THE RARE-GAS INVENTORY OF THE CONTINENTAL-CRUST, RECOVERED BY THE KTBCONTINENTAL DEEP DRILLING PROJECT

The rare gas inventory of the continental crust as mirrored in the con tinental metamorphic rocks from the ''KTB'' deep drilling site in NE B avaria, Germany, has been investigated through systematic isotope anal ysis 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 (less than or equal to 0.2% ). The presence of a mantle helium component is identified by He-3 exc ess leading to a range of 2 x 10(-8) less than or equal to He-3/He-4 l ess than or equal to 6.5 x 10(-6) for the He isotopic ratio. Excesses of Ne-21 and of heavy xenon isotopes demonstrate the presence of nucle ogenic and fissionogenic gases. The acquired data have enabled us to e stimate loss rates of He-4, Ne-21(nucl), Ar-40 and Xe-136(fiss) from t ypical crustal rocks. The global extension of these rates to the conti nents yields degassing fluxes of (2-3) x 10(10) atoms m(-2) s(-1) for He-4, 900-2100 atoms m(-2) s(-1) for Ne-21 and (1-6) x 10(9) atoms m(- 2) s(-1) for Ar-40, in good agreement with theoretical estimates from atmospheric models and from accumulated gases in crustal fluids. (C) 1 998 Elsevier Science B.V.