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Abstract Negative Cerium (Ce) anomalies are observed in chondrite-normalized Rare Earth Element patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks from the Mariana arc and have been interpreted as the result of addition of subducted sediments to the arc magma sources. This study combines 143Nd/144Nd and 138Ce/142Ce isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 to 0.90. The dataset includes sediments sampled immediately before subduction at the Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have Ce and Nd isotopic compositions that require <2.5% of a sediment component derived from the volcaniclastics. With this proportion of sediment, most of the Ce/Ce* range measured in lavas is reproduced. Thus, this study confirms that the origin of the Ce anomalies in the Mariana arc magmas can be principally attributed to recycling of trench sediments through active subduction. The participation of a component derived from biosiliceous sediments does not explain the Ce-Nd isotope composition of the lavas because the involved proportion is too high (up to 8%) in comparison to results obtained from other geochemical proxys. Using this end-member, the modeled Ce anomalies are also too high (0.91–0.84) in comparison to those measured in lavas. Various processes and conditions are able to generate Ce anomalies: oxygen fugacity, residual mineral phases, partial melting, fractional crystallization and tropical weathering. Their influence in the case of Mariana volcanic arc magmas seems to be very limited but partial melting effect may explain the lowest measured Ce/Ce* values. Magmatic processes cannot be definitely ruled out in producing Ce anomalies in other arc system environments. Additional experimental data, however, are needed for a better understanding of the behavior of cerium relative to its neighboring elements. Also, this study highlights the importance of using local depleted mantle and sediments to model the isotopic compositions of arc lavas. Highlights • Origin of negative Ce anomalies in Mariana arc magmas. • Coupled 138Ce/142Ce and 143Nd/144Nd isotope measurements in Mariana arc magmas, sediments and back-arc basalts. • Ce-Nd isotopic binary mixing models coupled with Ce/Ce* prove that volcaniclastics control the Ce/Ce* in Mariana lavas. [ABSTRACT FROM AUTHOR]

Abstract: Flatfish are a major component of the hydrothermal vent community on three seamounts of the northern Mariana Volcanic Arc in the northwest Pacific. Nikko, Kasuga-2 and Daikoku seamounts host vent fields between 375 and 480m depth where high temperature vents release molten sulphur. The small cynoglossid tonguefish, Symphurus thermophilus Munroe and Hashimoto, is ubiquitous in all vent habitats observed on these seamounts: among extensive fields of tubeworms and mussels and on solid sulphur surfaces on Nikko; on sulphur-rich sediments and barnacle-covered boulders on Kasuga-2; and on recent sulphur flows and on broad areas of loose and semi-consolidated sediments on Daikoku. We recorded repeated forays by individuals onto flows of molten sulphur as these surfaces cooled. Based on observations using ROVs, the mean density is 90fish/m2 with maximum counts over 200fish/m2 on Daikoku sediments. Compared to collected tonguefish from Daikoku and Kasuga-2, those from Nikko have significantly greater lengths and, on average, six times the mass. Otolith data indicate upper ages of 13 years with Nikko tonguefish growing significantly faster. Diets of tonguefish on the three seamounts reflect the different habitats and prey availability; in Daikoku specimens, small crustaceans and polychaetes are most common while on Nikko, gut contents are predominantly larger shrimp. We made the unusual observation of stunned midwater fish falling to the seafloor near the vents where S. thermophilus immediately attacked them. This tonguefish has a wide diet range and foraging behaviour that likely influence the differing growth rates and sizes of fish inhabiting the different vent sites. Limited genetic data suggest that larval exchange probably occurs among sites where the common habitat factor is high levels of elemental sulphur forming hard and partly unconsolidated substrata. Here, in the northern range of the Mariana Trench Marine National Monument, S. thermophilus, despite having an unusually broad habitat association, may be restricted in its overall range to this region of isolated volcanoes with active hydrothermalism. [Copyright &y& Elsevier]

Discrete Quaternary (<400 ka) tephra fallout layers (mostly <1 cm thick) within the siliceous oozes of the central Mariana Trough at 18°N are characterized by medium-K to high-K subalkalic volcanic glasses (KO=0.8-3.2 wt.%) with high large-ion lithophile elements (LILE)/high-field-strength elements (HFSE) ratios and Nb depletion (Ba/La≈35; Ba/Zr≈3.5; La/Nb≈4) typical for convergent margin volcanic rocks. Compositional zoning within layers ranges from basaltic to dacitic (SiO=48-71 wt.%; MgO=0.7-6.5 wt.%); all layers contain basaltic andesites. The tephra layers are interpreted as single explosive eruptive events tapping chemically zoned reservoirs, the sources being the Mariana arc volcanoes (MAV) due to their proximity (100-400 km) and similar element ratios (MAV: Ba/La=36±7; Ba/Zr=3.5±0.9). The glasses investigated, however, contrast with the contemporaneous basaltic to dacitic lavas of the MAV by being more enriched in TiO (≈1.2 wt.%; MAV≈0.8 wt.%), FeO (≈10 wt.%, MAV≈8-9 wt.%), KO (≈1.1 wt.%; MAV≈0.8 wt.%) and PO (≈0.4 wt.%; MAV≈0.2 wt.%). (Semi-)Incompatible trace elements (including Rare Earth Elements (REE)) of the basaltic-andesitic and dacitic glasses match those of the dacitic MAV lavas, which became enriched by fractional crystallization. Moreover, the glasses follow a tholeiitic trend of fractionation in contrast to MAV transitional trends and have a characteristic PO trend that reaches a maximum of 0.6 wt.% PO at ≈57 wt.% SiO, whereas MAV lavas increase linearly in PO from 0.1 to 0.3 wt.% with increasing silica. Both explosive and effusive series are interpreted to have evolved in common magma reservoirs by convective fractionation. Similar parental magmas are suggested to have separated into coexisting Si-andesitic to dacitic and basaltic melts by in situ crystallization. The differentiated melt is interstitial in an apatite-saturated crystalline mush of plag+px±ox±ol at the cooler chamber margins in contrast to the less differentiated basaltic to basaltic-andesitic magmas, which are not yet saturated in apatite and occupy the chamber interior. Reinjection of interstitial melt into the chamber interior and mixing with larger melt fractions of the interior liquid (mixing ratios about ≈1: 8-9) can explain the paradoxical behavior of apatite-controlled P and MREE variation in the basaltic andesite glasses and their MAV dacite-like fractionation patterns. The process may also account for the exclusively tholeiitic trend of fractionation of the glass shard series, but in situ crystallization alone cannot cause their absolute enrichment in (semi-)incompatible elements. The newly mixed melt is suggested to form the basaltic end member of the glass shard series. However, it must have become physically separated from the main MAV magma body (possibly by density-driven convective fractionation) in order to allow for further evolution of the contrasting geochemical paths as well as differentiation. [ABSTRACT FROM AUTHOR]