Thermal neutron dosimetry for fission track dating with metal activation monitors: A reinvestigation

Abstracts-6th International FTD Workshop and is in contact with the Sanbagawa belt at the MedIan Tectonic Lme (MTL), in order to reveal the emplacement hIstory. All the ages were determmed using the external detector method and the ages were calculated by the zeta calibration approach. FT zircon ages are approximately consistent around 58 Ma in the three traverses, suggesting uniform uplift and cooling of the region through the zircon isotherm ( - 240°C) FT apatite ages range from 60 to 10 Ma, and are divided into two groups: "old" apatites, located far away from MTL and consistent around 54 Ma in the three traverses, also indicating uniform uplift and cooling through the apatite isotherm ( - 105°C); "young" apatites, located near MTL, showing age differences among the three traverses as - liMa, - 17 Ma and - 29 Ma. FT length data of nine apatites reveal the later thermal overprinting events near MTL. The following has been concluded. (1) The studied region of the Ryoke belt cooled rapidly (cooling rate - 30°C Ma -I) from - 58 to - 54 Ma, and slowly ( - I.7C Ma - I) from - 54 Ma to the present. and subsequent thermal overprinting events along MTL existed from the Oligocene to the Miocene. (2) The cooling of the Ryoke belt is related to the uplift-erosion of the region. The rapid uplift from - 58 Ma to - 54 Ma should reflect normal subduction of the Pacific plate beneath the Eurasia plate with a convergence rate of - 10 cm yr - I The rapid uplifting was ceased at - 54 Ma, when the normal subduction changed mto left-lateral oblique subduction. (3) The thermal overpnnting events are very likely due to the shear heating by the strike-slip fault movements of MTL in the OlIgocene and the MIOcene. The reset apatite ages near MTL in each traverse give the timing of the last active fault movements of MTL; - 29 Ma in the eastern Kinki, - 17 Ma in the western Chubu and - II Ma In the eastern Chubu districts.

THERMAL NEUTRON DOSIMETRY FOR FISSION TRACK DATING WITH METAL ACTIVATION MONITORS: A REINVESTIGATION P. VAN DEN HAUTE! R JONCKHEERE* and F. DE CORTEt *Laboratorium voor Aardkunde, State Umversity of Gent, B-9000 Gent, Belgium and tinstituut voor Nuclealre Wetenschappen, State UniverSIty of Gent, B-9000 Gent, Belgium

DURING the course of the past decade direct thermal neutron dOSImetry with metal activation monitors has gradually come to be dIsfavoured by many FT-geochronologlsts. The argument that the calculated fluences are inaccurate is sustamed by the fact that the few calibration experiments using two or more independent metal activation monitors often failed to produce consistent results. Between November 1985 and March 1988, seventeen irradiations have been carried out in the THETIS nuclear reactor (State University of Gent). For each irradiation the thermal neutron fluence has been determined with Au and Cu fluence mom tors, while in thirteen IrradiatIOns a Co monitor has also been included. The fluences registered by the Au and Co monitors are very consistent, with a mean difference of only 1.3% and a maximum difference of 3.3%. The Cu monitors yielded fluences that are systematically some 8.6% lower than the corresponding Au values. This is ascribed to inaccuracies in the nuclear parameters of Cu and to problems mherent in the measurement of the 511 keV annihilation radIation of 64Cu. The excellent agreement between the Au and Co fluences m our opimon clearly sustains the accuracy of fluence determinatIOns WIth these monitors. An FT-analysls of two apatite age standards (Fish Canyon Tuff and Durango) by means of the population method and using combmed Au and Co dosimetry strongly supports a nommal ~rvalue of 8.5' 10- 17 yr -I Although a )'F-value closer to the nominal value of 7.0' 10- 17 yr-] corresponds to the Cu-fluences for these standards, this combination must be rejected outfight because of the unacceptabIlity of Cu as a Iluence monitor.

APATITE FISSION-TRACK AGES FROM KENYA BASEMENT ROCKS AND THEIR RELATIONS TO THE THERMO-TECTONIC EVOLUTION OF EAST AFRICA M WAGNER* and P VAN DEN HAUTEt *Instltut fUr Petrographle und Geochemie, Universitiit Karlsruhe, Kalserstr. 12, 0-7500 Karlsruhe, F R.G and tMax-Planck-Instltut fUr Kernphysik, Heidelberg, F.R.G.

ROCK SAMPLES from the basement of Kenya were dated by the apatite fiSSIOn-track method. The sample locatIOns are situated East and West of the Kenya-Rift, in the Northern Rift valley (Mozambique belt), and m the Proterozoic Nyanza shield (Western Kenya). As can be deduced from K/Ar- determinations on biotites, samples from the Mozambique belt cooled down below 300 C between 500 and 530 Ma ago. Apatite fission-track ages range from 30 to 300 Ma. There is a general increase in age with the topographic

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