THE NATURE OF MANTLE NEON CONTRIBUTIONS TO VIENNA BASIN HYDROCARBON RESERVOIRS

The abundances and isotopic compositions of He and Ne have been determined in various hydrocarbon gas reservoirs within the Vienna Basin, Austria. These continental gases are the first in which mantle-derived Ne has been resolved. They allow a comparison of the estimated mantle He/Ne ratio with that observed in basalts at mid-ocean ridges. Between 1.9% and 28% of the He-4 within these samples is mantle derived, the remainder is crustal-radiogenic. The contribution of mantle-derived Ne-21 to the total Ne-21 content ranges from < 3% to 34%, the contribution of Ne-21 produced nucleogenically in the crust ranges from 0% to 40%. The remaining Ne is atmospheric in origin. Ne-21/Ne-22 and Ne-20/Ne-22 ratios, corrected for atmosphere-derived contributions, correlate with the measured R/R(a) values. Analyses define a single hyperbolic mixing line between mantle and crustal-radiogenic isotopic end members. This line is defined by the constant [He-4/Ne-22]rad/[He-4/Ne-22]mntl = 94.7 +/- 9.4. For a crustal [He-4/Ne-22]rad ratio of 7.61 X 10(6) [1] the ratio [He-4/Ne-22]mntl = 8.04 X 10(4) for the Vienna Basin gases. This is a factor of 11.3 times lower than the best estimate of the mantle value [He-4/Ne-22]mntl = 9.10 X 10(5), measured in mid-ocean ridge basalts [2]. The magnitude of the mantle Ne enrichment in the Vienna Basin gases cannot be accounted for either by melt formation or degassing and must reflect either the transport of the mantle volatiles through the continental lithosphere or a mantle source relatively enriched in Ne over He.