Active Plume, Abandoned Ridge, or Ancient Slab? olivine-Hosted Melt Inclusions Record the Mantle Melting Conditions Below the East Pacific Revillagigedo Archipelago, Mexico

Abstract

The voluminous active volcanoes of the Revillagigedo Islands of Socorro and San Benedicto in the east Pacific Ocean exhibit hotspot geochemistry. However, the presence of a deep mantle plume has not been seismologically confirmed. Instead, a fossil subducted slab is identified in the lower mantle underneath the volcanoes. Moreover, the islands are located on the recently abandoned Mathematician Ridge. To test whether the mantle that melts carries signatures of a fossil mantle wedge, an extinct ridge, or a mantle plume, we present new combined major- and trace element data and Sr-Nd-Pb isotopic compositions on bulk rocks and olivine-hosted melt inclusions. We find broad trends between depleted mid-ocean ridge basalts (DMORBs) and enriched ocean island basalts (OIB)-type compositions. Highly variable melt inclusion compositions reflect melting of a locally heterogeneous source. The enriched melts do not carry fossil subduction wedge signatures, suggesting that the former wedge is no longer located above the deep slab remnant. From the melt inclusions, we derive melting P-T conditions between 1420–1493°C and 21–30 kbar, hotter and deeper than typical MORBs. This points at active mantle upwelling, which we confirm seismologically by a relatively deeper 410 km and shallower 660 km discontinuity, suggesting the presence of hot material to below the mantle transition zone. The enriched deep melts are thus likely plume-derived and are diluted by melts from a depleted component that melts shallow below the abandoned ridge. We hypothesize that a deep plume may be rising around the mid-mantle Socorro slab.