- Email: [email protected]
Contributions to the geothermal history of drill-core Urach III by apatite fission-track dating and annealing experiments
APPENDIX
437
Abstract reproduced from the unrefereed volume "The Fourth International
Fission-Track Dating Workshop."
CONTRIBUTIONS TO THE GEOTHERMAL HISTORY OF DRILL-CORE URACH III BY APATITE FISSION-TRACK DATING A N D A N N E A L I N G EXPERIMENTS M. WAGNERand G. A. WAGNER Max-Planck Institut fiir Kernphysik, P.O. Box 10 39 80, 6900 Heidelberg, F.R.G. APATITE fission-track dating was applied to core samples from the geothermal drilling Urach III (35 km southeast of Stuttgart, West Germany) to investigate their thermal history. The drilling site is situated in the center of a geothermal anomaly and a region of more than 300 Miocene volcanic pipes. Previous K - A r and Rb-Sr dating on fresh rocks from the crystalline section revealed an age of 325 Myr for all samples without any relation to their borehole position. This is due to the late Variscan period of rapid uplift. The Apatite FT-ages however decrease with increasing borehole depth (from 65.5 Myr at 937 m depth to 0.6 Myr at 3300 m depth) due to partial track fading. Actual borehole temperatures are 66°C at 1000 m depth and 143°C at 3300 m depth. The zero age was extrapolated to 150°C. Closure temperatures which were calculated with a constant cooling rate of 0.8°C/Myr range from 105°C (1021 m depth) to 143°C (3300 m depth). Measurements on projected lengths of spontaneous tracks reveal a relative shortening to 82~o for the sample from 1021 m depth, 70~ (2800 m depth) and 40~ (3000 m depth). Using the correction curve from Apatite T11 (Wagner 1973) corrected ages were calculated ranging from 81.2 Myr (1021 m) to 9.9 Myr (2900 m depth). This clearly shows that all tracks must have been erased during Mesozoic subsidence. Before 80 Myr ago the temperature exceeded 130°C (lab. experiments). For the Cretaceous an elevated gradient of 55-60°C/km can be deduced. During the following period the rock column cooled down slowly due to uplift. There is no indication for a measurable thermal pulse during Miocene volcanism. The annealing behavior of apatite FT under geological conditions in the borehole is compared with new experimental data.
437
Abstract reproduced from the unrefereed volume "The Fourth International
Fission-Track Dating Workshop."
CONTRIBUTIONS TO THE GEOTHERMAL HISTORY OF DRILL-CORE URACH III BY APATITE FISSION-TRACK DATING A N D A N N E A L I N G EXPERIMENTS M. WAGNERand G. A. WAGNER Max-Planck Institut fiir Kernphysik, P.O. Box 10 39 80, 6900 Heidelberg, F.R.G. APATITE fission-track dating was applied to core samples from the geothermal drilling Urach III (35 km southeast of Stuttgart, West Germany) to investigate their thermal history. The drilling site is situated in the center of a geothermal anomaly and a region of more than 300 Miocene volcanic pipes. Previous K - A r and Rb-Sr dating on fresh rocks from the crystalline section revealed an age of 325 Myr for all samples without any relation to their borehole position. This is due to the late Variscan period of rapid uplift. The Apatite FT-ages however decrease with increasing borehole depth (from 65.5 Myr at 937 m depth to 0.6 Myr at 3300 m depth) due to partial track fading. Actual borehole temperatures are 66°C at 1000 m depth and 143°C at 3300 m depth. The zero age was extrapolated to 150°C. Closure temperatures which were calculated with a constant cooling rate of 0.8°C/Myr range from 105°C (1021 m depth) to 143°C (3300 m depth). Measurements on projected lengths of spontaneous tracks reveal a relative shortening to 82~o for the sample from 1021 m depth, 70~ (2800 m depth) and 40~ (3000 m depth). Using the correction curve from Apatite T11 (Wagner 1973) corrected ages were calculated ranging from 81.2 Myr (1021 m) to 9.9 Myr (2900 m depth). This clearly shows that all tracks must have been erased during Mesozoic subsidence. Before 80 Myr ago the temperature exceeded 130°C (lab. experiments). For the Cretaceous an elevated gradient of 55-60°C/km can be deduced. During the following period the rock column cooled down slowly due to uplift. There is no indication for a measurable thermal pulse during Miocene volcanism. The annealing behavior of apatite FT under geological conditions in the borehole is compared with new experimental data.