Chemical and isotopic investigation of warm springs associated with normal faults in Utah

Thermal springs associated with normal faults in Utah have been analyzed for major cations and anions, and oxygen and hydrogen isotopes. Springs with measured temperatures averaging greater than 40°C are characterized by Na + K- and SO₄ + Cl-rich waters containing 10³ to 10⁴ mg/l of dissolved solids. Lower temperature springs, averaging less than 40°C, are more enriched in Ca + Mg relative to Na + K. Chemical variations monitored through time in selected thermal springs are probably produced by mixing with non-thermal waters. During the summer months at times of maximum flow, selected hot springs exhibit their highest temperatures and maximum enrichments in most chemical constituents.Cation ratios and silica concentrations remain relatively constant through time for selected Utah thermal springs assuring the applicability of the geothermometer calculations regardless of the time of year. Geothermometer calculations utilizing either the quartz (no steam loss), chalcedony or Mg-corrected Na/K/Ca methods indicate that most thermal springs in Utah associated with normal faults have subsurface temperatures in the range of 25 to less than 120°C. This temperature range suggests fluid circulation is restricted to depths less than about three kilometers assuming an average thermal gradient of about 40°C/km.Thermodynamic calculations suggest that most thermal springs are oversaturated with respect to calcite, quartz, pyrophyllite, (Fe, Mg)-montmorillonite, microcline and hematite, and undersaturated with respect to anhydrite, gypsum, fluorite and anorthite. Chalcedony and cristobalite appear to be the only phases consistently at or near saturation in most waters. Theoretical evaluation of mixing on mineral saturation trends indicates that anhydrite and calcite become increasingly more undersaturated as cold, dilute groundwater mixes with a hot (150°C), NaCl-rich fluid. The evolution of these thermal waters issuing from faults appears to be one involving the dissolution of silicates such as feldspars and micas by CO₂-enriched groundwaters that become more reactive with increasing temperature and/or time. Solution compositions plotted on mineral equilibrium diagrams trend from product phases such as kaolinite or montmorillonite toward reactant phases dominated by alkali feldspars.Isotopic compositions indicate that these springs are of local surface origin, either meteoric (low TDS, < 5000 mg/l) or connate ground water (high TDS, > 5000 mg/l). Deviations from the meteoric water line are the result of rock-water isotopic exchange, mixing or evaporation. Fluid source regions and residence times of selected thermal spring systems (Red Hill, Thermo) have been evaluated through the use of a σ D-contour map of central and western Utah. Ages for waters in these areas range from about 13 years to over 500 years. These estimates are comparable to those made for low-temperature hydrothermal systems in Iceland.     

Geochemistry and geothermometry of spring water from the Blackfoot Reservoir region, southeastern Idaho

The Blackfoot Reservoir region in southeastern Idaho is recognized as a potential geothermal area because of the presence of several young rhyolite domes (50,000 years old), Quaternary basalt flows, and warm springs. North- to northwest-trending high-angle normal faults of Tertiary to Holocene age appear to be the dominant structural control of spring activity. Surface spring-water temperatures average 14°C except for a group of springs west of the Reservoir Mountains which average 33°C. Chemical geothermometers applied to fifty water samples give temperatures less than 75°C except for eight springs along the Corral Creek drainage. The springs along Corral Creek have Na-K-Ca temperatures that average 354°C, a direct result of high potassium concentrations in the water. A correction for carbon dioxide applied to the Na-K-Ca geothermometer lowers the estimated temperatures of the anomalous springs to near the measured surface temperatures, and Na-K-Ca-Mg temperatures for the anomalous springs are near 100°C. Mixing model calculations suggest that hot water with a temperature of approximately 120°C may be mixing with cooler, more dilute water in the springs from the Corral Creek drainage, a temperature supported by Na-K-Ca-Mg temperatures and mineral saturation temperatures.Stability relations of low-temperature phases in the system indicate that the large concentrations of potassium in the eight anomalous springs are derived from reactions with the potassium-bearing minerals muscovite and K-feldspar. Carbon dioxide and hydrogen sulfide gases may be derived through the oxidation of organic matter accompanied by the reduction of sulfate. Concentrations of major and minor elements, and gases found in springs of the Blackfoot Reservoir region are due to water-rock reactions at temperatures less than 100°C.Based on spring geochemistry, a geothermal reservoir of 100°C up to 120°C may exist at shallow (less than 2 km) depths in the Blackfoot Reservoir region.     

Thermal segmentation along the N. Ecuador–S. Colombia margin (1–4°N): Prominent influence of sedimentation rate in the trench

Along the deformation front of the North Ecuador–South Colombia (NESC) margin, both surface heat flow and trench sediment thickness show prominent along-strike variations, indicating significant spatial variations in sedimentation rate. Investigating these variations helps us address the important question of how trench sedimentation influences the temperature distribution along the interplate contact and the extent of the megathrust seismogenic zone. We examine this issue by analysing 1/ a new dense reflection data set, 2/ pre-stack depth migration of selected multichannel seismic reflection lines, 3/ numerous newly-identified bottom-simulating reflectors and 4/ the first heat probe measurements in the region. We develop thermal models that include sediment deposition and compaction on the cooling oceanic plate as well as viscous corner flow in the mantle wedge. We estimate that the temperature from 60–150 °C to 350–450 °C, commonly associated with the updip and downdip limits of the seismogenic zone, extends along the plate interface over a downdip distance of 160 to 190 ± 20 km. We conclude that the updip limit of the seismogenic zone for the great megathrust earthquake of 1979 is associated with low-temperature (60–70 °C) processes. Our models also suggest that 60–70% of the two-fold decrease in measured heat flow from 3°N to 2.8°N is related to an abrupt southward increase in sedimentation rate in the trench. Such a change may potentially induce a landward shift of the 60–150 °C isotherms, and thus the updip limit of the seismogenic zone, by 10 to 20 km.     

Chemical and isotopic prediction of aquifer temperatures in the geothermal system at Long Valley, California

Temperatures of aquifers feeding thermal springs and wells in Long Valley, California, estimated using silica and Na-K-Ca geothermometers and warm spring mixing models, range from 160/dg to about 220°C. This information was used to construct a diagram showing enthalpy-chloride relations for the various thermal waters in the Long Valley region. The enthalpy-chloride information suggests that a 282 ± 10°C aquifer with water containing about 375 mg chloride per kilogram of water is present somewhere deep in the system. That deep water would be related to 220°C Casa Diablo water by mixing with cold water, and to Hot Creek water by first boiling with steam loss and then mixing with cold water. Oxygen and deuterium isotopic data are consistent with that interpretation. An aquifer at 282°C with 375 mg/kg chloride implies a convective heat flow in Long Valley of 6.6 × 10⁷ cal/s.     

On the evolution of the geothermal regime of the North China Basin

Recent heat flow and regional geothermal studies indicate that the North China Basin is characterized by relatively high heat flow compared with most stable areas in other parts of the world, but lower heat flow than most active tectonic areas. Measured heat flow values range from 61 to 74 mW m⁻². The temperature at a depth of 2000 m is generally in the range 75 to 85°C, but sometimes is 90°C or higher. The geothermal gradient in Cenozoic sediments is in the range 30 to 40°C/km for most of the area. The calculated temperature at the Moho is 560 and 640°C for surface heat flow values of 63 and 71 mW m⁻², respectively. These thermal data are consistent with other geophysical observations for the North China Basin. Relatively high heat flow in this area is related to Late Cretaceous-Paleogene rifting as described in this paper.     

Long-term underground temperature measurements at Syowa Station, East Antarctica

Underground temperature measurements in two shallow boreholes have been carried out by the Japanese Antarctic Research Expedition at Syowa Station, East Antarctica from April, 1981 to January, 1985. Two quartz thermometers were installed in the first borehole at depths of 2 and 5 m and three were in the second one at depths of 1, 4 and 6.8 m. The mean underground temperatures in the first borehole were −8.181 and −8.843°C at depths of 2 and 5 m, and in the second one −8.242 and −8.220°C at depths of 4 and 6.8 m. As the mean air temperature at Syowa Station was −10.8°C, the underground temperature in the 2 −6.8 m depth range is about two degrees higher than the air temperature. The thermal diffusivities of the observation area are determined by the same principle of the Ångström method, using long-term underground temperature records. The thermal diffusivity around Syowa Station is established to be about two times larger than those of ordinary igneous and metamorphic rocks measured in the laboratory. The thermal conductivity of the drilled cores and surrounding outcropping rocks are also measured by the transient method with temperature conditions at +23°C and −20°C. The thermal conductivities measured in various samples at −20°C are about 7% larger than those at +23°C. Thes tendency is consistent with the results of holocrystalline rock experiments.     

Microearthquake survey in the hot spring area in Parbati Valley (Himachal Pradesh, India)

A number of hot springs occur in the Parbati Valley in Himachal Pradesh in India. Temperatures range from 21 to 96°C, the boiling point of water at that altitude. Geological conditions, temperature variations and chemical composition of spring water in the Parbati Valley hydrogeological unit indicate that the deep thermal fluids are of meteoric origin. The maximum temperature acquired by water during its circulation is estimated to come close to 200°C. In order to assess the possibility of extracting geothermal energy, a seismic survey was arranged to locate the hypocentres of microearthquakes associated with the thermal source. A total of eight microearthquake units was set up at an interstation spacing of about 10 km and two months recording were obtained. During this period an average of 2–3 events per day was recorded with S—P interval less than 5 seconds. The data have been analysed with the help of Hypo 71, a Fortran IV computer program designed to determine the hypocentral parameters of earthquakes from seismic data. The results indicate faulting but there is no apparent spatial relationship to surface manifestations of geothermal energy.     

Chemical and physical studies of type 3 chondrites, VII. Annealing studies of the Dhajala H3.8 chondrite and the thermal history of chondrules and chondrites

Samples of the Dhajala H3.8 chondrite have been annealed for 10 hours at 600, 700, 800, 900 and 1000°C and at 1000°C for 1, 2, 20 and 100 h and their thermoluminescence (TL) properties measured. The TL sensitivity decreased by a factor of 2 after annealing at < 900°, but at higher temperatures fell by an order of magnitude. An abrupt increase in the temperature of the TL peak from 172 ± 9°Cto231 ± 8°C and a steady increase in the width of the peak from 169 ± 7°Cto212 ± 5°C were caused by the annealing treatment. The TL phosphor in Dhajala is thought to be feldspar predominantly in the high-temperature (disordered) form, but the present data indicate that a contribution from the low-temperature form is also present and that this low-temperature component is converted to the high form by the annealing treatment. The low-temperature feldspar is located in a few of the chondrules ( 20% of those separated from the meteorite) which are also noteworthy for having high TL sensitivities. These chondrules must have suffered greater crystallization of their mesostasis than the other chondrules, and equilibrated to lower temperatures. It is argued that, for compositional reasons, their mesostasis constituted less of a barrier to diffusion and therefore equilibration. Presumably the post-metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization, and the width and temperature of the TL peaks for petrologic types 3.5–3.9 are somehow related to cooling rate. Based on TL, there is no indication of a correlation between petrologic type and cooling rate for types 3.5–3.9; this is not consistent with a simple, single internally heated meteorite parent body.     

Diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks

Recently obtained data on oxygen diffusion in feldspars, quartz, and hornblende permit the prediction of the apparent¹⁸O¹⁶O temperatures that would be measured in a rock that consisted only of those three minerals, and cooled slowly from high temperature. The computed temperatures would be based on the differences in the¹⁸O¹⁶O ratios between coexisting pairs of minerals. The present calculation takes into account the diffusion rates for oxygen as a function of temperature, the cooling rate of the rock, the mineral grain sizes, and the mode of the rock. For mineral grains 1 mm in radius, and a cooling rate of 10°C/m.y., the minimum difference in apparent temperature between quartz-feldspar and feldspar-hornblende pairs will be 115°C, despite the assumption of a normal, uneventful, slow cooling history to room temperature. Further, the apparent quartz-hornblende temperature will range over 30°C (590–620°C) depending on the mode of the rock. For a cooling rate of 1000°C/m.y., the apparent difference in temperature can be as much as 400°C. Consequently, consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high-grade metamorphic rocks or igneous rocks which are cooled slowly. Departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature, or a complex history for the rock. For some minerals, including hornblende, the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens. The choice of the specimens used for such calibrations needs to be re-evaluated in light of these findings. This may result in a change in the equilibrium equation constants.An example from the literature, the San Jose tonalite, Baja California, Mexico, was modelled and yieldsδ¹⁸O concentrations in the minerals that correspond closely with the measured values. This suggests that the model used is appropriate, that the rock has had a simple thermal history, and that it cooled at 100–200°C/m.y. over the temperature range 800–500°C. The set of paleotemperatures obtained for a rock will, in general, yield neither the mineral closure temperatures nor the formation or crystallization temperatures. On the other hand, the cooling rate of the rock may be derived from the data. This, in turn, may have important tectonic implications with regard to denudation and uplift rates.     

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.     

Hydrogeochemical outline of thermal waters and geothermometry applications in Anatolia (Turkey)

The chemical compositions of a total of 120 thermal water samples from four different tectonically distinct regions (Central, North, East and West Anatolia) of Turkey are presented and assessed in terms of geothermal energy potential of each region through the use of chemical geothermometers. Na–Ca–HCO₃ type waters are the dominant water types in all the regions except that Na–Cl type waters are typical for the coastal areas of West Anatolia and for a few inland areas of West and Central Anatolia where deep water circulation exists. The discharge temperature of the springs ranges up to 100°C, and the bottom-hole temperatures in drilled wells up to 232°C. Geothermometry applications yield reservoir temperatures of about 125°C for Central Anatolia, 110°C for North Anatolia, 136°C for East Anatolia and 251°C for West Anatolia, the latter agreeing with some of the bottom hole temperatures measured in drilled wells. The results reveal that the highest geothermal energy potential in Turkey is associated with the West Anatolian extensional tectonics which provides a regional, deep-seated heat source and a widespread graben system allowing deep circulation of waters. The North Anatolian region, bounded to the south by the dextral North Anatolian Fault along which most of the geothermal sites are located, has the lowest energy potential, probably due to the restriction of the heat source to local magmatic activities confined to pull-apart basins. The East Anatolian region (undergoing contemporary compression) and the Central Anatolian region (where the compressional regime in the east is converted to the extensional regime in the west) have moderate energy potential. Although the recently active volcanoes suggest the presence, at depth, of still cooling magma chambers that are potential heat sources, the lack of well-developed fault systems is probably responsible for the comparatively low energy potential of these regions. Almost all the thermal waters of Turkey are saturated with respect to calcite and, hence, have a significant calcite scaling potential which is particularly high for West Anatolian waters.     

Emplacement temperatures of the November 22, 1994 nuée ardente deposits, Merapi Volcano, Java

A study of emplacement temperatures was carried out for the largest of the 22 November 1994 nuée ardente deposits at Merapi Volcano, based mainly on the response of plastic and woody materials subjected to the hot pyroclastic current and the deposits, and to some extent on eyewitness observations. The study emphasizes the Turgo–Kaliurang area in the distal part of the area affected by the nuée ardente, where nearly 100 casualties occurred. The term nuée ardente as used here includes channeled block-and-ash flows, and associated ash-clouds of surge and fallout origins. The emplacement temperature of the 8 m thick channeled block-and-ash deposit was relatively high, 550°C, based mainly on eyewitness reports of visual thermal radiance. Emplacement temperatures for ash-cloud deposits a few cm thick were deduced from polymer objects collected at Turgo and Kaliurang. Most polymers do not display a sharp melting range, but polyethylene terephthalate used in water bottles melts between 245 and 265°C, and parts of the bottles that had been deformed during fabrication molding turn a milky color at 200°C. The experimental evidence suggests that deposits in the Turgo area briefly achieved a maximum temperature near 300°C, whereas those near Kaliurang were <200°C. Maximum ash deposit temperatures occurred in fallout with a local source in the channeled block-and-ash flow of the Boyong river valley; the surge deposit was cooler (180°C) due to entrainment of cool air and soils, and tree singe-zone temperatures were around 100°C.     

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).     

Aquifer chemistry of the Puga and Chumatang high temperature geothermal systems in India

Results of a chemical study of the fluids from drill holes and hot springs of Puga and Chumatang areas in the northwestern part of the Himalaya are presented and discussed in this paper. The thermal waters of Puga and Chumatang are of Na-HCO₃-Cl and Na-HCO₃ types, respectively. A comparison between these waters, their chemical classification and activity studies suggest a flow path within a quartzitic-schistose basement, containing quartz, K-feldspar and illite, and in clayey terrains containing montmorillonite and illite.The chemistry of thermal waters also indicate their association with magmatic activity. The chemical geothermometers indicate the possible existence of a geothermal reservoir at Puga with temperature ≈250°C. The Chumatang area has a comparatively cooler reservoir with a temperature of 150–180°C.     

Geochemical investigations of submarine volcanic exhalations to the east of Panarea, Aeolian Islands, Italy

Results are presented on scubadiving investigations carried out on thermal manifestations in the area of Panarea (Aeolian Islands). The area investigated falls inside a caldera which extends from the main island to the group of islets located to the northeast. The distribution of the gaseous manifestations is regulated by the NE-SW, NW-SE and N-S regional tectonic directrices, through which the more recent basic magma intruded, giving rise to dikes and pillow lavas. f_O2-temperature relation of the gases sampled in the investigated area was calculated to be: logf_O2 = 11−24,593/T which indicates that a buffering mechanism acted on the gases as they cooled down during their ascent. The high ³He/⁴He ratio (6 × 10⁻⁶) and the δ¹³C = −3.2%. (PDB), suggest the presence of a magmatic component in the gas feeding the investigated manifestations. The above relations and the almost constant high He/N₂ ratio suggest that all the fumaroles are fed by the same deep hot fluids. On the basis of both the chemical characters of the fluids and the geothermo-barometric data, a deep geothermal body, having a temperature of about 240°C, is recognized. Two other shallower thermal aquifers, with a temperature of 170–210°C, are identified. A circulation pattern of the geothermal fluids is also proposed. On the basis of calculations regarding the convective energy released by the geothermal system of Panarea, and the magmatic mass responsible for the positive gravimetric anomaly of the area, it was estimated that the last volcanic activity took place less then 10,000 years ago.     

Modelling the effect of an assumed cosmic ray-modulated global cloud cover on the terrestrial surface temperature

We have used the thermodynamic model of the climate to estimate the effect of variations in the oceanic cloud cover on the surface temperature of the Earth in the North Hemisphere (NH) during the period 1984–1990. We assume that the variations in the cloud cover are proportional to the variation of the cosmic ray flux measured during the same period. The results indicate that the effect in the temperature is slightly noticeable when we consider the surface hemispheric temperature on July 1987, finding an average temperature anomaly between −0.06°C and −0.14°C, along a latitudinal band between 20° and 40°. The surface temperature averaged globally in the NH presents a decrease of 0.01°C in average, which is almost the same for continents and oceans. However, these values are not significant when compared to the overall variability of the time series with and without forcing.     

Evaluation of reservoir temperatures and local utilization of geothermal waters of the Konkan Coast, India

Geochemical studies on fifteen geothermal manifestations (38–70°C) from the Konkan coast geothermal province of India have been used to evaluate the reservoir temperatures. Activity studies of the minerals and the waters present in the reservoirs suggest that the thermal waters are in equilibrium with montmorillonite, kaolinite and quartz at about 100°C. Reservoir temperatures of these geothermal systems as estimated by geochemical thermometers, are 88 to 128°C, and thus too low for economic electricity production.     

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).     

Experimental evidence for the exsolution of cratonic peridotite from high-temperature harzburgite

Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite.The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed.Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.     

Geology, water geochemistry and geothermal potential of the jemez springs area, Canon de San Diego, new Mexico

Studies of the geology, geochemistry of thermal waters, and of one exploratory geothermal well show that two related hot spring systems discharge in Canõn de San Diego at Soda Dam (48°C) and Jemez Springs (72°C). The hot springs discharge from separate strands of the Jemez fault zone which trends northeastward towards the center of Valles Caldera. Exploration drilling to Precambrian basement beneath Jemez Springs encountered a hot aquifer (68°C) at the top of Paleozoic limestone of appropriate temperature and composition to be the local source of the fluids in the surface hot springs at Jemez Springs. Comparisons of the soluble elements Na, Li, Cl, and B, arguments based on isotopic evidence, and chemical geothermometry indicate that the hot spring fluids are derivatives of the deep geothermal fluid within Valles Caldera. No hot aquifer was discovered in or on top of Precambrian basement. It appears that low- to moderate-temperature geothermal reservoirs (< 100°C) of small volume are localized along the Jemez fault zone between Jemez Springs and the margin of Valles Caldera.