Peridotite vs pyroxenite source lithologies of primordial helium in the Iceland and Hawaiian plumes
Yaakov Weiss, Columbia University of New York, Palisades, NY, United States, Cornelia Class, Lamont -Doherty Earth Observatory of Columbia University, Palisades, United States, Steven L Goldstein, Lamont-Doherty Earth Observatory, Palisades, NY, United States, Albrecht W Hofmann, Max Planck Institute for Chemistry, Mainz, Germany and Alexander V Sobolev, University Grenoble Alpes, ISTerre, Institute of Earth Sciences, Grenoble, France
Abstract:
Recent studies have suggested that chemical compositions of olivine phenocrysts can be used to distinguish between olivine-rich and olivine-poor lithologies (peridotite and pyroxenite) in the mantle sources of MORB and OIB (Sobolev et al, 2005, 2007, Herzberg, 2011). Olivine phenocrysts are also used for 3He/4He analyses, allowing us to address linkages between basalt source lithologies and primordial helium storage in the mantle.
We report 3He/4He analyses of olivine phenocrysts from Iceland and Hawaii that were previously analyzed for major and minor elements (Sobolev et al., 2007, 2008). 3He/4He in Iceland samples vary between 7-26 (R/RA) and correlate negatively with Mn/Fe and positively with Ni/Mg of the olivine phenocrysts, as well as 187Os/188Os and Gd/Lu in host lavas. These relationships suggest mixing between a degassed and incompatible element depleted peridotitic source (MORB-type?), and a primordial 3He enriched plume source containing lithologies with lower modal olivine/garnet (pyroxenite?). Hawaiian samples, on the other hand, form a positive trend in 3He/4He vs Mn/Fe space, as compared to the negative trend of the Iceland samples. Thus, the Iceland and Hawaiian plumes give contrasting results for the high 3He/4He host lithology – more pyroxenite-rich for Iceland and peridotite-rich for Hawaii.
Plotting the Iceland and Hawaiian plume data together for 3He/4He vs Mn/Fe, the two divergent trends intersect at the composition of Loihi, characterized by intermediate pyroxenite/peridotite and 3He/4He of 9–28 RA. Baffin Island and West Greenland picrites have the highest known magmatic 3He/4He and high olivine Mn/Fe (Starkey et al., 2009, 2012), indicating high 3He/4He peridotites as well as pyroxenites in the Iceland plume. When plotted with the Iceland-Hawaii samples, they lie on an extension of the Hawaiian trend. This raises the possibility (pending more data from other plumes) that there is a global trend showing high 3He/4He residing in peridotite.
