NOBLE-GASES IN SUBMARINE GLASSES FROM MIDOCEANIC RIDGES AND LOIHI SEAMOUNT - CONSTRAINTS ON THE EARLY HISTORY OF THE EARTH

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 He-3/He-4 ratios (R/R(a) approximately 8), variable Ar-40/Ar-36 ratios from nearly the atmospheric ratio to 15,200, and the presence of excesses in Xe-129 and Xe-132-136. We also observed linearly correlated anomalies in Ne-20/Ne-22 ratios (up to approximately 13) and Ne-21/Ne-22 ratios (up to approximately 0.06), which are consistent with the MORB correlation found by SARDA et al. (1988). High Ne-20/Ne-22 ratios (up to 11.74) and relatively high Ar-40/Ar-36 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 He-3/He-4 vs. Ne-20/Ne-22 diagram also suggest that the high Ne-20/Ne-22 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 Ne-20/Ne-22 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 (Ne-20/Ne-22 approximately solar) and in the present atmosphere (Ne-20/Ne-22 = 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 (Ne-20/Ne-22 approximately 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.