Genesis of flood basalts from eclogite-bearing mantle plumes

On the basis of the geochemistry of flood basalts, we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core-mantle boundary. We present finite element models of the melting in starting plumes (plume heads) in both continental and oceanic regions. Starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt, or any melt at all, for reasonable plume head temperatures (1400-1600 degrees C) or lithosphere ages (6.25-100 m.y.). This result is in contrast to the results of Farnetani and Richards [1994] whose models require higher plume head temperatures (1600-1700 degrees C) and a weak, mobile lithosphere in order to generate volumes of melt similar to flood basalts. Plumes composed of 15 wt% eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings. The maximum extent of melting in these models is about 10-30%. This agrees with the model of Hofmann and White [1982], who argued that ocean island basalts are derived from subducted oceanic crust. If the embedded eclogite thermally reequilibrates with the surrounding plume material, then melt volumes and total extents of melting increase by a additional factor of 2-3. A simple two-dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume-related melting occurs over 50-60 m.y., a much longer period than for flood basalts.