Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: Assesment of shallow-level fractionation and characterization of source composition

To document the characteristics of volatiles in the terrestrial mantle, the abundances and the isotopic ratios of carbon, nitrogen, helium, and argon have been analyzed in 45 mid-ocean ridge basalts (MORB) glasses from the Mid-Atlantic Ridge between 24 degrees N and 36 degrees N, the East Pacific Rise at 21 degrees N, 13 degrees N, and 17-19 degrees S, the Red Sea (18-20 degrees N), the Indian Ocean near the Triple Junction, and the central North Fiji basin. Gases were extracted by crushing and subsequently were split for purification and analysis. Static mass spectrometry was used for N, He, and Ar, and conventional dynamic mass spectrometry was used for C. The data confirm the occurrence of near-constant He isotope ratios (He-3/He-4 = 8.53 +/- 0.79 Ra; n = 36) and C isotope ratios (delta(13)C = -5.2 +/- 0.7 parts per thousand vs. PDB; n = 21), and of a light nitrogen component in the convective mantle. Overall, the delta(15)N signature of MORE (mean delta(15)N = -33 +/- 1.0 parts per thousand vs. ATM, for samples with Ar-40/Ar-36 > 1000; n = 19) does not drastically differ from that of diamonds, some of which being presumably greater than or equal to 3 Ga, and is thought to represent the nitrogen isotopic ratio of the asthenospheric mantle. A common mantle source for diamond-bearing magmas and MORE magmas is unlikely, and this similarity may imply either active exchange of volatiles between the respective mantle sources or homogeneous distribution of nitrogen isotopes in these sources during most of Earth's history. Variations of the He-4/Ar-40* ratios as well as Ar-40/Ar-36 ratios are consistent with fractional crystallization-assimilation-degassing taking place in the depth range of 3 to 6 km. The corrected average C/N of the MORE mantle is 535 +/- 224, significantly higher than potential cosmochemical and geochemical end-members. C/He and C/N ratios corrected for fractional crystallization-assimilation-degassing fractionation increase with the degree of MORE enrichment (e.g., increasing K2O/TiO2) and are thought to reflect carbon heterogeneities in the mantle source, as a result of fractional recycling of a (fluid?) component extremely enriched in carbon (C/N > 3000). The N/He-3 ratios are less variable than the C/He-3 ratios, suggesting limited recycling of nitrogen. Such limited recycling is required to preserve a N isotope ratio in the convective mantle distinct from that of the atmosphere-hydrosphere sediments. Overall, neon, argon, nitrogen, and carbon abundances in the mantle appear to be chondritic, rather than solar, although the neon isotopic signature of the mantle indicates contribution of a solar component. This apparent discrepancy may reflect mixing between a solar-type component mostly seen at present in light rare gases and a chondritic-type component and has strong implications for the origin of terrestrial matter and the processes of their accretion. Copyright (C) 1999 Elsevier Science Ltd.