Systematics of rare gas isotopes in basic lavas and ultramafic xenoliths

Abstract

Mid‐ocean ridge (MOR) tholeiites and several ultramafic xenoliths from a variety of localities have identical 3He/4He ratios of 1.3×10−5 which indicate that a large portion of the mantle has a constant helium isotopic composition. In contrast, megacrysts from alkali basalts, phenocrysts from andesites, and some nodules have 3He/4He ratios of 0.3 to 1.3×10−5, and submarine tholeiites from Kilauea have ratios greater than 1.3×10−5. The low ratios of some nodules may be due either to loss of 3He by prior outgassing events that lowered the 3He/(U + Th) ratio or to interactions with metasomatic fluids having low 3He/4He or 3He/(U + Th) ratios. Andesites and nodules associated with subduction zones have 3He/4He ratios indicative of mixing between the helium in MOR tholeiites and subducted helium with a low 3He/4He ratio. High 3He/4He ratios of Hawaiian tholeiites, like the ratios measured in materials from other hot spots, reflect a deeper, less depleted source than do MOR tholeiites. Some xenoliths and lavas have greater 21Ne/22Ne and 20Ne/22Ne ratios than does the atmosphere. Correlation of excess 21Ne with 4He in most samples suggests that the 21Ne is nucleogenic. The excesses of 20Ne are probably the result of mass fractionation processes. Hawaiian submarine lavas, which may sample deeper, less depleted portions of the mantle than do MOR tholeiites, not only have high 3He/4He ratios but also low40Ar/36Ar ratios. Many nodules and megacrysts related to alkali basalts have lower 3He/4He ratios but higher 40Ar/36Ar ratios than do MOR tholeiites because of prior outgassing events that have lowered the 3He/(U + Th) and 36Ar/K ratios. Lavas and nodules associated with subduction zones can have variable 3He/4He and 40Ar/36Ar ratios because of the addition of subducted gases and the disturbance of the relative position of depleted and undepleted portions of the mantle.

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

“…Whereas the mantle generates negligible amounts of 20 Ne and 22 Ne (on the scale of Fig. 8a), significant quantities of 21 Ne are produced by nuclear reactions during the decay of radioactive U and Th (Kyser and Rison, 1982), thereby effecting measurable changes in 21 Ne/ 22 Ne. Honda et al (1993) (Honda et al, 1993).…”

Noble gases in the mantle wedge and lower crust: an inference from the isotopic analyses of xenoliths from Oki-Dogo and Ichinomegata, Japan.

“…The ratios for arc-volcanic gases, old oceanic crusts, Loihi and MORE are also in dicated in Fig. 2 (Kaneoka and Takaoka, 1980Kyser and Rison, 1982;Sarda et al, 1985;Allegre et al, 1986/87;Staudacher and Allegre, 1988). In Table 4, the noble gas composi tions of our rock samples have been compared with those of the deep-sea water, oceanic sediments, oceanic crust and upper mantle (Allegre et al, 1986/ 87;Staudacher and Allegre, 1988).…”

“…Although many noble gas data have been reported for mantle derived rock samples from the mid-oceanic ridges, oceanic-islands and continental areas (e.g. Kaneoka et al, 1978;Kyser and Rison, 1982;Lupton, 1983;Kaneoka and Takaoka, 1991;Porcelli et al, 1992; and the references therein), the data for island arc region are scarce, and all from Japanese islands: a lher zolite xenolith from Oki-Dogo (Kaneoka et al, 1978), JP-1 peridotite from Horoman, Hok kaido (Miura and Nagao, 1991), and lherzolite xenoliths from Ichinomegata (Porcelli et al, 1992). The 'He/'He ratios of (1.26 ± 0.02) x 10-5 for JP-1 and (0.95 -1.18) x 10-5 for xenoliths from the Ichinomegata were slightly higher than those (-< 1.05 x 10-5) of volcanic gases in Japan (e.g.…”

Xenon decay products of extinct radionuclides in the Navajo, New Mexico well gas.

“…Radio genic 129Xe has also been observed in a high purity. CO2 gas well of New Mexico (Butler et al, 1963;Boulos and Manuel, 1971;Hennecke and Manuel, 1975a;Phinny et al, 1978;Smith and Reynolds, 1981), and radiogenic 129Xe is frequently associated with CO2 in rocks from the upper mantle (Hennecke and Manuel, 1975 b;Kaneoka and Takaoka, 1978;Takaoka and Ozima, 1978;Kyser and Rison, 1982;Staudacher and Alldgre et al, 1983;Kirsten et al, 1983). However, there has been no clear identification of excess 111-136Xe from the spontaneous fission of 244Pu in any terrestrial sample.…”