Residence times and source ages of deep crustal fluids: interpretation of ¹²⁹I and ³⁶Cl results from the KTB‐VB drill site, Germany

We present here 129 I/I and 36 Cl/Cl ratios, together with halogen concentrations in crustal fluids from the continental deep drill site (KTB‐VB) in Germany, where fluids were collected from 4000 m depth during a pump test carried out in 2002 and 2003. Compared with seawater, the fluids are enriched by factors of 2, 8 and 40 for Cl − , Br and I, respectively, and show little variation over the test period. The 129 I/I ratios are between 1700 and 4100 × 10 −15 ; the 36 Cl/Cl ratios are below 10 × 10 −15 . Co‐variation between 129 I and 36 Cl concentrations in the fluids indicates that anthropogenic components are absent and that the ratios reflect an addition from crustal sources. The results suggest residence times of 10 Ma or more for the fluids in formations with uranium concentrations of 1 ppm. A minimum age of 30 Ma for the iodine source was derived from the correlation between 129 I and 36 Cl concentrations in the fluids. The results demonstrate that the halogen characteristics of the KTB fluids are very similar to those of other deep crustal fluids and that the combination of 129 I and 36 Cl systematics allows determination of residence times and source ages of such fluids.