Radiogenic Heat Production Research Articles

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.

Radiogenic heat production variations in the Gonghe basin, northeastern Tibetan Plateau: Implications for the origin of high-temperature geothermal resources

Hot dry rock is an almost inexhaustible source of geothermal energy. Since the first attempt to extract thermal energy from hot dry rock in America in the 1970s, successive studies have been conducted in many countries. Recently, high-temperature rock (exceeding 180 °C) was drilled in the Gonghe basin, northeastern Tibetan Plateau, marking a significant breakthrough in the exploration of hot dry rock resources in China. A proper understanding of the origin of the hot dry rock in the Gonghe basin is essential to the evaluation of the geothermal resource potential and to the establishment of an enhanced geothermal system in the basin.In the present research, we attempt to seek clues from the radiogenic heat production of the rocks. In all, 52 core samples collected from the main boreholes in the Gonghe basin were measured for the concentrations of radioisotopes. The heat production values were determined to be 1.21–2.02 μW m−3 for the sedimentary rocks, yielding an arithmetic mean of 1.67 ± 0.29 μW m−3, and 1.17–5.81 μW m−3 for the basal granitic rocks, yielding a mean of 3.20 ± 1.07 μW m−3. The results show that the radiogenic heat production of the granitic rocks is approximately that of global Mesozoic-Cenozoic granites, which is further corroborated by an extensive compilation of heat production data for granitic rocks in the immediate vicinity of the study area (average of 2.07 ± 0.97 μW m−3, n = 136). Based on the heat production data sets combined with available geological and geophysical information, the crustal heat production contribution is determined. The results show that the heat contribution from the crust is about 48.3 mW m−2, accounting for ∼47.3% of terrestrial heat flow. Compared with the regional background heat flow, the high heat flow in the Gonghe basin may be attributed to an additional heat flow contribution (∼27 mW m−2) from a heat anomaly (probably related to partial melting) at shallow depth, which is evidenced by the low-resistivity anomalies in the magnetotelluric survey results. Thus, this study emphasizes that the anomalous heat flow and the hot dry rock geothermal resources in the Gonghe basin may be the combined effects of the radiogenic heat contribution from the thickened crust and the heat contribution from a local heat anomaly beneath the Gonghe basin.

Geophysical Investigation of the Geothermal Potential Under the Largest Volcanic Cover in Anatolia: Kars Plateau, NE Turkey

In this study, Curie-point depth (CPD), geothermal gradient, radiogenic heat production, and heat flow maps were constructed based on different thermal conductivity coefficients using magnetic anomaly data for the Kars Plateau, which has the largest volcanic cover in Turkey. The bottom depths of the magnetic crust in the research area were revealed by the CPD map for the first time in this investigation. There are two apparent magnetic anomaly trends in the study area: the first is the Horasan–Senkaya–Sarikamis–Selim–Arpacay trend in the NE–SW direction, and the other is the Hanak–Ardahan–Arpacay trend in the NW–SE direction. Two other prominent elongations extend into the Ardahan–Gole–Senkaya and Kars–Digor axes. All these trends represent mountain chains and/or stratovolcanoes in the region, and no anomalies are observed around the non-volcanic outcrops. Curie depths are shallow, up to 14 km between Horasan and Kagizman towns, and 12 km in the northwestern part of the study area. Gradient values can reach 50 °C km−1 in the northwestern sector, together with the high heat flows represented by the 150 Wm−1 K−1 contours. The deepest CPD region lies between Gole and Susuz towns, where the geothermal gradient decreases to 27 °C km−1. Heat flows decrease 60 Wm−1 K−1 in the same area. An apparent gap around the Kars Plateau was observed in previous regional heat flow maps of Turkey by other authors (who used the bottom hole temperatures of boreholes and hot springs temperatures). This gap has been accurately filled from the results of this study, and geothermal exploration areas and the geothermal potential of the Kars Plateau have thus been determined for future exploration activity on the basis of the tectonic elements and earthquake data.

Modeling petroleum expulsion in sedimentary basins: The importance of igneous intrusion timing and basement composition

Abstract The concept of a critical moment in a petroleum system (the time of highest probability of entrapment and preservation of oil and gas) has underlain petroleum exploration for over 25 years. However, one area where understanding the critical moment is challenging is the Faroe-Shetland Basin (FSB; offshore UK). Isotopic dating of oils suggests that petroleum generation began between ca. 68 and 90 Ma; however, most basin models invoke an earlier generation beginning in the mid-Cretaceous at ca. 100 Ma, predating deposition of Paleocene and Eocene reservoirs. This time discrepancy has previously been explained by remigration from intermediary accumulations (“motel” hypothesis) and/or overpressure retardation of kerogen maturation. The FSB is characterized by a thick Cretaceous stratigraphic package (up to 5 km) that includes a large net thickness (up to 2 km) of Paleogene igneous material. In our model, separating sedimentary and igneous material and adding the igneous material at the correct time between ca. 58 and 55 Ma shallows the modeled burial depth of the Upper Jurassic source rocks during the Cretaceous sufficiently to delay maturation by 17 m.y. in comparison to results of previous studies. Additionally, previous studies have invoked crustal radiogenic heat production (RHP) based on the Phanerozoic crust averaging ∼2.8 µW/m3 in the North Sea (300 km to the east). However, the FSB basement is composed of significantly older, colder Neoarchean orthogneisses (ca. 2.7–2.9 Ga), reducing RHP by up to 50% to ∼1.6 µW/m3 (σ = 0.74). For the first time, our model unifies geological, geochronological, and geochemical observations, delaying the onset of petroleum expulsion by up to 40 m.y. in comparison to previous models.

Assessment of radiogenic heat generation in a flood plain of crystalline Basement rocks

Concentrations of radioelements in a flood plain of Crystalline Basement Complex are determined, with the aim of assessing the radiogenic heat produced in the study area. Ten soil samples are collected for analysis using Inductively Coupled Plasma Mass Spectrometry technique from ACME Laboratories. The radiogenic heat contributions by each radioelement in the flood plain varied from 95.70 to 393.37 ρw/kg; 71.68 to 642.56 ρw/kg; and 0.0088 to 0.0188 ρw/kg for uranium, thorium and potassium respectively. However, the total radiogenic heat production varied from 167.39 to 1034.9 ρw/kg, with thorium being the major contributor to the total heat generated in the study area. Nine locations in the study area are characterized by Low Heat Potential (LHP), while the remaining one is characterized by Moderate Heat Potential. On average, the study area could be classified as LHP, which is in agreement with some of the studies in crystalline terrain of Nigeria.

Radiogenic Heat Model in the southern axis of Ogbomoso, SW Nigeria

This study is aimed at estimating the radiogenic heat model in the southern part of Ogbomoso, Nigeria. In-situ measurements of activity concentrations of radioelements are randomly taken at seven locations. The individual data point is measured four times, while the average was recorded for accuracy. The results showed that the radiogenic heat produced per radionuclide varies from 28.71 to 143.55 ρW kg−1 for 238U; 89.60 to 217.60 ρW kg−1 for 232Th; 0.0010 to 0.0063 ρW kg−1 for 40K; with mean values of 73.83, 151.04 and 0.0033 ρW kg−1 for 238U, 232Th and 40K respectively. The total radiogenic heat production in the study area varies from 136.23 to 303.73 ρW kg−1, with mean value of 224.87 ρW kg−1. It is shown that thorium is the major contributor to the total heat generated in the study area. The study area is characterized by low heat production potential, which could be attributed to depletion of thorium and uranium in the geological rocks in the environs. Comprehensive radiometric survey to cover the entire city is recommended. This will help in characterization of the radiogenic heat produced per geological unit.

Thermal properties of sedimentary rocks in the Tarim Basin, northwestern China

Thermal properties of rocks are essential parameters for investigating the geothermal regime of sedimentary basins, and they are also important factors in assessments of hydrocarbon and geothermal energy resources. The Tarim Basin, the largest basin located in the north of the Tibetan Plateau, northwestern China, has great hydrocarbon resource potential and is an ongoing target for industry exploration. However, the thermal properties of sedimentary rocks within the basin are yet to be systematically investigated at a basin scale, thereby limiting our understanding of the thermal regime in the basin. Here, we collected 101 samples of sedimentary rocks and measured their thermal properties. Our results show that the ranges (and means) of thermal conductivity, radiogenic heat production, and specific heat capacity are 1.08–5.35 W/mK (2.52 ± 0.99 W/mK), 0.03–3.24 μW/m3 (1.24 ± 0.87 μW/m3), and 0.75–1.10 kJ/(kg·°C) (0.87 ± 0.07 kJ/(kg·°C)), respectively. Volumetric heat capacity and thermal diffusivity at the temperature of 40°C range from 1.61 to 2.79 MJ/(m3·K) (2.26 ± 0.25 MJ/[m3·K]) and 0.44–2.95 × 10−6 m2/s ((1.12 ± 0.53) × 10−6 m2/s), respectively. The thermal properties vary considerably for different lithologies, even within the same lithotype, indicating that thermal properties alone cannot be used to distinguish lithology. Thermal conductivity increases with increased burial depth, density, and stratigraphic age, suggesting the dominant influence is porosity variation on thermal conductivity. Furthermore, a strong contrast in the thermal properties of rock salt and other sedimentary rocks perturbs the geothermal pattern, which should be taken into consideration when performing basin modeling.

A 4 Ga record of granitic heat production: Implications for geodynamic evolution and crustal composition of the early Earth

The radiogenic heat produced by granites has a significant influence on the thermal state of the crust due to both their relatively high heat production with respect to most rock types and high abundance. However, the variations in present day heat production with age are generally based on relatively few measurements that are poorly distributed geographically. In this study, we construct a global model for the heat production of granitic rocks for the past 4 Ga using 13,400 geochemical analyses. We observe a nearly monotonic increase in radiogenic heat production from 4.0 to 2.0 Ga, which mirrors a shift from more TTG-like calcic to more alkalic compositions. This shift towards high-heat-producing granites post-2.0 Ga is often attributed to enrichment related to reworking and/or erosion. However, there is a strong correlation between granitic heat production with that of similarly-aged basalts and gabbros, which suggests a dominant mantle-level component to granite generation rather than crustal reworking. Secular cooling and mantle depletion may affect heat production, but the signal is complex and cannot easily explain the heat production with age profile. The most likely mechanism to describe the observed heat production–age pattern is one of selective preservation as a consequence of thermal stability. High heat producing terranes that were not stable during the Archean become increasingly stable towards the present. This selective preservation model has significant implications for the growth and composition of the continental crust. Ferroan, alkalic and felsic compositions were less thermally stable in the Archean due to their generally higher heat production and thus may have been more common in the early Earth than assumed by most compositional models. The temporal heat production model determined in this study can be used to improve geotherm models, particularly within ancient terranes.

U–Pb Geochronology of Hydrothermal Monazite from Uraniferous Greisen Veins Associated with the High Heat Production Mount Douglas Granite, New Brunswick, Canada

A combination of in situ laser ablation inductively coupled plasma–mass spectrometry (LA ICP–MS) analyses guided by Scanning Electron Microscope–Back-Scattered Electron imaging (SEM–BSE) was applied to hydrothermal monazite from greisen veins of the Late Devonian, highly evolved, uraniferous Mount Douglas Granite, New Brunswick, Canada. Understanding the uraniferous nature of the suite and characterizing the hydrothermal system that produced the associated mineralized greisen veins were the main goals of this study. The uraniferous nature of the Mount Douglas Granite is evident from previous airborne radiometric surveys, whole-rock geochemical data indicating high U and Th (2–22 ppm U; 19–71 ppm Th), the presence of monazite, zircon, xenotime, thorite, bastnaesite, and uraninite within the pluton and the associated hydrothermal greisen veins, as well as anomalous levels of U and Th in wolframite, hematite, and martite within greisen veins. New U–Pb geochronology of hydrothermal monazite coexisting with sulfide and oxide minerals yielded mineralization ages ranging from 344 to 368 Ma, with most of them (90%) younger than the crystallization age of the pluton (368 ± 3 Ma). The younger mineralization age indicates post-magmatic hydrothermal activities within the Mount Douglas system that was responsible for the mineralization. The production of uraniferous greisen veins by this process is probably associated with the High Heat Production (HHP) nature of this pluton, resulting from the radioactive decay of U, Th, and K. This heat prolongs post-crystallization hydrothermal fluid circulation and promotes the generation of hydrothermal ore deposits that are younger than the pluton. Assuming a density of 2.61 g/cm3, the average weighted mean radiogenic heat production of the Mount Douglas granites is 5.9 µW/m3 (14.1 HGU; Heat Generation Unit), in which it ranges from 2.2 µW/m3 in the least evolved unit, Dmd1, up to 10.1 µW/m3 in the most fractionated unit, Dmd3. They are all significantly higher than the average upper continental crust (1.65 µW/m3). The high radiogenic heat production of the Mount Douglas Granite, accompanied by a high estimated heat flow of 70 mW/m2, supports the assignment of the granite to a ‘hot crust’ (>7 HGU) HHP granite and highlights its potential for geothermal energy exploration.

Investigation of the thermal structure and radiogenic heat production through aeromagnetic data for the southeastern Aegean Sea and western part of Turkey

This paper presents a correlation of thermal structure with seismic activity of southeastern Aegean Sea and western part of Turkey. The reduce to pole method was applied to magnetic data and the magnetic map obtained from reduce to pole correction was divided into 132 overlapping blocks with the sizes 90 × 90 km for estimating Curie Point Depth. These depths were varied from 9.8–19.5 km whereas heat flow values were found to be 50–125 mW m−2. Generally, the region is characterized by shallow Curie Point Depth values because of the continental extensional regime in N–S directions and subduction zone. Furthermore, radiogenic heat production values were calculated as 0.21−0.51 μWm−3 and the distribution of these values showed that high radiogenic heat values associated with the region where high heat flow values were seen. Seismotectonic activity in the region implies that the deep-focus earthquakes (̴180 km) occurred in the subduction zone and these earthquakes were characterized with high heat flow zone in the Aegean Sea. All the results have shown good agreement with geological and tectonic regime of the Aegean Region.

Radiogenic heat production analysis of Fennoscandian Shield and adjacent areas in Sweden

Radiogenic heat production analysis of Fennoscandian Shield and adjacent areas in Sweden

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero: Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak P–T conditions of 5–7 kbar and 700–750 °C and a published age of ca. 2730 Ma based on U–Pb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving P–T conditions of 8–9 kbar and 550 °C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak P–T conditions of 7–8 kbar and 650–700 °C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak P–T conditions of 8–9 kbar and 650–700 °C on a clockwise P–T evolution. This sample has a U–Pb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak P–T conditions of 5–6 kbar and 650–700 °C on a near isobaric P–T path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon U–Pb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775–2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.

Conductive heat flow pattern of the central-northern Apennines, Italy

We analyzed thermal data from deep oil exploration and geothermal boreholes in the 1000-7000 m depth range to unravel thermal regime beneath the central-northern Apennines chain and the surrounding sedimentary basins. We particularly selected deepest bottom hole temperatures, all recorded within the permeable carbonate Paleogene-Mesozoic formations, which represent the most widespread tectono-stratigraphic unit of the study area. The available temperatures were corrected for the drilling disturbanceand the thermal conductivity was estimated from detailed litho-stratigraphic information and by taking into account the pressure and temperature effect. The thermal resistance approach, including also the radiogenic heat production, was used to infer the terrestrial heat flow and to highlight possible advective perturbation due to groundwater circulation. Only two boreholes close to recharge areas argue for deep groundwater flow in the permeable carbonate unit, whereas most of the obtained heat-flow data may reflect the deep, undisturbed, conductive thermal regime.

Paleo heat flow in areas of Sedimentary Exhalative (SEDEX) deposits of Eastern Brazil

Representative values of fluid inclusion temperatures and radiogenic heat production values have been compiled as part of an attempt to determine paleo heat flow in areas sedimentary exhalative (SEDEX) deposits in thirteen localities of eastern Brazil. The results obtained indicate heat flow in excess of 80 mW/m2in areas of mineral bearing sulphide ore deposits, during periods of ore forming processes. Such anomalously high heat flow are more than twice the present-day values for stable tectonic units of Precambrian age. There are indications that high heat flow values were sustained by circulation of hydrothermal fluids in the upper crust, during periods not exceeding a few hundred million years. The resulting geothermal episodes may be considered as constituting short-period “heat pulses” occurring in stable tectonic environments, generated by magma emplacements in the upper crust, leading to formation of areas of sulfide ore deposits. Model simulations indicate that subsidence episodes induced by stretching and magma under-plating constitute the mechanisms for high heat flow during the ore-forming processes.

A model for thermal gradient and heat flow in central Chile: The role of thermal properties

The aim of this work is to quantify variations in heat flow and thermal gradient patterns at the latitude of central Chile and to evaluate the role of thermal properties of macro-geological units within the lithosphere. We developed a numerical thermal model for a continental-scale cross section at 33∘S latitude, integrating available and new data on geometry and dynamics of subduction, as well as thermal and mechanical properties for the continental and oceanic lithosphere, and asthenosphere. The model compares heat flow and thermal gradient curves against homogeneous inputs for radiogenic heat production (RHP) and thermal conductivity. The results of this model were calibrated with results of thermal gradient measurements at different morpho-tectonic domains. The results show that both, cold slab subduction and mantle wedge advection, play major roles in the regional thermal structure. Variations with respect to regional tendency are due to changes in thermal properties. The fore-arc has an average thermal gradient from 12 to 16∘C/km, whereas in the High Andes Cordillera it reaches up to values in the range between 20 and 28∘C/km. Model results indicate that thermal gradient increases eastward up to ∼25∘C/km in the easternmost foreland, which is a stable domain. The consistency of the model with respect to seismic record is discussed. Variation in composition and thermal properties is common at subduction zones due to dynamic petrologic processes. The RHP of the exposed upper crust units was obtained from more than 1000 in-situ measurements, whereas thermal conductivity, specific heat capacity and density of samples were obtained in the laboratory. The calculated RHP of upper crust is ∼2.0 μW/m3, which is responsible for ∼15–40% of the heat flow that reaches the surface.

Determination of the radiation dose rate and radiogenic heat production of North Gabal Abu Hibban area, central Eastern Desert, Egypt

ABSTRACTAirborne gamma-ray spectrometric data for Gabal Abu Hibban area, central Eastern Desert, Egypt have been used for assessment of the radiation dose rate and recognize radiogenic heat production (RHP). The radioelement concentrations of eU (ppm), eTh (ppm) and K (%) were used to ascertain such rates for the rock units. Results showed several levels of radiation as follow; (less than 0.50 mSv/yr), (from 0.50 to 0.70 mSv/yr) and (from 0.70 to 2.5 mSv/yr). The dose rate more than 1 mSv/yr is considered the radioactivity hazard level which represented mainly with Mu'tiq group, Younger granite, Trachyte rocks and Duwi Formation. RHP calculations have been carried out for the various rock units to locate the highest radiogenic heat production. The rock units that possess relatively huge RHP are Mutiq group, Younger granite and Trachyte rocks with values more than 3 μWm-3.

Radiogenic heat production in the Coastal Plain Sands of Ipokia, Dahomey Basin, Nigeria

This study is aimed at establishing the radiogenic heat production in the Benin Formation of Dahomey Basin using in-situ measurements of elemental concentrations of uranium, thorium and potassium over a kaolin deposits in Ifonyintedo, Ipokia LGA, Ogun State. Super-Spec gamma ray spectrometer is used to obtain varied data of naturally occurring radionuclides over eleven different stations in the Coastal Plain Sands at 1 m height above the ground. The mean heat produced by uranium, thorium and potassium are 294.3, 410.5 and 0.001 ρW kg−1 respectively. The total heat produced in the study area varied from 544.0 to 902.1 ρW kg−1, which classified the study area as Low-to-Moderate Heat Production Potential zone. This classification corresponds to the radiogenic heat characteristics of Benin Formation in Niger Delta.•In-situ measurements of three radioisotopes were made over Coastal Plain Sands of Dahomey Basin.•The mean heat produced by uranium, thorium, potassium and the total radiogenic heat are 294.3, 410.5, 0.001 and 704.79 ρW kg−1 respectively.•The study area is characterized by Low-to-Moderate Heat Production Potential.

Enhancement of organic matter maturation because of radiogenic heat from uranium: A case study from the Ordos Basin in China

Carbonaceous debris (CD) within uranium-bearing strata has been studied in the Daying uranium deposit of the northern Ordos Basin, northern China. The influence of radiogenic heat from uranium on organic matter maturation was investigated through a series of tests including measurements of vitrinite reflectance (Ro), fission-track (FT) analysis in quartz grains, and the calculation of the radiogenic heat production rate of the samples. The results show that Ro in uranium-bearing strata generally increases as the burial depth increases, indicating that CD experienced normal burial coalification. However, Ro values of the samples rich in uranium are 0.062% Ro higher than those without uranium mineralization. Vitrinite reflectance bears a positive relationship with uranium content, and an inverse relationship with distance to the closest sandstone rich in uranium, indicating that uranium enrichment enhances organic matter maturation. The production of uranium decay makes FT observable in quartz grains, and the intensity of decay increases with proximity to the uranium ore body. The calculated radioactive heat production rate from the uranium ore body is 6.857 × 10−5 W/m3. During the long-term stable decay, as the uranium ore body theoretically results in an abnormal increase in temperature of 52°C without consideration of the loss of heat conduction, heat convection, and thermal radiation, this would yield a theoretical Ro increase of 0.209% Ro, reasonably greater than the observed. Therefore, the long-term stable radiogenic heat produced by uranium ore body can slightly enhance organic matter maturation, which is instructive in uranium prospecting.

The Formation of Continental Crust from a Physics Perspective

The generation of crustal material and the formation of continental crust with a thickness of ≈40 km involve different physical mechanisms operating over different time-scales and length-scales. This review focusses on the building of a thick crustal assemblage and on the vertical dimension where the consequences of gravity-driven processes are expressed most clearly. Continental crustal material is produced by a sequence of crust and mantle mlelting, fractionation of basaltic melts and sinking of dense mafic cumulates. The repeated operation of these mechanisms over tens of million years leads to a thick stably stratified crust. We evaluate the main mechanisms involved from a physics perspective and identify the key controls and constraints, with special attention to thermal requirements. To form magma reservoirs able to process significant magma volumes and to allow the foundering of mafic cumulates, melt must be fed locally at rates that are larger than that of average crustal growth. This requires the temporary focussing of magmatic activity in a few centers. In some cases, foundering of dense cumulates does not go to completion, leaving a deformed residual body bearing tell-tale traces of the process. Crust must be thicker than a threshold value in a 30–45 km range for mafic cumulates to sink into the mantle below the crust. Once that threshold thickness has been reached, further additions lead to increase the proportion of felsic material in the crust at the expense of mafic lithologies which disappear from the crust. This acts to enhance radiogenic heat production in the crust. One consequence is that crustal temperatures can be kept at high values in times of diminished melt input and also when magmatic activity stops altogether, which may lead to post-orogenic intracrustal melting and differentiation. Another consequence is that the crust becomes too weak mechanically to withstand the elevation difference with neighbouring terranes, which sets a limit on crustal thickening. The thermal structure of the evolving crust is a key constraint on the overall process and depends strongly on radiogenic heat production, which is surely one of the properties that make continental crust very distinctive. In the Archean Superior Province, Canada, the formation of juvenile continental crust and its thermal maturation 2.7 Gy ago can be tracked quite accurately and reproduced by calculations relying on the wealth of heat flow and heat production data available there. Physical models of magma ascent and storage favour the formation of magma reservoirs at shallow levels. This suggests that crustal growth proceeds mostly from the top down, with material that gets buried to increasingly large depths. Vertical growth is accompanied by lateral spreading in two different places. Within the crust, magma intrusions are bound to extend in the horizontal direction. Deeper down, lateral variations of Moho depth that develop due to the focussing of magmatic activity get relaxed by lower crustal flow. This review has not dealt with processes at the interface between the growing crust and the mantle, which may well be where dikes get initiated by mechanisms that have so far defied theoretical analyses. Research in this particular area is required to further our understanding of continental crust formation.

Numerical analysis of the role of radiogenic basement on temperature distribution in the St. Lawrence Lowlands, Qu\xe9bec

Regions with low or medium surface heat flow in stable cratonic areas, such as in eastern Canada, have received little attention for geothermal energy. In the presence of a high heat-producing basement overlain by a sedimentary cover, however, such areas might be prospective. Their potential will depend on various parameters such as heat production within the basement, thermal conductivity of sedimentary formations, and structural context. In this study, we aim at quantifying the importance of these parameters on temperature distribution at depth for a model representative of the St. Lawrence Lowlands, Québec, where locally anomalously higher heat flow has been observed and a 3D model of sedimentary cover is available. Scenarios involving physical properties from neighbouring Grenvillian domains are considered: Portneuf–Mauricie domain with radiogenic heat production of 0.94–5.83 upmu hbox {W m}^{-3}, 0.02–4.13 upmu hbox {W m}^{-3} for the Morin Terrane, and 0.34–1.96 upmu hbox {W m}^{-3} for the Parc des Laurentides domain. The impact of radiogenic heating on temperature distribution at depth was simulated using the Underworld2 numerical modeling code. Results show that at 5 km depth, the range of temperature difference is 22 °C for all modeled scenarios. In addition, the benefit of the thermal blanket effect of the sedimentary cover can be significant, but depends strongly on the contrast in thermal conductivity between the basement and the cover, as well as on the structural context, and less on heat production in the basement. Finally, depth of the 120 °C isotherm varies by up to 1 km for the scenarios considered; carefully assessing the boundary conditions therefore, appears critical in an exploration context.