Testing The Recycled Gabbro Hypothesis For The Origin Of "Ghost Plagioclase" Melt Signatures Using Sr-87/Sr-86 Of Individual Olivine-Hosted Melt Inclusions From Hawai'I

Melt inclusions with large, positive Sr anomalies have been described in multiple tectonic settings, and the origins of this unusual geochemical feature are debated. Three origins have been proposed, all involving plagioclase as the source of the elevated Sr: (i) direct assimilation of plagioclase-rich lithologies, (ii) recycled lower oceanic gabbro in the mantle source, and (iii) shallow-level diffusive interaction between present day lower oceanic crust (i.e., plagioclase-bearing lithologies) and the percolating melt. A "ghost plagioclase" signature (i.e., a large, positive Sr anomaly without associated high Al2O3) is present in melt inclusions from Mauna Loa. We present new Sr-87/Sr-86 measurements of individual olivine-hosted melt inclusions from three Hawaiian volcanoes, Mauna Loa, Loihi, and Koolau. The data set includes a Mauna Loa melt inclusion with the highest reported Sr anomaly (or highest (Sr/Ce)(N), which is 7.2) for Hawai'i. All melt inclusions have Sr-87/Sr-86 values within the range reported previously for the lavas from each volcano. Critically, the Sr-87/Sr-86 of the high (Sr/Ce)(N) melt inclusion lies within the narrow range of Sr-87/Sr-86 for Mauna Loa melts that lack high (Sr/Ce)(N) signatures. Therefore, to explain the high (Sr/Ce)(N) ratio of the ghost plagioclase signature using an ancient recycled gabbro, the gabbro-infused mantle source would have had to evolve, by chance, to have the same Sr-87/Sr-86 as the source of the Mauna Loa melts that lack a recycled gabbro (ghost plagioclase) signature. Alternatively, shallow-level diffusive interactions between Mauna Loa plagioclase-rich cumulates and a percolating mantle-derived melt provides a simpler explanation for the presence of the high (Sr/Ce)(N) Mauna Loa melts.