Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end-member: Evidence from the Samoan Volcanic Chain

[1] An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high Sr/Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing Pb/Pb with time from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu’u Seamount, (2) en-echelon arrays in Pb/Pb – Pb/Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai’i, (3) the highest Sr/Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in He/He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing Sr/Sr, and (5) mixing among four components which are best described as the ‘‘enriched mantle’’, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high U/Pb) mantle component. A theoretical, ‘‘pure’’ EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low Os/Os ratios and high He/He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.