Spatial and temporal variability in Marquesas Islands volcanism revealed by 3He/4He and the composition of olivine-hosted melt inclusions

The Marquesas Islands are a volcanic chain having an oblique orientation to Pacific plate motion. They represent the surface manifestation of a mantle hotspot having a low melt flux and a large intrinsic chemical and isotopic heterogeneity. Here we provide new helium isotope data for olivine separated from 23 basalts and one gabbroic xenolith, plus whole rock major and trace element data for the same lavas. We also carried out trace element analyses of 41 olivine-hosted melt inclusions (OHMIs) from 15 of the basalts, and 40Ar(/39)Ar age determinations for basalt groundmass from the islands of Fatu Hiva and Hiva Oa. He-3/He-4 shows a large range of values from < 5 R-A up to 14.5 R-A. Elevated He-3/He-4 ratios > 9 R-A are found at Nuku Hiva and Hiva Oa near the center of the archipelago, in agreement with previous findings (Castillo et al., 2007). The new ages for the Hiva Oa lavas indicate that He-3/He-4 increases from similar to 8 R-A to 14.5 R-A during the mid-late shield stage of volcanism (2.6-2.0 Ma), followed by a decrease during the post-shield stage (< 1.8 Ma) after collapse of the Temetiu caldera. Lavas from Fatu Hiva, Ua Huka, Ua Pou, Tahuata and Motane have He-3/He-4 < 8.3 R-A, with values typically between 5 and 7 R-A. Incompatible element ratios observed in the OHMIs show more variability than what is observed in the lavas because they represent a less extensive aggregation of melts derived from small scale chemical heterogeneities within the Marquesas mantle plume. The Cl/K ratio of OHMIs ranges from 0.02 to 0.4, potentially due to both mantle source heterogeneity and assimilation of brine or seawater-altered rocks at crustal levels. The geographic distribution of He-3/He-4 suggests the mantle plume is concentrically zoned in He isotope composition, with high He-3/He-4 material concentrated near its central axis that is sourced from hot, primitive material entrained near the core-mantle boundary region.