The rare gas inventory of the continental crust, recovered by the KTB Continental Deep Drilling Project

The rare gas inventory of the continental crust as mirrored in the continental metamorphic rocks from the “KTB” deep drilling site in NE Bavaria, Germany, has been investigated through systematic isotope analysis of 47 whole-rock samples and 10 mineral separates from the drill core. The measured concentrations confirm a very low crustal inventory of noble gases relative to the atmosphere (≤0.2%). The presence of a mantle helium component is identified by 3He excess leading to a range of 2×10−8≤3He/4He≤6.5×10−6 for the He isotopic ratio. Excesses of 21Ne and of heavy xenon isotopes demonstrate the presence of nucleogenic and fissionogenic gases. The acquired data have enabled us to estimate loss rates of 4He, 21Nenucl, 40Ar and 136Xefiss from typical crustal rocks. The global extension of these rates to the continents yields degassing fluxes of (2–3)×1010 atoms m−2 s−1 for 4He, 900–2100 atoms m−2 s−1 for 21Ne and (1–6)×109 atoms m−2 s−1 for 40Ar, in good agreement with theoretical estimates from atmospheric models and from accumulated gases in crustal fluids.