The composition of the incipient partial melt of garnet peridotite at 3GPa and the origin of OIB

We determined experimentally the composition of an incipient partial melt of a garnet lherzolite similar to KLB-1 peridotite at 3GPa using modified iterative sandwich experiments (MISE). The MISE method enables microbeam analyses of quenched liquids that are in equilibrium at the onset of melting (nominally, 0% melt fraction) with a specified peridotite bulk composition. The equilibrium incipient partial melt is a TiO₂-enriched (2.5±0.2wt.%) alkali olivine basalt with 44.8±0.2wt.% SiO₂ and 15.8±0.1wt.% MgO. Therefore, it has some compositional characteristics similar to a plausible parent to many alkalic oceanic island basalts. But in detail, several key components of this melt are different from parental liquids of typical primitive alkalic OIB, including lower FeO (9.7±0.1wt.%) and higher Al₂O₃ (12.7±0.2wt.%). It is also distinct from basalts derived from the EM and HIMU mantle end-members. We conclude that the vast majority of alkalic OIB cannot originate from volatile-poor partial melting of garnet peridotite at the base of the oceanic lithosphere unless there are contributions from non-peridotitic lithologies and/or enrichment in volatiles, iron, or other metasomatic components, and that such lithologic heterogeneities are intrinsic features of the HIMU and EM mantle reservoirs.