Heat flow and radiogenic heat production measurements have been obtained in different provinces of the Canadian Shield, ranging in age from 2700 to 400 Ma. There is no relation between heat flow and the age of the provinces. The average heat flow of the Early Proterozoic Trans-Hudson Orogen (1.8 Ga), 42±11 mWm-2, is identical to that of the Archean Superior Province (2.7 Ga), and of the younger Grenville Province (1.0 Ga). In each province, the correlation between heat flow and heat production is weak. The heat flow and heat production data are used with gravity and seismic data to constrain the crustal structure and composition. Heat flow variations across the Abitibi subprovince indicate significant differences in crustal composition that reflect the complex assemblages that make up the Archean crust. The heat flow exhibits a sharp increase from the Grenville Province to the Appalachians where the average heat flow is higher. This difference is due to higher heat production in the Appalachian upper crust with the same mantle heat flow as in the Shield (ca. 12 mWm-2). Precambrian crust is a complex assemblage of chemically and geologically distinct blocks, which is reflected in heat flow. As an example we review new data from the Trans-Hudson Orogen. High heat flow values are found in the Thompson Belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. The accumulation of sediments derived from older and enriched continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. Heat flow variations, that are related to the different history of magmatism and internal differentiation of the various belts, provide constrains on the crustal assemblage. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of island arc volcanic rocks. In the Flin Flon Belt, the low heat production and the lack of correlation with heat flow suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake Belt, the heat flow is lower than the average although the surface heat production is not low. The heat flow data require a thin surface layer overlying the mid and lower crust depleted in radioelements. The analysis of these data supports the view that the variability of the surface heat flow between the subprovinces can be entirely accounted for by changes in crustal composition and thickness.
A growing body of information regarding the seismic structure of the continental lithosphere has led to renewed interest in the detailed modeling of the thermal structure beneath Precambrian regions. We have approached this problem by applying uniform analysis to the heat flow data base of Pollack and others (1990). Specifically, for our global study, we have assumed steady state conduction and have used measured values of upper crustal heat production, and commonly accepted heat production values for the lower crust and sub-crustal lithosphere. The results are lithospheric geotherms for nearly all Precambrian regions that can be compared qualitatively, even if some uniform bias is introduced by our modeling assumptions. We find a global trend of thickening of the lithosphere with age from about 140±40 km in the mid-late Proterozoic, to 180±40 km in the early Proterozoic, and 250±70 km in the Archean lithosphere. We compare these estimates with those obtained from seismic tomography, and discuss the global variations in lithospheric thickness in terms of the evolution of the lithosphere from the Archean to the present.
We use the results of a geochemical survey (1150 outcrop samples) for heat production studies in a E-W oriented band (120 km x 500 km), located approximately between latitudes 62o and 63o N in Finland, the central Fennoscandian Shield. The study area covers formations from the Archaean granite-greenstone terrain in the east to Palaeoproterozoic autochtonous and allochtonous covers to the west of it, as well as Palaeoproterozoic mobile belts, major granitoid areas and schist belts in central and western Finland. In this study we review the (volumetric) radiogenic heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production values with major tectonic setting, lithological type, major composition and petrophysical properties of the rocks. The aim is to find out whether the heat production rate shows any systematic variation with these factors.
Generally, there is an increase in heat production rates from the Archaean to Proterozoic rocks in E-W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.2 Wm-3 in the Archaean domain, 1.4W m-3 in the Höytiäinen autochtonous Proterozoic domain, 1.2 Wm-3 in the Suvasvesi domain characterized by allochtonous Proterozoic cover, 1.1 Wm-3 in the Proterozoic Raahe-Ladoga mobile belt, 1.3 Wm-3 in the Proterozoic Rantasalmi-Haukivuori domain, 1.6 Wm-3 in the Central Finland Granitoid Complex and 1.5 Wm-3 in the Bothian Schist belt. The standard deviations of the arithmetic mean values are considerable and attain values of about 40-115% of the means. All distributions overlap.
The heat production rate of igneous rocks (mainly granitoids) and metavolcanites show a weak positive correlation with SiO2, whereas metasediments show a negative or no correlation. The heat production rates of igneous rocks and metavolcanites show negative correlations with density but in the metasediments the correlation is positive. This can be attributed to the affection of heat producing elements in the pelitic (mica-rich) rock types which have low SiO2 but high density. Generally, heat production shows only a very weak variation with P-wave velocity.
Variations within the igneous and metavolcanic rocks are influenced by the source material composition which is controlled by the tectonic setting for the mantle-derived component and the 'maturity' of the crust for the crust-derived component. The geochemical character and tectonic type of a granitoid is more decisive for heat production levels than the geological age (Achaean vs. Proterozoic).
The results obtained so far suggest that there is no simple way to determine the heat production levels of a shield terrain, and that crustal heat production models must include careful consideration of the evolutionary history of the area under study.
A large body of data provides unprecedented control on the relations between heat flow density (Q) and radioactive heat production rate (A) in Colorado and Wyoming in the western United States. Four Q-A lines (Q = Qo + AH) have been established: 1) the Southern Rocky Mountains-Wyoming Basin areas in southeastern Wyoming with Qo = 26-28 mWm-2 and H = 7-8 km; 2) the easterly Frontal Ranges of the Colorado mountains with Qo = 54-58 mWm-2 and H = 8-10 km; 3) the Rio Grande rift zone in Colorado with Qo = 89-93 mWm-2 and H = 8-10 km; and 4) "Western Ranges" in Colorado with Qo = 68 mWm-2 and H = 12 km. The transitions between these Q-A zones are narrow (¾50-60 km); therefore, crustal sources must explain parts of the different regional intercepts and calculated residual heat flow density values (Qr). In southeastern Wyoming, normal Qo and Qr values in Archean and early Proterozoic basement terranes likely reflect deep erosion that produced a "normal" 7-8 km thick granitic layer that overlies a low-radioactivity lower crust. In the Colorado Front Range immediately to the south, however, late Proterozoic and younger silicic rocks with relatively enriched radioactivity could constitute a 20-25 km thick granitic layer, and produce a large part of the above-normal Qo and Qr values. There, high surface and residual heat flow density values correlate with a mountain root with enriched radioactivity, assuming that a large part of the Colorado Front Range topography is isostatically compensated by low-density crystalline masses in the upper crust. The unusually high surface and residual heat flow density values (88-118 mWm-2) in the Rio Grande rift zone in Colorado imply unrealistically high equilibrium temperatures near the crust-mantle boundary, and the narrow borders (¾50-60 km) of the rift zone anomaly must be largely due to sources in the crust. But, crustal radioactivity is not, by itself, preferred to explain the shape and magnitude of this anomaly because the required values of A would be abnormally high. Therefore, late Miocene and younger intrusions in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the high Qo and Qr values in the rift zone.
The well-determined widths of the borders of the juxtaposed Q-A provinces in Colorado and Wyoming allow unique interpretations of the parameters of the regional heat flow density-radiogenic heat production rate lines. Perhaps future studies should better-define the widths of indicated transitions elsewhere, and thus provide new insight into the sources responsible for the intercepts and slopes of the regional Q-A relations.
Analysis of radioactive element (RAE) (U, Th, K) distribution and radiogenic heat production have been done for the Aldan and Anabar shields and the Angara-Kan uplift of the Enisey Ridge. Geological maps and geological-geophysical sections served as the basis of investigation. Heat production has been calculated by the use of near 10.000 samples. The sharp radiogeochemical heterogenity of different blocks of the Archean crystalline basement of the Siberian craton has been revealed. This heterogenity is typical to granite-greenstone and granulite-gneiss provinces. Within the granite-greenstone provinces Th content in the infracrustal grey gneiss complex of the Batomga block (east part of the Aldan shield) is 3 time lower than in similar complex of the Chara-Olekma block (western part of the Aldan shield). It results in difference of radiogenic heat production by a factor of one and a half (0,51 mW/m3 and 0,78 mW/m3, consequently). Among granulite-gneiss provinces the repeated granitizating charnokite-granulitic complexes of the Central Aldan megablock and the Angara-Kan uplift are most enriched in RAE. Their radiogenic heat production (1,8-2,4 mW/m3) is more than 4 times higher than other granulite-gneiss complexes of the Aldan and the Anabar shields. The Precambrian potassium granites and the Phanerozoic subalkaline volcano-plutonic suites enriched in RAE and metasomatic rocks bearing U-Th mineralization as well as greenstone belt metavolcanics and mafic intrusions depleted in RAE increase the differences in heat production of nearsurface layer. Differences of heat generation decrease with depth. Weighted average heat production of upper crustal layer (up to 12-20 km) mostly depending on radiogeochemical characteristics of infracrustal complexes varies between discrete blocks no more than 2-3 times (from 0,8 mW/m3 to 2,3 mW/m3). Model values of heat generation in middle (up to 30-35 km) (0,46-0,49 mW/m3) and lower (up to 45-50 km) (0,08-0,11 mW/m3) crustal layers have been evaluated by correlation equation between heat production and density. According with heat production the enderbire-gneiss complex of the Kurul'ta block (0,52 mW/m3) and the metabasite-plagiogneiss series of the Anabar shield (0,41 mW/m3) can be considered as analogs of the middle crustal layer. Overall heat generation in the Precambrian crust of the Siberian craton basement uplift changes from 0,38 to 1,0 mW/m3. These variations result from radiogeochemical differences of the upper crustal layer which contribution is in the range 60-80%. The comparison of the values of radiogenic heat production in the crust with the measured heat flow has showed that crustal heat flow component varies in the range from 60% (the Anabar shield) to 70-80% (the Aldan shield and the Angara-Kan uplift).
Heat production A is calculated from U, Th and K concentrations as measured in rock samples today. Even with the long halflifes of the natural radioisotopes involved, their concentrations were considerably higher in the geologic past. Also the isotope ratios 235U/238U, 40K/39K evolved with time. On the other hand, the decay schemes of the heat producting radioisotopes do not change with time, the heat production constants also remain unchanged; only the changes in radioelement contents during the geologic history must be considered. These effects can be treated by exponential functions and A can be calculated with the previously higher concentrations. E.g. for the 2780 Ma old Gaborone Granite Complex (SE Botswana) with 20 ppm U, 86 ppm Th and 4.5% K today (A = 10 µW/m3) heat production was > 20 µW/m3 at time of emplacement.Implications for granites of different ages and of higher heat flow and correspondingly higher geothermal gradients in continental crust in the past will be discussed.
Radioactive heat-production values of rocks cropping out in Liguria (northwestern Italy) were obtained from laboratory analyses of uranium, thorium and potassium on 145 specimens by means of a gamma-ray spectrometer. The sampled rocks belong to the thrust units at the boundary zone between the Alps and the Apennines. They are representative of the crystalline basement with associated covers of the Savona Massif (SM), and the ophiolitic sequences with metasediments and sediments of the Voltri Massif (VM), the Sestri-Voltaggio Zone (SVZ) and the Lavagna Nappe (LN). In sedimentary rocks the heat production rate increases from limestones (1.34 µW/m3) to shales (2.70 µW/m3). The heat production of ophiolites varies from a minimum of 0.04 µW/m3 (serpentinites) to a maximum of 0.20 µW/m3 (metabasalts). Orthogneisses of the crystalline basement, deriving from granitic or granodioritic parent rocks, and metasediments show the highest value of radiogenic heat production (2.89 µW/m3). By analysing the heat production of each radioelement, it turns out that K contributes on average by 15% to the total heat generation. The maximum contribution (42%) was observed in the SVZ metagabbros. In the metasedimentary and sedimentary rocks, Th accounts on average for 45%, whereas dolomites have an anomalous behaviour compared to the other sedimentary rocks, as 97% of the radiogenic heat is produced by uranium. In ophiolites, the uranium contribution is also high with a maximum value of 86% for SVZ serpentinites. The contribution of each radioelement to the total radiogenic heat production, depending on the rock type and possible alteration processes, was studied by using the Th/U, K/U and K/Th ratios. Calc-schists, shales, radiolarites and phyllites have a Th/U ratio of 3-4 that is very close to that of orthogneisses and typical of continental crust rocks. The Th/U ratio of limestones is obviously lower, ranging between 2 and 3. Almost all ophiolites and the dolomites have values lower than 1. This can be related to particular metamorphic and sedimentary processes. Most of the analysed rocks shows a K/Th ratio ranging between 2x103 to 4x103. In particular, limestones and marls have an average value of 3x103 typical of sedimentary rocks. The heat production data together with available thermal conductivities allowed us to construct a crustal thermal model. The model was adapted from geological cross-sections and integrated geophysical interpretations. Finite element simulations were used to analyse the effects on the heat flux and the temperature structure due to heterogeneities in thermal parameters.
It is well known that terrestrial heat flow is the sum of the radioactive heat generation of natural radioactive elements (uranium, thorium and potassium) in the crust and the heat flow from the mantle. If we can evaluate the radioactive heat generation in the crust more precisely, we can estimate the heat flow from the mantle and the thermal state in the crust more adequately. In this study, the authors measured the contents of radioactive elements by using gamma-ray spectral analysis technique in rocks at various tectonic settings in Japan (upper crustal rocks, lower crustal rocks, accretionaly prism rocks and active fault regions), and investigated the vertical distribution in the crust.
Especially for an active fault region, we measured core samples obtained from the Nojima fault, Southwest Japan where the devastating Kobe earthquake with magnitude 7.2 occurred in January 1995 (over 6,000 people were died).
At the Nojima fault, we obtained core samples to the depth of 1,800 m. These samples are basically granitic rocks and it reaches the main fracture zone at the bottom. In this borehole, we also made the detailed temperature logs by using an optical fiber temparature sensor. Therefore we can compare the relationship between temperature anomaly, which may correlate ground water flow, and distribution of the UTK elements. It may help to understand redistribution process of the UTK elements.
In 1970, Shaw (1970) wrote equations linking the composition of basaltic melts (Cl) to mantle source composition (C0), extent of melting (F) and crystal-liquid distribution coefficients (D) . Because C0 and F are unknown and considered independent, to be solved, these equations require an assumption about one of these two parameters. A solution to this severe problem is to take into account that heat producing elements (U, Th, K) link mantle composition and temperature (C0 is linked to F via T). U, Th, K are amongst the most incompatible elements. Consequently, they present the largest variations of concentration in the mantle. We propose a new model in which variations of mantle temperature are directly linked to mantle heterogeneity (Heterogeneous Source Melting or HSM). Following a fractionation event within the mantle, different mantle domains are characterized by different (U, Th, K) concentrations, therefore different heat production rates, resulting with time in different temperatures. The enriched, hotter mantle will cross the solidus deeper and will melt more than the depleted mantle. In other words, the temperature, the composition and melting extents are not independent parameters. The HSM model allows to solve partial melting equations with both C0 and F as variables. In contrast to previously proposed melting models, it is based on mantle heterogeneity. It is also a general model which holds in all melting contexts. The model has been applied along a ridge devoid of any hotspot influence (the Pacific-Antarctic Ridge between 56 and 66°S) and a ridge influenced by hotspots (the Mid-Atlantic-Ridge between 10 and 70°N). For a mantle age fractionation of 200 Ma, small mantle temperature variations (< 20°) are predicted far from hotspots whereas large thermal anomalies (> 200°) are confirmed for (or close to) hotspots. According to the HSM model, the age of mantle heterogeneities cannot greatly exceed 250 Ma.
Shaw, DM, Geochim. Cosmochim. Acta, 34, 237-243, (1970).
This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5-6 to 14-15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ~2.5 -3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.
The Klenov massif forms the largest subsidiary single body of the Eisgarn type in the Czech part of the Moldanubian batholith. On its northeastern margin lies the small uranium deposit Okrouhlá Radoun (1 340 t U). Three subtypes of monzogranites of the Eisgarn type have been identified within the Klenov massif. The main body of the Klenov massif consists of equigranular fine- to medium-grained two-mica monzogranites of the Destná subtype. Independent apophyses of the medium- to coarse-grained, porphyritic monzogranites of the Címer subtype occur on the northeastern margin of the Klenov pluton. Some of these apophyses are also several hundreds of meters thick. The leucocratic aplite granites that form a transitional subtype between the Destná subtype and the dyke aplites have been indentified in the mine workings of the Okrouhlá Radoun uranium deposit and in the surface boreholes. Highest concentrations of radioactive elements occur in the Címer subtype. Thorium contents decreases toward the monzogranites of the Destná subtype and aplite monzogranites. The fall in thorium content is best reflected by Th/U ratio - Címer subtype (2.32), Destná subtype (0.71), aplite granites (0.61). The radiogenic heat production decrease in the same direction. The highest radiogenic heat production values have been identified also in the Címer subtype (2.83 mWm-3). Distinctively lower are the values of heat production both in the Destná subtype (1.96 mWm-3) and aplite granites (1.94 mWm-3). This is according to the distribution of total activity as detected by aiborne gamma spectrometry.Correlation analysis has revealed a positive correlation between U, Y and Zr content in all subtypes of granites of Klenov massif and a positive correlation between Th content and the contents of TiO2, MgO, Ba, Zr and Ce. No correlation between the U and Th contents was established in the porphyritic granites of the Címer subtype and a positive correlation between the two elements was found in the even-grained and aplitic granites. The results of the correlation analysis show U and Th to be associated above all with monazite, sphene, zircon and xenotime. The ThO2 content in the monazite from the Destná subtype is 7-13%, content of ThO2 in the xenotime is 0.5-0.6%, content of UO2 in the xenotime is 1.06-1.59%. In the two-mica monzogranites of the Destná subtype was found also positive correlation between the content of U and that of muscovite and the content of Th and biotite.
The terrestrial heat flow map of the Bohemia Massif revealsa zone of higher geothermal activity in its northwestern part. To evaluate the sources of the increased heat flow, heat production of the near surface crustal rocks was investigated together with simulation of the Cenozoic-Quaternary magmatism. Seven combined heat flow (Q) -heat production (A) data yielded Q = 35 + 6.6 A, but showed a considerable scatter. More detailed interpretation of the Q-A relationship based on multi-component analysis applied to five specific regions in the NW part of the Bohemian Massif confirmed more pronounced differentiation of U and Th in the uppermost crust, while K does not show much vertical variation. The estimated value of the reduced heat flow is 42.7 mW.m-2, the characteristic logarithmic decrements D of U, Th and K are 7 km, 11.9 km and 25.7 km, respectively. The thermal aspects of the magmatism were modelled by the numerical solution of the heat conduction equation in 2-D cross-sections characteristic for existing geological-tectonic structures. The results suggest that the volcanism-related component of the heat flow attains a few mW.m-2 only even in the most disturbed cases.
Two observations indicate that heat flow decreases from the surface to the bottom of the crust. First, if all the heat loss measured at the surface originated from the mantle Earth would have cooled faster. Second, a downward continuation of surface heat flow into the crust would result in high temperatures causing partial melts far shallower than indicated by seismology. Heat flow (q) is increasing from the bottom of the crust (qB) to the top (q0) due to the cummulative contribution of radiogenic heat production (A). The kinetic energy of the radioactive decay of uranium, potassium and thorium is transformed into heat. The amount and distribution of these elements in the crust controls the changes of q with depth (z). A(z) must decrease with depth. Otherwise, values of q0 would imply zero or negative qB. Based on the observation of a linear q0-A0 relation in Sierra Nevada, an exponential law for A(z) was suggested. Variations of measured q0 within the same thermal province are then related to erosion: if upper layers of rocks with A(z) are eroded away the present-day surface q0 is lower. Knowing the distribution function, 'reduced' or 'basal' heat flow qB is calculated by substracting the cummulative contribution of A(z) from q0. Generally, boreholes are necessary to obtain A(z). Unfortunately, the deepest boreholes in crystalline rocks are not located in thermal provinces with a linear q0-A0 relation, the basis for an exponential A(z) distribution. The Russian Kola drill hole shows a steady increase of A(z) from the surface to 12 km depth. In the 9 km deep German KTB, A(z) is mainly dependant on lithology and the data do not provide a trend to be extrapolated to deeper sections. However, temperature extrapolations can provide constraints on the characteristics of A(z), using boundary conditions at the surface and the bottom of the crust. The extrapolated temperatures are equally dependent on the distribution of heat production and on the in-situ behavior of thermal conductivity k(z). The improvement of our knowledge about in-situ k(z) with sophisticated laboratory experiments will narrow the range of possible distributions for heat production - a property that can not be measured ahead of the drill bit.
The basic requirement for gamma-ray spectrometry measurements of uranium, thorium and potassium concentrations is that secular equilibrium occurs, especially in the 238U and 232Th series. The members of natural radioactive decay chains, in isolated ideal systems, reach radioactive equilibrium in long time periods. Moreover, several geological processes (erosion, alteration, weathering, etc.) can separate some elements from the system which could be seriously affected by radioactive disequilibrium. Therefore the deduced U and Th concentrations may differ from the actual concentration, especially in young rocks. Another problem arises from the production of 222Rn, which can diffuse through rocks into the atmosphere and then break the radioactive equilibrium between 214Bi and 238U. These problems were faced in a field study devoted to the determination of heat-producing elements in volcanic rocks of the Aeolian island arc (southern Tyrrhenian Sea). The investigated rocks span in age from 223 kyr to the Present and represent different volcanic sequences whose different petrochemical characteristics are related to an intricate interplay between crust- and mantle-derived melts. The entire magmatic suite is well represented on the Lipari and Vulcano islands. The former is characterised by calcalkaline to high-K calcalkaline volcanics while the latter is formed by shoshonitic and potassic series. The volcanic products were erupted during three main volcanic cycles, during which lavas and pyroclastites evolved from mafic to felsic composition. Field gamma-ray determinations of U, Th and K concentrations reflect this evolutionary trend. The lowest concentrations of heat producing elements were observed in basaltic-andesites and trachybasalts of the first two volcanic cycles, while rhyolites and obsidians of the third, younger cycle showed the highest concentrations. Most of these rocks have radiometric ages that are actually too young for secular equilibrium and may be also affected by 222Rn escapes. This problem was approached by comparing field gamma-ray results with laboratory analyses carried out by means of X-ray fluorescence and both NaI(Tl) and HpGe gamma spectrometry. Significant disequilibrium in the 238U series is observed on Vulcano, whereas thorium seems in secular equilibrium. The Lipari rocks instead show that secular equilibrium is apparently fulfilled. This could be explained by invoking some migration of daughter elements (214Bi and 208Tl) during melting, and enrichment in the glassy groundmass. The process would compensate the expected lack of U and Th in the neo-formed crystalline phases.
The study of existing correlations between the physical properties of rocks is very important in the construction of integrated geophysical models of the crust. To establish the interdependence between seismic velocity, density and thermal properties (heat conductivity and heat generation) of various rock types of the Upper Proterozoic age, we used a large amount of core samples obtained from drillhole in the central part of the Kola peninsula, north-eastern Baltic Shield. The most part of these samples was chemically analysed for determination of the main oxides in the rocks. The compiled data enabled us to construct the interdependence of velocity, density and heat generation at normal thermodynamic conditions as a two-dimensional probability functions. The approximations of the main tendencies for these interdependencies has a non-linear character and in general coincides with just the same proposed by L. Rybach. The heat conductivity has a good correlation with percentages contents of the main oxides in the rocks.
Experimental data allowed to consider some quantitative measures of the quality of these correlations. In general, entropy of the conditional density probability can be chosen as a quantitative measure of the accuracy of the studied interrelations. It characterizes the uncertainty of the interdependence between studied physical properties of the rocks for the general probability function. Calculated entropy functions for the velocity, density and heat generation interrelations of the rocks can be used as certain weighting functions in the combined inversion process. In this approach the results of the inversion in one geophysical method served as initial model for inversion task in another geophysical method and region of the unknown decision can be restricted by weighting functions which are proportional to entropy of the conditional probability.
More than 200 heat flow density determinations are available for the territory of Belarus at present. Geothermal investigations were conducted in both shallow and deep boreholes, located within all main tectonic units. Heat flow density ranges within the area from as low as 15-20 to as high as 100-110 mW/m2 within some of salt domes of the Pripyat Trough. Low heat flow values (20 -30 mW/m2) are typical for the Volyn-Orsha-Krestsy paleodepression and the Mikashevichi-Osnitsa Igneous Belt, as well as the main part of the Belarussian Anteclise and the Orsha Depression.
Mostly shallow boreholes were available for investigations within the Belarussian Anteclise and adjoining Polesskaya and Latvian Saddles. Fresh groundwater zone reaches the crystalline basement surface within the Central-Belarussian Massif, where the cooling effect of downward filtration was detected here. The similar situation exists within the uppermost 200-300 m of sediments of the Orsha Depression. An opposite situation exists in discharge areas.
Radiogenic heat production was studied in a few dozens of boreholes, mainly based on the content of the U and Th isotopes. The heat production values range from below 0.5 mkW/m3 for the crystalline basement blocks, where basic rocks prevail, to 2-3 and seldom up to 5 mkW/m3 within the Mosty Vygodsk and Martsinkonis granitoid massifs according to M. Zhuk (Garetsky et al., 1991). They exhibit slightly increased heat flow density values up to 45-48 mW/m2. The preliminary estimates along the Grodno - Starobin DSS profile, crossing the Belarussian Anteclise, show that the heat flow - heat production regression line is Q = 20 + 8.7 A, which is close to corresponding lines for the Baltic and Ukrainian Shields.
Radiogenic heat production is still poor studied within the Orsha and Brest depressions. Only a few data are available for the Pripyat Depression. High heat flow anomaly above 70 mW/m2 within its northern part resulted mainly not due to high radiogenic heat production but to the upper mantle diapir, revealed by the DSS method below this area.
Garetsky R G et al, Deep structure and dynamics of the Earth's interiors of Belarus (Russ.), 317 p, (1991).
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