Hot Springs Discharge Research Articles

Geospatial analysis of volcano-tectonic interactions: implications for spatiotemporal variations in water quality and quantity at Lake Beseka, East Africa, Ethiopia

ABSTRACT Water quality deterioration in the Main Ethiopian Rift Lakes is a problem affecting the area’s public health and socioeconomic development. The present study aimed to assess the water level rise in Lake Beseka using Landsat images from 1975 to 2023 and dilution trends at the pumping site from 2004 to 2022. In addition, the spatial water quality parameters, including pH, EC, TDS, and water surface temperature, for the years 2007, 2018, 2021, and 2023 were determined. The water level increased dramatically from 3 km2 in the 1960s to 27.5 km2 in 1980 following the construction of the Koka dam and the beginning of irrigation activity (sugarcane plantation) for the Metehara sugar estate factory. The water level reached a maximum (50 km2 in the dry season) in the mid of 1910s and become decreased and fluctuated after 2015. The observed pH, EC, and TDS were higher in the central portion of the lake, grouped under brackish water, and lower in the southwest, south, and northeast shores. The EC and TDS were higher in 2007, decreased in 2018, and fluctuated. The geothermal anomaly of the area varies from 23.1 to 47.9°C with the highest anomaly occurring at the northern side of the lake at a recent volcanic area that interlinked with geological structures. However, the observed and estimated water temperatures of the lake decreased from west to east due to hot spring discharge from the southwest shore of the lake. The series of hot springs in the southwest and west shores of the lake with high discharge rates indicate the presence of structural connectivity that leads to the emission of heat from a deep-seated source and heated groundwater of the area recharged from irrigation activity. Overall, spatiotemporal lake water quality variations were due to anthropogenic and geogenic factors, including hot springs and groundwater discharge to the lake, water level rise and bathymetric depth variation, evaporation, and irrigation activity. This study is significant in continuously estimating water level fluctuations and providing early warnings and preparedness to maintain the region's socioeconomic development.

Compositional Remote Sensing and Hyperspectral Laboratory Analyses of Sinters in Hydrothermal Fields in Chile, With Relevance to Astrobiological Targets on Mars

AbstractSiliceous hot spring deposits, or sinters, deposit from hot spring discharge at Earth's surface and are sites of exceptional preservation of biosignatures. Their macro‐ and micro‐textures are regarded as important evidence of past microbial activities in hydrothermal environments. However, biology mimics do occur, and bona fide microbial textures could be destroyed by subsequent diagenesis or other post‐depositional processes. Thus, it is paramount to narrow the search for prospective Martian silica‐rich deposits that may contain biosignatures from both orbital and rover‐based perspectives. This study investigates hydrothermal deposits in Chile, which are analogs of high‐silica deposits discovered in the Gusev crater on Mars, through remote sensing and laboratory analysis. Results indicate that compositional remote sensing based on multispectral data with a high spatial resolution of <4 m/pixel reflects various concentrations of silica, which assisted in identifying the direction of discharged hydrothermal flows from the vent to the apron. Micro‐infrared mapping of sinters from similar hydrothermal fields linked spectral features to specific textures revealed by scanning electron microscope and chemical compositions confirmed by electron microprobe analysis, indicating that sinters with no shift in their emissivity minimum in the thermal infrared range were more likely to preserve cellular structures. An instrument for collecting multispectral data with higher spatial resolution could aid in characterizing the geologic settings of potential hot springs on Mars. Locating emissivity minima in the infrared regions of silica that do not shift to a lower position would suggest the potential for well‐preserved microbial structures in Martian sinters, if life ever did exist there.

Electroneutralization with simultaneous highly-efficient desalination exemplified by acidic brackish wastewater

Chemical neutralization has commonly been employed for the treatment of acidic brackish wastewater through the addition of external alkaline chemicals. However, this approach can potentially compromise the health of aquatic environment and resource sustainability. In this study, an innovative electroneutralization stack incorporating highly-efficient desalination was developed as a green solution for acidic hot-spring discharge treatment. Two distinct stack configurations, i.e., AECE and AEBP, were designed to evaluate the performance of simultaneous neutralization and desalination under different operating conditions. The results demonstrated the effectiveness of neutralizing acidic discharges while concurrently achieving a significant reduction in salinity. The brine solution produced during the process also presented the possibility of being harnessed as a valuable resource. Notably, the design parameters for the process were determined using a first-order kinetic model integrated with the Nernst-Planck equation. The maximum rate constants for sulfate and chloride removal were found to be 0.129 min−1 and 0.125 min−1 for AECE, and 0.039 min−1 and 0.042 min−1 for AEBP, respectively. The corresponding chloride-to-sulfate selectivities were observed to range from 0.42 to 1.00 for AECE, and from 0.15 to 1.21 for AEBP. Furthermore, the optimal operational conditions were identified through a comprehensive multi-objective assessment, resulting in process energy consumption ranging from 1.5 to 2.2 kWh/m3, while maintaining a clean water productivity of ∼30 L/m2/h. Significantly, in comparison to conventional chemical neutralization methods, the developed electroneutralization process demonstrated a substantial reduction in operating costs by 2 to 5-fold, along with a remarkable decrease in CO2 emissions (by approximately 4-fold) and the avoidance of sludge generation.

Biogeochemical Responses to Mixing of Glacial Meltwater and Hot Spring Discharge in the Mount St. Helens Crater

AbstractEnvironments where geothermal waters and glacier meltwater mix are common on Earth yet little is known about the biogeochemical processes that occur when hot, reduced geothermal water mixes with cold, oxidized glacial meltwater in natural systems. Mount St. Helens provides an ideal location to study the interaction between geothermal and glacier waters since the water sources, and their mixing environment in Step Creek, are exposed in the volcanic crater. We find that the two water sources contain distinct major ion, trace element, dissolved organic matter (DOM), and biological signatures. The hot spring contains high concentrations of biogeochemically reactive components (e.g., siderophile and chalcophile trace elements and DOM) compared to the glacier discharge but a large fraction of these solutes do not remain in solution after the waters mix. In contrast, glacier discharge contains fewer solutes but most of these solutes remain in solution after the waters mix. The mixing of glacier and hot spring water in Step Creek supports seston and benthic ecosystems that have higher phototrophic and microbial biomass than those in the source waters, suggesting that the mixing environment in this high‐gradient stream provide a more comprehensive suite of soluble and essential nutrients that promote primary production and DOM cycling.

Biogeochemical responses to mixing of glacial meltwater and hot spring discharge in the Mount St. Helens crater

Biogeochemical responses to mixing of glacial meltwater and hot spring discharge in the Mount St. Helens crater

Submarine hot spring discharges along the coast of Beppu Bay as observed by radon towing survey

Submarine hot spring discharges along the coast of Beppu Bay as observed by radon towing survey

Kinetic Modelling of Electrokinetic Separation of Sulfate and Hydrogen Ions from Acidic Hot Spring Discharges

Electrokinetic separation has been widely utilized to remove ionic compounds from wastewaters due to its high energy-efficient feature. Hot spring dischages usually contain abundant amounts of sulfate and chloride with a strong acidity, e.g., the pH value is around 1.6. In this study, two stacks with different configurations of electrokinetic separation processes were designed and used to remove SO4 -2 and H+ from hot spring discharges. The results indicated that, with an applied voltage of 10 V for 60 min, the removal ratios of sulfate, chloride, and hydrogen ions were 99%, 98% and 98%, respectively. The kinetic models were established to evaluate and predict the performance of SO4-2 and H+ separation under different operating conditions using the developed electrokinetic processes. The kinetic rate constants of various applied voltages (ranged from 2.9 to 17.0 V) using electrokinetic separation were determined accordingly. The mechanisms of different configuration designs for ions removal using electrokinetic stacks were also elucidated. The obtained paramters of kinetics could be used for reactor designs, cost evaluation and performance improvement in large-scale demonstrations. It was thus concluded that the developed electrokinetic separation processes could be a potential solution to the acidic wastewater discharges with an significant amount of dissolved solids.

Curie point depth analysis of aeromagnetic data of Kasane region in northwest Botswana and surrounding regions for geothermal investigation of Kasane Hot Spring

The Kasane Hot Spring with discharge temperatures of 43–50 °C is characterized by intermittent bubbling accompanied by white precipitant within its vicinity. The hot spring discharges mainly along Chobe River which meets the Zambezi River at the tip of northwest Botswana. In this study the Curie point depth (CPD) was estimated using the spectral centroid method of aeromagnetic data at Kasane Hot Spring and the surrounding regions. The upward continued aeromagnetic data were analyzed to obtain depths to the bottom (CPD) of the magnetic sources. The CPD values were then used to compute the geothermal gradient, subsurface crustal heat flow including radiogenic heat flow distribution in the region.According to the obtained results, the depth to the magnetic bottom varies from 6 to 22 km. These depths are comparable to those obtained from recent regional study of the Okavango Rift Zone. The areas with shallow CPDs of demagnetized rocks at depth are located around Kasane Hot Spring, where high heat flow values in the range 70–90 mW/m2 were recorded. Moreover, the radiogenic heat production values between 0.50 and 0.60 μWm-3 within the same region may also provide an extra contribution to the geothermal potential of Kasane Hot Spring. The correlation of estimated shallow CPDs with high geothermal gradient, high heat flows, and radiogenic heat flow values at area within Kasane region indicate possible geothermal resource. The CPDs values obtained may be associated with the seismic events in the region and surroundings. The mean earthquake depth of events that occurred within Kasane and surrounding areas from 1987 to 1991 is 10 km, while the mean Curie Point Depth is approximately 11 km. These seismic events in the area could be responsible for possible thermal accumulation at Kasane Hot Spring, resulting in high heat flow values in the area.

The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida, Kenya Rift Valley

AbstractSaline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.

Deep groundwater discharge after the 2011 Mw 6.6 Iwaki earthquake, Japan

Hot spring discharge was linked to the 2011 Mw 6.6 Iwaki earthquake. Periodic surveys revealed that the discharge continued for more than 7 years, which is a rare and valuable long-term record of hot spring discharge triggered by an earthquake in a non-volcanic area. In terms of coseismic changes, based on a comparison of the spatial distribution of changes in the coseismic water head and calculated crustal volumetric strain using a fault model, hot spring water discharge was found to be caused by a change in the coseismic crustal volumetric strain. As for the postseismic changes, observations over 7 years revealed a gradual rise in the temperature and chloride ion concentration of the hot spring water. Such long-term hot spring discharge may be explained by the following two causes: the rise of thermal water from the deep part and the permeability changes along the hot spring channels.

Environmental isotopes as tracers of mining activities and natural processes: A case study of San Antonio de los Cobres River Basin, Puna Argentina

High concentrations of arsenic (As) in water have been detected in the San Antonio de los Cobres (SAC) River. Historically, the upper basin has been exposed to mining districts spread in the high altitudes of the Puna Argentina, intense evaporation and discharge of hot springs from a near geothermal area (Tuzgle-Tocomar Complex). This work evaluate possible sources of As in surface waters, applying major and minor ions and trace elements, as well as multi-isotope analysis (3H, 2H/H, 13C/12C,18O/16O and 34S/32S). Four main water types were defined: waters with no signs of contribution from mineralized areas or hot springs (P); waters with thermal origin (T); waters with natural sulphide oxidation (M); and acid mine drainage waters (AMD) waters from abandoned mine wastes. δ34S-SO42− used in combination with sulphate concentration, discriminate sulphate derived from sulphides oxidation and those from thermal waters inputs. All sampled surface water result from mixing between two or three end-members (P-M; P-AMD; P-M-T). The proposed mixing models estimate their relative contribution. δ18O-SO42− and δ18O-H2O corroborate the influence on the SAC River of water coming from AMD rather than from natural sulphides oxidation. δ13C-DIC identifies samples with thermal influence. δ18O-H2O and δ2H-H2O deviate from meteoric water, indicating evaporation of 7% to 17%. The distribution of arsenic along the San Antonio River together with the δ18O-SO42− values show that both, natural sulphides oxidation and thermal springs contribute to highly rise concentrations. The current drinking water for the town of SAC is not being affected by the AMD.

Aquatic biota in hot water: thermal gradients in rheocrene hot spring discharges as analogues for the effects of climate warming

Hot springs are characterised by water temperatures above 36.7 °C. Temperature decreases with distance in flow away from spring vents; this natural gradient provides a unique opportunity to investigate the influence of water temperature on aquatic biota. This study investigated the relationship between water temperature and the aquatic invertebrates and benthic diatoms in outflows from a hot spring complex in tropical north Queensland, Australia. Water temperature ranged from 62.7 °C at the vents to 26.0 °C at the location furthest downstream. Richness of benthic diatoms and aquatic invertebrates increased linearly in response to decreasing temperature, with no species present in the hot vents. Multivariate analysis showed that both community assemblages had a response to the temperature gradient. A drop in aquatic invertebrate richness and a change in assemblage composition occurred between 40 °C and 42 °C, indicating a threshold at this temperature. The nearby Einasleigh River has experienced several contemporary peaks in water temperature over 40 °C, which corresponds to this threshold level. The relationships indicate that consistent increases in water temperature expected under climate change could decrease biological richness and precipitate changes in the aquatic invertebrate and benthic diatom taxa of tropical aquatic ecosystems.

Geothermal system characteristics on Northern Ternate region, Indonesia based on interpretation of magnetic anomalies

Geothermal resources can be indicated by surface manifestations such as hot springs, hot mud puddles, and geysers. In the Northern Ternate region, in the villages of Tobololo and Sulamadaha, the manisfestation of hot springs is present. Therefore the purpose of this research is to study the geothermal characteristic in the Northern Ternate region, Indonesia, based on interpretation of magnetic anomalies in the hot surface temperature of hot springs. The method employed a fluxgate magnetometer to measure a sample of random temperature. Furthermore it examined for their chemical content to determine their resource type.The results show that the magnetic anomalies value in the Northern Ternate region is of -3678.63 nT up to 2679.57 nT. The contour map is then developed by indicating three levels of magnetic anomalies; low, medium, and high. This is an indication that the area is of geothermal manifestation and it is classified as a hypothetical resource that could become an unexpected reserve. The temperature is measured at 50 cm depth and it shows of 27-38.50 °C. The result of the temperature anomaly in the Tobololo area is due to the flow of hot springs along the coastline. Meanwhile the Sulamadaha area has a smaller hot spring discharge, which is due to hydrothermal activity below the surface. Based on the the geochemistry assessments, both areas have alkali chloride water type.

Hydrochemical and geological model of the Bañitos-Gollete geothermal system in Valle del Cura, main Andes Cordillera of San Juan, Argentina

The main characteristics of geothermal systems at flat-slab segments are not well understood. This work reports a geological and fluid geochemistry survey at the southern thermal areas of the isolated Valle del Cura, located at the Frontal Andes Cordillera over the Pampean flat-slab. Deep circulation along deep-rooted N-S trend faults and fractures likely heats meteoric water through a locally anomalous geothermal gradient related to scattering Quaternary volcanism. Hot springs discharge up to 60 °C Na-Cl bearing water with high TDS (>5000 mg/L) from meteoric origin, according to the isotopic composition. At the top of an 11 m height travertine fissure ridge, intense gas flux is mainly composed of CO2. Enthalpy model, δD, and B suggest a similar evolution trend for the Bañitos and Gollete thermal waters, clearly separated by the trend evidenced for thermal waters of the northern thermal sites of the Valle del Cura (e.g., Despoblados). Reservoir probably develops below 2–3 km depth hosted by the ignimbrites of the Choiyoi Group. Cation and multicomponent geothermometers estimate a temperature of ~140 °C and quartz + albite + calcite + chlorite ± beidellite-Na ± montmorillonite-K was inferred as the mineral assemblage. Very low tritium contents and δ18O isotopic shift might be because of the water-rock interaction process at low permeability conditions. These findings are consistent with a long residence, fault-controlled deep fluid circulation path. During their rise, the thermal waters likely interacted with cold waters at the base of the Oligocene volcanic deposits, where secondary processes such as calcite and silica precipitation may occur. Further investigations are required to assess the geothermal potential of the study areas, but we hope that this study may represent an incentive for other detailed investigations to evaluate the geothermal systems over flat-slab geothermal plays.

Evaluation of hot spring discharge in the Hikaruma District of Matsunoyama, Tokamachi City, Niigata Prefecture, using groundwater drainage wells installed for landslide mitigation

Evaluation of hot spring discharge in the Hikaruma District of Matsunoyama, Tokamachi City, Niigata Prefecture, using groundwater drainage wells installed for landslide mitigation

Near-surface study of a hot spring site in Fiji

A geophysical investigation of a hot spring system located in Rabulu, Fiji, was carried out from October 2014 to March 2015. The investigation covered a survey area of 6075 m2. Self-potential (SP), ground temperature and soil carbon dioxide (CO2) concentrations were measured and investigated for their distribution characteristics and inter-linkages. Results indicated obvious anomalous zone at the hot spring discharge site. The SP profile analysis highlighted thermal water upwelling zones and elevation-driven subsurface groundwater pathways. Measurement of subsurface temperatures up to 1 m depths revealed increasing temperatures, indicating potentially high thermal gradients in the area. Surface soil CO2 distributions also agreed with SP and ground temperature results. The overall result of the study demonstrated that synchronised measurements of SP, ground temperature and soil CO2 can be instrumental in identifying anomalous zones near the hot spring sites. Other parameters such as spring water temperature, discharge rate and energy flux estimates from the spring were calculated and analysed. The high-dense multi-parameter data coverage allowed interpretation of geothermal features at a scale never conducted in Fiji before. The near-surface investigations reported in this study corroborate previously suggested steady geothermal activity in the region, deserving further detailed investigation.

Geochemical characterization of the geothermal system at Villarrica volcano, Southern Chile; Part 1: Impacts of lithology on the geothermal reservoir

Besides temperature, mineralogy, residence time and dilution are crucial for assessing water-rock interaction intensity. The geothermal system at the Villarrica-Quetrupillán-Lanín volcanic chain, Southern Chile, is located across a prominent lithological transition from plutonic rocks of the North Patagonian Batholith (NPB) to volcano-sedimentary units. With the goal to investigate the impact of lithology on medium-enthalpy geothermal fluids, 15 hot spring discharges were sampled and analyzed for anthropogenic tracers and isotopic composition and compared to the analyses of 31 reservoir rock analogues. Comparison of strontium isotope signatures between rock analogues and hot spring discharges allow an allocation of associated reservoir rocks. Chlorofluorocarbons quantify the dilution of the geothermal springs by shallow groundwater, ranging from almost CFC-free samples to dilution with modern meteoric water by up to 50 %. Fluids discharging from plutonic rocks have low proportions of dilution with modern waters, while hot springs discharging from the volcano-sedimentary rocks have a higher and variable dilution with modern waters. The fractionation of oxygen isotopes of the SO4-H2O system reveals reservoir temperature estimates of 80–100 °C in the plutonic sequence matching discharge temperatures. For the springs discharging from volcano-sedimentary units higher reservoir temperatures of 100–140 °C are calculated. On basis of the analysis, a conceptual reservoir model can be derived. Fast fluid ascent is indicated along Liquiñe-Ofqui fault system in the NPB by similar discharge and reservoir temperatures and low surficial dilution rates. Large differences in discharge and reservoir temperatures in the Cura-Mallín formation along with a high influx of surficial water may be attributed to a more branched pathway pattern. In conclusion lithology maybe an important factor when coming to the utilisation of geothermal resources.

An unusual occurrence of the trace fossil Vagorichnus preserved in hydrothermal silica at Lake Baringo, Kenya Rift Valley: Taphonomic and paleoenvironmental significance

An unusual occurrence of the ichnogenus Vagorichnus is documented in the sublacustrine hydrothermal silica cement of conglomerate at Lake Baringo in the central Kenya Rift. The outcrop with ichnofossils lies at Soro on the NE shoreline of Ol Kokwe island, a Pleistocene volcanic centre in Lake Baringo. There, onshore and offshore hot-springs discharge hot and boiling (83–97.5°C), alkaline Na-HCO3 water, steam and volcanic gases (mainly CO2). The ichnofossils are preserved in full relief in diatomaceous opaline silica cement precipitated between immature gravel clasts. The trace fossils comprise horizontal and subhorizontal burrow-systems with straight, curved, and loosely meandering segments, which locally show poorly defined constrictions. The preserved features are diagnostic of the ichnospecies Vagorichnus anyao. Vagorichnus, a feeding structure (fodinichnion), may have been produced by insect larvae (Diptera, Coleoptera or other insects), which are common in modern East African lakes including Lake Baringo. A probable body fossil of a Diptera larva is preserved in one of the burrows. Hydrothermal silica is an unlikely host for macroscale ichnofossils, but the soft silica that precipitated near hot-spring vents on the shallow gravelly lake-floor was cool enough for colonisation by microbes and higher organisms. The trace-makers burrowed into cohesive, but still soft, silica; the trace fossils are therefore bioturbation structures rather than bioerosion structures. The soft silica contained organic matter from phytoplankton and detrital plant debris, and was locally covered by benthic microbial mats, all of which were potential food sources for the burrowers. The presence of Vagorichnus near modern and former hot springs confirms the broad ecological, hydrological and environmental conditions represented by this ichnogenus, and shows that Vagorichnus is not restricted to a specific environment, substrate, or type of lake basin.

Volcano–hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur

Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ~1000m above sea level (asl). The fumaroles are characterized by low-temperature, HCl- and S-rich gas and numerous hyper-acid pools (pH<1) without drains. At ~550m asl, in the Yurieva stream canyon, many hot (up to 87°C) springs discharge ultra-acidic (pH1–2) SO4–Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000–2000t/d of water vapor, <20t/d of SO2 and <5t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60–80t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano–hydrothermal system. Oxygen and hydrogen isotopic composition of water and Cl concentration for Yurieva springs show an excellent positive correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evaporation of the acidic brine. Temperatures calculated from gas compositions and isotope data are similar, ranging from 150 to 250°C, which is consistent with the presence of a liquid aquifer below the Ebeko fumarolic fields. Saturation indices of non-silicate minerals suggest temperatures ranging from 150 to 200°C for Yurieva springs. Trace elements (including REE) and Sr isotope composition suggest congruent dissolution of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compositions, suggesting complicated boiling–condensation–mixing processes.

Evolution of a dynamic paleo-hydrothermal system at Mangatete, Taupo Volcanic Zone, New Zealand

Recent quarrying and active faulting at Mangatete, Taupo Volcanic Zone (TVZ), New Zealand, illuminate a rare spatial and temporal window on a dynamic Late Quaternary geothermal system. Detailed geological mapping, stratigraphic logging, AMS 14C dating, and textural and mineralogical analyses were used to construct a complex history of hydrothermal, volcanological and tectonic activity from ~36 to 2ka. Extinct, surface hydrothermal manifestations occur over a ~2km2 area, and include in situ siliceous sinters distributed on normal fault terraces, an inferred hydrothermal eruption breccia (HEB) containing acid-etched sinter blocks, another probable HEB that was bathed in silicifying thermal fluids, and sinter clasts that were entrained in a debris flow associated with a volcanic ash event. Preserved sinter textures typical of near-neutral pH, alkali chloride spring discharge channels, aprons, terraces and affiliated marshes comprise plant-rich, palisade, tufted bubble mat, and domal stromatolitic fabrics. In addition, a packed fragmental sinter facies is shown herein to constitute silicified microbial mats that were broken, transported and deposited as point bar deposits in thermal spring-fed streams. Moreover, four unusual siliceous sinter fabrics—vuggy, globular spongy, scalloped, and arcuate wavy layered—are interpreted to have formed from local acid-sulfate-chloride thermal springs, possibly associated with paleo-fumaroles. The reconstructed history of paleo-hydrothermal activity indicates that the oldest sinters (~36ka) at Mangatete developed in alkali chloride hot springs, but then underwent post-depositional alterion/overprinting by acid-sulfate steam condensate and were dismembered, possibly by a hydrothermal eruption. Low pH hot-spring discharges forming the unusual, inferred acid sinter fabrics were localized in the same area. A shift in paleo-hydrology is evidenced by unaltered, alkali chloride sinters dated between ~22 and 3ka. A cluster of sinter dates between 22.7 and 20.7ka denote thermal fluid emissions around the time of the Okareka eruption (~22.1ka). An ~11kyr gap in sinter ages followed, which overlaps with a time of relative quiescence in local fault movement in between the Rotorua and Rotoma eruptions, as determined from previous paleo-seismicity studies on the Whirinaki Fault in the Mangatete area. At ~9.4ka, hot spring activity in thermally fed streams slightly post-dated the Rotoma eruption (~9.5ka). The locations and ages of the youngest (post-9.4ka) alkali chloride sinters and silicified paleosols in relation to Whirinaki Fault movements indicate an overall westward migration of siliceous spring discharge. Dated charcoal (~1.8kyr) in ash at the base of a locally extensive, volcaniclastic debris flow, containing sinter blocks of a range of ages and mineralogical maturity, suggests its affiliation with the Taupo eruption. Mangatete sinters occur along SW–NE striking faults parallel to the regional trend of the TVZ, which dissect the topography into stepped terraces. Inferred basement cross-faults in the study area, i.e. structural lineaments orthogonal to this trend, may have served as fluid pathways at depth, initiating and maintaining hydrothermal activity at Mangatete for more than 30kyr. Within the larger Ngakuru Graben (~150km2), the ages and distributions of sinters and hydrothermal eruption breccias, including those at Mangatete, show that significant hydrothermal activity occurred for more than 60kyr, in an area lacking such activity today. Thus, Mangatete retains a rare stratigraphic record within the TVZ sinter archive of changing geothermal fluids, resulting from shifts in paleo-phreatic water levels. Moreover, although limited, the presented age data imply a relationship among seismic activity, volcanic ash fall events, and the occurrence of permeable pathways for upward migration of thermal fluids. Finally, whereas currently active TVZ geothermal fields appear to be stable over tens of thousands of years or more, the fossil hydrothermal systems in the wider Mangatete area demonstrate the possibility of much greater temporal variability in the lateral expression of local surface manifestations of thermal fluid discharge through time.