Peridotite vs pyroxenite source lithologies of primordial helium in the Iceland and Hawaiian plumes
Wednesday, 17 December 2014
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.