Fluid-inclusion evidence for past temperature fluctuations in the Kilauea East Rift Zone geothermal area, Hawaii

Heating and freezing data were obtained for fluid inclusions in hydrothermal quartz, calcite, and anhydrite from several depths in three scientific observation holes drilled along the lower East Rift Zone of Kilauea volcano, Hawaii. Compositions of the inclusion fluids range from dilute meteoric water to highly modified sea water concentrated by boiling. Comparison of measured drill-hole temperatures with fluid-inclusion homogenization-temperature (T_h) data indicates that only about 15% of the fluid inclusions could have formed under the present thermal conditions. The majority of fluid inclusions studied must have formed during one or more times in the past when temperatures fluctuated in response to the emplacement of nearby dikes and their subsequent cooling. The fluid-inclusion data indicate that past temperatures in SOH-4 well were as much as 64°C hotter than present temperatures between 1000 and 1500 m depth and they were a maximum of 68°C cooler than present temperatures below 1500 m depth. Similarly, the data show that past temperatures near the bottoms of SOH-1 and SOH-2 wells were up to 45 and 59°C, respectively, cooler than the present thermal conditions; however, the remainder of fluid-inclusion T_h values for these two drill holes suggest that the temperatures of the trapped waters were nearly the same as the present temperatures at these slightly shallower depths. Several hydrothermal minerals (erionite, mordenite, truscottite, smectite, chlorite-smectite, chalcedony, anhydrite, and hematite), occurring in the drill holes at higher temperatures than they are found in geothermal drill holes of Iceland or other geothermal areas, provide additional evidence for a recent heating trend.     

Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapán geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapán area was carried out during August-November 1988 and October 1991–April 1992. The objective was to use the study of microearthquakes as an exploration tool to invvestigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapán geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991–1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapán geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests.     

Drilling and evaluation of Ascension #1, a geothermal exploration well on Ascension Island, South Atlantic Ocean

A deep (3126 m) geothermal exploration well (Ascension #1) was drilled on Ascension Island in the South Atlantic Ocean as the culmination of an exploration program that began in 1982. Ascension #1 encountered several geothermal fluid entries below a depth of 2400 m, and had a bottomhole temperature approaching 250°C. However, the fluid flow rate was limited. While attempting to improve production by drilling a second leg, a mechanical failure resulted in loss of the well. An analysis of the geologic controls on fluid production suggests that fracture permeability is oriented to the northeast and often associated with felsic dikes. The system may be sealed by a thick sequence of hyaloclastites that are mechanically unable to sustain open fractures. The reservoir intersected by Ascension #1 apparently lacks the permeability required for commercial fluid production.     

Isotopic and fluid inclusion study of hydrothermal and metamorphic carbonates in the Larderello geothermal field and surrounding areas, Italy

Fluid inclusions have been studied on six calcite veins from the shallow part (480 to 1515 m below ground level) of the Larderello geothermal field and outcropping in peripheral zones of the geothermal area. Oxygen and carbon isotopic analyses have been carried out on these carbonate veins, as well as on the dolostone layers found inside the Paleozoic metamorphic units of the deep part of the field (from 1939 to 3177 m below ground level). Fluid inclusion observations suggest that boiling processes probably occurred during calcite precipitation in most of the veins. The fluids that formed or interacted with the calcite veins below the uppermost reservoir (made up of Mesozoic marine carbonates), and with the calcite hydrothermal veins of Sassa, were characterised by an apparent salinity from 1.3 to 5.3 wt.% NaCl eq. and a homogenisation temperature from 137 to 245°C. The fluid inclusions related to the calcite veins hosted above the uppermost reservoir show a wide range of apparent salinity (from 1.7 to 22.2 wt.% NaCl eq.) and homogenisation temperatures from 224 to 296°C. Apparent salinity/homogenisation temperature covariations of the latter veins are interpreted as being the result of a mixing process between a low-temperature, high-salinity fluid and a higher-temperature, moderate-salinity fluid. The oxygen isotopic compositions of the calcite veins (δ¹⁸O from 10.34 to 11.45‰) located below the Mesozoic carbonates and in the outcrops (δ¹⁸O from 9.42 to 17.07‰) indicate that the vapour in equilibrium with these veins was isotopically similar to the present-day discharge steam. The aqueous fluids in equilibrium with these veins could be meteoric water that interacted with the Mesozoic carbonates of the upper reservoir. The δ¹³C values of the CO₂ produced at Larderello and the constant concentration of this gas over time are, however, indicative of a deep source inside the reservoir that is probably related to the decarbonation reaction within the metamorphic units that form the present-day deep reservoir. Fluid inclusion salinities (up to 22.2 wt.% NaCl eq.) and isotopic results (δ¹⁸O from 13.43 to 21.99‰, δ¹³C between −1.26 and −0.18‰) on the calcite veins hosted above the uppermost reservoir suggest that the water circulating in these veins has strongly interacted with Mesozoic carbonates or Neogene sediments containing evaporite layers. The isotopic values (δ¹⁸O from 14.09 to 19.91‰, δ¹³C from −4.09 to 1.90‰) of dolomite samples present in the Paleozoic metamorphic rocks indicate a reaction with fluid of variable temperatures under different water/rock ratios. The isotopic composition of one sample reveals equilibrium with present-day discharge fluids. This fact aside, the remaining data indicate that the Paleozoic dolomitic layers do not seem to contribute significantly to the production of CO₂.     

Fluid inclusion study of the Kirishima geothermal system, Japan

Gases from fluid inclusions in quartz and anhydrite were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite occur in hydrothermal veins in volcanic and pelitic rocks collected from geothermal wells in the Kirishima area, southwest Japan. The geothermal wells are located in a graben made up of Quaternary volcanic rocks underlain by sedimentary rocks of the Shimanto Group.Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H₂O and a variable but small amount of CO₂. CH₄ and other hydrocarbons are also detected in inclusions in a hydrothermal sample from the pelitic Shimanto Group. Peak ratios of CO₂/H₂0 in individual fluid inclusions are variable in some samples. This indicates that there is a difference in gas compositions of the fluid inclusions, and suggests that the inclusions were formed in multistages or trapped heterogeneous boiling fluids.Results of bulk analyses show that the inclusions are mainly composed of H₂O (98–99 mol%) with small amounts of non-condensable gases, mainly C0₂ and N₂, CH₄ and Ar. The proportion of N₂ is about one order of magnitude lower than C0₂, CH₄ is generally two orders of magnitude lower than C0₂ and Ar is just above the detection limit of the mass spectrometer. The gas concentration in the fluid inclusions is much higher than that in the present-day discharge fluids in this area. CO₂/N₂ and C0₂/CH₄ ratios of the fluid inclusions from the volcanic rocks are lower than those of the present-day discharge fluids. CO₂/N₂ and CO₂/CH₄ ratios in residual fluids increase with progressive degassing, because N₂ and CH₄ are released from the residual fluids more easily than CO₂. Thus, the difference in the CO₂/N₂ and CO₂/CH₄ ratios between the fluid inclusions and the present-day discharge fluids in the Kirishima area may be ascribed to the degree of degassing, and the fluid inclusions in the area were probably formed by trapping fluids that were weakly influenced by degassing. P_co2, values calculated from the gas compositions of the fluid inclusions are higher than that of buffer systems involving alteration minerals in the area. This suggests that the fluid inclusions might be trapped fluids which were not in equilibrium with the alteration mineral assemblages, that is, fluids prior to considerable degassing and alteration.     

An oxygen isotope study of silicates in the larderello geothermal field, Italy

Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The ∂¹⁸O values obtained for the most retentive minerals, quartz and tourmaline, are from + 12.0‰ to + 14.7‰ and 9.9‰, respectively, and indicate deposition from an ¹⁸O-rich fluid. Calculated ∂¹⁸O values for these fluids range from + 5.3‰ to + 13.4‰. These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.     

Genesis of the plutonic-hydrothermal system around quaternary granite in the kakkonda geothermal system, Japan

The Kakkonda plutonic-hydrothermal system has as its heat source the Quaternary Kakkonda granite. The Kakkonda granite has a thick (1.3 km) contact-metamorphic zone, known mainly from the geothermal survey well WD-1a (total depth: 3729 m) drilled by the New Energy and Industrial Technology Development Organization (NEDO). The Kakkonda granite is a stock several tens of square kilometers in area with an upper contact about 1.5–3 km deep. It is a composite pluton varying from tonalite to granite. The early-stage granitic rocks are slightly metamorphosed to biotite grade by late-stage granitic rocks. K-Ar ages of separated minerals from the granitic rocks in both stages show the same cooling ages of 0.24–0.11 Ma for hornblende, 0.21–0.02 Ma for biotite, and 0.14–0.01 Ma for potassium feldspar. These are the youngest ages for granite in the world. The K-Ar ages become almost zero at 580°C for biotite and potassium feldspar, and at 350°C for illite. The Kakkonda granite intruded into a regional stress field in which the minimum principal stress was ENE–WSW and nearly horizontal. The regional stress field coincides with that of a previously recognized F2 fracture system before 0.4–0.3 Ma. Both stages of the Kakkonda granite and the contact aureole are fractured by recent tectonism, resulting in a zone of hydrothermal convection from about 2.5–3.1 km depth up to the surface. The boundary between the zone of hydrothermal convection and the underlying zone of heat conduction occurs 250–550 m below the upper contact of the Kakkonda granite, and has a temperature of 380–400°C.     

The origin of hypersaline liquid in the quaternary kakkonda granite, sampled from well WD-1a, kakkonda geothermal system, Japan

Hypersaline metal-rich liquid (ca. 40 wt% total chloride species) was obtained from a depth of 3708 m in the Kakkonda geothermal system. Sampling of well WD-1a was conducted by reverse circulation after a standing time of about 196 hours (with temperature recovering to >500°C). Tritium content and the relationship between δD and δ¹⁸O showed that the river water that was circulated in the well had mixed with an isotopically heavy fluid during the standing time. Phase separation occurred during temperature recovery, concentrating the hypersaline liquid in the bottom of the well. This original hypersaline liquid has a salinity of about 55 wt% NaCl eq., consisting of Na–Fe–K–Mn–Ca chloride, rich in Zn and Pb but poor in Cu, Au and Ag. The fluid originates from the Kakkonda granite and mixed with circulating water from the well in a zone of fine fractures induced by thermal stress during drilling.     

Application of fluid inclusions to the study of Bagnore geothermal field (Tuscany, Italy)

A fluid inclusion study of the hydrothermal minerals in two breccias from two wells in the Bagnore geothermal field (Italy) has provided information on the evolution of the fluids, and has also demonstrated that fluid inclusions can be utilized as geothermometers in this geothermal field. Both breccias come from reservoir zones: one (Bagnore 3bis (Bg 3bis)) was cored at a depth of 3111 m below ground level (b.g.l.), whereas the other (Bagnore 22 (Bg 22)) was ejected during a blow-out, probably from a fractured zone present between 2200 and 2300 m b.g.l. The hydrothermal cement of the breccias is mostly made up of quartz, K-feldspar, Na-rich plagioclase, calcite, chlorite and illite. Fluid inclusion studies were carried out on quartz (Bg 3bis and Bg 22 breccias) and adularia (Bg 22 breccia). Three types of fluid inclusions were recognized in the Bg 3bis breccia. Type I (liquid-rich) inclusions trapped an aqueous fluid with a CO₂ concentration (1.7–2.7 mol/kg) that is significantly higher than present-day fluids (0.5 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with less CO₂ (0.6–1.0 mol/kg). Type III (vapor-rich) coexist with type I inclusions, and record an early fluid circulation under boiling conditions. The decrease of the CO₂ (and total gas) concentrations from type I inclusions to type II inclusions, and on to present-day conditions can be related to boiling with gas loss and/or mixing. Only one type of fluid inclusion (type II), with moderate CO₂ concentration (0.7–0.3 mol/kg), was found in the Bg 22 breccia. Boiling and/or mixing explain the variation of the CO₂ content in the Bg 22 reservoir fluid from inclusion formation to modern CO₂ concentration (0.3 mol/kg). The absence of any type I inclusions in Bg 22 breccia may be related to non-uniform CO₂ concentrations in different parts of the field. Present-day temperatures (295±10 °C for Bg 3bis and 320±10 °C for Bg 22) are close or equal to fluid inclusion average total homogenization temperatures (around 290 °C for Bg 3bis and 320 °C for Bg 22), suggesting that fluid inclusions can be useful for estimating local temperatures when direct measurements are not available or dubious.     

Isotopic zoning and origin of the aquifers in the discharge area of the geothermal fields of ahuachapán and Chipilapa, El Salvador

The northern discharge areas of the Ahuachapán, and Chipilapa geothermal fields can be subdivided into four different zones based on their structural position, and the isotopic and chemical composition of their waters. In general, the contribution of geothermal waters from these two fields was estimated to be less than 10%. Elevation effects are of little importance, whereas a slight trend towards higher isotopic values with increasing water temperatures may exist.The NNW-SSE-trending Escalante and Agua Caliente faults represent lateral groundwater barriers, and provide vertical conduits for the ascending geothermal waters. The western discharge areas seem to be more influenced by the Ahuachapán, geothermal field, whereas those to the east are more influenced by the Chipilapa field.Groundwaters in the Northern Plain are mainly from shallow northward-flowing aquifers. These waters show temperature effects, mixing with geothermal waters and are affected by the geology of the area. However, none of these factors alone can explain the isotopic variations observed in the waters of the northern discharge areas.     

Sustainability analysis of the Ahuachapán geothermal field: management and modeling

The Ahuachapán geothermal field (AGF) is located in north western El Salvador. To date, 53 wells (20 producers and 8 injectors) have been drilled in the Ahuachapán geothermal field and the adjacent Chipilapa area. Over the past 33 years, 550 Mtonnes have been extracted from the reservoir, and the reservoir pressure has declined by more than 15 bars. By 1985, the large pressure drawdown due to over-exploitation of the resource reduced the power generation capacity to only 45 MW_e. Several activities were carried out in the period 1997–2005 as part of “stabilization” and “optimization” projects to increase the electric energy generation to 85 MW_e, with a total mass extraction of 850 kg/s.     

Chemical and isotopic composition of geothermal discharges from the Puyehue-Cordón Caulle area (40.5°S), Southern Chile

The Puyehue-Cordón Caulle area (40.5°S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top (1500 m) and boiling springs at the northwest end (1000 m). In the latter, the alkaline-bicarbonate composition of the springs with low Mg (<0.06 mg/l) relative to the local meteoric waters (5 mg/l), low chloride (<60 mg/l), high silica (up to 400 mg/l) and δ¹⁸O–δD values close to the Global Meteoric Water Line (GMWL), in combination with the large outflow (100 l/s), suggest the existence of a secondary steam-heated aquifer overlying a main vapor-dominated system at Cordón Caulle. Subsurface temperatures of the secondary aquifer are estimated to be about 170–180 °C (corrected silica geothermometers). The Puyehue thermal area, on the other hand, includes Mg-rich hot springs discharging along stream valleys, with maximum temperatures of 65 °C and a δ¹⁸O–δD signature resembling the local meteoric composition, which suggests that the surface manifestations contain a reservoir component that is strongly diluted by meteoric waters. Topographic/hydrologic and chemical characteristics suggest that Cordón Caulle and Puyehue represent two separate upflows.     

Gas geochemistry of the Cordón Caulle geothermal system,Southern Chile

The Cordón Caulle geothermal system is located in a NW-trending volcano-tectonic depression of the Southern Andean Volcanic Zone of Chile. Outflows of low chloride water were previously interpreted as the surface expression of a shallow steam-heated aquifer, with subsurface temperatures of 150–170 °C. Gas data from fumaroles and hot springs have been used to assess the nature and temperature of the deeper, underlying geothermal reservoir. Fumaroles at the northeastern border of Cordón Caulle have ³He/⁴He ratios typical of subduction margins (6–7 R_A) and N₂/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N₂/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 °C, whereas those of hot spring waters suggest temperatures of about 160 °C. Based on these data, and comparisons with well documented liquid and vapor-dominated geothermal systems, a model is proposed of a boiling liquid-dominated geothermal system overlain by a secondary steam-heated aquifer.     

Hydrology,hydrochemistry and geothermal potential of El Chichón volcano-hydrothermal system,Mexico

Fluid and heat discharge rates of thermal springs of El Chichón volcano were measured using the chloride inventory method. Four of the five known groups of hot springs discharge near-neutral Na–Ca–Cl–SO₄ waters with a similar composition (Cl ∼ 1500–2000 mg kg⁻¹ and Cl/SO₄ ∼ 3) and temperatures in the 50–74 °C range. The other group discharges acidic (pH 2.2–2.7) Na–Cl water of high salinity (>15 g/L). All five groups are located on the volcano slopes, 2–3 km in a straight line from the bottom of the volcano crater. They are in the upper parts of canyons where thermal waters mix with surface meteoric waters and form thermal streams. All these streams flow into the Río Magdalena, which is the only drainage of all thermal waters coming from the volcano. The total Cl and SO₄ discharges measured in the Río Magdalena downstream from its junction with all the thermal streams are very close to the sum of the transported Cl and SO₄ by each of these streams, indicating that the infiltration through the river bed is low. The net discharge rate of hydrothermal Cl measured for all thermal springs is about 468 g s⁻¹, which corresponds to 234 kg s⁻¹ of hot water with Cl = 2000 mg kg⁻¹. Together with earlier calculations of the hydrothermal steam output from the volcano crater, the total natural heat output from El Chichón is estimated to be about 160 MWt. Such a high and concentrated discharge of thermal waters from a hydrothermal system is not common and may indicate the high geothermal potential of the system. For the deep water temperatures in the 200–250 °C range (based on geothermometry), and a mass flow rate of 234 kg s⁻¹, the total heat being discharged by the upflowing hot waters may be 175–210 MWt.     

Effects of the “pays de bray” fault on fluid paleocirculations in the Paris basin dogger reservoir, France

Clay fractions and fluid inclusions were studied in the Aulnay sous Bois and Cerneux (CER-1 and CER-P6) wells located 10 km, 1.3 and 0.6 km, respectively, from the “Pays de Bray” fault (Paris Basin, France). It was shown that a connection was probably established between the Dogger and Triassic reservoirs during the active period of the fault. Chemical and heat transfers have locally modified the diagenesis conditions in the Dogger formation, inducing an overmaturation of clay minerals. These effects, although attenuated, are still identifiable in the Aulnay and CER-1 wells. A maximum temperature of about 90°C was maintained in CER-P6 site even during the uplift stage of the basin.     

Tectonic and magmatic evolution of the active volcanic front in El Salvador: insight into the Berlín and Ahuachapán geothermal areas

In El Salvador, Central America, active deformation takes the form of a major dextral strike-slip fault system, the El Salvador Fault Zone, resulting from the oblique subduction of the Cocos Plate. The fault system is laterally discontinuous, being subdivided into different major en-echelon segments that partially overlap to form pull-apart structures. Volcanic activity is spatially confined to the fault segments and absent in the intervening pull-apart basins; no significant temporal gap exists in the erupted products, at least during the Plio-Quaternary. Detailed analyses within the geothermal fields of Berlín and Ahuachapán have revealed important volcano-structural and petrologic differences between the two areas. In the Berlín area active deformation is controlled by the regional transcurrent stress field, resulting in the development of systems of right-lateral E–W-trending strike-slip faults. Conversely, the structural setting of the Ahuachapán area is more complex, reflecting an interaction among different stress fields. Berlín products exhibit a marked geochemical and isotopic homogeneity indicating the presence of a single magmatic system. At Ahuachapán, on the other hand, the rocks display significant variations in both Sr isotopes and the LILE/HFSE ratios: this area is characterized by multiple volcanic centres, fed by different magma batches that reach the surface without reciprocal interactions in shallow reservoirs. Thus, the characteristics of the volcanic products at Berlín and Ahuachapán reflect their different tectonic settings, with important implications for geothermal investigations.     

High-temperature measurements in well WD-1A and the thermal structure of the kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their Deep Seated Geothermal Resources Survey project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414°C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500°C at 3500 m depth. Temperatures between 500°C and 510°C were also confirmed at 3720 m depth after 129–159 h standing time, using calibrated melting .tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230°C to 260°C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350°C to 360°C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.     

Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.     

Chemical description of spring waters from the Tutupaca and Rio Calientes (Peru) geothermal zones

Analysis of chemical data from 34 spring samples from the Tutupaca and Río Calientes (Republic of Perú) geothermal zones is presented in this paper. The main objective of the work was to characterize geothermal resources to be exploited in the future. The investigated zones were: Tutupaca and Río Calientes‐Ancocollo‐Challapalca. Thermal waters from Tutupaca showed low pH values and they were classified as sulphate type waters. Thermal springs from the Río Calientes zone showed almost neutral pH values and they were classified as sodium chloride type waters with a probably geothermal origin. Reservoir temperatures were estimated and the results indicated that water–rock equilibrium in the hydrothermal system was not completely attained. Taking into account the chemical composition of some equilibrated or ‘mature’ waters from the Río Calientes zone, reservoir temperatures in the range of 210–240°C were estimated. Mixing models based on silica, chloride and specific enthalpy of the samples were used to estimate the composition of the reservoir liquid. Isotopic data showed oxygen‐18 shift for the waters from Río Calientes‐Ancocollo regarding the waters from Tutupaca zone which were found on the world meteoric line. Copyright © 1999 John Wiley & Sons, Ltd.     

Magnetostratigraphy of the Ahuachapán-Chipilapa geothermal field, El Salvador

The volcanic stratigraphy for the Ahuachapán-Chipilapa geothermal field is defined on the basis of the magnetostratigraphic results on 156 oriented samples from 33 sites. The magnetostratigraphic sequence shows that the major volcanism associated with the Concepción de Ataco caldera and the Cuyanausul volcano took place during the middle Brunhes chron (Quaternary). Pre-caldera activity of small centers such as Empalizada and Apaneca in the southern sector of the field occurred during the early Brunhes (0.77±0.07 Ma). Basaltic-andesitec activity associated with the Cuyanausul volcano took place earlier, i.e. during the Matuyama chron, possibly around 1.3±0.15 and 1.7±0.3 Ma.The local igneous basement is composed of Late Miocene-Pliocene andesites, ignimbrites and volcano-sedimentary deposits. Normal polarities and a K---Ar date of 7.37±0.73 Ma indicate that the volcanic activity in the study area extends beyond the Gauss chron. The polarity of some of the units in the post-caldera sequence and in the Concepción de Ataco and Cuyanausul sequences suggests that they may have recorded short polarity subchrons.