Noble gases in submarine glasses from mid-oceanic ridges and Loihi seamount: Constraints on the early history of the Earth

Abstract

Isotopic and elemental compositions of noble gases in glass samples from mid-oceanic ridges (Mid-Atlantic Ridge, East Pacific Rise, and Galapagos Ridge) and from Loihi seamount (Hawaii) were analyzed with a sector-type mass spectrometer system. Our results for MORB (Mid-Oceanic Ridge Basalt) samples are consistent with the well-known MORB-type noble gas signatures: rather uniform 3He/ 4He ratios (R/R a ~ 8), variable 40Ar/ 36Ar ratios from nearly the atmospheric ratio to 15,200, and the presence of excesses in 129Xe and 132−136Xe. We also observed linearly correlated anomalies in 20Ne/ 22Ne ratios (up to ~ 13) and 21Ne/ 22Ne ratios (up to ~0.06), which are consistent with the MORB correlation found by Sarda et al. (1988). High 20Ne/ 22Ne ratios (up to 11.74) and relatively high 40Ar/ 36Ar ratios (up to 2780) are also found in some Loihi samples. Neon isotopic compositions for these samples are quite consistent with the Loihi-Kilauea correlation line recently found by Honda et al. (1991) but inconsistent with the MORB correlation line. Loihi data plotted in a 3He/ 4He vs. 20Ne/ 22Ne diagram also suggest that the high 20Ne/ 22Ne ratios found in Loihi samples are not the results of contamination of MORB-type neon but are indigenous to the source of Loihi. This suggests that the solar-type high 20Ne/ 22Ne ratios are not only the signature of the source mantle of MORB (upper mantle) but also that of the source of Loihi (possibly lower mantle), and hence, possibly the signature of the whole mantle. The fact that isotopic composition in the mantle ( 20Ne/ 22Ne ~ solar) and in the present atmosphere ( 20Ne/ 22Ne = 9.8) are different seems to contradict a simple degassing model of the atmosphere, which implies isotopically identical compositions for the mantle and the atmosphere. A possible scenario would be that neon in the present atmosphere had been fractionated from the original solar-type ratio ( 20Ne/ 22Ne ~ 13.6) down to the present ratio (9.8) by extensive loss of the (proto-) atmosphere at an early stage of the Earth's history. If the Earth's accretion was “heterogeneous,” mixing of a solar-type Ne (derived from most of the mantle) and a planetary-type Ne (derived from late-veneer, volatile-rich material) would be an alternative explanation for the present atmospheric Ne. In either case, the model should be consistent with other noble gas data, especially xenon isotopic data.