Mariana Volcanic Arc Research Articles

Archaeal community structure in sediments from a seamount in the Mariana Volcanic Arc

Seamounts are subsurface mountains in the ocean. Examination of the abundance and distribution of Archaea in seamount ecosystems may provide a better understanding of their ecological functions. Most studies of marine archaeal assemblages in seamount area have focused on hydrothermal vents or ferromanganese crusts. We investigated the archaeal communities from a seamount of the Mariana Volcanic Arc, in the tropical western Pacific Ocean by using high-throughput sequencing. Thaumarchaeota was dominant in the sediments of all sample stations. Community diversity and species richness were greatest at stations near the top of the seamount, and lowest at the deepest station. One sample station on the steep southeast slope that faced the Yap-Mariana trench had a unique composition of Archaea. In summary, depth has an important influence on archaeal community structure, and the geographic properties and sediment characteristics may explain the unique distribution patterns of Archaea in this seamount. This study provides a foundation for future research on Archaea in seamounts.

Origin of negative cerium anomalies in subduction-related volcanic samples: Constraints from Ce and Nd isotopes

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.

Bacterial community structure and novel species of magnetotactic bacteria in sediments from a seamount in the Mariana volcanic arc

Seamounts are undersea mountains rising abruptly from the sea floor and interacting dynamically with underwater currents. They represent unique biological habitats with various microbial community structures. Certain seamount bacteria form conspicuous extracellular iron oxide structures, including encrusted stalks, flattened bifurcating tubes, and filamentous sheaths. To extend our knowledge of seamount ecosystems, we performed an integrated study on population structure and the occurrence of magnetotactic bacteria (MTB) that synthesize intracellular iron oxide nanocrystals in sediments of a seamount in the Mariana volcanic arc. We found Proteobacteria dominant at 13 of 14 stations, but ranked second in abundance to members of the phylum Firmicutes at the deep-water station located on a steep slope facing the Mariana-Yap Trench. Live MTB dwell in biogenic sediments from all 14 stations ranging in depth from 238 to 2,023 m. Some magnetotactic cocci possess the most complex flagellar apparatus yet reported; 19 flagella are arranged in a 3:4:5:4:3 array within a flagellar bundle. Phylogenetic analysis of 16S rRNA gene sequences identified 16 novel species of MTB specific to this seamount. Together the results obtained indicate that geographic properties of the seamount stations are important in shaping the bacterial community structure and the MTB composition.

Endemicity of the cosmopolitan mesophilic chemolithoautotroph Sulfurimonas at deep-sea hydrothermal vents.

Rich animal and microbial communities have been found at deep-sea hydrothermal vents. Although the biogeography of vent macrofauna is well understood, the corresponding knowledge about vent microbial biogeography is lacking. Here, we apply the multilocus sequence analysis (MLSA) to assess the genetic variation of 109 Sulfurimonas strains with ⩾98% 16S rRNA gene sequence similarity, which were isolated from four different geographical regions (Okinawa Trough (OT), Mariana Volcanic Arc and Trough (MVAT), Central Indian Ridge (CIR) and Mid-Atlantic Ridge (MAR)). Sequence typing based on 11 protein-coding genes revealed high genetic variation, including some allele types that are widespread within regions, resulting in 102 nucleotide sequence types (STs). This genetic variation was predominantly due to mutation rather than recombination. Phylogenetic analysis of the 11 concatenated genes showed a clear geographical isolation corresponding to the hydrothermal regions they originated from, suggesting limited dispersal. Genetic differentiation among Sulfurimonas populations was primarily influenced by geographical distance rather than gas composition of vent fluid or habitat, although in situ environmental conditions of each microhabitat could not be examined. Nevertheless, Sulfurimonas may possess a higher dispersal capability compared with deep-sea hydrothermal vent thermophiles. This is the first report on MLSA of deep-sea hydrothermal vent Epsilonproteobacteria, which is indicative of allopatric speciation.

Comparative Analysis of Microbial Communities in Iron-Dominated Flocculent Mats in Deep-Sea Hydrothermal Environments.

It has been suggested that iron is one of the most important energy sources for photosynthesis-independent microbial ecosystems in the ocean crust. Iron-metabolizing chemolithoautotrophs play a key role as primary producers, but little is known about their distribution and diversity and their ecological role as submarine iron-metabolizing chemolithotrophs, particularly the iron oxidizers. In this study, we investigated the microbial communities in several iron-dominated flocculent mats found in deep-sea hydrothermal fields in the Mariana Volcanic Arc and Trough and the Okinawa Trough by culture-independent molecular techniques and X-ray mineralogical analyses. The abundance and composition of the 16S rRNA gene phylotypes demonstrated the ubiquity of zetaproteobacterial phylotypes in iron-dominated mat communities affected by hydrothermal fluid input. Electron microscopy with energy-dispersive X-ray microanalysis and X-ray absorption fine structure (XAFS) analysis revealed the chemical and mineralogical signatures of biogenic Fe-(oxy)hydroxide species and the potential contribution of Zetaproteobacteria to the in situ generation. These results suggest that putative iron-oxidizing chemolithoautotrophs play a significant ecological role in producing iron-dominated flocculent mats and that they are important for iron and carbon cycles in deep-sea low-temperature hydrothermal environments. We report novel aspects of microbiology from iron-dominated flocculent mats in various deep-sea environments. In this study, we examined the relationship between Zetaproteobacteria and iron oxides across several hydrothermally influenced sites in the deep sea. We analyzed iron-dominated mats using culture-independent molecular techniques and X-ray mineralogical analyses. The scanning electron microscopy-energy-dispersive X-ray spectroscopy SEM-EDS analysis and X-ray absorption fine structure (XAFS) analysis revealed chemical and mineralogical signatures of biogenic Fe-(oxy)hydroxide species as well as the potential contribution of the zetaproteobacterial population to the in situ production. These key findings provide important information for understanding the mechanisms of both geomicrobiological iron cycling and the formation of iron-dominated mats in deep-sea hydrothermal fields.

The October 16, 2013 rainfall-induced landslides and associated lahars at Izu Oshima Volcano, Japan

Intense rainfall related to the typhoon T1326 on October 15–16, 2013 (total 824mm; maximum hourly rainfall 118.5mm) triggered numerous landslides and associated lahars at Izu Oshima Volcano, the northernmost part of Izu Mariana volcanic arc, Japan. The landslides were concentrated mainly in a 2-km2 area located on the western slope of Izu Oshima Volcano. Most of the landslides were shallow soil slips (<2m thick) in unconsolidated fallout tephra layers overlying lava flows and pyroclastic rocks. The rupture surfaces of them were located near the base of Y1 tephra (AD 1777–1778) and/or the base of Y4 tephra (AD 1421). The Y1 and Y4 tephras differ from the underlying paleosols in permeability, grain size and degree of compaction. The saturated hydraulic conductivities of the paleosols were one to two orders of magnitude smaller than those of the overlying Y1 and Y4 tephras. Most landslides mobilized completely into lahars, traveling along stream channels or flat slopes and flooding at the foot of the volcano. The associated lahars severely damaged inhabited areas and caused thirty five fatalities. Although the lahars eroded slopes and transported boulders up to 1m in diameter and a large amount of woody debris, they contained more than 90% of sand-to-silt-size particles, similar in composition to the original sliding materials. Sediment discharge volumes from three basins were estimated at 1.8–4.1×104m3/km2, based on debris volumes trapped by sediment retention dams. The characteristics of rainfall-induced landslides and associated lahars at Izu Oshima Volcano in 2013 provide an important lesson about future non-eruption-related landslide and lahar hazards at tephra-rich volcanoes.

Temporal Evolution of the Mariana Arc: Mantle Wedge and Subducted Slab Controls Revealed with a Tephra Perspective

Tephras recovered by deep-sea drilling from fore-arc to back-arc locations across the Mariana Volcanic Arc record the last 34 Myr of the system’s evolution. Major and trace element abundances and Sr–Nd–Pb–Hf isotope ratios have been determined for tephra with high temporal precision and an average inter-tephra layer interval of � 1 Myr. Temporal variations of source-sensitive radiogenic isotopes and large-ion lithophile elements (LILE) are mostly decoupled from unchanging modes observed in silica and, with the exception of K, other archetypical crust-forming major elements. Modeling confirms that the temporal isotopic and elemental abundance trends are controlled by subducted slab and mantle sources. The Pb and Sr fluxes can be linked to fluids from altered oceanic crust (AOC), and are influenced by contributions from the mantle wedge and slab partial melts, whereas Hf mostly derives from the sub-arc mantle. Most plausibly, a K2O increase and fluctuations at � 10 Ma can be linked to collision of the leading trace of the Cretaceous-aged Western Pacific Seamount Province with the arc. This is inferred to have arrived at the Mariana Trench at � 15–16 Ma, coincident with the termination of spreading in the Parece Vela back-arc basin. A short period of slab melting followed, possibly induced by slab rollback that peaked at � 8–9 Ma and ended with incipient rifting in the Mariana Trough at � 7 Ma. Each of the periods of arc formation (52–24, 22–11 and 10–0 Ma) is characterized by a distinctive mixture of source materials that is not repeated through time. Mariana Arc crustal growth has occurred through the addition of predominantly mafic and silicic melts formed during relatively short time intervals, traceable via their chemically distinctive subducted slab inputs.

Molecular taxonomy and naming of five cryptic species of Alviniconcha snails (Gastropoda: Abyssochrysoidea) from hydrothermal vents

Large symbiont-hosting snails of the genus Alviniconcha (Gastropoda: Abyssochrysidae) are among the dominant inhabitants of hydrothermal vents in the Western Pacific and Indian oceans. The genus was originally described as monotypic, but unique DNA sequences for mitochondrial genes revealed six distinct evolutionary lineages that we could not distinguish based on external morphology. Subsumed under the name Alviniconcha hessleri Okutani & Ohta, the distinct allopatric and sympatric lineages have been assigned placeholder epithets that complicate scientific communications. Based on the present multi-gene sequence data, we hereby describe five Alviniconcha species (in the order of their discovery) – A. kojimai sp. nov., A. boucheti sp. nov., A. marisindica sp. nov., A. strummeri sp. nov. and A. adamantis sp. nov. Thus, we restrict application of the name A. hessleri to specimens that are genetically similar (≥95% for COI) to those found at localities in the Mariana Trough. Single distinct Alviniconcha species inhabit vent fields along the Central Indian Ridge, the Mariana volcanic arc, and the Mariana back-arc basin, whereas vents in the Manus, Fiji and Lau back-arc basins may host two or three additional species. Formal recognition of these species facilitates future attempts to assess their physiological differences and symbiont associations. Furthermore, their reported distributions have significant biogeographic implications, affecting estimates of the diversity within and overlap among Indo-Pacific vent localities.http://zoobank.org/urn:lsid:zoobank.org:pub:1E4B2E71-9F1D-479E-9A9A-22A9E303AAE5

Imaging of CO2 bubble plumes above an erupting submarine volcano, NW Rota‐1, Mariana Arc

AbstractNW Rota‐1 is a submarine volcano in the Mariana volcanic arc located ∼100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ∼560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 midwater data set shows: (1) bubble plumes were present on every pass over the summit and they rose 200–400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that midwater multibeam sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.

Population ecology of the tonguefish Symphurus thermophilus (Pisces; Pleuronectiformes; Cynoglossidae) at sulphur-rich hydrothermal vents on volcanoes of the northern Mariana Arc

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.

Submarine landslide triggered by volcanic eruption recorded by in situ hydrophone

Research Article| January 01, 2012 Submarine landslide triggered by volcanic eruption recorded by in situ hydrophone W.W. Chadwick, Jr; W.W. Chadwick, Jr * 1Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA *E-mail: bill.chadwick@noaa.gov. Search for other works by this author on: GSW Google Scholar R.P. Dziak; R.P. Dziak 1Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA Search for other works by this author on: GSW Google Scholar J.H. Haxel; J.H. Haxel 1Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA Search for other works by this author on: GSW Google Scholar R.W. Embley; R.W. Embley 2Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Newport, Oregon 97365, USA Search for other works by this author on: GSW Google Scholar H. Matsumoto H. Matsumoto 1Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (1): 51–54. https://doi.org/10.1130/G32495.1 Article history received: 25 May 2011 rev-recd: 18 Aug 2011 accepted: 23 Aug 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W.W. Chadwick, R.P. Dziak, J.H. Haxel, R.W. Embley, H. Matsumoto; Submarine landslide triggered by volcanic eruption recorded by in situ hydrophone. Geology 2012;; 40 (1): 51–54. doi: https://doi.org/10.1130/G32495.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract NW Rota-1 is a submarine volcano in the Mariana volcanic arc that is notable as the site where underwater explosive eruptions were first witnessed in A.D. 2004. After years of continuous low-level eruptive activity, a major landslide occurred at NW Rota-1 in August 2009, triggered by an unusually large eruption that produced 10 times the acoustic energy of the background level of activity. An anomalous earthquake swarm preceded the eruption, suggesting that the sequence started with a magmatic intrusion and associated faulting beneath the volcano. We quantify the size and extent of the landslide using bathymetric resurveys and interpret the timing of events using data from an in situ hydrophone. This is the first instrumental documentation of an earthquake-eruption-landslide sequence at a submarine volcano, and illustrates the close interaction between magmatic activity and mass wasting events in the growth of undersea arc volcanoes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Silicic Magmas in the Izu–Bonin Oceanic Arc and Implications for Crustal Evolution

Abundant rhyolitehaseruptedfromthe Izu^Bonin^Marianavolca-nic arc (IBM arc) from its earliest stage (Eocene) to the present.Geochemically, three types of Quaternary rhyolites exist in the Izu^Bonin arcfront, and theyare closely related tovolcanotypeand crus-tal structure.The dominantly basaltic islands of the volcanic frontproduce small volumes of rhyolites that we call R1.The submarinecalderas of the volcanic front erupt mostly rhyolite that we call R2.Seamounts, knolls, and pillow ridges in thebackarcextensionalzoneare mostly basaltic but also contain rhyolites that we call R3.Thethickest total crust, and the thickest intermediate compositionmiddle crust, occurs below the dominantly basaltic volcanoes,whereas the intermediate composition middle crust tends to be thin-nerbeneaththesubmarinecalderas. R1rhyolitesmaybederivedfromQuaternary andesitic sources whereas R2 and R3 rhyolites may bederivedfromOligoceneones.ThehigherCaO/Al

Three‐dimensional crustal structure of the Mariana island arc from seismic tomography

A three‐dimensional (3‐D) seismic refraction survey was acquired over the Mariana volcanic arc at 14.5–18.5°N and 145–147°E. First‐arrival traveltimes from this survey and from a separate 2‐D survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3‐D P wave velocity model using seismic tomography subject to smoothness constraints. The active arc, which initiated only 3–4 Ma ago, has an average crustal thickness of 18 km. Approximately 40 km to the east the inactive remnant of the rifted Eocene arc has an average crustal thickness of 21 km, due primarily to a thicker lower‐crustal layer with velocities of 6.5–7.0 km s−1. Crustal production clearly varies both temporally and spatially, with some crustal layers, including the igneous fore‐arc crust, varying in thickness by a factor of up to 2 along strike. Average P wave velocities within the upper crust of the modern arc are 240–360 m s−1 lower than in the Eocene arc but are 280 m s−1 higher within the lower crust. Middle crust with velocities of 6.0–6.5 km s−1 is best developed beneath the Eocene arc. These results suggest an evolution of arc structure with increasing age: We infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation. Although tonalitic rocks may predominate in the transition from upper to middle crust, the bulk of the crust is essentially basaltic.

Phylogenetic relationships of a tube worm (Lamellibrachia juni) from three hydrothermal vent fields in the South Pacific

The phylogenetic relationships were analysed based on nucleotide sequences of a mitochondrial gene for cytochrome c oxidase subunit I (COI) for the vestimentiferan tube worm Lamellibrachia juni, which inhabits three extremely acidic hydrothermal vent fields in the South Pacific: the TOTO Caldera in the South Mariana Volcanic Arc, the DESMOS site in the Manus Basin, and the Brothers Caldera in the Kermadec Arc. Six haplotypes were obtained from 34 lamellibrachiids from the three sites. Each haplotype was obtained only from a single site, which suggests that the three populations have been isolated from each other. Haplotypes from the DESMOS site formed a monophyletic cluster. All ten individuals from the TOTO Caldera were classified into the same haplotype, which was shown to form a monophyletic cluster with one of three haplotypes from the Brothers Caldera. The population in the Brothers Caldera was shown to consist of two genetically distinct groups.

Geomicrobiological exploration and characterization of a novel deep‐sea hydrothermal system at the TOTO caldera in the Mariana Volcanic Arc

Novel hydrothermal activities accompanying effluent white smokers and elemental sulfur chimney structures at the north-east lava dome of the TOTO caldera depression in the Mariana Volcanic Arc have been explored and characterized by geochemical and microbiological surveys. White smoker hydrothermal fluids were observed in the potential hydrothermal activity centre of the field and represented the maximal temperature of 170 degrees C and the lowest pH of 1.6. The chimney structures, all consisting of elemental sulfur (sulfur chimney), were also unique to the TOTO caldera hydrothermal field. Microbial community structures in a sulfur chimney and its formation hydrothermal fluid with a high concentration of hydrogen sulfide (15 mM) have been investigated by culture-dependent and -independent analyses. 16S rRNA gene clone analysis and fluorescence in situ hybridization (FISH) analysis revealed that epsilon-Proteobacteria dominated the microbial communities in the sulfur chimney structure and formed a dense microbial mat covering the sulfur chimney surface. Archaeal phylotypes were consistently minor components in the communities and related to the genera Thermococcus, Pyrodictium, Aeropyrum, and the uncultivated archaeal group of 'deep-sea hydrothermal vent euryarchaeotal group'. Cultivation analysis suggested that the chemolithoautotrophs might play a significant ecological role as primary producers utilizing gas and sulfur compounds provided from hydrothermal fluids.

Volcanologic and tectonic evolution of the Kasuga seamounts, northern Mariana Trough: Alvin submersible investigations

The Kasuga seamounts are active volcanoes situated wholly within the northern part of the backarc basin west of the active Mariana volcanic arc. They lie along a lineament that cuts across the northern portion of the Mariana arc from the trench axis to at least 60 km west of the volcanic front. The distribution of the major edifices probably represents a localized focusing of volcanic activity in the eastern backarc basin as a consequence of both cross-arc rifting and backarc basin formation. Structure of the edifices, distribution of satellite cones, and draping of the fault scarp by lava flows suggest continuing deformation of the volcanoes as a consequence of movement on the fault that bounds a graben to the east of the volcanic chain. Volcaniclastic deposits are common. Most are probably emplaced cold through the water column, but one of these deposits was probably emplaced hot. Hydrothermal processes are active at the summits of at least two of the volcanoes and produce compositionally distinctive fluids. The volcanoes have a broad range of lava compositions from basaltic to dacitic. Detailed studies of the volcanoes using the Alvin submersible show that the two southern edifices consist mainly of primitive, mantle-derived basalts, which range from relatively low-K calcalkaline basalts to high-K absarokites and shoshonites. The highest-K lavas lack many of the trace element characteristics of subduction-related volcanism (e.g. they have low Ba, Sr, Pb and B relative to other incompatible elements). We attribute this lack to derivation from old lithospheric sources during thinning of the arc as rifting proceeds. The intersection of cross-arc fractures related to forearc rifting and arc-parallel faults related to incipient backarc rifting permits primitive low-degree melts to erupt with minimal crustal-level differentiation. These melts preserve information about heterogeneities in the arc lithosphere.

The first evidence for MORB-like lavas from the outer Mariana forearc: geochemistry, petrography and tectonic implications

The Mariana forearc, the region between the active Mariana volcanic arc and the trench axis, is considered to be composed of rocks entirely of arc origin. This conclusion was based upon the lack of an accretionary wedge and recovery of arc volcanics from sites throughout the forearc (including the inner trench wall). New chemical data from dredged samples demonstrate that exposures of altered basalts similar to mid-ocean ridge basalt (MORB) and of ocean island basalt (OIB) exist in the Mariana forearc. Several forearc sites including a 2000 m scarp bounding a large graben at 19°30′N, two small scarps in the inner trench wall, and a seamount were dredged. MORB-like and OIB lavas in addition to island arc tholeiite (IAT) and boninite were recovered. All samples exhibit low-temperature seafloor alteration, and some are metamorphosed to lower greenschist facies. IAT samples have low TiO 2 (< 1%) and Zr (< 56 ppm). The MORB samples have TiO 2 of 1–2% and Zr of 60–93 ppm. OIB samples typically have high TiO 2 (> 3%) and Zr (> 180 ppm). Several OIB samples contain modal nepheline and all have high alkali contents. The presence of OIB and MORB-like lavas in the outer forearc may suggest either that entrapment of Philippine Sea or Kula Plate or that obduction of Pacific Plate has occurred in the evolution of the Mariana forearc. However, the intimate association of MORB-like lavas with arc lavas in the fault scarp requires that either IAT magma was injected into entrapped or obducted oceanic crust or that MORB-like magma was injected into an arc crust during forearc rifting or deformation.