Vincenzo Pasquale

Heat from radioactive elements in young volcanics by γ-ray spectrometry

Abstract Radioactive heat production data for volcanic rocks ranging in age from 430 ka to the historical time and cropping out on the three main islands of the Aeolian arc (Southern Italy) are presented. They were derived from uranium, thorium and potassium concentration measured from γ-radiations originating from the decay of 214Bi (238U series), 208Tl (232Th series) and the primary decay of 40K. Concentration results from a NaI(Tl) detector, compared with independent determinations by means of mass spectrometry, reveal significant secular radioactive disequilibrium between 214Bi and 238U, in the rhyolitic rocks, which is ascribable to their young age. The use of the low-energy portion of the spectrum, where there are a number of γ-rays produced by 234Th, minimises the disequilibrium problem in the uranium determination. The Th/U ratio remains almost constant at 3.3 in all the rock types. Due to their low isotopic content, basalts show a low radioactive heat production rate (0.6 μW m−3), whereas the highest values (6.6–7.1 μW m−3) are found in trachytic and rhyolitic lavas.

Heat Conduction and Thermophysical Parameters

This chapter presents the basic equations for conductive heat transfer and the main thermal parameters of the rocks, in particular the thermal conductivity and radiogenic heat. Also, it outlines the most commonly used techniques for measuring these parameters. Models involving the application of mixing laws for a mineral aggregate are discussed together with techniques for estimating the in situ thermal conductivity and volumetric heat capacity . Finally, methods for determining the radiogenic heat in the crust are introduced.

Subsurface Temperature and Climate Change Reconstruction

Subsurface temperature field forms an independent archive of past climate changes that is complementary to both the surface air temperature observations and the traditional climate proxy data, each with particular strengths and limitations. In this chapter, we focus on the contribution of temperature measurements in deep boreholes to the inference of climate history. The ground surface temperature variations can be reconstructed by inverting the borehole temperature perturbations. Moreover, the joint analysis of temperature recorded in borehole and surface air temperature time series yields estimates of the pre-observational mean of surface air temperature. We also show how the subsurface temperature variations by a short wavelength climate change can be modelled with a known time-dependent temperature boundary condition.

Heat Conduction and Thermal Parameters

This chapter presents the basic equations for conductive heat transfer and the main thermal parameters of the rocks, in particular the thermal conductivity and radiogenic heat. Also, it outlines the most commonly used techniques for estimating these parameters. Models involving the application of mixing laws for a mineral aggregate are discussed together with techniques for estimating the in situ thermal conductivity and volumetric heat capacity. Finally, methods for determining the radiogenic heat in the crust are introduced.

Heat in the Groundwater Flow

The presence of groundwater flow implies other heat transfer mechanisms rather than pure conduction. Several strategies have been developed to explore the heat transport associated with water flow. This chapter presents some analytical methods and shows how subsurface temperatures can provide a quantitative tool for inferring water flow in permeable layers and heat advection in hydrothermal systems. Thermal convection in deep aquifers and its potential are then analyzed by means of the dimensionless Rayleigh number.