We investigate the magma plumbing system of the 2.8-ka Saruana Lava at Chokai volcano, northern Honshu, based on mineral chemistry and textures. The Saruana Lava is a 0.6 km 3 basaltic andesite containing mafic enclaves with porphyritic and equigranular textures. Whole-rock compositions define linear trends on Harker diagrams, with Ni decreasing to near-zero concentrations at ~60 wt% SiO₂. Phenocrysts include olivine, plagioclase, augite, orthopyroxene, titanomagnetite, and rare amphibole. Olivine shows only normal zoning, whereas plagioclase and pyroxenes exhibit both normal and reverse zoning. Crystals are classified into felsic antecrysts, mafic antecrysts, and autocrysts based on zoning patterns, mineral chemistry, and inclusion assemblages. Felsic antecrysts, forming the cores of reversely zoned crystals, originated from an andesitic crystal mush stored in the mid-crust. Thermobarometric estimates indicate crystallization at approximately ~850 °C and mid-crustal pressures, consistent with a partially molten mush reservoir. Mafic antecrysts include Fo85–88 olivine with spinel derived from primitive basalt magma. Equigranular mafic enclaves represent crystal accumulation in basalt magma at deeper crustal levels prior to interaction with the felsic system, consistent with amphibole-based pressure estimates. Reverse zoning records partial dissolution of mush-derived crystal cores followed by overgrowth in basalt magma. These textures indicate rapid thermal interaction between hot basalt and a cooler felsic crystal mush, resulting in simultaneous partial melting of mush minerals and crystallization in the mafic magma. Diffusion profiles in olivine constrain the recharge-to-eruption timescale to weeks to months. The Saruana Lava preserves a crystal-scale record of basalt recharge into a felsic crystal mush and the development of a chemically and texturally heterogeneous magma prior to lava emplacement. • Phenocrysts were classified to reconstruct stages of basalt recharge and crystal mush interaction. • Thermal diffusion promoted resorption of mush-derived crystals and overgrowth in basalt magma. • Equigranular enclaves record deep crystal accumulation in basalt magma (~900 MPa). • Diffusion profiles constrain the recharge-to-eruption timescale to weeks to months. • Zoning patterns and glass compositions document the development of a chemically heterogeneous magma.
Ohba et al. (Sun,) studied this question.