| Abstract |
Ireland is located within stable lithosphere, unaffected by recent tectonism and volcanism. The upper crust under Ireland includes several thick Carboniferous sedimentary sequences where measured geothermal gradients are, for the most part, moderate (less than 25°C/km), though viable sources of geothermal energy may exist beneath them. Exploration for deep-drilled, low enthalpy geothermal energy can benefit from knowing the variation in heat production rates (HPR) in 3D. Generally extrapolation in depth is challenging. However, Ireland is fortunate in this regard since the surface geology provides a series of quasi-3D sections as a result of several episodes of deformation and exhumation during the early and late Palaeozoic Caledonian and Variscan orogenies. Effectively, measurements of HPR at outcrop are a valid proxy for rocks at depth. By extrapolating the HPR of the major stratigraphic units combined with a consideration of structural geology, borehole data and geophysical data, including new magnetotelluric surveys undertaken as part of the IRETHERM project, a 3D model of the Irish upper crust is in progress. The first step towards achieving this has been to calculate heat production rates of rocks exposed at the surface and obtained from drilling. Published whole-rock geochemical data have been combined with new analyses, using XRF and field-based gamma-ray spectrometry of a wide range of crustal rocks from across Ireland. Concentrations of 238U, 232Th and 40K of over 3300 samples have been used to produce a map of bedrock radiogenic heat production. The data show that heat production generally corresponds to rock type. Of the large volume lithologies, basalts yield the lowest HPR with a mean of 0.6 µW/m3 and, as might be expected, granitoid rocks are generally hotter than other major lithologies. For example, the Cenozoic Mourne Granite records the highest mean HPR of granitoids of 6.83 µW/m³. Other high heat production rates are found in the mid-Caledonian Carnsore Granite (6.42 µW/m³) and the late Caledonian Costello Murvey Granite (6.33 µW/m³). Granite petrogenesis likely controls heat production but universal relationships between HPR and bulk composition and tectonic setting are not seen, although a weak correlation with crystallisation age is apparent. The highest mean HPR for any formation is recorded in the Clare Basin where high-uranium, phosphatic shales of Upper Carboniferous age have a mean HPR of 27.7 µW/m³. Such high heat production suggests that these shales and others elsewhere in Ireland should be investigated as potential geothermal prospects. Extrapolation into the crust reveals that basement rocks that represent Laurentia in the north of Ireland have a higher HPR (1.64 µW/m³) than Avalonian continental basement in the far south (0.94 µW/m³). Lower Palaeozoic rocks that form the basement of much of the south and the midlands, also show a similar divergence in heat production values to their age equivalents north of the ISZ. On a regional scale HPR at shallow crustal levels beneath Ireland is in line with global averages, though compilation of the 3D map is ongoing. |