Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan

Volcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanic rocks. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period.

Micropaleontological proxies as tool to date serpentinite mud volcanisms and seamount subduction and to reconstruct paleoenvironmental conditions in the Mariana convergent margin system (IODP Expedition 366)

<p>The Mariana forearc system represents the only known currently active serpentine mud volcanism in a convergent margin setting. Here, International Ocean Discovery Program (IODP) Expedition 366 recovered material from three serpentinite mud volcanoes at increasing distances from the Mariana trench subduction zone along a south-to-north transect: Yinazao (Blue Moon), Fantangisña (Celestial), and Asùt Tesoru (Big Blue). Cores contain serpentinite mud with lithic clasts from the subducting Pacific Plate, forearc crust and mantle. Furthermore, at almost all drilled sites, a thin cover of pelagic sediment containing planktic and benthic foraminifera, calcareous nannofossils, radiolaria and sponge spicules was recovered, constraining the most recent mud volcano activity. The base of the seamounts overlies pelagic sediment and volcanic ash/tephra layers which establish a maximum age for the mud activity. Additionally, separate serpentinite mud flows are intercalated by distinct sedimentary layers.</p><p>Integrated biostratigraphy, based on planktonic foraminifera and calcareous nannofossils, is used to assess the minimum and maximum age of mud flow activity and of the distinct sedimentary layers and serpentinite mud flow layers. Biostratigraphic information will also provide time indications on lower plate dehydration and serpentinization of the forearc wedge. Preliminary results from Fantangisña seamount (Site U1497 and U1498) reveal the existence of biostratigraphic marker species for both planktonic foraminifera and calcareous nannofossils. Specifically, the presence of <em>Globigerinella calida</em>, <em>Globorotalia flexuosa</em>, <em>Globorotalia truncatulinoides</em>, <em>Globorotalia tumida</em>, <em>Sphaeroidinella dehiscens</em> (amongst planktic foraminifera) and <em>Gephyrocapsa</em> spp., <em>Pseudoemiliania lacunosa</em>, <em>Reticulofenestra asanoi</em>, <em>Discoaster deflandrei</em>, <em>Discoaster variabilis</em> (amongst calcareous nannofossils) allow a possible age assessment from Late Pleistocene to Late Miocene.</p><p>Planktonic assemblages are dominated by (sub)tropical Globigerinoides forms such as <em>G. conglobatus</em>, <em>G. ruber</em>, <em>G. elongatus</em>, <em>G. sacculifer</em>, <em>G. trilobus</em>. Other common (sub)tropical species detected are G<em>. menardii</em>, and <em>O. universa</em>, whereas <em>G. siphonifera</em>, <em>N. dutertrei</em>, <em>S. dehiscens</em> and <em>P. obliquiloculata</em> are less common.</p><p>Benthic foraminifera are less abundant but show high diversity. Forms of <em>Lagena</em>, <em>Cibicidoides</em>, <em>Fissurina</em>, <em>Ehrenbergina</em>, <em>Gyroidina</em>, <em>Melonis</em>, <em>Pullenia</em>, <em>Osangularia</em>, <em>Favulina</em>, <em>Reophax</em>, <em>Rhabdammina</em>, <em>Saccorhiza</em>, and <em>Hormosinella</em> are present. To the best of our knowledge, the occurrence of benthic forms in such environments is highly unusual and has not been recorded in detail so far.</p><p>Quantitative and statistical analyses on foraminifera assemblages will provide information on water column and bottom water conditions. Moreover, a detailed comparison between assemblages pre- and post-volcanism may reflect possible changes in the ecological conditions.</p><p>Collected data will not only allow to constrain in time the evolution of submarine volcanoes in the Mariana convergent system but also to investigate foraminifera ecology in such an extreme environment.</p><p> </p>

Chevreuxiopsis franki gen. n., sp. n. (Crustacea, Amphipoda, Thoriellidae) from the deep sea southwest of Tasmania

A new amphipod species and genus, Chevreuxiopsisfranki, found in a pelagic sediment trap southwest of Tasmania is described. The new species can be recognized by its unique antenna 2, which consists of a narrow peduncle, and a 4-articulate flagellum, which has a massively developed, article 1, large, posteriorly drawn out articles 2 and 3, and an elongate lanceolate 4th article. The pereopod 1 basis surrounds large maxillipedal plates. Pereopod 3 to 6 are equipped with subchelate propodus dactylus arrangements. The bases of pereopods 5–7 are narrow.