Tracing magma generation and evolution in a multi-level magma plumbing system at the Izu rear-arc

Abstract Volcanism in the Izu rear-arc is bimodal in composition and occurs in an extensional zone associated with thickened arc crust. IODP Expedition 350 sampled a 4.0 to 4.4 Ma pumiceous felsic succession at 700 m below seafloor from Site U1437 in the Izu rear-arc. Named Unit II, this 45 m thick succession exhibits multiple homogeneous pumice-rich beds that are complexly graded and interbedded. Minor tuffaceous mudstone represents hiatus in eruptions when background sedimentation resumed. Using plagioclase crystal chemistry, we investigate the generation and evolution of the magmas and reconstruct the architecture of their corresponding magma plumbing system. Plagioclase is compositionally bimodal with peak frequencies at An 30–60 and An 75–95. Plagioclase crystals have 87 Sr/ 86 Sr of 0.703096 to 0.703380, and ẟ 18 O values of 5.2 to 6.4‰. We interpret the chemical zonation within individual crystals to represent fractional crystallisation, recharge, magma mixing and partial melting of altered crust. From the plagioclase-liquid crystallisation pressures, we identified deep mafic magma pockets in the oceanic lithosphere (30 to 38 km) and below the mantle-crust transition (20 to 24 km). During ascent, the magmas differentiated and stalled in the lower (10 to 12 km) and upper crust (4 to 6 km). We infer the source of the Unit II magmas to be depleted mantle wedge, associated with rifting of the Philippine Sea Plate, enriched by a uniform contribution of mélange and variable fluid flux from the subducted slab.