Radiogenic heat production in Ireland
PhD Candidate: Nicola Willmot Noller
Supervisor: Professor Stephen Daly
Funded by: Science Foundation Ireland (SFI)
Heat production values in the crust and mantle rock inform heat flow density data to provide crucial information about the structure of the Earth’s lithosphere. In addition, accurate models of horizontal and vertical distribution of heat production can help to define geothermal exploration targets. Low-enthalpy district scale space heating and Enhanced Geothermal Systems (EGS) using hot, dry rock may provide sustainable energy resources in regions currently perceived as having low geothermal energy potential.
Ireland is located within stable lithosphere, unaffected by recent tectonism and volcanism, and has an estimated heat flow range below the measured global continental average. Borehole data indicate that heat production is variable across the island, with anomalously high rates observed, for example, in Cavan, Meath and Antrim. Data coverage is, however, poor and other locations may also feature high heat production rates, with buried granites of particular interest.
With the objective of compiling the first comprehensive database of information about radioelement distribution in the Irish lithosphere, in three dimensions, this project, part of the IRETHERM, has undertaken an extensive literature search to obtain data from published and unpublished whole-rock major and trace element analyses of Irish rocks, specifically to extract measurements of potassium, uranium and thorium. These radiogenic elements have half-lives which are geologically significant and their radioactive isotopic decay produces heat. By using established heat production constants and known concentrations of unstable isotopes of 40K, 238U and 232Th, along with rock density values, a heat production rate in µW m-3 is obtained.
Collation of data from Ireland's bedrock has shown that geochemical analyses are absent for many locations and an extensive programme of fieldwork utilizing portable gamma ray spectrometry and supplementary XRF has been undertaken. Even so the absence of adequate lithological exposure in central areas in particular requires significant extrapolation of interpreted heat production rates, for which the presence of systematic trends correlating heat production to properties such as age and lithology are investigated.
Offering insight into the vertical component of heat production distribution, reliable heat data from borehole measurements may be used. However, here too there is an unequal geographical distribution and depths only extend beyond 2,500 metres for two wells in all Ireland. Mapping core lithology is compromised similarly and stratigraphic data is not often available. Other methods of vertical extrapolation involve downward projection of surface lithology (and associated heat production), as well as interpreting seismic tomography, magnetotellurics and gravity data.
Irish xenoliths emplaced in Lower Carboniferous volcanics are regarded as a reliable proxy for the present-day lower crust. Their geochemical composition gives heat production values that are higher than expected for the depths indicated by their thermobarometric data, suggesting that heat production rates do not simply reduce with depth.
The data show that heat production generally corresponds to rock type. Of the large volume lithologies, basalts yield the lowest heat production rate (HPR) 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. Other high HPR are found in the mid-Caledonian Carnsore Granite and the late Caledonian Costello Murvey Granite. 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 Shannon Basin in high-uranium, phosphatic shales of Upper Carboniferous age. Such high heat production suggests that these shales and others elsewhere in Ireland should be investigated as potential geothermal prospects.
Extrapolation into the crust suggests that basement rocks that represent Laurentia in the north of Ireland have a higher HPR than Avalonian continental basement in the far south. 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 Iapetus Suture Zone.
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