Resolving the mantle He/Ne and crustal 21Ne/22Ne in well gases

He and Ne isotope data from 48 mantle-rich well gases (He-3/He-4 > 0.1R(a)) in 6 different regions of crustal extension around the world are used to derive both the average mantle He/Ne elemental ratio beneath the extensional systems, and the regional average crustal Ne-21/Ne-22 nucleogenic production ratio, (Ne-21/Ne-22)(crust). Assuming a mantle with R/R-a = 6.0-8.0R(a), on a plot of measured He-3/He-4 vs. air-corrected Ne-21/Ne-22, 44 of the 48 data points fall within error of a well-defined mixing line between mantle and crust isotopic end-member values. The mixing line requires a mantle He-3/Ne-22 range of 0.6-1.9 and is less than half that estimated for the upper mantle, assuming steady state and a Solar-like lower mantle [1,2]. The derived (Ne-21/Ne-22)(crust) is 0.47-0.76. This is significantly smaller than the theoretical value of 3.5 [3]. The low (Ne-21/Ne-22)(crust) confirms that mineral scale preferential siting of U and Th with respect to O and F controls the regional (Ne-21/Ne-22)(crust) [4]. The difference between the derived well-gas mantle He/Ne and estimated upper-mantle values can be reconciled by a gas/melt solubility fractionation process. This requires that the gas/melt volume ratio is small (< 0.001) in all of the rifting environments in order to produce the observed magnitude of He/Ne fractionation. These small gas/melt volume ratios can only be produced in the lithospheric mantle. Furthermore, this requires that the mantle He/Ne composition is similar to or less than models invoking a Solar-like upper mantle, with (He-3/Ne-22) less than or equal to 4.4. These results are consistent with a model in which small melt fraction asthenospheric melts are emplaced into the subcontinental lithospheric mantle. These melts partially degas, to a similar magnitude in all regions, lose a maximum of similar to 15% of their He content by gas/melt phase partitioning, and form the dominant source of mantle-derived volatiles in regions of crustal extension. During migration of the magmatic volatiles through the crust the fractionated mantle-derived gases mix with crustal radiogenic noble gases on a regional scale, at or near their elemental production ratios. (C) 1997 Elsevier Science B.V.