Melt inclusions reveal geochemical cross‐arc variations and diversity within magma chambers feeding the Higashi‐Izu Monogenetic Volcano Field, Izu Peninsula, Japan

The Higashi‐Izu Monogenetic Volcano Field (HIMVF) consists of 70 subaerial monogenetic volcanoes on the northeastern Izu Peninsula, Japan, and 50 submarine monogenetic volcanoes that extend offshore toward Izu‐Oshima on the Izu‐Bonin Arc volcanic front. We investigate olivine‐hosted glassy silicate melt inclusions erupted in scoria from two of the subaerial monogenetic volcanoes, Takatsukayama and Sukumoyama. Major element data (and volatile elements S and Cl) have been collected from 85 inclusions, of which 26 and 41 have been analyzed for trace and volatile (H 2 O, CO 2 ) elements, respectively. The trace element compositions of the melt inclusions, together with those of the whole rocks from volcanoes within the HIMVF, vary systematically across the HIMVF, even over 5 km between Takatsukayama and Sukumoyama. These variations reflect a decrease in the contribution from slab fluids that leads to melting of the mantle with distance from the volcanic front, and increasing subducting slab depth. The Takatsukayama and Sukumoyama inclusions also reveal an intra‐volcano geochemical diversity; most are low TiO 2 (0.81–1.18 wt.%), high Al 2 O 3 (15.88–19.34 wt.%) basalts (low TiO 2 ‐HABs), with a few high TiO 2 (1.11–2.19 wt.%), low Al 2 O 3 (11.75–14.04 wt.%) basalts (high TiO 2 ‐LABs) that are sometimes trapped in the same crystal. As both groups show the cross‐HIMVF geochemical variability, we conclude that the heterogeneity does not reflect mantle processes or the true composition of melts in the magma chambers; instead we relate it to interstitial melts within the crystal mush of the magma chambers experiencing distinct crystallization histories.