Noble gas systematics of the hawaiian volcanoes based on the analysis of Loihi, Kilauea and Koolau submarine rocks

Noble gas isotopes were analyzed for glass and olivine phenocrysts of Hawaiian submarine volcanic rocks collected at depths of 2300-5700 m using submersibles "Shinkai 6500" and "Kaiko". Both crushing and heating methods were used for extracting noble gases. Loihi samples had ³He/⁴He of 21-32 Ra, ²⁰Ne/²²Ne of 10-12 and ⁴⁰Ar/³⁶Ar of 300-2700, while Kilauea samples had ³He/⁴He of 13-15 Ra, ²⁰Ne/²²Ne of 10.6-11.1, and ⁴⁰Ar/³⁶Ar of 800-3600. For all samples analyzed, the ¹²⁹Xe/¹³⁰Xe cannot be distinguished from the atmospheric value within analytical uncertainty. Olivine separates from two Koolau basaltic rocks collected at depths of about 3600 m and 3100 m have ³He/⁴He of about 19 Ra, whereas other Koolau samples collected at depths of less than 3000 m have ³He/⁴He of 14-15 Ra like subaerial Koolau rocks. The samples collected from Loihi, in the pre-shield stage, contain both higher and more variable values of 3He/He compared to the shield-building stage of other volcanoes in the Hawaiian chain. The post-shield and rejuvenated stages show the lowest ³He/⁴He, overlapping values obtained for MORB. We interpret this age trend to reflect an increasing degree of interaction between the rising plume magma and the uppermost part of asthenosphere. The extreme He isotope variation shown at Loihi may represent additional interaction of the plume magma with the old, altered oceanic crust. The variation of heavier noble gases may indicate incorporation of seawater into the magma before eruption.