High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts

Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike siticic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO2 (< 48 wt%), low Al2O3 (< 12 wt%) and high CaO (> 12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIXIG or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825degreesC, respectively, which are less than 50degreesC cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725degreesC, indicating a narrow melting interval (similar to100degreesC). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550degreesC, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts. (C) 2003 Elsevier B.V. All rights reserved.