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Pia Söderlund, GeoBiosphere Science Centre (Sweden)
Laurence M. Page, GeoBiosphere Science Centre (Sweden)
Joaquim Juez-Larré, Vrije Universiteit (Netherlands)
Finlay M. Stuart, Scottish Universities Environmental Research Centre (United Kingdom)
Paul A.M. Andriessen, Vrije Universiteit (Netherlands)
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The Swedish Nuclear Fuel and Waste Management Company (SKB) has recently completed investigations at Forsmark and Oskarshamn, south-eastern Sweden, aimed at locating and characterizing a site for the disposal of highly radioactive nuclear waste. The safety analysis of a selected site requires an understanding of the past geological evolution. In this context, the knowledge of the younger tectonothermal history is of vital importance. The bedrock in both areas consists of crystalline rocks that are predominantly Palaeoproterozoic in age. Apatite from several levels along c. 1000 m deep boreholes and a number of surface samples have been dated using apatite (U-Th)/He thermochronology. A few borehole samples at both sites were also dated with the apatite fission-track (AFT) technique.
The age/depth slopes for the apatite (U-Th)/He and AFT data at Oskarshamn are in excellent agreement. At Forsmark, these two slopes are similar. However, at both sites, the corrected apatite (U-Th)/He ages are older than the AFT ages, i.e. are incompatible with the generally accepted closure temperatures for the two systems of c. 70 °C and 110 °C, respectively. The age discrepancy is clearly systematic, and may be related to: 1) enhanced He retention in U-rich apatite, which may increase the closure temperature of the (U-Th)/He system, and/or 2) enhanced annealing of fission tracks in U-rich apatite that may lower the closure temperature of the AFT system, and/or 3) an invalid FT-correction factor in the (U-Th)/He system due to heterogeneous distribution of U. The latter could have a profound effect in slowly cooled terrains. Considerable intra-sample (U-Th)/He age variations were obtained on apatite grains from Forsmark, most probably due to zonation of U or grain morphology. Etched apatite internal grain surfaces show that many grains are strongly zoned in U concentration. This phenomenon appears to be less pronounced for apatite in samples from Oskarshamn. Furthermore, apatite grains at Forsmark are often small, anhedral, elongate or broken, whereas most of the apatite grains at Oskarshamn are unbroken and have an euhedral shape. These factors are also inferred to potentially affect (U-Th)/He ages.
The thermal models based on the AFT ages indicate that both Oskarshamn and Forsmark experienced temperatures higher than 100 °C prior to 200 Ma, consistent with a several km thick sedimentary cover at this time. This is in agreement with earlier studies in these areas, and probably reflects Palaeozoic sedimentation, firstly in a passive continental margin and, thereafter, in a Caledonian foreland basin setting. Subsequent uplift may be linked to far-field effects of transtensional tectonics along the Sorgenfrei-Tornquist Zone in southernmost Sweden and doming effects from extensive volcanism in areas to the west at c. 300 Ma, and to the south during the Mesozoic. Uplift waned at c. 100 Ma and, thereafter, denudation and exhumation have occurred at a much slower rate.
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