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High-Mg Kroenke-type basalts containing 9–11 wt% MgO from the Ontong Java Plateau (OJP) include spinels. These spinels have nearly identical Cr/(Cr + Al) ratios (0.45–0.54), and are generally hosted by olivine phenocrysts with relatively primitive compositions, with Fo contents [100 × Mg/(Mg + Fe2+) in atomic ratios] as high as 88. This implies that the primary OJP magmas were in equilibrium with refractory peridotites (i.e., harzburgites) and were homogenized by large-scale melting and magmatic evolutionary processes. Oxygen geobarometry indicates that the OJP primary magmas record uniform oxygen fugacity (fO2) conditions and are slightly more oxidized (0.6 log units) than normal mid-ocean ridge basalts, but record less-oxidized fO2 conditions than ocean island basalts. These data are consistent with previous studies that suggest the OJP primary magmas were generated by large-scale and extensive melting of a mantle source region under relatively oxidized conditions.

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