Geochemical constraints on magma sources and mixing processes in Easter Microplate MORB (SE Pacific): a case study of plume–ridge interaction

The Easter Microplate is bounded in the east and west by two large overlapping spreading centres called the East and West Rift, respectively. The East Rift is located close to the Easter Hotspot, the site of young intraplate volcanism above a mantle plume. Easter Hotspot material creates a geochemical anomaly on the East Rift spreading axis of the microplate and mainly northward-directed material flow occurs beneath the propagating East Rift. The incompatible element-depleted mantle of the Easter Microplate consists of two different sources, one of which mixes with enriched Easter Hotspot material forming the East Rift Mid-ocean ridge basalt (MORB). The other depleted mantle source is present beneath the West Rift and mixes with an enriched endmember apparently distinct from the Easter Hotspot source. While the Easter Hotspot material leads to a bathymetric anomaly on the East Rift, no increased degree of melting is observed at the point of maximum inflow of enriched plume material, implying low excess temperatures (<50°C) of the mantle. The mixing processes between the Easter Hotspot and MORB material probably take place in the partially molten asthenosphere during the horizontal flow of the plume material. Melting above the plume and at the spreading axes in the Easter Microplate region occurs over a range of pressures but there is no sign of melts from enriched garnet pyroxenite.