Cathy J. Janik

Geochemical surveillance of magmatic volatiles at Popocatépetl volcano, Mexico

Surveillance of Popocatepetl volcanic plume geochemistry and SO 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, S total , F, CO 2 , Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ 34 S-S total in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ 13 C-CO 2 of trap solutions were greatly contaminated with atmospheric CO 2 and affected by absorption kinetics. When trap data are combined with SO 2 flux measurements made through November 1996, Popocatepetl released about 3.9 Mt SO 2 , 16 Mt CO 2 , 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H 2 O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatepetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.

Elevated carbon dioxide flux at the Dixie Valley geothermal field, Nevada; relations between surface phenomena and the geothermal reservoir

Abstract In the later part of the 1990s, a large die-off of desert shrubs occurred over an approximately 1 km 2 area in the northwestern section of the Dixie Valley (DV) geothermal field. This paper reports results from accumulation-chamber measurements of soil CO 2 flux from locations in the dead zone and stable isotope and chemical data on fluids from fumaroles, shallow wells, and geothermal production wells within and adjacent to the dead zone. A cumulative probability plot shows three types of flux sites within the dead zone: locations with a normal background CO 2 flux (7 g m −2 day −1 ); moderate flux sites displaying “excess” geothermal flux; and high flux sites near young vents and fumaroles. A maximum CO 2 flux of 570 g m −2 day −1 was measured at a location adjacent to a fumarole. Using statistical methods appropriate for lognormally distributed populations of data, estimates of the geothermal flux range from 7.5 t day −1 from a 0.14-km 2 site near the Stillwater Fault to 0.1 t day −1 from a 0.01-km 2 location of steaming ground on the valley floor. Anomalous CO 2 flux is positively correlated with shallow temperature anomalies. The anomalous flux associated with the entire dead zone area declined about 35% over a 6-month period. The decline was most notable at a hot zone located on an alluvial fan and in the SG located on the valley floor. Gas geochemistry indicates that older established fumaroles along the Stillwater Fault and a 2-year-old vent in the lower section of the dead zone discharge a mixture of geothermal gases and air or gases from air-saturated meteoric water (ASMW). Stable isotope data indicate that steam from the smaller fumaroles is produced by ≈100°C boiling of these mixed fluids and reservoir fluid. Steam from the Senator fumarole (SF) and from shallow wells penetrating the dead zone are probably derived by 140°C to 160°C boiling of reservoir fluid. Carbon-13 isotope data suggest that the reservoir CO 2 is produced mainly by thermal decarbonation of hydrothermal calcite in veins that cut reservoir rocks. Formation of the dead zone is linked to the reservoir pressure decline caused by continuous reservoir drawdown from 1986 to present. These reservoir changes have restricted flow and induced boiling in a subsurface hydrothermal outflow plume extending from the Stillwater Fault southeast toward the DV floor. We estimate that maximum CO 2 flux in the upflow zone along the Stillwater Fault in 1998 was roughly seven to eight times greater than the pre-production flux in 1986. The eventual decline in CO 2 flux reflects the drying out of the outflow plume.

The origin of the Cerro Prieto geothermal brine

Abstract The Cerro Prieto geothermal brine may have originated from mixing of Colorado River water with seawater evaporated to about six times its normal salinity. This mixture circulated deeply and was heated by magmatic processes. During deep circulation, Li, K, Ca, B, SiO 2 and rare alkalis were transferred from rock minerals to the water, and Mg, SO 4 , and a minor quantity of Na were transferred to the rock. Similar alteration of seawater salt chemistry has been observed in coastal geothermal systems and produced in laboratory experiments. After heating and alteration the brine was further diluted to its present range of composition. Oxygen isotopes in the fluid are in equilibrium with reservoir calcite and have been affected by exploitation-induced boiling and dilution.

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

Abstract Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO 2 , 0.5 mol% H 2 S, and 1 mol% other components. 3 He/ 4 He ratios indicate a deep magmatic source ( R / R a up to 6) whereas δ 13 C–CO 2 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 CO 2 and He, but depleted in H 2 S 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 CO 2 are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH 4 , H 2 , and H 2 S 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 N 2 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 CH 4 , N 2 and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).

Carbon isotope systematics and CO2 sources in The Geysers-Clear Lake region, northern California, USA

Abstract Carbon isotope analyses of calcite veins, organic carbon, CO 2 and CH 4 from 96 rock and 46 gas samples show that metamorphic calcite veins and disseminated, organically-derived carbon from Franciscan Complex and Great Valley Sequence rocks have provided a primary carbon source for geothermal fluids during past and present hydrothermal activity across The Geysers-Clear Lake region. The stable isotope compositions of calcite veins vary widely on a regional scale, but overall they document the presence of 13 C-poor fluids in early subduction-related vein-precipitating events. δ 13 C values of calcite veins from the SB-15-D corehole within The Geysers steam field indicate that carbon-bearing fluids in the recent geothermal system have caused the original diverse δ 13 C values of the veins to be reset. Across The Geysers-Clear Lake region the carbon isotope composition of CO 2 gas associated with individual geothermal reservoirs shows a general increasing trend in δ 13 C values from west to east. In contrast, δ 13 C values of CH 4 do not exhibit any spatial trends. The results from this study indicate that regional variations in δ 13 C–CO 2 values result from differences in the underlying lithologies. Regional CO 2 contains significant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH 4 . CO 2 from degassing of underlying magma chambers is not recognizable in this region.

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Abstract Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km 2 region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are ∼180° C , whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are ∼310° C . An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.

A geochemical model of the Platanares geothermal system, Honduras

Abstract Results of exploration drilling combined with results of geologic, geophysical, and hydrogeochemical investigations have been used to construct a geochemical model of the Platanares geothermal system, Honduras. Three coreholes were drilled, two of which produced fluids from fractured Miocene andesite and altered Cretaceous to Eocene conglomerate at 450 to 680 m depth. Large volume artesian flows of 160–165°C, predominantly bicarbonate water are chemically similar to, but slightly less saline than widespread boiling hot-spring waters. The chemistry of the produced fluid is dominated by equilibrium reactions in sedimentary rocks at greater depths and higher temperatures than those measured in the wells. Chemical, isotope, and gas geothermometers indicate a deep fluid temperature of 200–245°C and reflect a relatively short residence time in the fractures feeding the wells. Chloride-enthalpy relations as well as isotopic and chemical compositions of well discharges, thermal springs, and local cold waters support a conceptual model of ascending high-temperature (minimum 225°C) parent fluid that has cooled conductively to form the 160–165°C shallow (to 680 m) fluid encountered by the wells. The hot-spring waters are formed by boiling and steam loss from more or less conductively cooled parent fluid. The more dilute boiling spring waters (Cl = ∼32 mg/kg) have cooled from > 225°C to about 160°C by conduction and from 160°C to 98°C by boiling. The most concentrated boiling spring waters (Cl = 37 mg/kg) have cooled from > 225°C to about 200°C by conduction and from 200°C to 98°C by boiling. Intermediate concentrations reflect mixed cooling paths.

A geochemical reconnaissance of the Alid volcaniccenter and geothermal system, Danakil depression, Eritrea

Geological and geochemical studies indicate that a high-temperature geothermalsystem underlies the Alid volcanic center in the northern Danakil depression of Eritrea Alid is avery late-Pleistocene structural dome formed by shallow intrusion of rhyolitic magma some of which vented as lavas and pyroclastic flows Fumaroles and boiling pools distributed widelyover an area of ∼10 km2 on the northern half of Alid suggest that an activehydrothermal system underlies much of that part of the mountain Geothermometers indicate thatthe fumarolic gases are derived from a geothermal system with temperatures >225°C Theisotopic composition of condensed fumarolic steam is consistent with these temperatures andimplies that the source water is derived primarily from either lowland meteoric waters or fossilRed Sea water or both Some gases vented from the system (CO2 H2Sand He) are largely magmatic in origin Permeability beneath the volcanic center may be high given the amount of intrusion-related deformation and the active normal faulting within theDanakil depression

Geothermal gas compositions in yellowstone National Park, USA

Abstract Gas samples collected between 1974 and 1986 have been analysed for the ten major components. Samples have been collected almost exclusively from the tops of pools, which has degraded the value of the data, and limited inter-comparisons to the relatively insoluble components, Ar, N 2 , CH 4 , H 2 and He. A general gas distribution pattern in the park, in terms of these components, shows the major heat source(s) to underlie the Gibbon and Mud Volcano areas with all other geothermal areas having gas compositions consistent with a general north-south water flow. Shoshone Basin gases show a large range of compositions and these are analysed in detail. The patterns conform to that which would be expected from an east-west flow or fluid with progressive boiling and subsequent dilution.

Tecuamburro Volcano, Guatemala: exploration geothermal gradient drilling and results

Results of geological, volcanological, hydrogeochemical, and geophysical field studies conducted in 1988 and 1989 at the Tecuamburro geothermal site, Guatemala, indicate that there is a substantial shallow heat source beneath the area of youngest volcanism. Gases from acid-sulfate springs near Laguna Ixpaco consistently yield maximum estimated subsurface temperatures of 300°C. To obtain information on subsurface temperatures and temperature gradients, stratigraphy, fracturing, hydrothermal alteration, and hydrothermal fluids, a geothermal gradient core hole (TCB-1) was drilled to 808 m low on the northern flank of the Tecuamburro Volcano complex. The hole is located 300 m south of a 300m-diameter phreatic crater. Laguna Ixpaco, dated at 2910 years. TCB-1 temperature logs do not indicate isothermal conditions at depth and the calculated thermal gradient from 500–800 m is 230°C/km. Bottom hole temperature is close to 240°C. Calculated heat flow values are around 350–400 mW/m2. Fluid-inclusion and secondary-alteration studies indicate that veins and secondary minerals were formed at temperatures equal to or slightly less than present temperatures; thus, the Tecuamburro geothermal system may still be heating up. The integration of results from the TCB-1 gradient core hole with results from field studies provides strong evidence that the Tecuamburro area holds great promise for geothermal resource development.