Reaction between MORB-eclogite derived melts and fertile peridotite and generation of ocean island basalts

We performed reaction experiments between partial melt of volatile-free MORB-eclogite and volatile-free fertile peridotite at 2.5-3GPa, 1375°C and 1440°C. The fraction of added basaltic andesite melt was varied from ~8 to 50wt.%. Melt was introduced either as a separate layer or mixed homogeneously with peridotite to simulate channelized and porous flow, respectively. Layered experiments produced a zone of orthopyroxene-rich garnet-websterite separating the reacted melt pool from a residual four phase lherzolite while mixed experiments produced a residual assemblage of orthopyroxene±clinopyroxene±olivine±garnet co-existing with reacted melt where residual olivine was absent only in the experiments with 50wt.% added melt. It is observed that the reacted melts display a continuous spectrum from tholeiitic to alkalic melts with increasing extent of wall-rock reaction for the layered runs and decreasing melt:rock ratio for the mixed experiments. The reacted melts at ~10-16wt.% MgO match better with natural alkali basalts and basanite from intraplate ocean islands in terms of SiO ₂ (44-48wt.%), TiO ₂ (2.2-4.1wt.%), Al ₂O ₃ (12.6-14.3wt.%), CaO (~8-11wt.%), Na ₂O (~2-4wt.%), and CaO/Al ₂O ₃ (0.52-0.81) as compared to partial melts of volatile-free peridotite and MORB-eclogite. FeO* content (~9-11wt.%) of the reacted melts, however, remains poorer compared to most ocean island basalts (OIBs). We demonstrate that both alkalic and tholeiitic melts are produced in the process of MORB-eclogite partial melt and fertile peridotite reaction. We also demonstrate that near-primary alkali basalt can form at a temperature distinctly below the peridotite solidus and mantle potential temperature (T _P) of ~1350°C may be sufficient to generate near-primary alkalic OIBs. Our study obviates the necessity for exotic lithologies, such as silica-deficient garnet pyroxenites, in the solid state mantle to explain the genesis of alkalic OIBs.