Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source. There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases. Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The 3He/4He in the East Pacific Rise shows a remarkably uniform ratio of (1.21 +/- 0.07)*10**-5, while the40Ar/36Ar ranges from 700 to 5600.

(Table 3) Noble gas composition of dissociated gas hydrate specimens from ODP Leg 164 sites

Fractionation of the noble gases should occur during formation of a Structure I gas hydrate from water and CH4 such that CH4 hydrate is greatly enriched in Xenon. Noble gas concentrations and fractionation factors (F[4He], F[22Ne], F[86Kr], and F[132Xe] as well as R/Ra) were determined for eight gas hydrate specimens collected on Leg 164 to evaluate this theoretical possibility and to assess whether sufficient quantities of Xe are hosted in oceanic CH4 hydrate to account for Xe "missing" from the atmosphere. The simplest explanation for our results is that samples contain mixtures of air and two end-member gases. One of the end-member gases is depleted in Ne, but significantly enriched in Kr and Xe, as anticipated if the source of this gas involves fractionation during Structure I gas hydrate formation. However, although oceanic CH4 hydrate may be greatly enriched in Xe, simple mass balance calculations indicate that oceanic CH4 hydrate probably represents only a minor reservoir of terrestrial Xe. Noble gas analyses may play an important role in understanding the dynamics of gas hydrate reservoirs, but significantly more work is needed than presented here.

(Table 1) Downhole variation of potassium, apparent K-Ar age, and inert gas abundances at DSDP Holes 69-504B, 70-504B and 83-504B

Ne, Ar, Kr, Xe, and K2O were measured in representative samples of holocrystalline basalt from DSDP Hole 504B. No hiatus in inert gas abundance is recognized at the base of the "oxic" alteration zone and the extent rather than the nature of alteration appears to determine these abundances. When the inert gas abundances are separately plotted against K2O, two distinct trends of loss emerge, one for alteration involving K-gain, the other for K-loss. Apparent whole-rock K-Ar ages are anomalous in the upper 50 m of basement, and below 300 m sub-basement. In the intervening zone of basement, celadonization adds sufficient potassium and eliminates enough "primary" 40Ar early in the history of the basalts for "excess" 40Ar to become subordinate to radiogenic 40Ar in basalts showing potassium enrichment greater than 0.2%. Stratigraphically correct K-Ar ages are obtained, therefore, from K-enriched basalts of the oxic alteration zone.