Radiogenic Heat Production Values Research Articles

High radiogenic heat production in the Kerala Khondalite Block, Southern Granulite Province, India

In situ radioelemental (K, U and Th) analysis and heat production estimates have been made at 59 sites in the Kerala Khondalite Block (KKB) of the Southern Granulite Province (SGP) of India. Together with the in situ analyses on granulites and gneisses previously reported from 28 sites, and heat production estimated from the published geochemical analyses on granites and syenites of the KKB, the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the KKB. Garnet biotite gneisses are characterized by high levels of Th and U, with mean values of 60 and 3 ppm, respectively. Khondalites, leptynites and charnockites have slightly lower levels of Th (23, 20 and 22 ppm, respectively) and U (2.9, 2.4 and 0.9 ppm, respectively). The mean K, U, Th abundances for the granites, leucogranites and granitic gneisses ranges from 3.9 to 4.3%, 2.6 to 4.3 ppm, 22 to 50 ppm respectively, and for the syenites 4.8%, 2 ppm and 5.7 ppm. Mean radiogenic heat production values for garnet–biotite gneiss, khondalite, leptynite and charnockite are 5.5, 2.7, 2.4 and 2.2 μW m−3, respectively. For the granites, leucogranites, granitic gneisses and syenites it is 2.6, 3.4, 4.6 and 1.4 μW m−3, respectively. Heat production of granulite facies rocks, which are the most abundant rocks in KKB, correlate well with Th, but less with U, suggesting that variation is caused by Th and U bearing accessory minerals such as monazite and zircon. The high heat production of the KKB granulites are in contrast to the low heat production of the Late Archaean granulites of the Northern Block (NB) of the SGP which are highly depleted in radioelements and also the granulites of Madurai Block (MB) that have higher radioelemental abundances than in the granulites of the NB. The high heat production of the KKB granulites could be due to the nature of protoliths and/or metasomatism associated with Neoproteroic- to- Pan African alkaline magmatic activity represented by alkali granite and syenite–carbonatite emplacements and emplacement of pegmatites.

One-dimensional thermal models constrained by seismic velocities and surface radiogenic heat production for two main geotectonic units in southern Portugal

New heat flow density data and radiogenic heat production values obtained in southern Portugal, along with lithologic and structural data inferred from vertical seismic velocity distributions from deep seismic refraction surveys, are used to construct one-dimensional (1D) geotherms for the crust in the two main geotectonic units of the region, i.e., the Ossa–Morena Zone (OMZ) and the South Portuguese Zone (SPZ). Surface radiogenic heat production values calculated using potassium, uranium and thorium concentrations measured in different rock types were used to constrain the heat production values. In the Ossa–Morena Zone and South Portuguese Zone the surface radiogenic heat production does not exceed 3 μW/m 3. Geotherms were constructed considering that the crust in the region has a three-layer structure, as inferred from seismic refraction surveys, but with different thicknesses in the OMZ and SPZ. For both zones two 1D models with constant and exponential heat productions were constructed. The models suggest that temperatures are higher in the SPZ than in the OMZ, the difference reaching values of about 300°C at Moho depths; however, heat production values in the former are less constrained than in the latter and so temperatures in the SPZ are probably overestimated. For each geotectonic unit, no significant difference is observed in the results when constant heat production is assumed for the upper crust or an exponential decrease is assumed for the upper crust.

Terrestrial heat flow in the alkaline intrusive complex of Poços de Caldas, Brazil

Terrestrial heat flow measurements have been made in five localities within the alkaline intrusive complex of Poços de Caldas (Brasil) making use of exploratory boreholes drilled for uranium. Heat flow is found to be variable, from as low as 53 mW/m 2 to as high as 99.4 mW/m 2. The non-mineralized zones are found to be characterized by high heat flow values (70–99.4 mW/m 2) relative to those in mineralized areas (53–62 mW/m 2). The lower values in mineralized zones arc believed to be due to heat removal associated with groundwater circulation. Hence the mean value of 85.6 mW/m 2 for the non-mineralized zone is considered as representative of the true heat flow within the intrusion. The overall mean for the five localities within the intrusion is 74.4 mW/m 2, significantly higher than the mean value for seven localities in the surrounding shield areas. Thus the intrusive complex is characterized by a positive thermal anomaly relative to the adjacent shield. Radiogenic heat production values were calculated based on radiometric measurements of U, Th and K on thirteen samples of drill cores. Mean heat production value (excluding those samples from mineralized zones) calculated is 7.0 μW/m 3. On a reexamination of earlier heat flow and heat generation measurements on three other sites situated within the same age province (the Brazilian fold belt), along with the present values for Poços de Caldas, a case can be made for a linear heat flow-heat generation relation. This new relation defines a reduced heat flow of 53 mW/m 2 and a depth parameter of 4.8 km. The adjacent but older Trans-Amazonian fold belt seem to be characterized by a lower reduced heat flow ( Q o = 28.3 mW/m 2 ) and a higher depth parameter ( D = 13.1 km ).