A helium, argon, and nitrogen record of the upper continental crust (KTB drill holes, Oberpfalz, Germany): implications for crustal degassing

An extensive set of bulk rock samples and mineral separates from the German Deep Drilling Project (KTB) has been analyzed for nitrogen, helium, and argon concentrations. The samples comprise metabasites, paragneisses, and hornblende–gneisses from a 7-km section in the crystalline basement metamorphosed during the Variscan orogeny. The data indicate a dependence of the N and Ar concentrations on the mica content of the rocks. The He–N2–Ar composition of fluid inclusions in quartz from felsic veins suggests that the fluids are old basement brines with large proportions of radiogenic He and Ar. There is no systematic down-hole variation in N and Ar concentrations, but He concentrations appear to increase with depth. Calculated degassing rates suggest near complete degassing of He from the gneisses, but some He retention in the metabasites. Nonetheless, overall degassing fluxes of He (1.2–4.0×1010 atoms m−2 s−1) are not in conflict with estimates of the He flux in continental crust based on the steady-state degassing model. In contrast, our estimates indicate that Ar fluxes are significantly lower than predicted by a steady-state degassing model which is thus probably inappropriate to describe Ar fluxes correctly. The down-hole trend of increasing He concentration is interpreted as a degassing profile. A model of simultaneous radiogenic ingrowth and diffusive loss of He has been applied to determine the bulk transport coefficient of He in crystalline basement, which was found to be between 10−10 and 10−7 m2 s−1. This is much higher than reasonable diffusion rates, suggesting that episodes of enhanced degassing, probably related to hydrothermal or tectonic activities, played an important role in the degassing history of the crust. The relationship between the transport of heat and He in the crust in the KTB area is examined. Under the assumption that the ratio of heat flow and He flux from the mantle is the same as in ocean basins, the He isotopic composition of the basement fluids in the KTB boreholes is consistent with the inferred reduced heat flow at the KTB site. On a global scale, however, heat flow and flux of mantle He are probably not correlated.