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Here we report (210Pb/226Ra), (226Ra/230Th), (230Th/238U) and (234U/238U) disequilibria for eleven lavas from the ABE vent site in the Lau Basin. Most ABE lavas have (210Pb/226Ra) > 1 and (226Ra/230Th) > 3. These results indicate that most of these lavas erupted within the past 100 years. Model ages calculated assuming initial (210Pb/226Pb) = 1.8–2.0 further constrain the timing of eruption, suggesting that more than half of the lavas erupted within the past 60 years. When combined with complementary data (side-scan sonar, lava flow morphology, tectonic mapping), this high-resolution record provides fundamental time constraints for interdisciplinary studies examining oceanic crustal construction and development of the hydrothermal system in the ABE vent field. Notably the youngest samples cluster around the active vent sites indicating that the ABE vent site’s location is a direct consequence of this concentrated young volcanism. This study is the first high resolution U-series study of a seafloor vent site and demonstrates the potential of using (210Pb/226Ra) for the determination of lava ages for young submarine lavas in spreading environments with active hydrothermal venting. As such these (210Pb/226Ra) measurements hold the promise for addressing in far greater detail the connections between spreading ridge eruptive and hydrothermal activity on the decadal to century time scales.

Abstract Subduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Abstract International Ocean Discovery Program Expedition 352 to the Bonin forearc drilled the sequence of volcanic rocks erupted in the immediate aftermath of subduction initiation along the western margin of the Pacific Plate. Pristine volcanic glasses collected during this expedition were analyzed for major and trace elements, halogens, sulfur, and H and O isotopes with goals of characterizing the fluids and melts of subducted materials that were involved in generating the nascent upper plate crust. Incompatible trace element compositions of the oldest lavas (forearc basalts [FAB]) are similar to those of the most depleted mid‐ocean ridge basalts globally. Most FAB were generated by decompression melting during seafloor spreading in a near‐trench, supra‐subduction zone environment with only minor involvement of diverse and generally dilute water‐rich fluids from the subducting plate. Boninite series glasses are enriched in incompatible trace elements mobilized from the subducting plate, but strongly depleted in other elements, such as the middle‐heavy rare‐earth elements. These traits are attributed to generation of boninites largely by flux melting involving water‐rich melts first derived from the leading edge of subducted basaltic crust and then from both subducted crust and sediment. These melts were generated at low pressures as the shallow, embryonic slab extracted heat from hot asthenosphere near the trench. The progressive depletion of the mantle source for the FAB‐through‐boninite sequence suggests that the asthenospheric mantle remained trapped above the nascent subducting plate for the first several million years of subduction beneath the Philippine Sea Plate.

Abstract International Ocean Discovery Program Expedition 352 to the Izu‐Bonin forearc cored over 800 m of basement comprising boninite and boninite‐series lavas. This is the most extensive, well‐constrained suite of boninite series lavas ever obtained from in situ oceanic crust. The boninites are characterized as high‐silica boninite (HSB), low‐silica boninite (LSB), or basaltic boninite based on their SiO2‐MgO‐TiO2 relations. The principal fractionation products of all three series are high‐Mg andesites (HMA). Lavas recovered >250 meters below seafloor (mbsf) erupted at a forearc spreading axis and are dominated by LSB and HMA. Lavas recovered from

An 40Ar/39Ar age of 45·1 Ma determined for lavas from northern Saipan confirms that these high-silica rhyolites erupted during the ‘proto-arc’ stage of volcanism in the Izu–Bonin–Mariana system, which is characterized elsewhere by eruption of boninitic lavas. Incompatible trace element concentrations and Sr, Hf, Nd, and Pb isotope ratios for these rhyolites are transitional between those of c. 48 Ma boninitic lavas and post-38 Ma ‘first-arc’ andesites and dacites from Saipan and Rota that have typical subduction-related compositions. These transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust. A second stage of Rayleigh fractionation in the upper crust is required by SiO2 concentrations that exceed 77 wt % and near-zero compatible element concentrations. First-arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first-arc magmas themselves. The mantle sources of the proto-arc and first-arc lavas from Saipan and Rota are similar to those of Philippine back-arc basin basalts based on Nd and Hf isotopic compositions. The Pb isotope compositions of these lavas are between those of Pacific sea-floor basalts and Jurassic and younger cherty and clay-rich sediments. This contrasts with the boninitic proto-arc volcanic rocks from Guam and Deep Sea Drilling Project Sites 458 and 459 that have Pb isotope compositions similar to Pacific basin basalts and volcaniclastic sediments. The preferred explanation for the difference in the nature of proto-arc volcanism between Saipan and other fore-arc locations is that the crust ceased extending 3–4 Myr earlier beneath Saipan. This was caused by a change from mantle upwelling, fore-arc extension, and shallow melting to an environment dominated by more normal mantle wedge convection, stable crust, and deeper melting. [ABSTRACT FROM AUTHOR]