Positioning the titanite fission-track partial annealing zone

As part of the KTB (Kontinentale Tiefbohrung) programme in Germany, titanite (sphene) fission-track data were acquired for samples to a depth of 9000 m, where the host rock temperature is similar to 265 degrees C. At this depth, age and confined track length information suggest that the partial annealing zone for the fission tracks in titanite was not reached. By reconstructing the thermal history and 'backstacking' of an uplifted partial annealing zone, it is suggested that the in situ partial annealing zone is likely to be found in the region 265-310 degrees C. This value is more precise than previous estimates of the titanite partial annealing zone, and it is also higher than previous estimates. Microprobe analysis indicates that the chemical composition of the titanites is rather homogeneous throughout the whole drilled section. (C) 1998 Elsevier Science B.V. All rights reserved.