Oxygen isotope evidence for past and present hydrothermal regimes of Long Valley caldera, California

Whole-rock oxygen isotope compositions of cores and cuttings from Long Valley exploration wells show that the Bishop Tuff has been an important reservoir for both fossil and active geothermal systems within the caldera. The deep Clay Pit-1 and Mammoth-1 wells on the resurgent dome penetrate mildly to strongly altered Bishop Tuff with δ¹⁸O_WR values as low as −2.6% (vs V-SMOW). The idfu 44-16 well intercepts a thinner Bishop Tuff section with δ¹⁸O_WR values of +0.4 to +2.3%. in the western caldera moat, where milder and more sporadic ¹⁸O depletions occur in Tertiary volcanic rocks of the western caldera floor (δ¹⁸O_WR = +2.2 to +6.4‰). Bishop Tuff samples from deeper parts of the 715 m rdo-8 (Shady Rest) well in the SW moat are also strongly depleted in ¹⁸O (δ¹⁸O_WR = −1.5 to +0.6‰). Four shallow thermal gradient wells (469–715 m td drilled in the western moat did not penetrate Bishop Tuff, but Early Rhyolites from two of these holes are depleted in ¹⁸O (δ¹⁸O_WR = −1.2 to +6.0‰ inplv-1 +4.6 to +5.3%. inmlgrap-1), compared to lithologic equivalents from the other two holes (δ¹⁸O_WR = +6.3 to +8.0‰ inplv-2 andmlgrap-2).Whole-rock oxygen isotope profiles for the resurgent dome wells are unlike profiles calculated assuming alkali feldspar-H₂O fractionation behavior and total O-isotopic equilibration with −14.3‰ fluids at measured temperatures. The sense of this divergence implies an earlier hydrothermal episode within the central caldera driven by one or more shallow intrusions. Geochemical similarities between an intrusive granophyre at the bottom of the Clay Pit-1 well and a nearby Moat Rhyolite dome with a K/Ar cooling age of 0.5 Ma suggest that vigorous hydrothermal activity beneath the central resurgent dome may have occurred as much as 0.5 m.y. ago. Calculated and measured O-isotope profiles are similar for deep wells that penetrate the western moat of the caldera, where steep temperature gradients and low δ¹⁸O_WR values in Early Rhyolites from plv-1 are attributed to an active hydrothermal aquifer that has descended slightly from earlier, shallower elevations. Similarly, severe ¹⁸O depletions in Bishop Tuff samples from the idfu 44-16 and rdo-8 wells reflect active convection beneath the western moat, whereas milder ¹⁸O depletions in Early Rhyolites from mlgrap-1 were apparently caused by hydrothermal alteration at lower temperatures. The O-isotope profiles imply that surface discharge within and around the resurgent dome results from shallow, eastward-directed outflow from a zone of higher enthalpy hydrothermal upflow beneath the western caldera moat. Intrusive magmatic heat source(s) are inferred to exist beneath the western moat, perhaps beneath Mammoth Mountain.     

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.     

Gas geochemistry of the Valles caldera region, New Mexico and comparisons with gases at Yellowstone, Long Valley and other geothermal systems

Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO₂, 0.5 mol% H₂S, and 1 mol% other components. ³He/⁴He ratios indicate a deep magmatic source (R/R_a up to 6) whereas δ¹³C–CO₂ values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO₂ and He, but depleted in H₂S compared to Valles gases. Regional gases have R/R_a values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO₂ are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH₄, H₂, and H₂S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N₂ than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH₄, N₂ and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).     

The stable isotope geochemistry of volcanic lakes, with examples from Indonesia

Stable isotope compositions (δD, δ¹⁸O and δ³⁴S) of volcanic lake waters, gas condensates and spring waters from Indonesia, Italy, Japan, and Russia were measured. The spring fluids and gas samples plot in a broad array between meteoric waters and local high-temperature volcanic gas compositions. The δD and δ¹⁸O data from volcanic lakes in East Indonesia plot in a concave band ranging from local meteoric waters to evaporated fluids to waters heavier than local high-temperature volcanic gases. We investigated isotopic fractionation processes in volcanic lakes at elevated temperatures with simultaneous mixing of meteoric waters and volcanic gases. An elevated lake water temperature gives enhanced kinetic isotope fractionation and changes in equilibrium fractionation factors, providing relatively flat isotope evolution curves in δ¹⁸O–δD diagrams. A numerical simulation model is used to derive the timescales of isotopic evolution of crater lakes as a function of atmospheric parameters, lake water temperature and fluxes of meteoric water, volcanic gas input, evaporation, and seepage losses. The same model is used to derive the flux magnitude of the Keli Mutu lakes in Indonesia. The calculated volcanic gas fluxes are of the same order as those derived from energy budget models or direct gas flux measurements in open craters (several 100 m³ volcanic water/day) and indicate a water residence time of 1–2 decades. The δ³⁴S data from the Keli Mutu lakes show a much wider range than those from gases and springs, which is probably related to the precipitation of sulfur in these acid brine lakes. The isotopic mass balance and S/Cl values suggest that about half of the sulfur input in the hottest Keli Mutu lake is converted into native sulfur.     

An oxygen isotope study of hydrothermal alteration in the Lake City caldera, San Juan Mountains, Colorado

A 23-m.y.-old, fossil meteoric-hydrothermal system in the Lake City caldera (11 × 14 km) has been mapped out by measuring δ ¹⁸O values of 300 rock and mineral samples. δ ¹⁸O varies systematically throughout the caldera, reaching values as low as −2. Great topographic relief, regional tilting, and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system, from the surface down to a depth of more than 2000 m. The initial δ ¹⁸O value of the caldera-fill Sunshine Peak Tuff was very uniform (+7.2 ± 0.1), making it easy to determine the exact amount of ¹⁸O depletion experienced by each sample during hydrothermal alteration. Also, we have excellent stratigraphic control on depths beneath the mid-Tertiary surface, quantitative information on mineralogical alteration products, and accurate data on the shape of the central resurgent intrusion, which was the principal ‘heat engine’ that drove the hydrothermal circulation. Major conclusions are: (1) Although pristine mid-Tertiary meteoric waters in this area had δ ¹⁸O −14, these fluids were ¹⁸O-shifted upward to about δ¹⁸O = −8 to −5 prior to entering the shallow convective system associated with the resurgent intrusive rocks. Although there was undoubtedly radial inflow toward the caldera from all directions, the highly fractured Eureka Graben, southwest of the caldera, was probably the principal source of recharge groundwater for the Lake City system. (2) Fluid flow within the caldera was dominated by three major categories of permeable zones: the porous megabreccia units (which dip outward from the resurgent dome), vertical fractures and faults related to resurgence, and the caldera ring fault itself. All of these zones exhibit marked ¹⁸O depletions, and they are also typically intensely mineralogically altered. (3) The resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water/rock ratios lower than the permeable zones; however, they have similarly low δ ¹⁸O values because they were altered at higher temperatures. (4) Throughout the caldera, the δ ¹⁸O of Sunshine Peak Tuff decreases with increasing depth (about 6 per mil/km), indicative of a shallow thermal gradient, typical of a convective hydrothermal system. The near-surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid. (5) Deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid-Tertiary surface. These fluids were drawn into the shallow convective system through the lower, porous, megabreccia units. Near the resurgent intrusions, fluid flow was again directed upward where resurgence-related, near-vertical fractures intersect the megabreccia units.     

Chemical equilibrium and mass balance relationships associated with the Long Valley hydrothermal system, California, U.S.A.

Recent drilling and sampling of hydrothermal fluids from Long Valley permit an accurate characterization of chemical concentrations and equilibrium conditions in the hydrothermal reservoir. Hydrothermal fluids are thermodynamically saturated with secondary quartz, calcite, and pyrite but are in disequilibrium with respect to aqueous sulfide-sulfate speciation. Hydrothermal fluids are enriched in ¹⁸O by approximately 1‰ relative to recharge waters. ¹⁸O and Cl concentrations in well cuttings and core from high-temperature zones of the reservoir are extensively depleted relative to fresh rhyolitic tuff compositions. Approximately 80% of the Li and 50% of the B are retained in the altered reservoir rock. Cl mass balance and open-system ¹⁸O fractionation models produce similar water-rock ratios of between 1.0 and 2.5 kg kg⁻¹. These water-rock ratios coupled with estimates of reservoir porosity and density produce a minimum fluid residence time of 1.3 ka. The low fluid Cl concentrations in Long Valley correlate with corresponding low rock concentrations. Mass balance calculations indicate that leaching of these reservoir rocks accounts for Cl losses during hydrothermal activity over the last 40 ka.     

Oxygen and hydrogen isotopes in the volcano-sedimentary complex of Huelva (Iberian Pyrite Belt): example of water circulation through a volcano-sedimentary sequence

The minerals of basic and acidic rocks from the volcano-sedimentary sequence in the Huelva area, Spain, Iberian Pyrite Belt, display an extendedδ¹⁸O enrichment. Quartzδ¹⁸O values from quartz-keratophyres vary from +10.5 to +17.0 and feldsparδ¹⁸O values from +14.4 to +16.0. For the spilite or spilitized doleritesδ¹⁸O values vary from +9.9 to +13.4 for feldspar, from +6.4 to +9.8 for chlorite, from +3.7 to +4.3 for ilmenite and from +13.6 to +14.0 for quartz, but pyroxene exhibits magmatic values, from +5.3 to +6.1 with an exception at +7.5. The chloriteδD values vary from −34 to −43‰.This is attributed to hydrothermal alteration with seawater enriched inδ¹⁸O by circulation through sediments.The temperatures of interaction determined from isotopic fractionations between minerals range from 400° to 520°C.CalculatedδD andδ¹⁸O values for water in equilibrium with the minerals at isotopic temperatures range from −16 to +5 and from +8.3 to +12.8, respectively.A model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high¹⁸O compositions of the rocks from the Huelva District. Water/rock mass ratios calculated from this model range between 0.3 and 0.7 for the determined range of temperatures.     

Earthquake-induced variations in the composition of the water in the geothermal reservoir at Vulcano Island, Italy

During 1979–1989, variations were observed in the oxygen composition of the water contained in the geothermal reservoir at Vulcano Island, Italy.The reservoir water, that has a magmatic origin, showed an oxygen composition of +1.0±0.5‰ δ¹⁸O during periods without local tectonic earthquakes, and an oxygen composition of +3.4±0.5‰ δ¹⁸O after the highest-energy seismic activity that occurred recently near the island. A slight increase of the δ¹⁸O value in the reservoir water was also observed after a low-energy sequence of tectonic earthquakes that occurred at very shallow depth just beneath Vulcano Island. These ¹⁸O variations in the reservoir water are consistent with earthquake-induced increases in the contribution from high-temperature δ¹⁸O-rich magmatic condensate to the geothermal reservoir, and with subsequent decreases in the δ¹⁸O value due to ¹⁸O exchanges at the temporarily increased reservoir temperature during reactions between the highly reactive magmatic condensate and the local rocks.Only minor changes in the deuterium composition of the reservoir water occurred with time, as the δD value in the magmatic condensate released from the magma after major local earthquakes quickly approached the δD value of the water contained in the geothermal reservoir.Also the chloride concentration in the reservoir water appears to be linked to the contribution from the magmatic fluid. This chloride content seems not to have undergone major changes with time, as it may be buffered by temporary increases in the reservoir temperature up to values >300°C induced by major local earthquakes. This mechanism may possibly occur also in other magmatic–hydrothermal systems.     

Chemical and stable isotopic models for boundary creek warm springs, southwestern Yellowstone National Park, Wyoming

Thermal springs of the Boundary Creek hydrothermal system in the southwestern part of Yellowstone Park outside the caldera boundary vary in chemical and isotopic composition, and temperature. The diversity may be accounted for by a combination of processes including boiling of a deep thermal water, mixing of the deep thermal water with cool meteoric water and/or with condensed steam or steam-heated meteoric water, and chemical reactions with surrounding rocks. Dissolved-silica, Na⁺, K⁺ and Ca²⁺ contents of the thermal springs could result from a thermal fluid with a temperature of 200 ± 20°C. Chloride-enthalpy and silica-enthalpy mixing models suggest mixing of 230°C, 220 mg/l Cl⁻ thermal water with cool, low-Cl⁻ components. A 350 to 390°C component with Cl⁻ ≥ 300 mg/l is possibly present in thermal springs inside the caldera but is not required to fit observed spring chemical and isotopic compositions. Irreversible mass transfer models in which a low-temperature water reacts with volcanic glass as it percolates downward and warms, can account for observed pH and dissolved-silica, K⁺, Na⁺, Ca²⁺ and Mg²⁺ concentrations, but produces insufficient Cl⁻ or F⁻ for measured concentrations in the warm springs. The ratio of a_Na/a_H, and Cl⁻ are best accounted for in mixing models. The water-rock interaction model fits compositions of acid-sulfate waters observed at Summit Lake and of low-Cl⁻ waters involved in mixing.The cold waters collected from southwestern Yellowstone Park have δD values ranging from −118 to −145 per mil and δ¹⁸O values of −15.9 to −19.4 per mil. Two samples from nearby Island Park have δD values of −112 and −114 per mil and δ¹⁸O values of −15.1 and −15.3 per mil. All samples of thermal water plot significantly to the right of the meteoric water line. The low Cl⁻ and variable δD values of the thermal waters indicate isotopic compositions are derived by extensive dilution with cold meteoric water and by steam separation on ascent to the surface. Many of the hot springs with higher δD values may contain in addition a significant amount of high-D, low-Cl⁻, acid-sulfate or steam-heated meteoric water. Mixing models, Cl⁻ content and isotopic compositions of thermal springs suggest that 30% or less of a deep thermal component is present. For example, the highest-temperature springs from Three Rivers, Silver Scarf and Upper Boundary Creek thermal areas contain up to 70% cool meteoric water and 30% hot water components, springs at Summit Lake and Middle Boundary Creek spring 57 are acid-sulfate or steam-heated meteoric water; springs 27 and 48 from Middle Boundary Creek and 49 from Mountain Ash contain in excess of 50% acid-sulfate water; and Three Rivers spring 46 and Phillips could result from mixing hot water with 55% cool meteoric water followed by mixing of acid-sulfate water. Extensive dilution by cool meteoric water increases the uncertainties in quantity and nature of the deep meteoric, thermal component.     

Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia

The crater lake of Kawah Ijen volcano contains extremely low pH (<0.4) waters with high SO₄ (70000 mg/kg), Cl (21000 mg/kg), F (1500 mg/kg), Al (5000 mg/kg), Fe (2000 mg/kg) and trace metal (Cu 0.5, Zn 4, Pb 3 mg/kg) contents. These brines seep outward through the western crater rim and reappear on the other side as streamlets, which form the headwaters of the Banyupahit stream. The Banyupahit first mixes with fresh rivers and thermal springs in the Ijen caldera and then irrigates a coastal agricultural plain which is 30 km from the summit crater.We discuss the downstream composition changes affecting the Banyupahit waters by using stable isotope, chemical and mineralogical data collected from sites along the stream length. The saturation of the stream waters with respect to minerals was evaluated with SOLVEQ and WATEQ4F and compared with the geochemical observations. An aluminous mineralogy (alunogen, pickeringite, tamarugite and kalinite) develops in the upper part of the Banyupahit due to concentration of the headwaters by evaporation. Downstream attenuation of dissolved element concentrations results principally from dilution and from mineral precipitation. The stream pH changes from 0 at the source to >4 close to the mouth. The δD and δ¹⁸O values and the relative SO₄–Cl–F contents of the Banyupahit waters indicate that the tributaries are mostly meteoric. Dissolved SO₄ in the acidic stream come only from the crater lake seepages and are not involved later in microbially mediated reactions, as shown by their δ³⁴S and δ¹⁸O values. Re-equilibration of the stream SO₄ oxygen-isotope composition with H₂O from tributaries does not occur.Calcium, SiO₂, Al, Fe, K and SO₄ behave non-conservatively in the stream waters. Gypsum, silica (amorphous or poorly ordered), a basic aluminum hydroxysulfate (basaluminite?), K-jarosite and amorphous ferric hydroxide may exert a solubility control on these elements along the entire stream length, or in certain stream sections, consistent with the thermochemical model results. Downstream concentration trends and mineral saturation levels suggest that precipitation of Sr-, Pb-rich barite and celestite consume Ba, Sr and Pb, whereas dissolved Cu, Pb and Zn may adsorb onto solid particles, especially after the junctions of the acidic stream with non-acidic rivers. We calculated that significant fluxes of SO₄, F, Cl, Al, SiO₂, Ti, Mn and Cu may reach the irrigation system, possibly causing serious environmental impacts such as soil acidification and induration.     

Fumaroles in ice caves on the summit of Mount Rainier—preliminary stable isotope, gas, and geochemical studies

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to failure. The argillic alteration results from the neutralization of acidic magmatic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62°C) in the caves were sampled for stable isotopic, gas, and geochemical studies.Stable isotope data on fumarole condensates show significant excess deuterium with calculated δD and δ¹⁸O values (−234 and −33.2‰, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic–hydrothermal steam. Between 1982 and 1997, δD of the fumarole vapor may have decreased by 30‰.The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO₂ and CH₄. The elevated CO₂ contents (δ¹³C_CO2=−11.8±0.7‰), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H₂S were detected in the fumaroles, most notably in a sample which had an air δ¹³C_CO2 signature (−8.8‰), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H₂S (or possibly SO₂) concentrations in past fumarolic gases.Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level).The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal. The extent to which hydrothermal alteration is currently occurring at depth, and its possible influence on future edifice collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves.     

Hydrothermal vent waters at 13°N on the East Pacific Rise: isotopic composition and gas concentration

Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples ( 5 × 10⁻³ moles/kg) the gas concentrations are: 3–4.5 × 10⁻⁵ cm³ STP/kg helium, 0.62–1.24 cm³ STP/kg CH₄, 10.80–16.71 × 10⁻³ moles/kg CO₂. The samples contain large quantities (95–126 cm³/kg) of H₂ and some carbon monoxide (0.26–0.36 cm³/kg) which result from reaction with the titanium sampling bottles. δ¹³C in methane and CO₂ (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ¹³C_CO is compatible with formation by reduction of CO₂ on Ti at 350°C close to the sampling temperature.³He/⁴He are very homogeneous at (7.5 ± 0.1)R_A(³He/⁴He = 1.0 × 10⁻⁵) and very similar to already published data as well as CH₄/³He ratios between 1.4 and 2.1 × 10⁶.¹⁸O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct¹⁸O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The³He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and³He mantle flux.     

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.     

O/O mapping and hydrogeology of a short-lived (≈10 years) fumarolic (>500°C) meteoric–hydrothermal event in the upper part of the 0.76 Ma Bishop Tuff outflow sheet, California

¹⁸O/¹⁶O data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (≈10 years), high-temperature, fumarolic meteoric–hydrothermal event. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low-¹⁸O groundmass/glass (δ¹⁸O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ¹⁸O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of ¹⁸O/¹⁶O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ¹⁸O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low-¹⁸O, stratigraphic zone that is sandwiched between the essentially unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic ¹⁸O/¹⁶O values. Shallow-dipping columnar joints and other fumarolic features (i.e., subhorizontal tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low-¹⁸O zones. The base of the low-¹⁸O zone is extremely sharp (3‰ per meter) and is located directly above the transition from partially welded tuff to densely welded black tuff. The observed average whole-rock ¹⁸O-depletions within this low-¹⁸O zone are about 6–7‰, requiring meteoric water/rock ratios in excess of 0.24 in mass units. Rainfall on the surface of the tuff would not have been high enough to supply this much H₂O in the short lifetime of fumarolic activity, suggesting that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1°–5°) top of the impermeable lower zone. This is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern. Some of this recharge may in part have been from the lake that filled Long Valley caldera, which was dammed by the Bishop Tuff up to the level of this boundary between the partially and densely welded zones (≈7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibits exactly the same kind of ¹⁸O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; ¹⁸O/¹⁶O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic ¹⁸O signature (i.e., analogous δ¹⁸O-depth profiles, as well as low-¹⁸O groundmass coexisting with high-¹⁸O feldspar phenocrysts). This fumarolic ¹⁸O/¹⁶O signature is observed to much greater depths (≈650–750 m) in the intracaldera tuffs (≈1500 m thick) than it is in the ≈200-m-thick Bishop Tuff outflow sheet (≈80 m depth).     

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.     

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.     

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.     

Orographic controls on rain water isotope distribution in the Mount Lofty Ranges of South Australia

Hydrogen and oxygen isotopes of water are common environmental tracers used to investigate hydrological processes, such as evaporation, vegetation water use, surface water–groundwater interaction, and groundwater recharge. The water isotope signature in surface water and groundwater evolves from the initial rain signature. In mountain terrain, rain water stable isotope composition spatially varies due to complex orographic precipitation processes. Many studies have examined the isotope–elevation relationships, while few have quantitatively investigate the terrain aspect and slope effect on rain isotope distribution. In this paper, we examine the orographic effects more completely, including elevation, terrain slope and aspect, on stable isotope distribution in the Mount Lofty Ranges (MLR) of South Australia, using a multivariate regression model. The regression of precipitation isotope composition suggests that orographic effects are the dominant controls on isotope spatial variability. About 75% of spatial variability in δ¹⁸O and deuterium excess is represented by the regression using solely orography-related variables (elevation, terrain aspect and slope), with about 25% of δ¹⁸O spatial variability attributed to the terrain aspect and slope effect. The lapse rate is about −0.25‰ for every 100 m at both windward and leeward slopes. However, at the same elevation, δ¹⁸O at the leeward slope (eastern MLR) is 0.5‰ larger than that at the windward slope. The difference can be explained by different mechanisms – continuous rain-out processes on the windward side and sub-cloud evaporation on the leeward side. Both δ¹⁸O and deuterium excess maps (1 km resolution) are constructed based on the regression results for the MLR. Both maps are consistent with groundwater of local precipitation origin, and useful to examine groundwater recharge.     

Very high-frequency and seasonal cave atmosphere PCO2 variability: Implications for stalagmite growth and oxygen isotope-based paleoclimate records

Cave air P_CO2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO₂ production, resulting in high summer P_CO2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO₂ degassing rates from drip water, by considering both the seasonal and high-frequency cave air P_CO2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h^− 1) and minima in late June/early July (0.033 μm h^− 1). For well-mixed ‘diffuse flow’ cave drips such as those that feed CC-Bil, high summer cave air P_CO2 depresses summer calcite deposition, while low winter P_CO2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ¹⁸O, integrated annual stalagmite calcite δ¹⁸O is unaffected; however, seasonality in cave air P_CO2 may influence non-conservative geochemical climate proxies (e.g., δ¹³C, Sr/Ca). Stalagmites fed by ‘seasonal’ drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air P_CO2 may increase relative to cave air P_CO2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ¹⁸O records towards isotopically heavier summer drip water δ¹⁸O values, resulting in elevated calcite δ¹⁸O values compared to the ‘equilibrium’ values predicted by calcite–water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air P_CO2 variability and the resulting intra-annual variability in calcite deposition rates.     

O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs

Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ¹⁸O = +5.7 to +6.6 (‰ SMOW) andδ³⁴S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ¹⁸O = +6.3 ± 0.2 are slightly enriched in¹⁸O with respect to unfractionated basalts of<53%SiO₂ with meanδ¹⁸O = +6.0 ± 0.1. This¹⁸O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ¹⁸O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ¹⁸OSiO₂ trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO₂; therefore, the meanδ³⁴S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region.The enrichment of¹⁸O and³⁴S in Mariana Arc parental magmas relative to ocean floor basalts withδ¹⁸O ca. + 5.7‰ andδ³⁴S = ca.0.3‰ is attributed to the recycling of¹⁸O- and³⁴S-rich crustal components (sediment withδ¹⁸O = ca. + 25‰ and seawater sulfate withδ³⁴S = ca. +20‰ into the upper mantle source region for these arc magmas. This interpretation is consistent with enrichments of radiogenic Sr and Pb in the same lavas relative to ocean-floor basalts erupted either side of the arc, which are presumed to share a common upper mantle source. This enrichment is considered to reflect the mixing of two components, one having a typical upper mantle composition and the other having a more radiogenic character similar to that of western Pacific pelagic sediments.