AR-40 AR-39 THERMOCHRONOMETRY OF K-FELDSPAR FROM THE KTB BOREHOLE, GERMANY/

The great depth and relatively steep geothermal gradient (similar to 2 9 degrees C/km) of the KTB deep crustal borehole, located in the Bavar ian region of Germany, provides the basis for our study of diffusive a rgon loss in K-feldspar. The observed borehole temperature at 9 km is similar to 265 degrees C which reaches well into the argon closure int erval for K-feldspar (similar to 125-350 degrees C). Four microcline a nd two adularia samples distributed relatively evenly down the 9.1-km deep borehole were analyzed by the Ar-40/Ar-39 method. The thermal his tories of the shallow samples are consistent with existing thermochron ologic data from the borehole. After correction for chlorine correlate d excess Ar-40, the deepest two samples result in age spectra that rec ord zero apparent ages over the first few percent of Ar-39 released, w hich is predicted by diffusion theory where a range of diffusion volum es are present. Inverse modeling of the argon kinetics of these two de ep samples reveals that the present elevated geothermal gradient in th e borehole has existed for only the last approximately one million yea rs. Recent volcanism within 30 km of the borehole is likely the source of this present-day thermal pulse. (C) 1998 Elsevier Science B.V. All rights reserved.