Reaction between basaltic melt and orthopyroxene at 3.0-4.5 GPa: Implications for the evolution of ocean island basalts in the mantle

Interactions between basaltic melt and orthopyroxenite (Opx) were investigated to gain a better understanding of the consequences of the residence and transport of ocean island basalts (OIBs) within the mantle. The experiments were conducted using a DS-3600 six-anvil apparatus at 3.0-4.5 GPa and 1300-1450 ℃. The basaltic melt and Opx coexisted at local equilibrium at these pressures and temperatures; the initial melts dissolved Opx, which modified their chemical composition, and clinopyroxene (Cpx) precipitated with or without garnet (Grt). The trace-element contents of Grt, Cpx, and melt were measured and the mineral-melt distribution coefficients (D) of Cpx-melt and Grt-melt were calculated, which can be used to assess the distribution of trace elements between basalt and minerals in the mantle. Two types of reaction rim were found in the experimental products, Cpx, and Cpx + Grt; this result indicates that residual rocks within the mantle should be pyroxenite or garnet pyroxenite. Both rock types are found in mantle xenoliths from Hawaii, and the rare-earth-element (REE) pattern of Cpx in these mantle pyroxenites matches those of Cpx in the experimental reaction rims. Furthermore, residual melts in the experimental products plot in similar positions to Hawaiian high-SiO₂ OIBs on major-element Harker diagrams, and their trace-element patterns show the signature of residual Grt, particularly in runs at ≤1350 ℃ and 4.0-4.5 GPa. Trace-element concentrations of the experimental residual melts plot in similar positions to the Hawaiian OIBs on commonly used discrimination diagrams (Ti vs. Zr, Cr vs. Y, Cr vs. V, Zr/Y vs. Zr, and Ti/Y vs. Nb/Y). These results indicate that reaction between basaltic melt and pyroxenite might contribute to the generation of Hawaiian high-SiO₂ OIBs and account for their chemical variability.