Ultra-magnesian igneous olivine is thought to crystallize from ultramafic melts and can therefore provide information about physicochemical conditions and the geochemistry and lithology of the source in the context of extremely hot mantle activity. This study focuses on the petrology and mineralogy of the Shizuoka meimechites, a series of ultra-magnesian olivine-bearing ultramafic lava blocks embedded in middle Eocene–early Miocene accretionary sediments at Shizuoka, Japan. Olivine phenocrysts within the Shizuoka meimechites have Fo compositions of ca. 81–92.8 mol%, where Fo = 100 Mg/(Mg + Fe). These ultra-magnesian olivines have trace-element compositions that are indicative of derivation from ultramafic melts that were in equilibrium with peridotitic mantle olivine. Al-in-olivine geothermometry yielded crystallization temperatures of ~1450 °C for Fo92.5–92.8 olivine, higher than the potential temperature of present-day mid-ocean ridge basalt source mantle material and similar to the potential temperatures of some large igneous provinces and ocean island basalts. Primary melt compositions in equilibrium with Fo92.5–92.8 olivine were estimated using the whole-rock compositions of the ultramafic lavas and are meimechitic, with MgO concentrations of ~21.4–27.0 wt% and SiO2 concentrations of 45.4–44.5 wt%. The potential temperature of the source mantle region for these meimechites (Tp) is estimated to be ~1486 °C–1705 °C, which is consistent with the Al-in-olivine geothermometry when considering adiabatic cooling during magma ascent. The relationship between Tp and Al-in-olivine temperature estimates for these lavas is consistent with global examples of ultra-magnesian olivine-bearing lavas. These lavas provide evidence of hitherto unknown Late Cretaceous–early Miocene deep mantle activity beneath the western Pacific region.
All Science Journal Classification (ASJC) codes