Basaltic volcaniclastics from the C hallenger D eep forearc segment, M ariana convergent margin: Implications for tectonics and magmatism of the southernmost I zu– B onin– M ariana arc

Convergent margin igneous activity is generally limited to 100–200 km from the trench except where spreading ridges are subducted or in association with S ubduction‐ T ransform E dge P ropagators ( STEP faults). The southernmost M ariana forearc, facing the C hallenger D eep, subducts M esozoic seafloor and is not in a STEP fault setting but includes at least one site where tholeiitic basalts recently erupted close to the trench, the SE M ariana F orearc R ift ( SEMFR ). We present evidence of young basaltic volcanism from ca. 100 km west of SEMFR . Shinkai 6500 diving during YK 13‐08 ( D ive 1363) recovered volcaniclastics from 5.5 to 6 km deep in the inner wall of the Mariana Trench, 50 km NE of the C hallenger D eep. Glassy fragments are tholeiitic basalts similar to MORB except for much higher contents of magmatic water (approx. 2% H 2 O vs. <0.2% H 2 O in MORB ) and enrichments in trace elements Rb ‐ Cs ‐ Ba , K , Pb , and Sr . D ive 1363 glasses are similar to basalts from SEMFR erupted near the trench and to M ariana T rough backarc basin basalts. Basalt fragments and palagonitized matrix dominate the studied samples, but small xenocrysts and xenoliths derived from mantle peridotite and N eogene volcanics are also present, probably torn from the vent walls. D ive 1363 hyaloclastites erupted at 3–6 km water depth accompanied by vigorous degassing of volatiles, most likely CO 2 . These results provide further evidence that the forearc adjacent to the C hallenger D eep has been invaded by asthenospheric mantle and derivative hydrous melts. Extension, hydration, and melt invasion combine to further weaken C hallenger D eep forearc lithosphere. Combined effects of: (i) absence of strong, cold lithosphere of the overriding plate; (ii) rapid rollback of a narrow, short subducted slab; and (iii) weak coupling between the subducting P acific plate and the overriding Mariana plate may be responsible for the great depth of the C hallenger D eep.