Variation in the mantle sources of the northern Izu arc with time and space — Constraints from high-precision Pb isotopes

We present new ages and geochemical data for back-arc lavas from the northern Izu–Bonin arc 33–35° N including high-precision double-spike Pb isotope measurements. The northern part of the Izu–Bonin arc is distinct from the rest of the arc as it lacks active rifting behind the volcanic front but it does have Quaternary volcanoes (e.g. Niijima). However, in common with the rest of the arc the northern section has back-arc seamount chains and NE–SW volcanic ridges. 9Ar dating of volcanic rocks has revealed that Quaternary volcanism is limited to within 40 km of the volcanic front. Miocene and Pliocene volcanism extended as far as 120 km west of the current volcanic front along the back-arc seamounts and ridges. emical characteristics of back-arc volcanism are significantly different in the Pliocene–Quaternary compared to the Miocene. Opx–cpx andesite and hornblende andesite are dominant in Miocene volcanic centres, while Pliocene and Quaternary centres are characterized by basalt and rhyolite. e volcanic centres show a marked correlation between Th/Ce and Pb and Nd isotopes. Generally, these lavas have higher Δ7/4 and lower 143Nd/144Nd with increasing Th/Ce. In contrast, the Pliocene and Quaternary lavas show little, if any, Th enrichment relative to potential mantle sources and no correlation with isotopes. These correlations suggest that partial melt of sediment from the subducting slab was an important component in the Miocene, whereas, the Pliocene–Quaternary volcanic centres show little evidence of sediment melt and are restricted to a contribution of fluid from altered oceanic crust and fluid from sediment. nary volcanoes at similar distances from the volcanic front are calculated to have similar compositions and amounts of slab-derived fluid in their sources. However, on Pb–Pb isotope plots, they lie closer to the NHRL towards south (i.e., Δ8/4 decreases towards south). Almost parallel but distinct trends on Pb–Pb plots imply differing mantle sources depending on the northward position within the arc. This variation in mantle wedge composition is seen to continue into the central part of the arc [Ishizuka, O., Taylor, R.N., Milton, J.A., Nesbitt, R.W., Fluid–mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes, Earth Planet. Sci. Lett. 211 (2003a) 221–236]. The observed along-arc isotopic variation can be explained by the presence of two mantle components; a MORB source observed in the back-arc basins of the Philippine Sea Plate (e.g. [Hickey-Vargas, R., Origin of the Indian Ocean-type isotopic signature in basalts from Philippine Sea Plate spreading centers: An assessment of local versus large-scale processes, J. Geophys. Res. 103 (1998a) 20963–20979]) and a more Pacific MORB-like source. northern part of the Izu–Bonin arc, Miocene to Pliocene volcanism was active over a much wider back-arc region than present, and these lavas show the greatest influence of sediment melt generated at slab depths of > 250 km. However, in the Quaternary active volcanism was constrained to the vicinity of the volcanic front. This Quaternary magmatism can be related to the release of fluid from the slab at relatively shallow depths (110–200 km). These spatial and temporal chemical variations could be related to either the change of the angle of subducting Pacific Plate and/or changes in the regional stress regime (e.g. resulting from the collision of the Izu–Bonin arc with the Honshu arc or in association with initiation of rifting in the south).