hydrothermal system below crater lakes where magmatic gases condense is responsible for high Δ_HSO4⁻–S values, whereas contribution of HSO₄⁻ produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower Δ_HSO4⁻–S values. Currently, Noboribetsu crater lake contains no HSO₄⁻ of magmatic origin. A 40-year period observation of δ³⁴S_HSO4⁻ and δ³⁴S_S values at Yugama indicated that the isotopic variations reflect changes in the supply rate of SO₂ to the magmatic hydrothermal system. This implies a possibility of volcano monitoring by continuous observation of δ³⁴S_HSO4⁻ values. The δ¹⁸O values of HSO₄⁻ and lake water from the studied lakes covary, indicating oxygen isotopic equilibration between them. The covariance gives strong evidence that lake water circulates through the sublimnic zone at temperatures of 140±30°C.     

Isotope geochemistry of minerals and fluids from Newberry volcano, Oregon

Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The δ¹⁵O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The δ¹⁸O values of quartz and calcite from the andesite and basalt flows (700–932 m) have isotopic values which require that the equilibrated water δ¹⁸O values increase slightly (− 11.3 to −9.2‰) with increasing measured temperatures (150–265°C). The lithic tuffs and brecciated lava flows (300–700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76–100°C). The δ¹⁸O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water (− 10.5 ‰) with meteoric water (− 15.5 ‰) recharged within the caldera. The δ¹³C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO₂ of about −8 ‰.The δ¹⁸O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100°C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The δ¹⁸O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water.The calculated δ¹⁸O and δD values of bottom hole water determined from the fluid produced during the 20 hour flow test are −10.2 and −109‰, respectively. The δD value of the hydrothermal water indicates recharge from outside the caldera.     

Petrography and uplift history of the Quaternary Takidani Granodiorite: could it have hosted a supercritical (HDR) geothermal reservoir?

The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3–5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: porphyritic granodiorite, porphyritic granite, biotite-hornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by >70 wt% SiO₂ at its inferred margin and <67 wt% SiO₂ at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquid-dominated, convective hydrothermal system, with <380°C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. ‘Healed’ microfractures also trapped >600°C, hypersaline (35 wt% NaCl_eq) fluids of magmatic origin, with inferred minimum pressures of formation being 600–750 bar, which corresponds to fluid entrapment at 2.4–3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7–9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at 1.25 Ma) and shallower (6.5–7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1–5.9 mm/yr (although uplift was about 7.5 mm/yr prior to 1.2 Ma), which is faster than average uplift rates in the Japan Alps (3 mm/yr during the last 2 million years). A temperature–depth–time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500°C to 400°C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature–time–depth window is proposed: for 500°C at 1.54–1.57 Ma and 5.2±0.9 km (drilling) depth; and 400°C at 1.36–1.38 Ma and 3.3±0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature–depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan).     

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO₄–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and ¹⁸O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and ¹⁸O content. The large Δ_SO4–S⁰ (23.8–26.4‰) measured in the lake suggest that dissolved SO₄ forms during disproportionation of magmatic SO₂ in the hydrothermal conduit at temperatures of 250280°C. The lake δ¹⁸O_SO4 and δ¹⁸O_H2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO₂-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO₄–Cl waters on the lakeshore and neutral pH HCO₃–SO₄–Cl–Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO₄/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO₄–Cl water and ground water.The lakeshore fumarole discharges (T=170245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is 260°C. The proportions of the oxidized, SO₂-dominated magmatic vapor and of the reduced, H₂S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO₂-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H₂S/SO₂ ratio deduced for the gas producing the SO₄–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H₂S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H₂S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ³⁴S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.     

Geochemistry of the acid Kawah Putih lake, Patuha Volcano, West Java, Indonesia

Kawah Putih is a summit crater of Patuha volcano, West Java, Indonesia, which contains a shallow, 300 m-wide lake with strongly mineralized acid–sulfate–chloride water. The lake water has a temperature of 26–34°C, pH=<0.5–1.3, S^tot=2500–4600 ppm and Cl=5300–12 600 ppm, and floating sulfur globules with sulfide inclusions are common. Sulfur oxyanion concentrations are unusually high, with S₄O₆²⁻+S₅O₆²⁻+S₆O₆²⁻=2400 – 4200 ppm. Subaerial fumaroles (<93°C) on the lake shore have low molar SO₂/H₂S ratios (<2), which is a favorable condition to produce the observed distribution of sulfur oxyanion species. Sulfur isotope data of dissolved sulfate and native sulfur show a significant ³⁴S fractionation (ΔSO₄–S_e of 20‰), probably the result of SO₂ disproportionation in or below the lake. The lake waters show strong enrichments in ¹⁸O and D relative to local meteoric waters, a result of the combined effects of mixing between isotopically heavy fluids of deep origin and meteoric water, and evaporation-induced fractionation at the lake surface. The stable-isotope systematics combined with energy-balance considerations support very rapid fluid cycling through the lake system. Lake levels and element concentrations show strong seasonal fluctuations, indicative of a short water residence time in the lake as well.Thermodynamic modeling of the lake fluids indicates that the lake water is saturated with silica phases, barite, pyrite and various Pb, Sb, Cu, As, Bi-bearing sulfides when sulfur saturation is assumed. Precipitating phases predicted by the model calculations are consistent with the bulk chemistry of the sulfur-rich bottom sediments and their identified mineral phases. Much of the lake water chemistry can be explained by congruent rock dissolution in combination with preferential enrichments from entering fumarolic gases or brines and element removal by precipitating mineral phases, as indicated by a comparison of the fluids, volcanic rocks and lake bed sediment.Flank springs on the mountain at different elevations vary in composition, and are consistent with local rock dissolution as a dominant factor and pH-dependent element mobility. Discharges of warm sulfate- and chloride-rich water at the highest elevation and a near-neutral spring at lower level may contain a small contribution of crater-lake water. The acid fluid-induced processes at Patuha have led to the accumulation of elements that are commonly associated with volcano-hosted epithermal ore deposits. The dispersal of heavy metals and other potentially toxic elements from the volcano via the local drainage system is a matter of serious environmental concern.     

Authigenic carbonates in sediments from the Gulf of Mexico

The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between “normal-marine” (δ¹³C ca. 0‰) and −14.6‰ which suggests that biogenic CO₂ contributed to the carbonate formation. The δ¹³O values of dolomite and coexisting calcite are very similar but variable down-core.Dolomite and calcite precipitated early from pore water where SO₄²⁻ was not reduced. However, during (and after?) SO₄²⁻ reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.     

Nitrogen isotope geochemistry of basaltic glasses: implications for mantle degassing and structure?

The nitrogen isotope geochemistry of 15 basaltic glasses has been investigated using stepped heating and high sensitivity static vacuum mass spectrometry. At low temperature (< 600°C) the glasses release small amounts of nitrogen with δ¹⁵N_AIR, averaging −0.3‰, suggesting surficial adsorption of atmospheric nitrogen. At high temperature, usually with a maximum at 1000°C, indigenous nitrogen with a concentration ranging from 0.2 to 2.1 ppm is released. The δ¹⁵N values of this high temperature release show a wide range from −4.5‰ to +15.5‰. There is no correlation between N ppm and δ¹⁵N, and the samples apparently form 3 groups with distinctive δ¹⁵N. Six MORB glasses from the Mid-Atlantic Ridge, East Pacific Rise and Juan de Fuca Ridge define a group with δ¹⁵N = +7.5 ± 1.3‰. In contrast two Indian Ocean MORB glasses (Carlsberg Ridge and Gulf of Aden) gave negative δ¹⁵N averaging −3.2‰. Glasses from Loihi Seamount have high δ¹⁵N averaging +14.0 ± 1.0‰. Comparison of the δ¹⁵N data with the mantle models derived from helium and argon isotope studies suggests that the wide range in δ¹⁵N may reflect in part heterogeneities in the mantle related to its degassing history. It is possible, however, that magmatic degassing processes have also affected nitrogen isotopic compositions, and the data cannot yet be unambiguously interpreted in terms of source variations.     

Chemical geothermometry and fluid–mineral equilibria for the Ömer–Gecek thermal waters, Afyon area, Turkey

Thermal waters of the Ömer–Gecek geothermal field, Turkey, with temperatures ranging from 32 to 92°C vary in chemical composition and TDS contents. They are generally enriched in Na–Cl–HCO₃ and suggest deep water circulation. Silica and cation geothermometers applied to the Ömer–Gecek thermal waters yield reservoir temperatures of 75–155°C. The enthalpy–chloride mixing model, which approximates a reservoir temperature of 125°C for the Ömer–Gecek field, accounts for the diversity in the chemical composition and temperature of the waters by a combination of processes including boiling and conductive cooling of deep thermal water and mixing of the deep thermal water with cold water. It is also determined that the solubility of silica in most of the waters is controlled by the chalcedony phase. Equilibrium states of the Ömer–Gecek thermal waters studied by means of the Na–K–Mg triangular diagram, Na–K–Mg–Ca diagram, K–Mg–Ca geoindicator diagram, activity diagrams in the systems composed of Na₂O–CaO–K₂O–Al₂O₃–SiO₂–CO₂–H₂O phases, log SI diagrams, and finally the alteration mineralogy indicate that most of the spring and low-temperature well waters in the area can be classified as shallow or mixed waters which are likely to be equilibrated with calcite, chalcedony and kaolinite at predicted temperature ranges similar to those calculated from the chemical geothermometers. It was also observed that mineral equilibrium in the Ömer–Gecek waters is largely controlled by CO₂ concentrations.     

Chemistry and mineralogy of high-temperature gas discharges from Colima volcano, Mexico. Implications for magmatic gas–atmosphere interaction

Gases, condensates and silica tube precipitates were collected from 400°C (Z2) and 800°C (Z3) fumaroles at Colima volcano, Mexico, in 1996–1998. Volcanic gases at Colima were very oxidized and contain up to 98% air due to mixing with air inside the dome interior, close to the hot magmatic body. An alkaline trap method was used to collect gas samples, therefore only acidic species were analysed. Colima volcanic gases are water-rich (95–98 mol%) and have typical S/C/Cl/F ratios for a subduction type volcano. δD-values for the high-temperature Z3 fumarolic vapour vary from −26 to −57‰. A negative δD–Cl correlation for the Z3 high-temperature fumarole may result from magma degassing: enrichment in D and decrease in the Cl concentration in condensates are likely a consequence of input of “fresh” batches of magma and an increasing of volcanic activity, respectively.The trace element composition of Colima condensates generally does not differ from that of other volcanoes (e.g. Merapi, Kudryavy) except for some enrichment in V, Cu and Zn. Variations in chemical composition of precipitates along the silica tube from the high-temperature fumarole (Colima 1, fumarole Z3), in contrast to other volcanoes, are characterized by high concentrations of Ca and V, low concentration of Mo and a lack of Cd. Mineralogy of precipitates differs significantly from that described for silica tube experiments at other volcanoes with reduced volcanic gas. Thermochemical modelling was used to explain why very oxidized gas at Colima does not precipitate halite, sylvite, and Mo- and Cd-minerals, but does precipitate V-minerals and native gold, which have not been observed before in mineral precipitates from reduced volcanic gases.     

Iron-magnesium order-disorder in an orthopyroxene crystal from the Johnstown meteorite

Equilibrium reversals of Fe²⁺Mg distribution between the M1 and M2 sites of an orthopyroxene from the Johnstown meteorite were achieved at several temperatures between 700 and 1000°C. One single crystal was used for the whole thermal treatment and for collecting all the X-ray data after quenching. The intracrystalline ion exchange for the bulk chemical composition: (Mg_1.453Fe_0.441Cr_0.024Ca_0.054Mn_0.015Fe_0.005Ti_0.003Al_0.005)(Si_1.960Al_0.040)O₆ is given by: ln K_D = −3027(±39)/T(K) + 0.872(±0.013)> where K_D is the distribution coefficient for the reaction: Fe_M2²⁺ + Mg_M1 = Mg_M2 + Fe_M1²⁺.A transmission electron microscopy (TEM) study of part of the crystal showed the presence of very thin augite lamellae and Guinier-Preston zones indicating a relatively rapid cooling of the host rock at temperatures close to 1000°C. The new temperature scale yields a relatively high quenching temperature of 379 (±8)°C for the pyroxene which appears consistent with a rapid cooling (estimated few degrees per hundred years) of a magmatic cumulate excavated by an impact on its parental body.     

A reconnaissance geochemical study of La Primavera geothermal area, Jalisco, Mexico

The Sierra La Primavera, a late Pleistocene rhyolitic caldera complex in Jalisco, México, contains fumaroles and large-discharge 65°C hot springs that are associated with faults related to caldera collapse and to later magma insurgence. The nearly-neutral, sodium bicarbonate, hot springs occur at low elevations at the margins of the complex, whereas the water-rich fumaroles are high and central.The Comisión Federal de Electricidad de México (CFE) has recently drilled two deep holes at the center of the Sierra (PR-1 and Pr-2) and one deep hole at the western margin. Temperatures as high as 285°C were encountered at 1160 m in PR-1, which produced fluids with 820 to 865 mg/kg chloride after flashing to one atmosphere. Nearby, PR-2 encountered temperatures to 307°C at 2000 m and yielded fluids with chloride contents fluctuating between 1100 and 1560 mg/kg after flashing. Neither of the high-temperature wells produced steam in commercial quantities. The well at the western margin of the Sierra produced fluids similar to those from the hot springs. The temperature reached a maximum of 100°C near the surface and decreased to 80°C at 2000 m.Various geothermometers (quartz conductive, Na/K, Na-K-Ca, δ¹⁸O(SO₄-H₂O) and D/H (steam-water) all yield temperatures of 170 ± 20°C when applied to the hot spring waters, suggesting that these spring waters flow from a large shallow reservoir at this temperature. Because the hot springs are much less saline than the fluids recovered in PR-1 and PR-2, the mixed fluid in the shallow reservoir can contain no more than 10–20% deep fluid. This requires that most of the heat is transferred by steam. There is probably a thin vapor-dominated zone in the central part of the Sierra, through which steam and gases are transferred to the overlying shallow reservoir. Fluids from this reservoir cool from 170°C to 65°C by conduction during the 5–7 km of lateral flow to the hot springs.     

Low-temperature opal-CT precipitation in Antarctic deep-sea sediments: evidence from oxygen isotopes

Porcellanite has been found in Antarctic deep-sea sediments of shallow burial depth at four different sites in host sediments younger than 4 Ma. Oxygen isotope analysis shows that the opal-CT samples are extremely rich in¹⁸O (δ¹⁸O= 41.2to44.7‰ rel. SMOW). According to the quartz/water fractionation the calculated isotopic opal-CT formation temperature is in the range of 0 to 4°C. This agrees well with sediment temperature measurements. The low opal-CT precipitation temperature contrasts with current ideas about later diagenetic formation of opal-CT at higher temperatures of 18 to 56°C.     

Fluids in the lithosphere, 1. Experimentally-determined wetting characteristics of CO2H2O fluids and their implications for fluid transport, host-rock physical properties, and fluid inclusion formation

The equilibrium distribution of CO₂H₂O fluids in synthetic rock samples (principally dunite and quartzite) has been characterized by measurements of the dihedral wetting angle, θ, resulting from 5-day annealing periods at 950–1150°C and 1 GPa. For fluids in equilibrium with polycrystalline quartz, θ varies systematically from 57° for pure H₂O to 90° at X_CO2 0.9. Similarly, for San Carlos olivine, θ varies from 65° for pure H₂O to 90° at X_CO2 0.9. The addition of solutes (NaCl, KCl, CaF₂, Na₂CO₃) to H₂O causes a major decrease in θ in the quartz/fluid system (to values as low as 40°), but has no effect on fluid wetting in dunite. Reconnaissance experiments on other mono- and polymineralic aggregates indicate universally high wetting angles (θ 60°) in upper mantle assemblages and for CO₂ in felsic compositions. For diopside + H₂O, θ 80°, with large variation due to crystalline anisotropy. In no case does θ approach 0°, the condition necessary for fluid to be present along all grain boundaries.Because a value of θ greater than 60° precludes the existence of an interconnected fluid phase in a rock, our results have important implications not only for fluid transport but also for the physical properties of the bulk fluid/rock system. Any static fluid present in the upper mantle must exist as isolated pores located primarily at grain corners, and transport can occur only by hydrofracture. In the continental crust, aqueous fluids (especially saline ones) are likely to form an interconnected network along grain edges, thus contributing to high electrical conductivity and allowing the possibility of fluid transport by porous flow or surface energy-driven infiltration.