High Geothermal Gradient Research Articles

Logistic-Based Translation of Orogenic Gold Forming Processes into Mappable Exploration Criteria for Fuzzy Logic Mineral Exploration Targeting in the Kushaka Schist Belt, North-Central Nigeria

This paper applied a logistic-based fuzzy logic inference system to integrate critical factors that could control orogenic gold mineralization in part of the Kushaka schist belt, north-central Nigeria to develop a process-based mineral potential mapping (MPM) of the area. The critical factors from geophysical and geological dataset were weighted using logistic functions. The fuzzy logic inference system provides the capability to handle complex geological processes that culminated in orogenic gold mineralization as well as minimizing systemic uncertainties/fuzziness that often plague MPM. The results of this work show that granitic intrusions with fuzzy scores of 0.67–0.90 played a major role in generating high geothermal gradient in the area. Seventy percent of the existing gold mine sites in the area spatially coincide with metasedimentary rocks, having fuzzy scores of 0.7–0.9; this suggests metasedimentary rocks as being responsible for the production of gold fluid and ligands in the area. The evidence of hydrothermal activity, with fuzzy scores of 0.53 and 0.91, confirms the occurrence of mineralization associated with quartz veins and granite rocks. Lithological contacts and faults, having fuzzy scores of 0.60–0.80, presumably contribute to the localization of orogenic gold mineralization in the area. Emerging from the results, favorable zones for primary orogenic gold mineralization in the area occurred predominantly on granite gneiss and quartz veins. The mineral potential map was found consistent with the local geology, structural styles and hydrothermal alteration signatures in the area, and its validation using the existing locations of geochemical anomalies and prediction–area rate curve in the study area showed 75 and 72% agreement, respectively, thus confirming the reliability of the developed mineral potential map for resource management.

GEOPHYSICAL INVESTIGATION TO ESTIMATE THE CURIE POINT DEPTH, HEAT FLOW AND GEOTHERMAL GRADIENT IN SOKO AND ANKPA, BENUE TROUGH NIGERIA

This work presents the interpretation of the aeromagnetic data over Soko and Ankpa area using spectral analysis method. The study area was divided into eight (8) equal spectral blocks in order to estimate the depth to the top boundary, centroid, Curie point depth, heat flow and geothermal gradient of the study area. The result of the analysis shows the range of the depths to the top boundary and centroid varies between 1.085 to 1.984 km and 6.151 to 8.730 km respectively. The Curie temperature isotherm ranges between 11.112 km and 15.476 km and the geothermal gradients associated with it ranges from 39.967 and 52.196 0 𝐶⁄𝑘𝑚. The corresponding values of heat flow ranges from 93.697 𝑚𝑊𝑚􀀀 and 130. 49􀀁 𝑚𝑊𝑚􀀀. From this analysis, it was observed that areas with high geothermal gradient correspond to high heat flow and an inverse relationship exists between the heat flow and the Curie point depth. With the high geothermal gradient especially at the southeastern part of the study area, there is a possibility of enough geothermal energy for exploration in order to boost and generate clean energy for electricity.

Impact of crust–mantle mechanical coupling on the topographic and thermal evolutions during the necking phase of ‘magma-poor’ and ‘sediment-starved’ rift systems: A numerical modeling study

We used high-resolution (500 × 250 m) two-dimensional lithospheric-scale thermo-mechanical numerical modeling to unravel the unexpected topographic and thermal evolutions recorded during the necking phase of several rift systems worldwide. Through a systematic analysis we studied how the lithosphere rheology impacts the topographic and thermal evolutions across the entire width of magma-poor and sediment-starved rift systems until their crust is locally thinned to 10 km. We quantified the evolution of topography, uplift and subsidence rates, accommodation and emerged space creation, temperature and surface heat flow for a wide panel of crustal and mantle rheologies to provide an overview of possible rifting behaviors. Extension of a lithosphere for which the crust and mantle are mechanically decoupled by a weak lower crust generates complex morphotectonic evolutions, with the formation of temporarily restricted sub-basins framed by uplifted parts of the future distal margin. Mechanical decoupling between the crust and mantle controls also largely the thermal evolution of rift systems during the necking phase since, for equivalent extension rates and initial geotherms: (i) weak/decoupled lithospheres have a higher geothermal gradient at the end of the necking phase than strong/coupled lithospheres; and (ii) weak/decoupled lithospheres show intense heating of the lower crust at the rift center and intense cooling of the crust on either side of the rift center, unlike strong/coupled lithospheres. These behaviors contrast with the continuous subsidence and cooling predicted by the commonly used depth-uniform thinning model. Accommodation space in the evolving basins is first generated by vertical crustal velocities and subsequently by horizontal velocities causing the widening of the earlier formed basin. Processes such as strain softening and mantle fertilization have a limited impact on the primary morphology and thermal state of rift systems before the crust is thinned to 10 km but may locally amplify relief and thermal heterogeneities.

3-D Geothermal Model of the Lurestan Sector of the Zagros Thrust Belt, Iran

The Zagros thrust belt is a large orogenic zone located along the southwest region of Iran. To obtain a better knowledge of this important mountain chain, we elaborated the first 3-D model reproducing the thermal structure of its northwestern part, i.e., the Lurestan arc. This study is based on a 3-D structural model obtained using published geological sections and available information on the depth of the Moho discontinuity. The analytical calculation procedure took into account the temperature variation due to: (1) The re-equilibrated conductive state after thrusting, (2) frictional heating, (3) heat flow density data, and (4) a series of geologically derived constraints. Both geotherms and isotherms were obtained using this analytical methodology. The results pointed out the fundamental control exerted by the main basement fault of the region, i.e., the Main Frontal Thrust (MFT), in governing the thermal structure of the crust, the main parameter being represented by the amount of basement thickening produced by thrusting. This is manifested by more densely spaced isotherms moving from the southwestern foreland toward the inner parts of orogen, as well as in a lateral variation related with an along-strike change from a moderately dipping crustal ramp of the MFT to the NW to a gently dipping crustal ramp to the SE. The complex structural architecture, largely associated with late-stage (Pliocene) thick-skinned thrusting, results in a zone of relatively high geothermal gradient in the easternmost part of the study area. Our thermal model of a large crustal volume, besides providing new insights into the geodynamic processes affecting a major salient of the Zagros thrust belt, may have important implications for seismotectonic analysis in an area recently affected by a Mw = 7.3 earthquake, as well as for geothermal/hydrocarbon exploration in the highly perspective Lurestan region.

Estimation of Curie point depth, geothermal gradient and heat flow within the lower Benue trough, Nigeria using high resolution aeromagnetic data

Aeromagnetic data have been used in estimating the Curie point depth (CPD), geothermal gradient and heat flow within the lower Benue trough, Nigeria, using the spectral analysis method. The area covered is approximately 227,225 km2. It is bounded by latitudes 6°30′ to 8°00′N and longitudes 6°30′ and 8°00′E. The results of this study show that the depth to the centroid (Zo) varies between 6.268 and 12.556 km, and the estimated depth to top boundary (Zt) varies between 6.268 and 12.556 km. The depth to the top boundary (Zt) of magnetic sources ranges from 0.796 to 2.171 km beneath the level of the sea. Furthermore, Curie point depth obtained varies between 10.5 and 22.5 km, and the geothermal gradient ranges from 25.527 to 54.063 °C/km having an average value of 37.821 °C/km. On the other hand, the calculated heat flow values ranges from 63.818 to 135.160 mW m−2 having an average value of 96.2198 mW m2. The graph of Curie point depth plotted against the values of heat flow shows a linear and inverse relationship. This relationship indicates that the Curie point depth increases as the heat flow decreases in the study area. With the high geothermal gradient in the study area, there is a possibility of the geothermal energy potential in the study area. These results demonstrate that the contribution of knowledge of Curie depth point and thermal heat flow to geoscientists could be traced to the thermal history of the study area.

Reassessing the PT conditions of Neoproterozoic collisional metamorphism and partial melting in southernmost Brazil

Partially equilibrated textures and partial melting products are key features in the study of high-grade metamorphic complexes, but their complexity limits the advancement of knowledge about the tectono-thermal history of ancient orogens. This difficulty is apparent in the wide interval of conditions (720–1000 °C/3–10 kbar) determined by previous studies in a high-grade complex formed during the amalgamation of West Gondwana in southernmost Brazil. In order to understand the orogen, we reanalyse the metamorphic evolution of the Várzea do Capivarita Complex through field data, petrography, whole-rock and mineral chemistry, and thermodynamic modelling. Peak metamorphic conditions (M1) are constrained with diverse mineral assemblages (Bt-Crd-Sil-Grt-Ilm/Bt-Sil-Grt-Ilm/Bt-Grt-Ilm) and overlap at 800 °C/4.5 kbar, while the partially equilibrated domain (Crd-Hc-Sil) indicates post-peak conditions (M2) of 680 °C/2.8 kbar. Models predict leucosome volume up to 20%, in agreement with field and petrographic estimates, attesting the migmatitic character. Leucogranitic veins correlating petrographically and compositionally with leucosome and melt estimates reinforce this character and the thermodynamic modelling, even with low correlations, which are explained by inherited minerals. Thus, M1 and M2 conditions indicate exhumation from depth of 16 to 10 km and high geothermal gradient, which are interpreted as due to anomalous mantle heat flow and rapid exhumation facilitated by large volumes of melt.

Estimation of Geothermal Gradient, Geothermal Heat Flux and Thermal Conductivity of Rocks in Western Niger Delta Using Well Log Data

This study presents the result of the estimation of heat flow from six (6) wells (Well X:001 to 006) in South-Western Niger Delta using values of Geothermal gradient (GG), Geothermal heat flux (Q) and thermal conductivity (K) computed from Sonic and continuous temperature log data for each well. Geothermal gradient was computed from continuous temperature logs using the simple gradient method while geothermal heat flux and thermal conductivity of the rocks in the wells were computed from the sonic log data, using the Relative Heat Flow Model and Fourier One-dimensional Heat Flow Law respectively. The results were analysed and interpreted to investigate the thermal structure and pattern of heat flow distribution of the basin. Results showed that geothermal gradient ranges from 1.45 0 C/100m to a value of 1.61 0 C/100m, with a simple average of 1.55 0 C/100m. Geothermal gradient contour map computed from this result, showed a low thermal gradient at the northern part of the study area where we have Well X-006 and increases outwards in all direction as we move further offshore. These differences reflect changes in thermal conductivity of rocks, ground water movement and endothermic reaction during diagenesis, since geothermal gradient is influenced by lithology or differential rate of sedimentation. Therefore, it was inferred that sediments with a relatively high geothermal gradient (1.55 to 1.61 0 C/100m) will mature earlier (low oil window) than those with low thermal gradient values. By implication, a high geothermal gradient enhances the early formation of oil at relatively shallow burial depths, but causes the depth range of the oil window to be narrow, while low geothermal gradient causes the first formation of oil to begin at fairly deep subsurface levels, but makes the oil window broad. Geothermal heat flux estimated from subsurface temperature and one-way sound travel time, shows heat flux varying between 33.16 mWm -2 to 72.73 mWm -2 with a simple average of 48.43 mWm -2 . Low heat flux was observed at the central part of the study area which increases towards the western and eastern parts of the area with Well X-005 characterized by a higher geothermal heat flux. Therefore, it was inferred that the western and eastern parts of the study area with higher heat flux values may be characterized as zones with maximum sediment thickness and are characterized as having depressions (gravity low) on the geoid which is characteristics of a basin, while the central part of the study area with low heat flux values correspond with zones of minimum sediment thickness. Also, thermal conductivity of rocks in the study area computed directly from heat flux and geothermal gradient results, ranges from 2.28W/m 0 C to 4.76 W/m 0 C with an average of 3.19 W/m 0 C. Thermal conductivity contour map computed from this result, showed low thermal conductivity values observed at the central part of the study area, and increases outwards towards the west and eastern parts. This pattern of thermal conductivity variation suggests probably there exists heavy crude oil at the central part of the study area and lighter crude oil as we move outward in all direction. It was also observed that within each well, thermal conductivity increased with depth and decreased with porosity which may be caused by difference in lithology and fluid content, due to the fact that all pore fillers (i:e gases and liquids) are poor conductors. The estimated values of geothermal gradient, heat flux and thermal conductivity obtained in this study are similar to the results obtained from previous studies in the region and with other passive continental margins of the world. Keywords: Heat flow, Geothermal Gradient, Geothermal Heat Flux, Thermal Conductivity, Continuous Temperature and Sonic log. DOI: 10.7176/JETP/10-2-04 Publication date: April 30 th 2020

Hydrochemistry and geological features of a new geothermal field, Bayatcık (Afyonkarahisar/Turkey)

The thermal waters in Bayatcık geothermal field (Afyonkarahisar/Turkey) discharge from a Palaeozoic-aged fissured and karstic marble aquifer. The temperatures and electrical conductivities of the thermal waters are 65 °C and 7305–7778 μs/cm, respectively. Thermal waters have Na-Ca-Cl-HCO3 type. The most important parameters affecting the chemical content are the cation exchange process and carbonate-rich reservoir rocks. Meteoric water from precipitation is heated at depth by the high geothermal gradient and ascends to the surface through fractures and fault zones. Isotope data (δ18O, δ2H and tritium) indicates meteoric origin, and residence time of more than 50 years. Chemical geothermometers calculated the reservoir temperature as around 72–146 °C, whereas the mixing model diagrams produce temperatures of 106–191 °C. The results show that the thermal waters in the Bayatcık region have the same characteristics as the neighbouring Ömer-Gecek region and gained their current composition after possible cooling effect and/or water-rock interaction in the colder parts of the reservoir.

Geothermal characteristics of two deep U-shaped downhole heat exchangers in the Weihe Basin, China

Geothermal resources are abundant in the Weihe Basin, which provides great potential for winter district heating in the Guanzhong area in western China. As a demonstration project for deep geothermal exploitation and utilization, four boreholes of over 2,000 m were drilled and paired to form two U-shaped downhole heat exchangers (DHEs) in the Xi’an Depression of the Weihe Basin. In this paper, the borehole equilibrium temperatures of these two DHEs, measured two years after the systems had been completed, are presented. The analysis of the borehole temperatures show that: (1) the thermal and hydrological regimes of the study area are stable, as indicated by the highly consistent borehole temperatures among the four boreholes, both horizontally and vertically. (2) the geothermal gradient range down to the depth of 2,000 m in this area is 3.45 ˜ 3.47°C/hm with an average of 3.46 ± 0.01°C/hm, substantially higher than the regional mean and implying a great geothermal energy exploitation potential; the high geothermal gradient anomaly can be attributed to the rather thin crust in the Weihe Basin with a relatively high mantle heat flow and hydrothermal convection systems involving fluid circulation in nearby discordogenic fault zones. (3) the Lantian-Bahe Formation has the features of stable distribution, good aquifer yield, high transmissivity, high permeability and high thermal conductivity, and hence, it is a good hosting stratum to the horizontal section of U-shaped DHEs in this area to maximize heat exchange efficiency and sustainability.

Application of Tikhonov Regularization for 1D Geothermal Heat Flux Ill-Posed Inverse Problem: A Case Study on Chad Sedimentary Basin

Subsurface heat flux information is important in geothermal exploration. With the information, geophysicists can map exactly the thermal potential in a particular area. Based on the surface heat flux, inverse modeling produces the 1D subsurface heat flux distribution. However, inverse problems in the geothermal system are generally ill-posed. Small changes in the data can cause large changes in the solution and the solution may not be unique. To solve the mentioned non-linear and ill-posed equation above, Tikhonov regularization is a choice for stabilizing the inverse calculation. This paper demonstrates how Tikhonov regularization is useful to solve subsurface heat flux distribution both in the synthetic model and real model. Based on surface heat flux distribution from the direct problem, the preconditioned conjugate gradient algorithm calculates the subsurface heat flux. With the correct choice of the regularization parameter, the inverse model fits the initial model. For the testing purposes in real-world conditions, Chad sedimentary basin located in Chad and Nigeria is used as a model. A high geothermal gradient is found in this area. Therefore, geothermal explorations are on the rise recently. Its thermal conductivity, heat production, and stratigraphy data from previous researches provide information about the initial model. The heat flux curve generated from inversion matches the initial noisy model with the error of around 10−9 mW/m2. Therefore, to answer the increasing energy demand, this method can be highly applicable to future geothermal prospecting.

Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

From extension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a first structural and RSCM thermometric study

From extension to compression: high geothermal gradient during the earliest Variscan phase of the Moroccan Meseta; a first structural and RSCM thermometric study

Numerical simulation of rock breaking by PDC bit in hot dry rocks

Penetrating through hot dry rocks for geothermal resources with high geothermal gradients, high degrees of hardness and abrasivity will be extremely difficult, and a reasonable tooth distribution is a key factor to improve its rock-breaking efficiency. In order to explore an available cutter arrangement design of PDC cutters in hot dry rock drilling, we based on elastic–plastic mechanics and rock mechanics, established a dynamic 3D numerical simulation model of rock breaking with PDC cutters by using the Drucker–Prager yield criterion as the rock strength judgment principle. On the basis of this, we studied, under the confining pressure of 60 MPa, the effects of cutting depth, temperature, back rake angle and cutting speed on the mechanical specific energy of PDC cutters. The following results were achieved. (1) When a PDC cutter cuts a rock with the speed of 0.5 m/s and the back rake angle of 5–25°, the rock destruction specific energy of the cutter decreases significantly with the rise of cutting depth, while increases first and then decreases with the increasing temperature. And the critical temperature is 200 °C. (2) When a PDC cutter cuts a rock with the speed of 0.5 m/s and the cutting depth of 1–3 mm, the rock destruction specific energy of the cutter decreases first and then increases with the increase of back rack angle. And the optimal rack angle is 20°. (3) Within the temperature range of 20–300 °C, a PDC cutter cuts a rock with the back rake angle of 5°, the rock destruction specific energy of the cutter increases with the cutting speed but decreases with the increase of cutting depth.

Decadal‐Scale Aquifer Dynamics and Structural Complexities at a Municipal Wellfield Revealed by 25 Years of InSAR and Recent Groundwater Temperature Observations

AbstractOver the past 35 years the Buckman wellfield near Santa Fe, New Mexico, experienced production well drawdowns in excess of 180 m, resulting in ground subsidence and surface cracks. Increased reliance on surface water diversions since 2011 has reduced pumping and yielded water level recovery. To characterize the impact of wellfield management decisions on the aquifer system, we reconstruct the surface deformation history through the European Remote Sensing Satellite, Advanced Land Observing Satellite, and Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) time series analysis during episodes of drawdown (1993–2000), recovery (2007–2010), and modern management (2015–2018) in discontinuous observations over a 25‐year period. The observed deformation generally reflects changes in hydraulic head. However, at times during the wellfield recovery, the deformation signal is complex, with patterns of uplift and subsidence suggesting a compartmentalized aquifer system. Recent records of locally high geothermal gradients and an overall warming of the system (~0.5°C during the water level recovery) obtained from repeat temperature measurements between 2013 and 2018 constrain a conceptual model of convective heat transfer that requires a vertical permeable zone near an observed fault. To reproduce observed temperature patterns at monitoring wells, high basal heat flow and convective cooling associated with downward flow of water from cool shallow aquifers during the drawdown period is necessary. The fault, however, appears to die out southward or may be locally permeable, as conceptual cross‐sectional hydrologic modeling reproduces the surface deformation without such a structure. Our work demonstrates the importance of incorporating well‐constrained stratigraphy and structure when modeling near‐surface deformation induced by, for instance, groundwater production.

Thermal structure of the Cappadocia region, Turkey: a review with geophysical methods

This study is a review paper collecting all previous investigations on the thermal structure and the geothermal potential of the Cappadocia region. The main purpose of this review is to reveal the geothermal potential of the research area and stimulate the geothermal exploration. The scientific objective of this paper is to present a compilation of previous studies on the thermal behavior of the crust in the Cappadocia region and to indicate the geothermally prospective areas in the region. Authors of this study commenced the search of potential geothermal areas in Turkey using the aeromagnetic anomalies, starting from the Central Anatolia. They initiated the investigations of shallow crustal structure of Cappadocia, first and determined recently formed depressions down to 2 km. In the second stage of these research activities, detailed CPD calculations were performed and mapped throughout Cappadocia. They found an elliptical shallow CPD anomaly up to 7 km where the geothermal gradient is obtained as 68 °C/km in the apex of the anomaly. In connection with these values, heat flows were estimated up to 210 mW/m, while the radiogenic heat production is being observed as maximum of 0.70/μWm in this area. These calculations are different and more encouraging than the general regional heat-flow map of Turkey calculated previously from the hot springs and bottom-hole temperatures of shallow wells. A possible magma chamber beneath the caldera-like circular depression between Nevsehir–Aksaray–Nigde–Yesilhisar cities is thought to be the main reason for high heat flow and high geothermal gradient in association with the young volcanism in and around Cappadocia. Due to lack of reservoir units, hydrothermal–geothermal energy possibility is not expected in this region. On the other hand, large anomalous area between Nigde, Aksaray and Nevsehir presents very high hot dry rock (HDR) energy potential which is considered as the most prospective geothermal energy type in the future.

Using a Paleosurface to Constrain Low‐Temperature Thermochronological Data: Tectonic Evolution of the Cuevas Range, Central Andes

AbstractDispersion of low‐temperature thermochronologic data from nine samples collected on a deformed paleosurface preserved on the Cuevas range (Central Andes) can be exploited to unravel complex thermal histories. The nine samples yielded data that have both intersample and intrasample dispersions; the data set includes apatite fission‐track ages (180–110 Ma), mean track lengths (11–13 μm), apatite helium (10–250 Ma), and zircon helium ages (180–348 Ma). We ran inverse thermal history models for each sample that reveal spatial variations of the Miocene reheating along the paleosurface. Next, we ran a multiple‐sample joint model to infer a common form for thermal history for all samples. Our results suggest that initial exhumation during the Famatinian orogeny was followed by a residence between ~2.5 and 7.0 km depth during the Paleozoic and the Triassic. The onset of Mesozoic rifting was responsible for an increase of the geothermal gradient and extensive horst exhumation, which brought the basement of the Cuevas range close to the surface (~1–2 km) in the Late Jurassic. Between the Late Cretaceous and the Paleocene, the combination of low relief, a humid climate, and low erosion rates (0.006–0.030 km/Ma) facilitated the development of the Cuevas paleosurface. During the Miocene, this paleosurface experienced differential reheating with a high geothermal gradient (>25 °C/km) due to the sedimentary cover and local magmatic heat sources. During the Andean orogeny, in the Pliocene, the Cuevas paleosurface was deformed, exhumed, and uplifted.

Transition of subduction-related magmatism from slab melting to dehydration at 2.5 Ga

The Archean plate tectonics and subduction-related magmatism have been concerned for several decades. Here we investigate the Archean plate subduction from a new perspective, using the secular records of global-scale non-TTG (tonalite-trondhjemite-granodiorite) felsic rocks with arc-like geochemical affinities. The statistical results show that the La/Yb and Sm/Yb of non-TTG arc felsic rocks evolved at high values in the Archean, and decreased sharply at 2.5 Ga. This abrupt transition indicates a significant change of the petrogenesis. During the Archean, the oceanic crust subducted into the mantle along a high geothermal gradient and reached the conditions of partial melting with garnet stable in the residue, resulting in high La/Yb and Sm/Yb in the arc felsic rocks. As a consequence of the secular cooling of the mantle, the geothermal gradient of a post-Archean subducting cold oceanic crust intersected with the stability limits of hydrous minerals before it reached the wet solidus of basalts, so that the dehydration occurred preferentially and resulted in a drier crust with a much higher temperature of solidus. As such, the post-Archean arc felsic rocks were mostly originated from melting of the hydrated mantle wedge overlying the subducted oceanic slab. The magma source of the arc felsic rocks thereafter became much shallower, corresponding to the low La/Yb and Sm/Yb of those rocks as a consequence of the less stable garnet in the residue. We thus conclude that the subduction-related magmatism changed from slab melting to dehydration model at 2.5 Ga.

Secular cooling and crystallization of partially molten Archaean continental crust over 1 Ga

The protracted tectonic and magmatic record of cratons over the Archaean Eon has been classically interpreted in terms of long-lived shallow-dipping subduction or repeated mantle plumes. In this paper, we use the 1D conductive heat equation to model the evolution of the geotherm of a generic felsic-dominated Archaean cratonic nuclei solely considering the secular decay of radioactive isotopes (238U, 235U, 232Th, and 40K), responsible for heat production in the crust. Using a range of plausible parameters for crustal thickness, lithospheric thickness, and surface heat flux, this modelling shows that Archaean crust was characterized by an initially high geothermal gradient at 3.5 Ga, with a Moho temperature close to 900 °C, and that it might have remained partially molten for about one billion years. The existence of a partially molten crust for an extended period of time offers an alternative option to shallow-dipping subduction or repeated mantle plumes for the understanding of the peculiar tectonic evolution of Archaean cratons marked by (i) protracted high-temperature metamorphism and magmatism associated with crustal differentiation, and (ii) widespread deformation characterized by structural domes attributed to the development of crustal-scale gravitational instabilities.

Hydrogeochemistry, isotopes and geothermometry of Ixtapan de la Sal–Tonatico hot springs, Mexico

The Ixtapan de la Sal–Tonatico hot springs are located in the geological setting of the Trans-Mexican Volcanic Belt, on the border of the Morelos–Guerrero Basin, which is carbonated region with limestone and silicic rocks. The temperature, electrical conductivity and pH of the thermal waters are 25.7–37.2 °C, 9520–13,920 µS/cm and 5.90–6.88, respectively. The hydrochemical family is NaCl (Cl− mean concentration of 2198 mg/L and Na+ mean concentration of 1396 mg/L). The minor and trace elements that are notable for their high concentrations are B (19.69–23.58 mg/L), Li (6.09–8.06 mg/L), Sr (4.80–6.18 mg/L) and As (0.30–1.97 mg/L). Isotope data (δ18O, δ2H) show δ18O enrichment of the thermal waters, which may be a result of isotopic exchange with the host rock at high temperatures, although the value was very small, which may be due to mixing with groundwater near the surface. The water samples were oversaturated with calcite, dolomite and quartz, verified in the field by the presence of precipitated rocks (travertines). The samples were undersaturated with chalcedony, silica gel, cristobalite, celestite and strontianite. Several chemical geothermometers and the results from multicomponent equilibrium suggest a reservoir temperature between 90 and 130 °C. Meanwhile, a temperature of 129 °C was estimated with the Cl–enthalpy mixing model. According to the conceptual model of the functioning of this system, considering that flow is characterized by the heating of water at deep zones due to the high geothermal gradient caused by recent tectonic activity. This water rises to the surface through fractures and faults. As it rises, thermal water mixes in different proportions with Ca-HCO3 groundwater.

Springwater provenance and flowpath evaluation in Blue Lake, Bonneville basin, Utah

Water in Utah and Nevada is important for agriculture, municipal use, solute transport, and ecosystem preservation. Large spring wetland systems occur on the playa margin of the Bonneville basin, including Blue Lake, 15 km south of the Bonneville Salt Flats on the Utah-Nevada state border. Large spring systems have historically been studied as water budgets don't apparently balance from direct mountain front recharge (Nelson and Mayo, 2014; Gardner and Heilweil, 2014; and others). Blue Lake is no exception; prior studies here have suggested discharge rates 0.04 - 0.05 km3/yr (Louderback and Rhode, 2009), greater than expected for modelled recharge in the surrounding mountain range (0.03 km3/yr). Three hypothesized recharge mechanisms for Blue Lake are tested: mountain-front recharge, interbasinal groundwater flow, and infiltration from historic Lake Bonneville. Remote sensing suggests that a conservative estimate of Blue Lake discharge constitutes 50-64% of modelled mountain-front recharge (Flint et al., 2011). Major ions, δ18O, δ2H, dissolved gases (14C, 3He, 4He, Ne, Ar, Kr, Xe) and trace elements (Sr and 87Sr/86Sr isotopes) comprehensively constrain recharge conditions, water-rock interactions, flowpaths, and groundwater provenance of this large spring system. 14C signatures suggest that Blue Lake discharge has a transit time between 5,600 and 12,200 years, older than that of Fish Springs. Noble gas concentrations in Blue Lake water suggest an elevated recharge temperature greater than 19°C and low salinity, indicating a deep water table and high geothermal gradient in the recharge area. 87Sr/86Sr ratios of playa-margin springs are elevated from that of mid-playa groundwaters, springwaters from the adjacent mountain range, and alluvial fill groundwater from the valley directly south of Blue Lake. Playa-margin spring Sr isotope values (0.713-0.714) are most similar to direct runoff from the Deep Creek Range granodiorite outcrop (>0.713). Interbasinal groundwater flow in combination with mountain-front recharge is best supported by chemical data rather than mountain-front recharge alone or the slow discharge of regional aquifers recharged by lacustrine infiltration from Lake Bonneville.