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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.

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