A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

Basement outcrops sampled by submersible and dredge from the inner slope of the Izu–Bonin trench at 32°N and 6200–6700 m water depth have a distinct mid-ocean ridge basalt (MORB) chemistry unlike any other rocks previously sampled in the Izu–Bonin arc. They are low K tholeiites with moderate TiO2 (0.7–1.8 wt%), extremely low Ba (1.5–7 parts per million), low Ba/La (1.2–3) and are depleted in light rare-earth elements. These samples could represent either an accreted piece of subducting Pacific plate or a trapped remnant of Philippine Sea plate on which the Izu–Bonin arc was built. Although their major and trace element chemistry do not help to distinguish their source, the Sr, Nd and Pb isotopes clearly support a Philippine Sea plate origin. The isotopic signature of the inner trench slope samples matches that of Philippine Sea plate lavas, with 87Sr/86Sr=0.70321–0.70373, 143Nd/144Nd=0.513057–0.513077 and 206Pb/204Pb=18.2–18.5. The samples have elevated 207Pb/204Pb (15.3–15.5) and 208Pb/204Pb (38.0–38.2) values compared to the Northern Hemisphere reference line (NHRL) and their isotopic signature is distinct from the Mesozoic Pacific MORB being subducted. These are the first samples of trapped Philippine Sea oceanic crust discovered in the Izu–Bonin–Mariana arc. They require that models for the formation of intra-oceanic arc crust account for pre-existing oceanic crust and that estimates of arc magma production rates are lowered accordingly.