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 2.5–3 GPa, 1375 °C and 1440 °C. The fraction of added basaltic andesite melt was varied from ~ 8 to 50 wt.%. 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 50 wt.% 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 ~ 10–16 wt.% MgO match better with natural alkali basalts and basanite from intraplate ocean islands in terms of SiO2 (44–48 wt.%), TiO2 (2.2–4.1 wt.%), Al2O3 (12.6–14.3 wt.%), CaO (~ 8–11 wt.%), Na2O (~ 2–4 wt.%), and CaO/Al2O3 (0.52–0.81) as compared to partial melts of volatile-free peridotite and MORB-eclogite. FeO* content (~ 9–11 wt.%) 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 (TP) 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.