Low-chlorinity springs sampled from ten sites on nine serpentinite mud volcanoes show systematic chemical gradients across the outer Mariana forearc that result from progressive devolatilization of the subducting Pacific plate. Sites range from 50 to 90 km from the trench axis corresponding to depths to the top of the plate of similar to 15-29 km. Dissolved sulfate, Na/Cl, K, Rb, Cs, and B in the springs all increase regularly with distance from the trench, leached from subducting sediment and altered basalt in response to increasing temperature at depth from similar to 80 to 350 degrees C. Sites nearer the trench have high Ca (up to 76 mmol/ kg) and Sr, low alkalinity, and pH 10.7, whereas sites farther from the trench have almost no Ca and Sr, alkalinity (some carbonate but mostly hydroxyl) as high as 69 meq/kg, and pH 12.5. Springs with high alkalinity also have high methane (> 44 mmol/kg) that feeds sulfate-reducing archaeal communities in the shallow subsurface and macrofauna at the seafloor. These distal springs form chimneys and crusts of CaCO3, whereas the proximal springs form chimneys of brucite. High alkalinity at the distal sites apparently results from carbonate dissolution at the top of the subducting plate; because serpentinization during ascent generates both high pH and H-2, the resulting dissolved carbonate is reduced to methane such that carbonate alkalinity is replaced by hydroxyl alkalinity: 4H(2) + HCO3- -> CH4 + 2H(2)O + OH-. This reaction can account for the much higher pH of the distal springs. Chlorinity of the springs varies from 234 to 546 mmol/kg and is related to latitude N-S rather than distance from the trench. Distal springs have otherwise similar compositions over this entire range of chlorinity, implying that chloride derives from depth rather than from mixing with seawater within the seamounts themselves. The range in chlorinity can readily be explained by serpentinization at reasonable water/rock mass ratios of 0.2-1.0 if 30-40% of the spring water originates as residual pore water in subducted sediment and basalt rather than as H2O+ of dehydration. The pH, alkalinity, and methane content of the springs increase abruptly, while Ca and Sr decrease, because carbonate dissolution joins dehydration as a major process at the top of the subducting plate at similar to 70 km from the trench, where metabasites recovered from the serpentinite mud indicate the transition from lawsonite blueschist facies to epidote blueschist facies also occurs. Replacement of lawsonite by epidote drastically depletes the solution in Ca and shifts the equilibrium toward massive dissolution of subducted carbonate. Fluxes of sulfate, C, Na, K, Rb, Cs, B, Ca, and Sr in the forearc springs represent only a few percent of the amounts subducted, consistent with continued supply at greater depth. (c) 2022 Published by Elsevier Ltd.

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ∼25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K 2 O contents (median = 4.6 wt%) and loss on ignition (median = 5.3 wt%, as a proxy for H 2 O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO 2 for all samples, and increased MgO and Na 2 O at one but losses of MgO and Fe 2 O 3 * at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts. The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K 2 O, H 2 O, and fluid-mobile elements to sub-arc depths. These findings demonstrate that mass transfer from the subducting lithosphere starts at low P / T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (<30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.

This study's goal is to understand the structural events and episodes of fluid flow recorded in calcium carbonate precipitation sampled in Mariana forearc serpentine mud volcanoes. Those active mounts provide a unique window to deep structural and fluid flow events affecting the subduction channel m 'elange zone and the subducted Pacific Plate. To build a conceptual model of vein precipitations from the subduction zone to the mud volcanoes edifice, we unravelled the origin, timing, and mechanisms of those precipitations with a multidisciplinary study of calcium carbonate veins, from samples drilled in the flank of Fantangisna Seamount during the International Ocean Discovery Program (IODP) Expedition 366. Structural analysis of the carbonate vein network revealed stages of precipitation and dissolution. The most ancient calcium carbonate minerals underwent high tectonic stress and several degrees of silicification. U-Pb dates ranged from Early Cretaceous to present day, recording potential pre-convergence events and calibrating an episodic building of the Mariana mud volcanoes from the start of the convergence to present day. The UCC-normalized LREE pattern, Sr-87/Sr-86 ratios ranging from 0.704977 to 0.705798 and delta O-18 signature 18.7 to 19.8 (%, V-SMOW) of the veins indicate a mixed origin of the precipitated fluids influenced by the serpentine mud signature. Those mud volcanoes were episodically built from the start of the subduction to present day in function of the forearc tectonic activity. They are complex systems internally composed of a fault network providing multiple circulation pathways that can be successively opened or closed.

Mafic and ultramafic clasts (mostly similar to 1-5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program(IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (similar to 80mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite-pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite-blueschistfacies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearcmantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation. (C) 2020 Elsevier B.V. All rights reserved.

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life. This article is part of a discussion meeting issue 'Serpentine in the Earth system'.

Aphyonids are poorly-known, live-bearing brotulas (Ophidiiformes, Bythitidae) that until recently were considered to be in a distinct family, Aphyonidae. A single, ca. 9.3cm total length aphyonid observed during a remotely-operated vehicle survey in the Mariana Archipelago at 2504.2m on Explorer Ridge (20.68152°N, 145.08750°E) is the first seen alive in its natural habitat. Collection to verify its identification was not possible, but based on observations it was a species of either Barathronus or Nybelinella. The fish swam 1–10cm over sediment between rocks and small boulders on a 45° talus slope. Swimming speeds were consistently slow, 0.33 ± 0.15 body lengths per second, and the fish appeared to be neutrally buoyant. Although there are few other records of aphyonid-clade fishes in the Pacific away from continental margins, this observation suggests that they will be found elsewhere in the basin when appropriate methods are used to detect these small fishes in the high-relief, rugose habitats of central Pacific oceanic islands and seamounts.

Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly depicted by pole figures for the [100], [010], and [001] axes, and they can be categorized into five well-known fabric types: A, B, C, D, and E. These fabric types can be related to olivine slip systems: A with (010)[100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (001)[100]. In addition, an AG type is commonly found in nature, but its origin is controversial, and could involve several contributing factors such as complex slip systems, non-coaxial strain types, or the effects of melt during plastic flow. In this paper we present all of our olivine fabric database published previously as well as new data mostly from ocean floor, mainly for the convergent margin of the western Pacific region, and we introduce a new index named Fabric-Index Angle (FIA), which is related to the P-wave property of a single olivine crystal. The FIA can be used as an alternative to classifying the CPOs into the six fabric types, and it allows a set of CPOs to be expressed as a single angle in a range between −90° and 180°. The six olivine fabric types have unique values of FIA: 63° for A type, −28° for B type, 158° for C type, 90° for D type, 106° for E type, and 0° for AG type. We divided our olivine database into five tectonic groups: ophiolites, ridge peridotites, trench peridotites, peridotite xenoliths, and peridotites enclosed in high-pressure metamorphic rocks. Our results show that although our database is not yet large enough (except for trench peridotites) to define the characteristics of the five tectonic groups, the natural olivine fabrics vary in their range of FIA: 0° to 150° for the ophiolites, 40° to 80° for the ridge peridotites, −40° to 100° for the trench peridotites, 0° to 100° for the peridotite xenoliths, and −40° to 10° for the peridotites enclosed in high-pressure metamorphic rocks. The trench peridotites show a statistically unimodal distribution of FIA consisting of the high peak equivalent of the A type, but with some FIAs close to the AG and D types. The variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate.

Deep-sea trenches remain one of the least explored ocean ecosystems due to the unique challenges of sampling at great depths. Five submersible dives conducted using the DEEPSEA CHALLENGER submersible generated video of undisturbed deep-sea communities at bathyal (994m), abyssal (3755m), and hadal (8228m) depths in the New Britain Trench, bathyal depths near the Ulithi atoll (1192m), and hadal depths in the Mariana Trench Challenger Deep (10908m). The New Britain Trench is overlain by waters with higher net primary productivity (~3-fold) than the Mariana Trench and nearby Ulithi, and receives substantially more allochthonous input from terrestrial sources, based on the presence of terrestrial debris in submersible video footage. Comparisons between trenches addressed how differences in productivity regime influence benthic and demersal deep-sea community structure. In addition, the scavenger community was studied using paired lander deployments to the New Britain (8233m) and Mariana (10918 m) trenches. Differences in allochthonous input were reflected in epibenthic community abundance, biodiversity, and lifestyle representation. More productive locations were characterized by higher faunal abundances (~2-fold) at both bathyal and hadal depths. In contrast, biodiversity trends showed a unimodal pattern with more food-rich areas exhibiting reduced bathyal diversity and elevated hadal diversity. Hadal scavenging communities exhibited similar higher abundance but also ~3-fold higher species richness in the more food-rich New Britain Trench compared to the Mariana Trench. High species- and phylum-level diversity observed in the New Britain Trench suggest that trench environments may foster higher megafaunal biodiversity than surrounding abyssal depths if food is not limiting. However, the absence of fish at our hadal sites suggests that certain groups do have physiological depth limits. Submersible video footage allowed novel in situ observation of holothurian orientation, jellyfish feeding behavior as well as lifestyle preferences for substrate, seafloor and overlying water. This study documents previously unreported species in the New Britain Trench, including an ulmariid scyphozoan (8233m) and an acrocirrid polychaete (994m), and reports the first observation of an abundant population of elpidiid holothurians in the Mariana Trench (10908m). It also provides the first megafaunal community analysis of the world׳s deepest epibenthic community in the Mariana Trench Challenger Deep, which was composed of elpidiid holothurians, amphipods, and xenophyophores.

Petrographic and geochemical characteristics of drill core and surface rock samples from the four hydrothermal vent sites of the Yamanaka, Snail, Archaean, and Pika sites at the Southern Mariana Trough (SMT) are described in order to clarify the geological background of hydrothermal activities. The core samples were drilled by the Benthic Multi-coring System (BMS), penetrating up to ~8 m below seafloor. The recovered samples included both basement rocks and sulfide ores. The basement rocks are further subdivided into basaltic andesite and andesite, most of which are recovered from on-axis and off-axis regions of the SMT, respectively. All of the rocks are characterized by relative enrichment of large-ion lithophile elements with noticeable depletion of Nb and Ta, suggestive of significant influence of subducted slab-derived components into the basement rocks both at the on- and off-axis hydrothermal vent sites. Major and trace element variations of the samples suggest that the Yamanaka, Snail, and Archaean rocks can be explained by a sequence of fractionation of an on-axis magma, whereas only the Pika rocks may be influenced by a different magma component, i.e., off-axis magmatism.

Major and minor chemistry, Sr, Nd and Pb isotope ratios, water content and hydrogen isotope ratios of the backarc basin volcanic rocks taken from the active spreading ridge and surrounding area between 12°40′N and 13°15′N in the southern part of the Mariana Trough indicate a complex regional interplay of suprasubduction-zone magmatic sources. In this area, active backarc spreading occurs along the eastern side of the Mariana Trough, and discrete seamounts align on two parallel chains east of the backarc spreading center. The spreading-center rocks originated from a typical mid-ocean ridge basalt (MORB)-like source, influenced progressively southwestward by proximity to the arc magmatic source and range from basaltic to dacitic in composition. The volcanic rocks from the seamount chains are explained by the mixing of two types of magmatic sources; one is originally a depleted mantle similar to the arc source for magmas farther north and the other is the MORB-like source of the spreading ridge rocks. The influence of the arc source on magma composition of the seamount chains is higher north of 13°N, indicating heterogeneity of the mantle wedge in this region and the merging of the two sources toward the southwest.

The 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.

Extremophilic archaeal communities living in serpentinized muds influenced by pH 12.5 deep-slab derived fluids were detected and their richness and relatedness assessed from across seven serpentinite mud volcanoes located along the Mariana forearc. In addition, samples from two near surface core sections (Holes D and E) at ODP Site 1200 from South Chamorro were subjected to SSU rDNA clone library and phylogenetic analysis resulting in the discovery of several novel operational taxonomic units (OTUs). Five dominant OTUs of Archaea from Hole 1200D and six dominant OTUs of Archaea from Hole 1200E were determined by groups having three or more clones. Terminal-restriction fragment length polymorphism (T-RFLP) analysis revealed all of the dominant OTUs were detected within both clone libraries. Cluster analysis of the T-RFLP data revealed archaeal community structures from sites on Big Blue and Blue Moon to be analogous to the South Chamorro Hole 1200E site. These unique archaeal community fingerprints resulted from an abundance of potential methane-oxidizing and sulfate-reducing phylotypes. This study used deep-sea sediment coring techniques across seven different mud volcanoes along the entire Mariana forearc system. The discovery and detection of both novel Euryarchaeota and Marine Benthic Group B Crenarcheaota phylotypes could be efficacious archaeal indicator populations involved with anaerobic methane oxidation (AMO) and sulfate reduction fueled by deep subsurface serpentinization reactions.