Reaction between MORB-Pyroxenite-derived Partial Melts and Subsolidus Peridotite at 3 GPa and Generation of Alkalic Ocean Island Basalts

Major element characteristics of ocean island basalts (OIBs) source heterogeneities are a subject of major debate that primarily stems from the fact that neither volatile-free fertile peridotite nor subducted oceanic crust (silica-excess MORB-pyroxenite at mantle depths) produce partial melts that match the major element chemistry of alkalic OIBs [1]. This has led to exploring many exotic lithologies as OIB sources. No studies, however, have systematically explored the evolution of MORB-pyroxenite partial melts owing to reaction with peridotite. Because the solidus of subducted crust intersects with mantle adiabat deeper than the peridotite solidus [2,3], partial melt extracted from MORB-pyroxenite will react with subsolidus peridotite and form secondary pyroxenite [4,5]. Here we explore whether such a melt-rock reaction between pyroxenite partial melt and peridotite can produce alkalic OIBs. We performed reaction experiments between partial melt of the anhydrous MORB-like pyroxenite [3] and fertile peridotite KLB-1. Experiments were performed using a piston-cylinder apparatus and Pt/Gr capsules at 1375°C and 3 GPa, a condition where volatile-free MORB-pyroxenite is ~10% melted [4]. The added fraction of andesitic melt varied from ~8-33%. Melt was introduced either in a layered geometry or mixed homogeneously with peridotite to simulate channelized and porous flow, respectively. All the experiments produced a residual assemblage of olivine+opx+cpx±garnet coexisting with reacted partial melts. Layered experiments produced a zone of (± garnet-)websterite separating the pool of melt from four-phase lherzolite. With increasing opx mode from ~16 to 35 wt.% in the residue, reacted melts in the layered experiments show a systematic decrease in SiO2 (~55 to 45 wt%) and Al2O3 (~15 to 13 wt%), and increase in MgO (~5.4 to 14.9 wt%) and CaO (~7 to 14 wt%). TiO2 varies from ~2.4 to 5.6 wt%, Na2O from ~2.2 to 4.2 wt% and FeO* from ~ 6.8 to 8.1 wt%. Experiments with homogeneous melt-rock mixtures produced partial melts with ~44-48 wt% SiO2, ~2.8-6.7 wt% TiO2, ~12-13 wt% Al2O3, 8.5-11.2 wt% FeO*, 13-17 wt% MgO, 7.8-11.0 wt% CaO, and 2.1 to 4.2 wt% Na2O. Our experiments demonstrate that andesitic partial melts of MORB-pyroxenite can evolve to alkalic basalts with low SiO2 and high MgO owing to reaction with subsolidus peridotite. Most importantly, the reacted melts match the primitive alkalic OIBs (MgO ~9-15 wt%) better compared to partial melts of fertile peridotite in terms of TiO2, Al2O3, and CaO/Al2O3 ratio. Thus, our study precludes the need of exotic silica-deficient lithologies in the source of many alkalic OIBs. [1] Jackson M.G. and Dasgupta, R. 2008, EPSL 276, 175-186. [2] Yasuda et al. 1994, JGR., 99(B5), 9401-9414. [3] Pertermann, M. and Hirschmann, M.M. 2003, JPet 44, 2173-2201. [4] Sobolev A.V. et al. 2005, Nature 434, 590-597. [5] Herzberg C. 2006, Nature 444, 605-609.