What CO ₂ well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

Abstract

Study of commercially produced volcanic CO 2 gas associated with the Colorado Plateau, USA, has revealed substantial new information about the noble gas isotopic composition and elemental abundance pattern of the mantle. Combined with published data from midocean ridge basalts, it is now clear that the convecting mantle has a maximum 20 Ne/ 22 Ne isotopic composition, indistinguishable from that attributed to solar wind-implanted (SWI) neon in meteorites. This is distinct from the higher 20 Ne/ 22 Ne isotopic value expected for solar nebula gases. The non-radiogenic xenon isotopic composition of the well gases shows that 20 per cent of the mantle Xe is 'solar-like' in origin, but cannot resolve the small isotopic difference between the trapped meteorite 'Q'-component and solar Xe. The mantle primordial 20 Ne/ 132 Xe is approximately 1400 and is comparable with the upper end of that observed in meteorites. Previous work using the terrestrial 129 I-129 Xe mass balance demands that almost 99 per cent of the Xe (and therefore other noble gases) has been lost from the accreting solids and that Pu-I closure age models have shown this to have occurred in the first ca 100 Ma of the Earth's history. The highest concentrations of Q-Xe and solar wind-implanted (SWI)-Ne measured in meteorites allow for this loss and these high-abundance samples have a Ne/Xe ratio range compatible with the 'recycled-aircorrected' terrestrial mantle. These observations do not support models in which the terrestrial mantle acquired its volatiles from the primary capture of solar nebula gases and, in turn, strongly suggest that the primary terrestrial atmosphere, before isotopic fractionation, is most probably derived from degassed trapped volatiles in accreting material.By contrast, the non-radiogenic argon, krypton and 80 per cent of the xenon in the convecting mantle have the same isotopic composition and elemental abundance pattern as that found in seawater with a small sedimentary Kr and Xe admix. These mantle heavy noble gases are dominated by recycling of air dissolved in seawater back into the mantle. Numerical simulations suggest that plumes sampling the core-mantle boundary would be enriched in seawater-derived noble gases compared with the convecting mantle, and therefore have substantially lower 40 Ar/ 36 Ar. This is compatible with observation. The subduction process is not a complete barrier to volatile return to the mantle.

Variation in the terrestrial isotopic composition and atomic weight of argon (IUPAC Technical Report)

“…Marty and Humbert [43] report values as high as 42 366 ± 9713 from similar samples crushed under vacuum. Subsequent reviews indicate values of around 40 000 or slightly higher may be typical of the 40 Ar/ 36 Ar ratio in large regions of the Earth's mantle from which MORB magmas were derived [44][45][46]. 38 Ar/ 36 Ar ratios in such samples generally are similar to that of atmospheric Ar, although the uncertainties of these measurements are large compared to those used in mass-dependent fractionation studies.…”

Disequilibrium degassing model determination of the 3He concentration and 3He/22Ne of the MORB and OIB mantle sources

“…However, if the OIB source mantle contains significant recycled oceanic crust (White, 1995;Hofmann, 1997;Holland and Ballentine, 2006;Brandenburg et al, 2008), then low He concentrations would be unsurprising. A small volatile rich ( 3 He) component, perhaps sampled from the deep mantle, would then account for high OIB 3 He/ 4 He (Ballentine and Holland, 2008). The recycled nature of the OIB-mantle source could also explain the higher major volatile concentrations that lead to disequilibrium degassing paths giving lower than MORB 4 He/ 40 Ar ratios (Fisher, 1985).…”

“…Noble gas isotope ratios are well studied within the mantle, although disentangling the different noble gas origins is still ongoing (Holland and Ballentine, 2006;Stroncik et al, 2007;Ballentine and Holland, 2008;Holland et al, 2009;Parai et al, 2009;Mukhopadhyay, 2012;Parai et al, 2012). Elemental noble gas ratios from different mantle reservoirs are also important for the understanding of mantle system.…”

Primary and secondary processes constraining the noble gas isotopic signatures of carbonatites and silicate rocks from Brava Island: evidence for a lower mantle origin of the Cape Verde plume

“…It is now clear that the Earth's mantle evolves through bidirectional mass transfer with the external reservoirs (crust, hydrosphere, and atmosphere; e.g., Hofmann 2003), but there is still an intense debate about whether the crustal and atmosphere-derived noble gases are re-introduced into the mantle through subduction (cf., Staudacher and Allègre 1988;Matsumoto et al 2001;Mohapatra and Honda 2006;Holland and Ballentine 2006;Ballentine and Holland 2008;Moreira and Raquin 2007). Considering: (1) the revision of arc magmas data suggesting that a very important amount of the altered oceanic crust noble gases budget is not deeply recycled, returning back to the crust/ atmosphere through subduction-related magmatism; (2) the enormous difference between the 40 Ar/ 36 Ar ratios reported for the atmosphere (298.6; Lee et al 2006) and the upper mantle (25,000; (3) reasoning based on the 129 Xe/ 130 Xe and 40 Ar/ 36 Ar ratios of Archean rocks and on the Ne-Ar systematics of oceanic basalts, leading to the proposition that the evolution of some rare gas isotopic ratios from solar to atmospheric ratios occurred before or during the Earth's accretion (Moreira and Raquin 2007;Raquin and Moreira 2009), we believe that noble gases recycling is not a major player constraining the mantle composition.…”