Noble gas isotopes including 3He 4He, 40Ar 36Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount. Glass and coexisting olivine crystals have similar 3He 4He ratios (2.8-3.4) ?? 10-5, 20 to 24 times the atmospheric ratio (RA), but different 40Ar 36Ar ratios (400-1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not. The apparent high 3He 4He ratio (3 ?? 10-5; = 21 RA) coupled with a relatively high 40Ar 36Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma. Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component. Excess 129Xe has not been observed due to apparent large mass fractionation among Xe isotopes. ?? 1983.
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Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount