Concentrations Of Heat-producing Elements Research Articles

Effects of magma-generation and migration on the expansion and contraction history of the Moon

Geological and geodetic observations of the Moon from spacecraft revealed that it expanded by a few km for the first several hundred million years and then contracted later. The period when the planet expanded most coincides with that when the mare volcanism of the Moon was active. Given the high initial temperature of the deep mantle inferred from the giant impact and mantle overturn hypotheses of the Moon, the observed early expansion is difficult to account for by thermal expansion only. To understand the observed radial change of the Moon, we numerically calculated the thermal evolution of a one-dimensional spherically symmetric mantle caused by transport of heat, mass, and incompatible heat-producing elements (HPEs) by migration of magma that is generated by internal heating. The mantle is assumed to be enriched in HPEs at its base in the initial condition. The calculated mantle expands for the first several hundred million years by melting of the deep mantle and upward migration of the generated magma to the uppermost mantle; the top of the partially molten region rises to the depth level of around 300 km, which is shallow enough to generate mare basalts of the Moon. The migrating magma, however, extracts HPEs from the deep interior, and the planet then contracts gradually by cooling and solidification of the partially molten mantle. We obtained a thermal history model that is consistent with the observed history of radial change of the Moon when the initial mid-mantle temperature T_{{text{M}}} approx 1600 ,{text{K}} and the initial ratio of the concentration of HPEs in the crust to that of the mantle F_{{{text{crst}}}}^{*} le 12. This model suggests that melting of the deep mantle and upward migration of the generated magma strongly affect the thermal history of the Moon. The model we developed here is a good starting point for constructing more realistic models of the thermal history of the Moon where the effects of heat and mass transport by mantle convection are also considered.Graphical

Heat flow in Rajasthan Craton, North–Western Indian Shield and its Implications

A synthesis and reinterpretation of surface heat flow values of the Rajasthan Craton(RC), north-western Indian Shield has been carried out. The results reveal that the q values are relatively high, varying from 52 to 96 mWm-2. Appreciable variations in the magnitude of q are found between and within its geotectonic units, but with considerable overlap of values. Paleo–Meso Proterozoic Aravalli Super Group is generally associated with high q values, varying from 52 to 67 mWm-2, with a mean value of 60 mWm-2. Values of still higher order varying from 52 to 76 mWm-2 occur in North Delhi Fold Belt of Meso–Neo Proterozoic Delhi super Group, Tusham area, which is located towards north west corner of the Delhi Fold Belt in the Trans-Aravalli Province of RC, is characterized by a very high heat flow value. This is the highest of all heat flow values so far reported for the Proterozoic terrains of the Indian Shield, which is ascribed as due both to enhanced crustal radioactivity and renewed tectonic activity beneath its region. The heat flow values reported in areas of the Delhi Super Group and Aravalli Super Group are of short wavelength, naturally these are due to shallow sources. We infer that these are mostly due to varying degrees of concentration of heat producing elements in the upper crust. There are also no evidences of major tectono-thermal events after the Neo-Proterozoic times (~600Ma) in the study area. Hence, we conclude that observed variations in heat flow do not carry thermal transients of any recent tectonic activity.

The thermal equation of state of (Mg, Fe)SiO3bridgmanite (perovskite) and implications for lower mantle structures

AbstractThe high‐pressure/high‐temperature equation of state (EOS) of synthetic 13% Fe‐bearing bridgmanite (Mg silicate perovskite) is measured using powder X‐ray diffraction in a laser‐heated diamond anvil cell with a quasi‐hydrostatic neon pressure medium. We compare these results, which are consistent with previous 300 K sound speed and compression studies, with a reanalysis of Fe‐free Mg end‐member data from Tange et al. (2012) to determine the effect of iron on bridgmanite's thermoelastic properties. EOS parameters are incorporated into an ideal lattice mixing model to probe the behavior of bridgmanite at deep mantle conditions. With this model, a nearly pure bridgmanite mantle composition is shown to be inconsistent with density and compressibility profiles of the lower mantle. We also explore the buoyant stability of bridgmanite over a range of temperatures and compositions expected for Large Low‐Shear Velocity Provinces, concluding that bridgmanite‐dominated thermochemical piles are more likely to be passive dense layers externally supported by convection, rather than internally supported metastable domes. The metastable dome scenario is estimated to have a relative likelihood of only 4–7%, given the narrow range of compositions and temperatures consistent with seismic constraints. If buoyantly supported, such structures could not have remained stable with greater thermal contrast early in Earth's history, ruling out formation scenarios involving a large concentration of heat producing elements.

Probing the Earth’s Deep Interior Through Geochemistry

Basaltic volcanism provides a window into the Earth’s mantle. Seeing through this window requires seeing through the processes of magma genesis and evolution that distort the view. Radiogenic isotope ratios, and to a lesser extent incompatible elements and stable isotope ratios, allow us to see through these distortions and infer the nature and evolution of various mantle magma sources. Geochemical studies of mid-ocean ridge basalts (MORB) reveal that the upper mantle was long ago depleted in incompatible elements through previous episodes of melt extraction, likely associated with crust formation. Oceanic island basalts (OIB) are the products of mantle plumes and hence provide us with a view of the deepest parts of the Earth’s mantle. These plumes can be divided into several genera, EMI, EM II, and HIMU, most of which contain several species. Little or no mixing is observed between these genera, but all mix with a prevalent mantle component, PREMA, that contributes to many if not all plumes. Although the mantle comprising plumes also show evidence of previous melt extraction events, the more prominent chemical signature is of material anciently recycled from the Earth’s surface through subduction of oceanic crust and sediment, subduction erosion, and foundering of lower continental crust. This is most unambiguously manifest in variations in stable isotope ratios that could only have been produced at or near the surface of the Earth. In addition to a recycled component, many plumes also contain a quite primitive component, manifested primarily in their noble gas isotope ratios. The primitive component seems most closely associated with PREMA. In contrast to hot spot and mid-ocean ridge volcanism, subduction-related volcanism allows an assessment of the flux into the Earth’s mantle. It was apparent from early Pb isotope data that sediment was subducted along with oceanic crust, an observation later dramatically confirmed by the discovery of cosmogenic 10Be in subduction zone magmas. Because of this sediment, the flux into the mantle at subduction zones is more incompatible-element enriched than MORB. Although some fraction of this material is extracted into subduction zone magmas, a variety of approaches reveal that the flux into the deep mantle remains incompatible-element enriched. Other studies, particularly geophysical ones, indicate that this subduction flux is substantially augmented by subduction erosion of the overriding plate. Over Earth’s history these processes together with continental crustal founding may have delivered a volume of material equal to the present volume of the continents. The discovery that all modern terrestrial rocks have an excess of 142Nd, which is the decay product of the extinct radionuclide 146Sm, compared to chondrites, reopens the question of the Earth’s composition. One important implication is that the concentrations of heat producing elements, U, Th, and K, may be 25 % or more lower than previously thought. Another implication is that the depleted source of MORB may constitute a very large fraction of the mantle, perhaps nearly all of it. On the other hand, the heat flow from the core may be substantially greater than previously thought. This implies that the lowermost mantle and the core-mantle boundary layer may play a dominant role in driving mantle convention, and ultimately all geologic processes. Geophysical evidence is revealing that the lowermost mantle is indeed a dynamic and interesting region. The challenge for mantle geochemists is to integrate their observations with those of geophysicists and progress is slowly being made in this respect.

Evolution of the Corsica–Sardinia Batholith and late-orogenic shearing of the Variscides

The Corsica–Sardinia Batholith formed in the late Carboniferous–Permian along the northern Gondwana margin. One or more of the following processes may have raised the Variscan geotherm enhancing melting of the crust and uppermost mantle, such as: i) break-off and detachment of the Rheic oceanic slab, ii) mantle delamination due to gravitational re-equilibration of thickened crust, iii) shear heating, iv) advection of mantle-derived melts, and v) concentration of heat-producing elements. In this paper, we present a simple one-dimensional thermal model to explain the origin of the batholith in a geodynamic setting consistent with intraplate shearing between Gondwana and Cadomian–Avalonian blocks. Input parameters, and boundary and initial conditions of the model crust are derived from a careful re-examination of large petrological, geochronological, and structural datasets. All parameters are varied within a range of geologically realistic values to reproduce thermal histories related to different tectonic processes, and the reliability of models is quantitatively evaluated by comparing the simulated geotherms with a large dataset of pressure–temperature–time (P–T–t) constraints. The best fit to P–T–t constraints is obtained for: i) break-off in the Late Devonian–Early Carboniferous, ii) decreasing exhumation rates from 340 to 280Ma, and iii) transpression at high average strain rates, mostly coeval with the peak of high-T–low-P metamorphism.

Estimating concentrations of heat producing elements in the crust near the Sudbury Neutrino Observatory, Ontario, Canada

Abstract Because the concentrations of uranium and thorium in the crust must be determined precisely for the future geoneutrino observations planned at the Sudbury Neutrino Observatory, we investigate whether airborne radiometric surveys can be used to constrain crustal radioactivity. The regional airborne surveys cover a wide area with high spatial resolution ( σ ) μW m − 3 for more than 176,000 values. The outcrop samples collected along a transect in the Superior Province yield an average heat production of 2.9 ± 2.4 ( σ ) μW m − 3 and core samples from drill holes yield an average of 2.5 ± 0.8 (σ) μW m − 3 . The high heat production in the rock samples is consistent with surface heat flux measurements near Sudbury with a mean value that is 12 mW m − 2 higher than the average Canadian Shield. The study shows that airborne aeromagnetic surveys give useful information on lateral variations in surface heat production but are unlikely to provide the reliable values of heat production needed to calculate the crustal geoneutrino flux. Crustal heat production will be best calculated from heat flux data complemented by heat production measurements on rock samples. The high mean heat production in Sudbury Igneous Complex samples (≈ 1.5 μW m − 3 ) suggests that the main source of the melt sheet was the very radioactive upper crust of the Superior Province or that the melt sheet was extremely enriched relative to a lower crustal source.

Geology, geochemistry, and geophysics of the Moon: Status of current understanding

The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH− and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3Ga, starting at about 3.9–4.0Ga and ceasing at ∼1.2Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

Crustal recycling, mantle dehydration, and the thermal evolution of Mars

We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650 K, preferably a primordial crustal thickness of 30 km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500 Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core–mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100 ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14 m thick global surface layer, suggesting that volcanic outgassing of H 2O could have significantly influenced the early martian climate and increased the planet’s habitability.

Thermal, radioactive and magnetic properties of the lavas of the Mt Melbourne Volcanic Field (Victoria Land, Antarctica)

We present the results of measurements of physical properties carried out on mafic lavas from the Mt Melbourne Volcanic Field, useful for interpretation of geophysical surveys designed to shed light on the structure of the crust. The thermal conductivity is comparable to that of glass and shows a clear negative dependence on porosity. The volume heat capacity and the thermal diffusivity are less variable. The concentration of the thermally important natural radioactive isotopes was determined by gamma-ray spectrometry. Lavas denoted a rather low heat-production rate, and the largest concentration of heat-producing elements (potassium, uranium, thorium) was found in the trachyte samples. The magnetic susceptibility is more variable than the other physical properties and, among the several iron-titanium oxides, it appears primarily controlled by the ulvospinel-magnetite solid solution series.

Ancient Lunar Crust: Origin, Composition, and Implications

Samples from the Apollo (USA) and Luna (Soviet) missions and from lunar meteorites, coupled with remote sensing data, reveal that the ancient highlands of the Moon are compositionally diverse. The average surface material contains 80 vol% plagioclase. A major suite of rocks, the ferroan anorthosites, averages 96 vol% plagioclase. The feldspathic composition reflects plagioclase flotation in a magma ocean. Late-stage REE-rich magma pooled in the Procellarum region of the lunar nearside. The concentration of heat-producing elements in this region triggered mantle melting and overturn of the cumulate pile, forming two more suites of chemically distinct highland rocks, the magnesian and alkali suites.

The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift system

Two distinct stages of extension are recognized in the West Antarctic Rift system (WARS). During the first stage, beginning in the Late Cretaceous, extension was broadly distributed throughout much of West Antarctica. A second stage of extension in the late Paleogene was focused primarily in the Victoria Land Basin, near the boundary with the East Antarctic craton. The transition to focused extension was roughly coeval with volcanic activity and strike–slip faulting in the adjacent Transantarctic Mountains. This spatial and temporal correspondence suggests that the transition in extensional style could be the result of a change in plate motions or impingement of a plume. Here we use finite element models to study the processes and conditions responsible for the two-stage evolution of rifting in the WARS. Model results indicate that the transition from a prolonged period of broadly distributed extension to a later period of focused rifting did not require a change in the regional stress regime (changes in plate motion), or deep mantle thermal state (impingement of a plume). Instead, we attribute the transition from diffuse to focused extension to an early stage dominated by the initially weak accreted lithosphere of West Antarctica, and a later stage that concentrated around a secondary weakness located at the boundary between the juvenile West Antarctica lithosphere and Precambrian East Antarctic craton. The modeled transition in extension from the initially weak West Antarctica region to the secondary weakness at the West Antarctic–East Antarctic boundary is precipitated by strengthening of the West Antarctica lithosphere during syn-extensional thinning and cooling. The modeled syn-extensional strengthening of the WARS lithosphere promotes a wide-rift mode of extension between 105 and ∼ 65 Ma. By ∼ 65 Ma most of the extending WARS region becomes stronger than the area immediately adjacent to the East Antarctic craton and extension becomes concentrated near the East Antarctic/West Antarctic boundary, forming the Victoria Land Basin region. Mantle necking in this region leads to syn-extensional weakening that promotes a narrow-rift mode of extension that becomes progressively more focused with time, resulting in formation of the Terror Rift in the western Victoria Land Basin. The geodynamic models demonstrate that the transition from diffuse to focused extension occurs only under a limited set of initial and boundary conditions, and is particularly sensitive to the pre-rift thermal state of the crust and upper mantle. Models that predict diffuse extension in West Antarctica followed by localization of rifting near the boundary between East and West Antarctica require upper mantle temperatures of 730 ± 50 °C and sufficient concentration of heat producing elements in the crust to account for ∼ 50% of the upper mantle temperature. Models with upper mantle temperatures < ca. 680 °C and/or less crustal heat production initially undergo diffuse extension in West Antarctica, and quickly develop a lithospheric neck at the model edge furthest from East Antarctica. Models with upper mantle temperatures > ca. 780 °C do not develop focused rifts, and predict indefinite diffuse extension in West Antarctica.

Modes of active intraplate deformation, Flinders Ranges, Australia

The Flinders Ranges form one of the most seismically active zones within the Australian continent with seismogenic strain rates over the last 30 years of ∼10−16s−1. Active deformation in the region reflects late Neogene increases in stress levels in the Indo‐Australian plate as a response to increased plate boundary forcing from collision zones with the neighboring Asian and Pacific plates. Geological and geophysical observations suggest two modes of active deformation in operation in the Flinders Ranges over the last several million years: (1) low‐amplitude (∼200–500 m), long‐wavelength (∼200 km) lithospheric flexure and (2) active reverse faulting along the range front with fault slip rates of up to ∼50 m Myr−1. Numerical models are developed to explore the contribution of each of these deformation modes to the observed geophysical signals. An elastic mode of deformation is suggested by a distinctive long‐wavelength positive correlation between gravity and topography in which the Flinders Ranges are bordered by anomalous topographic and gravity lows, now occupied by playa‐lake systems, centered some 50 km outboard of range‐bounding faults. Numerical models show that flexural instabilities localized by vertical loads arising from older tectonic structuring produce a first‐order match with observed topography and gravity. Numerical models are also used to illustrate how the localized failure evident in the contemporary seismicity and Quaternary faulting record within the Flinders Ranges reflects thermal weakening associated with extraordinary concentrations of heat producing elements in the crust, as reflected in modern‐day heat flows of ∼90 mW m−2.

A thermo-chemical regime in the upper mantle in the early Earth inferred from a numerical model of magma-migration in a convecting upper mantle

A numerical model of mantle magmatism in a convecting upper mantle has been developed to study the thermo-chemical evolution of the upper mantle of the early Earth. The solid-state convection in the upper mantle is modeled by a convection of a binary eutectic material with a Newtonian temperature-dependent rheology in a two-dimensional rectangular box placed on a heat bath as a model of the lower mantle. The density depends on the chemical composition and melt-content as well as temperature of the material. The material contains heat-producing elements that are incompatible and exponentially decay with time. Mantle magmatism is modeled by a permeable flow of melt generated by a pressure-release melting induced by the solid-state convection through matrix. The permeable flow is driven by a buoyancy due to the density difference between the melt and the matrix. The thermo-chemical evolution in the box occurs in two stages if the deeper part of the box is not so strongly depleted in heat-producing elements in spite of the upward migration and concentration of heat-producing elements into a crustal layer along the top surface boundary due to magmatism. In the earlier stage, active magmatism occurs because of a strong internal heating due to the heat-producing elements, a chemically stratified structure develops well in the box with dense magmatic products in the deeper part and less dense residual materials in the shallower part, and the temperature distribution becomes strongly superadiabatic over the entire box. The temperature at the base of the box becomes as high as the solidus temperature. The chemically stratified structure is, however, suddenly destroyed by convective mixing and the temperature in the deeper part of the box suddenly drops by several hundred degrees when the internal heat source becomes too weak owing to the decay of heat-producing elements which sustain the active magmatism and hence keep the effect of chemical differentiation due to the magmatism stronger than the effect of convective mixing. In the later stage of the evolution, the box becomes chemically homogeneous and magmatism occurs only mildly. If heat-producing elements are efficiently transported into the crustal layer and the deeper part of the box becomes strongly depleted in heat-producing elements owing to the magmatism, only a mild magmatism occurs even at the beginning, a chemically stratified structure does not develop well, and the temperature in the box rapidly decreases to a stationary value. The regime of hot and chemically stratified upper mantle suggested from the earlier stage of the case with mild depletion of heat-producing elements at depth fits in with many observations of the Archean continental crust.

Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements

We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m −3; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m −3; Kapuskasing granulites = 0.44 μW m −3. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade. The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m −3. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m −3, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m −2 lower than the reduced heat flow calculated from the heat flow-heat production regression.

Moderate-Temperature Geothermal Resources Beneath Atlantic Coastal Plain: ABSTRACT

The most promising geothermal resource in the eastern United States is warm water stored in the permeable sediments of Cretaceous and younger age beneath the Atlantic Coastal Plain. Optimum sites for high gradients are locations where the heat flow is high and sediment thermal conductivity is low. Low conductivity is characteristic of most of the Coastal Plain sediments. Heat flow in the eastern United States varies from about 33 mW/m2 to 100 mW/m2. All variations in heat flow in the eastern United States are caused either by differences in the concentrations of the heat-producing isotopes of uranium, thorium, and potassium in crystalline rocks, or by differences in thickness of heat-producing crystalline rocks. The highest concentrations of heat-producing elements, and the highest heat flows, are in the relatively young (ca. 300 m.y.) unmetamorphosed granite stocks and batholiths. Similar granites also are present in the southeast in the basement beneath the Coastal Plain. Thus, optimum sites for the development of moderate-temperature geothermal resources beneath the Coastal Plain require a knowledge of the (1) distribution and thickness of heat-producing granites in the basement, (2) thermal conductivity and thickness of sediments above basement, and (3) nature and extent of aquifers in the sediments above basement. A site for the first deep geothermal test on the Atlantic Coastal Plain was chosen at Crisfield, Maryland. A temperature of 57°C was found at a depth of 1.26 km. Economic analyses at this site and elsewhere by the Applied Physics Laboratory of Johns Hopkins University, aquifer pump tests, and numerical modeling of the thermal lifetime of a reservoir suggest that geothermal energy may be an important resource at some locations on the Atlantic Coastal Plain. End_of_Article - Last_Page 1660------------