The missing half of the subduction factory: Shipboard results from the Izu Rear Arc, IODP Expedition 350

IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although severalsites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand theevolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporalrecord (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chainsup to ~100-km long roughly perpendicular to the arc front. Dredge samples from these aregeochemically distinct from arc front rocks, and drilling was undertaken to understand this arcasymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-kmwest of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery.We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected(~60%), and the remaining fraction of the section is much finer-grained than predicted from itsposition within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size(clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% ofthe section, and only one igneous unit was encountered, a rhyolite peperite intrusion at~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagneticdatum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zirconconcordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed tothe fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arcasymmetry is not recorded in rocks older than ~13 Ma.