The composition of the incipient partial melt of garnet peridotite at 3 GPa 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 3 GPa 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 ± 0.2 wt.%) alkali olivine basalt with 44.8 ± 0.2 wt.% SiO2 and 15.8 ± 0.1 wt.% 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 ± 0.1 wt.%) and higher Al2O3 (12.7 ± 0.2 wt.%). 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.

► 3 GPa near-solidus partial melt of garnet peridotite broadly resembles alkalic OIB. ► Some key characteristics are different, particularly greater Al2O3, and lower FeO*. ► Bulk partitioning limits TiO2 contents of fertile peridotite-derived OIB to < 4%. ► Most OIB require non-peridotitic or metasomatically enriched peridotite sources. ► HIMU-flavored, but not EM-flavored, OIB may indicate peridotite enriched in CO2.