Southern Mariana Forearc Research Articles

Evaluation of nutrient and energy sources of the deepest known serpentinite-hosted ecosystem using stable carbon, nitrogen, and sulfur isotopes.

The Shinkai Seep Field (SSF) in the southern Mariana forearc discovered in 2010 is the deepest (~5,700 m in depth) known serpentinite-hosted ecosystem dominated by a vesicomyid clam, Calyptogena (Abyssogena) mariana. The pioneering study presumed that the animal communities are primary sustained by reducing fluid originated from the serpentinization of mantle peridotite. For understanding the nutrient and energy sources for the SSF community, this study conducted four expeditions to the SSF and collected additional animal samples such as polychaetes and crustaceans as well as sediments, fragments of chimneys developing on fissures of serpentinized peridotite, seeping fluid on the chimneys, and pore water within the chimneys. Geochemical analyses of seeping fluids on the chimneys and pore water of the chimneys revealed significantly high pH (~10) that suggest subseafloor serpentinization controlling fluid chemistry. Stable isotope systematics (carbon, nitrogen, and sulfur) among animals, inorganic molecules, and environmental organic matter suggest that the SSF animal community mostly relies on the chemosynthetic production while some organisms appear to partly benefit from photosynthetic production despite the great depth of SSF.

A very deepProvanna(Gastropoda: Abyssochrysoidea) discovered from the Shinkai Seep Field, Southern Mariana Forearc

The ‘Shinkai Seep Field’ is a serpentinite-hosted chemosynthetic ecosystem in the Southern Mariana Forearc. In June 2015 the site was revisited and a number of rissoiform gastropods were collected. Taxonomic investigations revealed that these specimens represent a hitherto undescribed species ofProvanna(Gastropoda: Abyssochrysoidea), described herein asProvanna cingulatan. sp. This new species is characterized by numerous spiral keels, lack of significant axial sculpture, rounded and inflated whorls, and large size for the genus. With the shell height exceeding 16.5 mm (may reach 20 mm), it is the largestProvannaspecies known thus far. Phylogenetic analysis using 411 bp of the cytochrome oxidase c subunit I (COI) gene confirmed its systematic placement within the genusProvanna. This is the only gastropod from a family endemic to chemosynthetic ecosystems thus far known from the ‘Shinkai Seep Field’. Furthermore, with a collection depth of 5687 m, it represents the deepest known bathymetric range for the superfamily Abyssochrysoidea as a whole.

Brucite chimney formation and carbonate alteration at theShinkaiSeepField, a serpentinite‐hosted vent system in thesouthernMariana forearc

Brucite-carbonate chimneys were discovered from the deepest known (∼5700 m depth) serpentinite-hosted ecosystem – the Shinkai Seep Field (SSF) in the southern Mariana forearc. Textural observations and geochemical analysis reveal three types (I-III) of chimneys formed by the precipitation and dissolution of constitutive minerals. Type I chimneys are bright white to light yellow, have a spiky crystalline and wrinkled surface with microbial mat and contain more brucite; these formed as a result of rapid precipitation under high fluid discharge conditions. Type II chimneys exhibit white to dull brown coloration, tuberous textures like vascular bundles, and are covered with grayish microbial mats and dense colonies of Phyllochaetopterus. This type of chimney is characterized by inner brucite-rich and outer carbonate rich zones and is thought to have precipitated from lower fluid discharge conditions than type I chimneys. Type III chimneys are ivory colored, have surface depressions and lack living microbial mats or animals. This type of chimney mainly consists of carbonate, and is in a dissolution stage. Stable carbon isotope compositions of carbonates in the two types (I and II) of active chimneys are extremely 13C-enriched (up to +24.1‰), which may reflect biological 12C consumption under extremely low dissolved inorganic carbon concentration in alkaline fluid. Type III chimneys have 13C compositions indicating re-equilibration with seawater. Our findings demonstrate for the first time that carbonate chimneys can be form below carbonate compensation depth and provide new insights about linked geologic, hydrologic, and biological processes of the global deep-sea serpentinite-hosted vent systems. This article is protected by copyright. All rights reserved.

Textures and mineralogy of chimneys at the Shinkai Seep Field, Southern Mariana Forearc

Textures and mineralogy of chimneys at the Shinkai Seep Field, Southern Mariana Forearc

Geochemical and isotopic study of a plutonic suite and related early volcanic sequences in the southern Mariana forearc

AbstractThe forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81‐06‐26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well‐studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48–43 Ma. This is the first report of boninite‐like plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58–1.44, Nb/La: 0.18–0.79) and other features uniquely connected with boninites: TiO2 < 0.15 wt % and Zr/Sm > 25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO2, forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low‐velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites.

A serpentinite-hosted ecosystem in the Southern Mariana Forearc

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H(2)- and CH(4)-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the "Shinkai Seep Field (SSF)." The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.

Trace element composition of peridotites from the southern Mariana forearc: Insights into the geochemical effects of serpentinization and/or seafloor weathering

Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging. These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab, and later interacted with seawater after emplacement at or near the seafloor. This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites. We measured trace element concentrations of peridotites from the southern Mariana forearc. The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly, and La shows good correlation with other REEs (except Ce). In addition, there is a great enrichment of U, Pb, Sr and Li elements, which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram. For the seawater-like REE pattern, we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor, by serpentinization or by marine weathering. Furthermore, the anomalous behavior of Ce, compared with other rare earth elements in these samples, may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater. Positive U, Pb, Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.

Talc-bearing serpentinized peridotites from the southern Mariana forearc: implications for aseismic character within subduction zones

The serpentinized peridotites overlying the subducted zones in the Izu-Bonin-Mariana (IBM) arc system have been interpret as the cause of the low-velocity layer identified beneath the IBM froearc, in turn few earthquakes occurred along the plate boundary. Chrysotile, which is a low temperature and highly hydrated phase of serpentine with low frictional strength, has been suggested as the low velocity material in the serpentinized peridotites, besides, brucite is inferred to be likely conducive to stable sliding. However, such idea encounters challenging in our serpentinized peridotites from the southern Mariana forearc, which absent both the above minerals. The presence of talc, which characterized by its weak, low-friction and inherently stable sliding behavior, provides new clue. Here we report the occurrence of talc in serpentinized peridotites collected from the landward trench slope of the southern Mariana forearc. We infer that talc is mainly forming as a result of the reaction of serpentine minerals with silica-saturated fluids released from the subducting slab, and talc also occurs as talc veins sometimes. Due to its unique physical properties, talc may therefore play a significant role in aseismic slip in the IBM subduction zone.

Serpentinization of peridotites from the southern Mariana forearc

A detailed petrologic and mineralogic study was carried out on serpentinized peridotites dredged from the southern landward slopes of the Mariana Trench, in order to reveal the serpentinization process of these unusual rocks and to identify the sole presence of the mineral lizardite. The constituent minerals of these southern Mariana forearc peridotites are olivine, amphibole and spinel, as well as serpentine, chlorite and talc. Compared with serpentinite seamounts, the serpentinized peridotites from the southern Mariana forearc are characterized by the absence of magnetite and brucite, and the common presence of talc; besides, the serpentine mineral variety is simplex, only lizardite. Combining mineral chemistry and mineral phase relationships, we conclude that (1) the absence of magnetite in the serpentinized peridotites is due to incomplete serpentinization, other than magnetite, the iron end-member in olivine forms Fe-rich brucite and Fe-rich serpentine; (2) brucite is not stable with high silica activity, reacting with later SiO 2-rich fluid and then forming lizardite, leading to a lack of brucite in these serpentinized peridotites; (3) the occurrence of talc is the result of later SiO 2-rich fluid reactions with lizardite; and (4) the reason for the sole occurrence of lizardite is that the temperature condition of our study area was not high enough for the formation of antigorite (which is stable at >500 °C). Despite the broad overlap of lizardite and chrysotile in growth temperature, differences in the modes of occurrence of lizardite and chrysotile, such as the scarcity of H 2O, low porosity and permeability, as well as the actual situation of initial serpentinization in the study area, result in the absolute prevalence of lizardite over chrysotile in the area.

Peridotites from a ductile shear zone within back‐arc lithospheric mantle, southern Mariana Trench: Results of a Shinkai 6500 dive

Two N–S fault zones in the southern Mariana fore arc record at least 20 km of left‐lateral displacement. We examined the eastward facing slope of one of the fault zones (the West Santa Rosa Bank fault) from 6469 to 5957 m water depth using the submersible Shinkai 6500 (YK06‐12 Dive 973) as part of a cruise by the R/V Yokosuka in 2006. The dive recovered residual but still partly fertile lherzolite, residual lherzolite, and dunite; the samples show mylonitic, porphyroclastic, and coarse, moderately deformed secondary textures. Crystal‐preferred orientations of olivine within the peridotites show a typical [100](010) pattern, with the fabric intensity decreasing from rocks with coarse secondary texture to mylonites. The sampled peridotites therefore represent a ductile shear zone within the lithospheric mantle of the overriding plate. Peridotites were probably exposed in association with a tear in the subducting slab, previously inferred from bathymetry and seismicity. Furthermore, although the dive site is located in the fore arc close to the Mariana Trench, spinel compositions within the sampled peridotites are comparable to those from the Mariana Trough back arc, suggesting that back‐arc basin mantle is exposed along the West Santa Rosa Bank fault.

Peridotites from the southern Mariana forearc: Heterogeneous fluid supply in mantle wedge

Detailed petrological work was carried out on serpentinized peridotite dredged and sampled by submersible from the southern part of the Mariana Trench to reveal the nature of the mantle wedge in the southern Mariana forearc. The southern part of the Mariana Trench is important in that we should expect to find a transect of a typical island arc structure; that is, from east to west, the Mariana forearc, the Mariana arc proper, the Mariana Trough (active back‐arc spreading center), and the West Mariana Ridge (remnant arc). The most striking feature of peridotites from the southern part of the trench is that primary hornblende is a major constituent mineral in many specimens. Thus, the peridotite samples are divided into anhydrous (A‐type), hydrous (H‐type) and intermediate (I‐type) groups. Petrological data suggest that each type of peridotite is a residue of extensive partial melting in the upper mantle. It is argued here that the I‐ and H‐type peridotites were modified from `proto‐A‐type peridotite' by fluid infiltration. The fluid was enriched in Al, Ti, Fe, and alkalis, and may have caused changes in mineral and bulk chemical compositions of the peridotites. A‐type peridotite derives from the `proto‐A‐type peridotite' directly, without any fluid contamination. After the formation of the `proto‐A‐, I‐, and H‐type peridotites', lower‐temperature fluids, probably of seawater origin, produced retrograde metamorphism and alteration including serpentinization. The mantle wedge in the southern Mariana forearc was heterogeneous in fluid supply.

Peridotites from the southern Mariana forearc: Heterogeneous fluid supply in mantle wedge

Peridotites from the southern Mariana forearc: Heterogeneous fluid supply in mantle wedge

Volcanoes of the southwestern extension of the active Mariana island arc: New swath‐mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back‐arc basin intersects the line of arc volcanoes. They develop well‐defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back‐arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault‐controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.