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| Titel |
Constraining heat production rates in Ireland’s basement rocks: measurements of exposed basement and correlations from across the Caledonides |
| VerfasserIn |
Nicola Willmot Noller, Stephen Daly |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250113136
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| Publikation (Nr.) |
 EGU/EGU2015-13331.pdf |
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| Zusammenfassung |
| Ireland is situated on stable lithosphere and much of its surface geology features thick Upper
Palaeozoic sedimentary sequences, and a few shallow Permo-Triassic basins, for which
measured geothermal gradients are generally moderate. Nevertheless, crystalline rocks
beneath these basins might produce enough heat for a viable deep-drilled, low enthalpy
geothermal resource. Accurate knowledge of the lateral and vertical distribution of radiogenic
heat production is, therefore, important in helping to define geothermal exploration
targets.
The crystalline basement of Ireland is interpreted as an assemblage formed from
the convergence of Laurentia and Gondwanan terranes during the closure of the
Iapetus Ocean and the Caledonian orogenic event. Despite the extensive sedimentary
cover observed today, folding and faulting episodes during the Caledonian and the
subsequent Variscan orogenies enabled exhumation of a wide range of Precambrian and
Palaeozoic rocks, albeit exposed at relatively few sites across Ireland. A mean calculated
heat production rate (HPR) derived from these outcrops is used as a proxy for the
equivalent stratigraphic unit at depth. This has been achieved using established heat
production constants, rock density and known concentrations of uranium, thorium and
potassium, combined with a knowledge of geological mapping and geophysical
data.
To further constrain the vertical component of heat production distribution, Irish
metapelitic xenoliths emplaced in Lower Carboniferous volcanics in the Iapetus Suture Zone
(ISZ) in central Ireland are regarded as a reliable representation of the present-day lower crust
there. The xenoliths have a mean HPR of 1.7 μW/m3; this is similar to a mean HPR of 1.9
μW/m3 measured in exposed Ordovician sedimentary rocks in the south east of Ireland. The
slightly lower HPR in the xenoliths is a consequence of reduced uranium concentrations,
probably owing to the radioelement’s mobility. It is likely that these Ordovician rocks are
subjacent to the Mid/Late Palaeozoic sedimentary basin between the ISZ and their outcrop in
SE Ireland.
In addition, Newfoundland Appalachians are interpreted as part of the relict collision
zone from the Caledonian orogenic event, separated from the Irish Caledonides by rifting as
the Atlantic opened. This region offers extensive exposure of Precambrian, Lower Palaeozoic
supracrustal and plutonic rocks many of which can be regarded as equivalent to those in
Ireland. The Canadian geochemical dataset thus provides an opportunity to test the validity of
assigning to Ireland’s basement, heat production rates obtained from otherwise limited
exposures. |
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