Systematics of rare gas isotopes in basic lavas and ultramafic xenoliths

Mid‐ocean ridge (MOR) tholeiites and several ultramafic xenoliths from a variety of localities have identical 3 He/ 4 He 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 3 He/ 4 He 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 3 He by prior outgassing events that lowered the 3 He/(U + Th) ratio or to interactions with metasomatic fluids having low 3 He/ 4 He or 3 He/(U + Th) ratios. Andesites and nodules associated with subduction zones have 3 He/ 4 He ratios indicative of mixing between the helium in MOR tholeiites and subducted helium with a low 3 He/ 4 He ratio. High 3 He/ 4 He 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 21 Ne/ 22 Ne and 20 Ne/ 22 Ne ratios than does the atmosphere. Correlation of excess 21 Ne with 4 He in most samples suggests that the 21 Ne is nucleogenic. The excesses of 20 Ne 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 3 He/ 4 He ratios but also low 40 Ar/ 36 Ar ratios. Many nodules and megacrysts related to alkali basalts have lower 3 He/ 4 He ratios but higher 40 Ar/ 36 Ar ratios than do MOR tholeiites because of prior outgassing events that have lowered the 3 He/(U + Th) and 36 Ar/K ratios. Lavas and nodules associated with subduction zones can have variable 3 He/ 4 He and 40 Ar/ 36 Ar ratios because of the addition of subducted gases and the disturbance of the relative position of depleted and undepleted portions of the mantle.