Systematics of rare gas isotopes in basic lavas and ultramafic xenoliths

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