Izu-Bonin-Mariana Rear Arc: The Missing Half of the Subduction Factory

4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layers and enclosing sediments and to better constrain the thickness of the mafic ash layers. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the IzuBonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory”; this was because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear part of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf

Novel genetic loci associated with hippocampal volume

The hippocampal formation is a brain structure integrally involved in episodic memory, spatial navigation, cognition and stress responsiveness. Structural abnormalities in hippocampal volume and shape are found in several common neuropsychiatric disorders. To identify the genetic underpinnings of hippocampal structure here we perform a genome-wide association study (GWAS) of 33,536 individuals and discover six independent loci significantly associated with hippocampal volume, four of them novel. Of the novel loci, three lie within genes (ASTN2, DPP4 and MAST4) and one is found 200 kb upstream of SHH. A hippocampal subfield analysis shows that a locus within the MSRB3 gene shows evidence of a localized effect along the dentate gyrus, subiculum, CA1 and fissure. Further, we show that genetic variants associated with decreased hippocampal volume are also associated with increased risk for Alzheimer's disease (rg =-0.155). Our findings suggest novel biological pathways through which human genetic variation influences hippocampal volume and risk for neuropsychiatric illness

Tracing of Cl input into the sub-arc mantle through the combined analysis of B, O and Cl isotopes in melt inclusions

co-auteur étrangerInternational audienceThe effect that recycling crust and sediments have on the composition of the mantle wedge, in particular in terms of volatiles, is still debated. Chlorine, an important fluid mobile element that has stable isotopes with different concentrations in the terrestrial reservoirs, has the potential to be used to trace slab-derived fluids.Olivine-hosted melt inclusions (OHMIs) provide a first order constraint on the δ37Cl of primary magmas, since they are unaffected by near surface processes. In this study, δ37Cl were coupled with δ11B and δ18O analyses in samples from the Lesser Antilles, Vanuatu, Aeolian, NE Japan and Izu-Bonin arcs. This unique dataset is used to better understand the large δ37Cl variation in melt inclusions from a single sample. OHMIs from the Vulcano (Aeolian arc) and Sukumoyama (Izu-Bonin arc) samples have similar δ37Cl (−2.5 ±0.5and −2.6 ±0.8, respectively). These are different from δ37Cl in OHMIs from the other three localities (δ37Cl of −0.7 ±0.6for Aoba (Vanuatu arc) and St. Vincent (Lesser Antilles arc), −1 ±0.9for Iwate (NE Japan)). Vulcano OHMIs also have statistically different B and O isotope compositions compared to those from the other locations: average δ11B of −5.1 ±2.9for Vulcano OHMIs, compared to 2.5 ±3.7, 5.2 ±1.4, 7.0 ±2.2, 3.8 ±7.5for Sukumoyama, Iwate, Aoba and St. Vincent OHMIs, respectively. All OHMIs have δ18O between 4.0 and 7.4, except for those from Vulcano, which are significantly different, with δ18O from 7.2 to 9.1. Combining these three stable isotope systems suggests that the large variation (>2) of δ37Cl in OHMIs from a sample reflects inputs from different sources of Cl rather than heterogeneities in a single main source. Variability between arcs might reflect different major sources of Cl.Comparing OHMIs Cl isotope data from the Aeolian and Izu-Bonin arcs with existing bulk rock Cl isotope data suggest that OHMIs preserve the source signature of Cl input whereas this signal can be lost in whole rocks as a result of Cl isotope diffusive fractionation during Cl degassing. SIMS measurements of Cl isotopes in OHMIs could thus help refine models of Cl cycles in the mantle

First identification of a Cathaysian continental fragment beneath the Gagua Ridge, Philippine Sea, and its tectonic implications

International audienceThe tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized

Highly heterogeneous mantle caused by recycling of oceanic lithosphere from the mantle transition zone

Geochemical heterogeneities observed in the mantle are usually attributed to recycling of oceanic lithosphere through subduction. However, it remains hotly debated where recycled material stagnates, and how quickly it can be liberated back to surface. This knowledge gap hinders our understanding of mantle circulation and the chemical evolution of the Earth. Here we address these questions using a combination of geochronology and geochemistry from South China Sea (SCS) seamounts. The Shixingbei seamount lavas formed during active seafloor spreading at c. 19.1 Ma show limited geochemical variability, whereas the Zhenbei-Huangyan seamount chain formed during the post-spreading stage at c. 7.8 Ma and displays a wide range of compositions. However, melt inclusions in olivine and plagioclase from the Zhenbei-Huangyan basalts show considerably greater isotopic variability than seen in the whole rock compositions of both the SCS syn- and post-spreading lavas. A previously unidentified third mantle source component (FOZO) revealed by olivine-hosted melt inclusions along with both depleted (DMM) and enriched (EMII) mantle components is required in the source region to explain the observed isotopic and chemical variability. On the basis of our results, the age of the recycled ocean crust and sediments in this region are estimated to be c. 120 – 350 Ma. We infer that these enriched components in the SCS lavas come from the mantle transition zone. Variations in mantle source heterogeneity coupled with melting process control spatial–temporal (spreading vs. post-spreading stage) geochemical variations of lavas from the SCS and surrounding areas. Together with the results from published studies, we propose that marginal basins are one of the major locations on Earth where oceanic and/or continental lithosphere is transferred into the upper mantle and transition zone, representing an important source of upper mantle heterogeneity. We provide a simple conceptual model linking plate subduction and upper mantle heterogeneity and the volcanism in the SCS and surrounding areas