40Ar/39Ar thermochronometry of K-feldspar from the KTB borehole, Germany

The great depth and relatively steep geothermal gradient (∼29°C/km) of the KTB deep crustal borehole, located in the Bavarian region of Germany, provides the basis for our study of diffusive argon loss in K-feldspar. The observed borehole temperature at 9 km is ∼265°C which reaches well into the argon closure interval for K-feldspar (∼125–350°C). Four microcline and two adularia samples distributed relatively evenly down the 9.1-km deep borehole were analyzed by the 40Ar/39Ar method. The thermal histories of the shallow samples are consistent with existing thermochronologic data from the borehole. After correction for chlorine correlated excess 40Ar, the deepest two samples result in age spectra that record zero apparent ages over the first few percent of 39Ar released, which is predicted by diffusion theory where a range of diffusion volumes are present. Inverse modeling of the argon kinetics of these two deep samples reveals that the present elevated geothermal gradient in the borehole has existed for only the last approximately one million years. Recent volcanism within 30 km of the borehole is likely the source of this present-day thermal pulse.