Radiogenic Heat Production Research Articles

Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 1: Uncertainty analysis of the thermal-conductivity parameterization

The subsurface temperature field and the geothermal conditions in sedimentary basins are frequently examined by using numerical thermal models. For those models, detailed knowledge of rock thermal properties are paramount for a reliable parameterization of layer properties and boundary conditions. Despite the state-of-the-art in other research fields (e.g. hydrogeological ground-water models) where the spatial permeability variations within geological layers is often considered, parameterization of the major rock thermal properties (in particular thermal conductivity, to minor extent radiogenic heat production and specific heat capacity) is almost always conducted by applying constant parameters for each modelled layer. Moreover, initial parameter values are usually obtained from only few core measurements and/or literature values, which raise questions for their representativeness. Only some rare studies have considered detailed lithological composition or well log information, still with constant layer properties.This study presents a thermal-modelling scenario analysis in which we demonstrate how the use of both different parameter input type (from literature, lithology and well logs) and parameter input style (constant or spatially varying layer values) affects model temperature predictions in sedimentary basins. It is a case study located in the Danish-German border region at the northern margin of the North German Basin. To conduct the scenario analysis, rock thermal properties are determined from lithological descriptions and standard petrophysical well logs for several boreholes in the area of study. Statistical values of rock thermal properties are derived for each geological formation at each well location and, furthermore, for the entire dataset. The thermal model is validated against known observed temperatures of good quality.Results clearly show that the use of location-specific well-log derived rock thermal properties and the integration of laterally varying input data (reflecting changes of lithofacies) significantly improves the temperature prediction. The parameterization from boreholes always prevails over the parameterization based on literature values, and it allows for reducing uncertainty of model temperatures by up to 80%.

Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 2: A case study from the Danish-German border region

We present a 3D numerical crustal temperature model and analyse the present-day conductive thermal field of the Danish-German border region. The model region covers the northernmost part of the North German Basin, a transition zone between the deep-reaching sedimentary rocks of the Glückstadt Graben (including complex salt structures) and the shallow crystalline basement of the Ringkøbing-Fyn High. The modelling approach is novel as it implements for the first time a comprehensive analysis of well-log data on a regional modelling scale. Those logs were used both to derive the spatial distribution of rock thermal conductivity across the study area, and to calculate heat flow values. New values of terrestrial surface heat flow are reported for eight deep boreholes in the North German Basin ranging from 72 to 84mW/m2 with a mean of 80±5mW/m2. The values are computed from continuous temperature logs, carefully corrected BHT values, drill-stem tests and well-log derived rock thermal properties (thermal conductivity, radiogenic heat production) and were included in the setup of the numerical lower boundary conditions. New surface heat flow is up to 20mW/m2 higher than low values reported in some previous studies for this region. Heat flow from the mantle is 33–40mW/m2.The model temperature predictions are validated against 59 temperature observations from 24 wells. The prediction uncertainties between observed and modelled temperatures at deep borehole sites are small (rms=3.5°C, ame=2.1°C). Pronounced lateral temperature variations are predicted and found to be caused mainly by complex geological structures, including a large amount of salt structures and marked lateral variations in the thickness of basin sediments. The associated variations in rock thermal conductivity generate significant variations in model heat flow and large variations in temperature gradients.With regard to the utilization of geothermal energy, the Rhaetian and the Middle Buntsandstein sandstone reservoirs are found with temperatures within the range of 40–80°C, suitable for low enthalpy heating purposes, in most of the area and locally also with higher temperatures. Temperatures above 120°C, of interest for the production of electricity, are observed only in the very southeastern part of the study area.

Focused radiogenic heating of middle crust caused ultrahigh temperatures in southern Madagascar

Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens. This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at ~630 Ma, (ii) became incorporated into the Gondwanan collisional orogen by ~580 Ma, and (iii) were exhumed during crustal thinning at 525–510 Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000 km^2, mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enough—locally >5 μW/m^3—to cause regional UHTM in <60 Myr. We conclude that, due in large part to the stability and insolubility of monazite at high crustal temperatures, RHP was the principal heat source responsible for UHTM, obviating the need to evoke external heat sources. Focused RHP probably thermally weakened portions of the middle crust, gravitationally destabilizing the orogen and facilitating thinning via lateral extrusion of hot crustal sections.

Role of uranium in controlling radiogenic heat production based on gamma ray spectrometry and thermal remote sensing data, southwestern Sinai, Egypt

This study is a trial to reveal a new relationship among important three scientific terms: gamma-ray spectrometry, processed remotely sensed data and radiogenic heat production (RHP). In addition, the uranium migration history represents a new key that controls the amount and flow of RHP over exposed igneous and metamorphic rocks. Solaf-Umm Graifat area in the west of Sinai pluton has the oldest metamorphic rocks and granites in Sinai Peninsula. This gives the study area a special case which helps in binding among these terms. Land surface temperature-radioelements composite images reflect the relationship between the amount of thermal flow and rock units, which have radiometric print (uranium content) as source rock or received rock. The total count and uranium anomalies located in the areas of highest surface temperature range from 35 to >40 °C give the highest RHP range from 13.15 to 11.27 μW m−3 rather than elevation, while some elevated localities show high radioactive content and RHP. This study reveals that the uranium migration history represents another important factor that judges the amount and the flow of RHP, rather than lithology, in the study area. Uranium is the most radioelement able to change the amount and flow of RHP according to its mobility; this is clearly shown in some received rocks of uranium which have the highest values of RHP such as quartzo-feldspathic gneiss (13.15 μW m−3). In addition uranium concentration shows strong correlation with heat production, compared with the weaker correlations with thorium and potassium concentrations.

Potential groundwater and heterogeneous heat source contributions to ice sheet dynamics in critical submarine basins of East Antarctica

AbstractWe present the results of two numerical models describing contributions of groundwater and heterogeneous heat sources to ice dynamics directly relevant to basal processes in East Antarctica. A two‐phase, one‐dimensional hydrothermal model demonstrates the importance of groundwater flow in vertical heat flux advection near the ice‐bed interface. Typical, conservative vertical components of groundwater volume fluxes (from either topographical gradients or vertically channeled flow) on the order of ±1–10 mm/yr can alter vertical heat flux by ±50–500 mW/m2 given parameters typical for the interior of East Antarctica. This heat flux has the potential to produce considerable volumes of meltwater depending on basin geometry and geothermal heat production. A one‐dimensional hydromechanical model demonstrates that groundwater is mainly recharged into saturated, partially poroelastic (i.e., vertical stress only; not coupled to a deformation equation) sedimentary aquifers during ice advance. During ice retreat, groundwater discharges into the ice‐bed interface, which may contribute to water budgets on the order of 0.1–1 mm/yr. We also present an estimated map of potentially heterogeneous heat flow provinces using radiogenic heat production data from East Antarctica and southern Australia, calculated sedimentary basin depths, and radar‐derived bed roughness. These are overlaid together to delineate the areas of greatest potential effect from these modeled processes on the ice sheet dynamics of the East Antarctic Ice Sheet.

High-quality heat flow determination from the crystalline basement of the south-east margin of North China Craton

Very few of heat flow data have come from the crystalline basement in the North China Craton but rather from boreholes in the sedimentary cover of oil–gas basins. Explorations for hot dry rock (HDR) geothermal resources and porphyry gold deposits in eastern China offer now valuable opportunities to study the terrestrial heat flow in the crystalline basement. In this study, we obtained continuous temperature logs from two boreholes (the LZ borehole with a depth of 3471m and the DR borehole with a depth of 2179m) located in the south-east margin of the North China Craton. The boreholes have experienced long shut-in times (442days and 261days for the LZ borehole and DR borehole, respectively); thus, it can be expected that the temperature conditions have re-equilibrated after drilling and drill-mud circulation. Rock thermal conductivity and radiogenic heat production were measured for 68 crystalline rock samples from these two boreholes. The measured heat-flow density was determined to be 71.8±2.3mWm−2 (for the LZ borehole) and 91.5±1.2mWm−2 (for the DR borehole). The heat flow for the LZ borehole is close to the value of 75mWm−2 determined in the Chinese Continental Scientific Drilling main hole (CCSD MH), both being in the Sulu-Dabie orogenic belt and thus able to verify each other. The value for the DR borehole is higher than the above two values, which supports former high heat-flow values determined in the Bohai Bay Basin.

Geo-neutrino results with Borexino

Borexino is a liquid scintillator detector primary designed to observe solar neutrinos. Due to its low background level as well as its position in a nuclear free country, Italy, Borexino is also sensitive to geo-neutrinos. Borexino is leading this interdisciplinary field of neutrino geoscience by studying electron antineutrinos which are emitted from the decay of radioactive isotopes present in the crust and the mantle of the Earth. With 2056 days of data taken between December 2007 and March 2015, Borexino observed 77 antineutrino candidates. If we assume a chondritic Th/U mass ratio of 3.9, the number of geo-neutrino events is found to be 23.7+6.5 -5.7(stat) +0.9-0.6 (syst). With this measurement, Borexino alone is able to reject the null geo-neutrino signal at 5.9σ, to claim a geo-neutrino signal from the mantle at 98% C.L. and to restrict the radiogenic heat production for U and Th between 23 and 36 TW.

Curie point depth, heat-flow and radiogenic heat production deduced from the spectral analysis of the aeromagnetic data for geothermal investigation on the Menderes Massif and the Aegean Region, western Turkey

In this study, the Curie point depth (CPD), heat-flow and radiogenic heat production maps of the Menderes Massif (MM) and the Aegean Region-Turkey were prepared for the thermal crustal structure from the aeromagnetic data by means of a spectral analysis method. The estimated CPDs in the study area vary from 6.21 to 12.41km. The heat-flow values were calculated for two (2) different thermal conductivities of 2.5–2.7Wm−1K−1. The radiogenic heat production was also calculated between 0.38 and 0.80μWm−3 on the high-grade metamorphic units of the study area. The estimated heat-flow and radiogenic heat production maps indicate that a high temperature in the upper crust and thin lithosphere in the observation area are probably resulted from the crustal extension, active deformation and magmatism. All of the produced maps in this study were correlated with the analytic signal (AS) map which indicates the locations of the bodies causing magnetic anomalies. In addition, maxspots of the horizontal gradients from the magnetic anomalies were also emplaced onto the produced maps to indicate the boundaries of critical magmatic units in the study area. These correlations allow reasonable and meaningful interpretation for the reasons of high geothermal gradient and high heat-flow. In addition, a longitudinal seismic wave (Vp velocity) map was presented from the relationship between seismic velocity and radiogenic heat production in the western Anatolia, for the first time and they were correlated with the previous investigations. The estimated Vp velocities range from about 6 to 6.4km/sn and low Vp values may be related to the partial melting of subducted structures in the western Anatolia. As a result, two new potential geothermal areas were determined by the integration of different methods used in this study.

Deep structure of the Argentine margin inferred from 3D gravity and temperature modelling, Colorado Basin

Following previous work on the Colorado Basin using a 3D crustal structural model, we now investigate the presence of lower crustal bodies at the base of the crust using 3D lithospheric gravity modelling and calculations of the conductive thermal field.Our first study highlighted two fault directions and depocentres associated with thinned crust (NW–SE in the West and NE–SW at the distal margin). Fault relative chronology argues for two periods of extension: (1) NW–SE faulting and thinning in the western Colorado Basin and (2) NE–SW faulting and thinning related to the continental breakup and formation of the NE–SW-striking volcanic margins of the Atlantic Ocean.In this study, the geometry of modelled high-density Lower Crustal Bodies (LCBs) enables the reproduction of the gravimetric field as well as of the temperature measured in wells down to 4500m. The modelled LCBs correlate with geological observations: (1) NW–SE LCBs below the deepest depocentres in the West, (2) NE–SW LCBs below the distal margin faults and the seaward dipping reflectors. Thus the proposed poly-phased evolution of the margin could as well correspond to two emplacement phases of the LCBs. The calculated conductive thermal field fits the measured temperatures best if the thermal properties (thermal conductivity and radiogenic heat production) assigned to the LCBs correspond to either high-grade metamorphic rocks or to mafic magmatic intrusions. To explain the possible lithology of the LCBs, we propose that the two successive phases of extension are accompanied by magma supply, emplaced (1) in the thinnest crust below the older NW–SE depocentres, then (2) along the NE–SW continentward boundary of the distal margin and below the volcanic seaward dipping reflectors. The South African conjugate margin records only the second NE–SW event and we discuss hypotheses which could explain these differences between the conjugate margins.

Geothermal gradients of the northern continental shelf of the Gulf of Mexico

A wide, systematic variation of sedimentary geothermal gradients has been previously observed along the northern continental shelf of the Gulf of Mexico. From east to west, geothermal gradients change from 25 to 30 °C/km off Alabama to lower values (15–25 °C/km) off eastern Louisiana and to higher values (30–60 °C/km) off Texas. In order to assess the mechanism responsible for this variation, the present study first compiled an extensive bottom-hole temperature database from over 6000 wells in the northern continental shelf and constructed a more detailed geothermal gradient map than those published previously. Second, basin models were then constructed for three areas within the continental shelf (off Texas, Louisiana, and Alabama) that show differing geothermal gradients. A basin model is a mathematical model that simulates the heat transport through the crust and the sediments of a basin in the context of its geologic evolution. Previous researchers proposed two possible causes for the observed geothermal gradient variation in the northern continental shelf. The first was the thermal effect of sedimentation: areas with faster sediment accumulation result in low geothermal gradients, and vice versa. The second was that basal heat flow (heat flow that enters from the igneous crust to the bottom of the sediments) varied across the continental shelf. The present study finds that sedimentary geothermal gradients in these areas are primarily impacted by two competing mechanisms associated with sediment accumulation. One is the radiogenic heat production within the sediment that adds to the total heat budget upward through the sedimentary column. The other is the transient effect of fast sediment accumulation, which results in reduction in the upward heat flow. Off Louisiana, the transient effect prevails, and hence the area shows the lowest geothermal gradients. Off Texas, due to slower sedimentation, the positive contribution by radiogenic heat is most significant. Off Alabama, because the sediments there are not as thick, the overall contribution of radiogenic heat is less. The models show that the thermal effects of sedimentation are large enough to explain the observed variation in geothermal gradients. Therefore, corresponding variation in basal heat flow is not required.

Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

AbstractSeismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low‐velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat flow data and mantle‐xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one‐dimensional shear‐velocity profiles that are consistent with phase‐velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1‐D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity‐depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low‐velocity zone in the upper mantle and that temperature structures that are consistent with constraints from mantle xenoliths yield phase‐velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km.

Generation of felsic crust in the Archean: A geodynamic modeling perspective

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.

A review for genesis of continental arc magmas: U, Th, K and radiogenic heat production data from the Gümüşhane Pluton in the Eastern Pontides (NE Türkiye)

In this study, in situ radioelemental (238U, 232Th and 40K) measurements and heat production rate evaluation have been conducted at 132 sites in the Gümüşhane Pluton of the Eastern Pontides Orogenic Belt (EPOG) of Northern Türkiye. The average K, U and Th values for granites are 4.35±0.71%, 5.85±2.74ppm, 22.13±5.55ppm, respectively; microgranites 4.33±1.37%, 7.12±2.23ppm, 24.11±9.59ppm; metagranite 3.93±0.68%, 6.24±2.75ppm, 12.87±3.92ppm; andesite/basalt 1.75±0.74%, 3.22±1.07ppm, 9.01±5.14ppm. Based on the abundances of K, U and Th, the radiogenic heat production (RHP) values, which is a reflection of the geological rock types, are estimated as 3.52±1.03μWm−3, 3.86±1.31μWm−3, 2.93±0.89μWm−3, and 1.68±0.63μWm−3 for granites, microgranites, metagranites and andesite/basalt, respectively. The average heat production value for the study area is estimated as 2.83±1.39μWm−3, which is closer to the upper continental crustal value; contrary to the highest heat production value of granitic basement and the lowest heat production values of the sedimentary rocks.The study region is depicted with very high level of Th/U value ranging from 0.11 to 23.19 with mean value of 3.53±2.5, which is comparatively close to the upper continental crustal estimate of 3.8. The numerical results verify that mantle-derived magmas are subjected to contamination with both middle and lower crustal material constituted by fragments of southerly dipping subduction of Tethys Ocean during the late Mesozoic–Cenozoic.

How hot are the Cairngorms?

Heat flow measured over the East Grampians batholith in the 1980s was found to be unexpectedly low and at odds with high radiogenic heat production within the outcropping granites and a very large volume of granite predicted from an interpretation of gravity data. Past climate variations perturb temperature gradients in the shallow subsurface leading to erroneous estimates of heat flow. A reconstruction of the surface temperature history during the last glacial cycle has enabled a rigorous palaeoclimate correction to be applied to the heat flow that shows an increase of 25% over previously reported values; revised to 86 ± 7 mW m −2 . An interpretation of recent mapping reveals that the surface exposures of the East Grampians granites are the roof zones of a highly evolved magma system. Rock composition, therefore, is likely to become more mafic with depth and the heat production will decrease with depth. This petrological model can be reconciled with the gravity data if the shape of the batholith is tabular with deep-seated feeder conduits. The increased heat flow value leads to revised predictions of subsurface temperatures of 129°C at 5 km depth and 176°C at 7 km depth, increases of 40% and 49%, respectively, compared to previous estimates. These temperatures are at the lower end of those currently required for power generation with Engineered Geothermal Systems, but could potentially be exploited as a direct heat use resource in the Cairngorm region by targeting permeable fractures with deep boreholes.

Spectroscopy of geoneutrinos from 2056 days of Borexino data

We report an improved geo-neutrino measurement with Borexino from 2056 days of data taking. The present exposure is $(5.5\pm0.3)\times10^{31}$ proton$\times$yr. Assuming a chondritic Th/U mass ratio of 3.9, we obtain $23.7 ^{+6.5}_{-5.7} (stat) ^{+0.9}_{-0.6} (sys)$ geo-neutrino events. The null observation of geo-neutrinos with Borexino alone has a probability of $3.6 \times 10^{-9}$ (5.9$\sigma$). A geo-neutrino signal from the mantle is obtained at 98\% C.L. The radiogenic heat production for U and Th from the present best-fit result is restricted to the range 23-36 TW, taking into account the uncertainty on the distribution of heat producing elements inside the Earth.

Heterogeneous Crystalline Crust Controls the Shallow Thermal Field – A Case Study of Hessen (Germany)

We present a seismic- and 3D-gravity-constrained lithospheric-scale 3D structural model of Hessen that differentiates 7 sedimentary units, 5 Variscan upper crustal bodies, the lower crystalline crust and the lithospheric mantle. To predict the present- day subsurface temperatures, we solve the steady-state conductive heat equation by using a 3D FE method and assigning lithology-dependent thermal properties. We show that the thermal field is mainly controlled by the varying radiogenic heat production in the crystalline crust, which results in a colder NW and a warmer SE domain. Locally, this regional trend is superimposed by thermal blanketing of low-conductive sediments leading to higher temperatures.

2-D thermal modeling of HT–LP metamorphism in NW and Central Iberia: Implications for Variscan magmatism, rheology of the lithosphere and orogenic evolution

A model for the P–T–t evolution of NW and Central Iberia is presented to better understand its metamorphic evolution from initial Barrovian stage through HT–LP conditions. The model is constrained by the well-known succession of structural events and geochronology of the Variscan deformation, standard physical parameters, and conservative values for the surface heat flow and radiogenic heat production. The goals are to test an orogenic model derived from observations against a physical reality constrained by the thermal properties of the lithosphere, and to explore the implications for Variscan orogenic evolution. A600km section, parallel to the axial surface of the Central Iberian arc is modeled to the scale of the continental lithosphere. The model outcomes are presented as sections showing the evolution of isotherms over 65Ma, as the variation of the lithospheric geotherm and as P–T–t paths for significant parts of the model section and for points originating at different depths. Profiles of lithosphere strength have been constructed combining the maximum shear stress for brittle fracture with published flow–stress–depth profiles of characteristic rocks and temperatures obtained for the different depths in the model.Quantitative constraints from 2-D thermal models suggest that the metamorphic and magmatic evolution can be explained by the succession of three orogenic stages, namely crustal thickening, a period of thermal relaxation, and subsequent extensional collapse, providing that the first stage doubles the thickness of the continental crust. Thrusting of a thick nappe stack, well preserved in NW Iberia, seems to be the likely cause of the high temperatures reached later during the HT–LP event that occurred during late orogenic extension. The late addition of heat from asthenospheric upwelling remains a possibility, but the model results suggest that it is not necessary to explain the Variscan P–T–t paths.

Preliminary geological, geochemical and numerical study on the first EGS project in Turkey

Geothermal energy is attracting more and more attention due to its large capacity and lack of dependency on the weather. Currently, many countries have planned enhanced geothermal system (EGS) projects. In this paper the first EGS project in Turkey, which is being implemented at the license area of SDS Energy Inc., in Dikili of the Izmir province, is introduced. Extensive geological, paleostress (279 fault-slip data from 33 locations), geophysical (magnetotelluric and vertical electrical sounding at 80 and 129 locations, respectively) and geochemical studies as well as paleostress measurements have been conducted in this area within the scope of this project. In the light of all these studies, it has been determined that the Dikili region is remarkable in terms of its high thermal gradient of about 7 °C/100 m. The geothermal reservoir formation “the Kozak granodiorite” is a homogeneous, crystalline volcanic rock mass with high radiogenic heat production, and suitable for an EGS application. The analysis shows that the dominating fault system is normal, and the corresponding primary stress regime is extensional. Based on the geological, geophysical surveys and the estimated in situ stresses, numerical studies were carried out to assess the results of the hydraulic fracturing and geothermal energy production using the numerical codes FLAC3Dplus and TOUGH2MP, respectively, in the area A of the Dikili site. The simulation results show that the stimulated reservoir volume and area could reach 44.5 million m3 and 1 km2, respectively, with an injection volume of 122,931 m3. Assuming the fractured zone has a height of 1000 m and a half-length of 1200 m (the distance between injection and production wells being 1000 m), an average overall geothermal capacity of 83.7 MWth in 20 years could be reached with an injection rate of 250 l/s. The injection strategy and design parameters of the reservoir stimulation and geothermal production will be further optimized with the project running.

Surface heat flow and lithosphere thermal structure of the Rhenohercynian Zone in the greater Luxembourg region

Comprehensive knowledge of surface heat flow and subsurface temperature distribution is indispensable for the interpretation and quantification of crustal/mantle processes as well as for the evaluation of the geothermal potential of an area. In cases where subsurface temperature data are sparse, thermal modeling may be used as a tool for inferring the geothermal resource at depth but requires profound structural, geological, and petrophysical input data. The study area encompasses the Trier–Luxembourg Basin and the western realm of the Rhenish Massif, itself subdivided into the Ardennes region in the west as well as the Eifel and Hunsrück regions in the east. For the study area, 2-D steady-state and conductive thermal models were established based on geological models of lithosphere-scale which were parameterized using thermal rock properties including thermal conductivity, radiogenic heat production, and density. The thermal models are constrained by surface heat flow (qs) and the geophysically-estimated depth of the lithosphere–asthenosphere boundary (LAB). A qs of 75±7 (2σ)mWm−2 was determined in the area. A LAB depth of 100km, as seismically derived for the Ardennes, provides the best fit with the measured qs. Modeled temperatures are in the range of 120–125°C at 5km depth and of 600–650°C at the Moho, respectively. The mantle heat flow amounts to ∼40mWm−2. Possible thermal consequences of the 10–20Ma old Eifel plume, which caused elevation of the LAB to 50–60km depth, were modeled in a steady-state thermal scenario resulting in a qs of 91mWm−2 in the Eifel region. Available qs values (65–80mWm−2) are significantly lower and do indicate that the plume-related heating has not yet reached the surface in its entirety.

Cooling history of Earth’s core with high thermal conductivity

Thermal evolution models of Earth’s core constrain the power available to the geodynamo process that generates the geomagnetic field, the evolution of the solid inner core and the thermal history of the overlying mantle. Recent upward revision of the thermal conductivity of liquid iron mixtures by a factor of 2–3 has drastically reduced the estimated power available to generate the present-day geomagnetic field. Moreover, this high conductivity increases the amount of heat that is conducted out of the core down the adiabatic gradient, bringing it into line with the highest estimates of present-day core–mantle boundary heat flow. These issues raise problems with the standard scenario of core cooling in which the core has remained completely well-mixed and relatively cool for the past 3.5Ga. This paper presents cooling histories for Earth’s core spanning the last 3.5Ga to constrain the thermodynamic conditions corresponding to marginal dynamo evolution, i.e. where the ohmic dissipation remains just positive over time. The radial variation of core properties is represented by polynomials, which gives good agreement with radial profiles derived from seismological and mineralogical data and allows the governing energy and entropy equations to be solved analytically. Time-dependent evolution of liquid and solid light element concentrations, the melting curve, and gravitational energy are calculated for an Fe–O–S–Si model of core chemistry. A suite of cooling histories are presented by varying the inner core boundary density jump, thermal conductivity and amount of radiogenic heat production in the core. All models where the core remains superadiabatic predict an inner core age of ≲600Myr, about two times younger than estimates based on old (lower) thermal conductivity estimates, and core temperatures that exceed present estimates of the lower mantle solidus prior to the last 0.5–1.5Ga. Allowing the top of the core to become strongly subadiabatic in recent times pushes the onset of inner core nucleation back to ∼1.5Gyr, but the ancient core temperature still implies a partially molten mantle prior to ∼2Ga. Based on these results, the scenario of a long-lived basal magma ocean and subadiabatic present-day core seems hard to avoid.