Serpentine Mud Research Articles

Serpentinite‐hosted chemosynthetic community of South Chamorro Seamount, Mariana Forearc

AbstractDeep‐sea chemosynthetic ecosystems are ‘oases’ of life powered by reducing geofluids, of which serpentinite‐hosted seeps are among the least studied. South Chamorro Seamount, a serpentine mud volcano on the Mariana Arc, has been known to host chemosynthesis‐based assemblages since 1996, but no detailed information on the fauna was published. Here, we revisited South Chamorro to characterise its biodiversity. We located two regions of chemosynthetic communities dominated by bathymodioline mussels, vesicomyid clams, and chaetopterid parchment worms: one on the northwestern flank (‘Fryer Site’) and one on the southern summit (‘Summit Site’). We sampled a total of 20 species including 13 molluscs, five annelids, and two crustaceans – all present on the more active Summit Site but only a subset being found at Fryer Site. A mussel bed surrounding the Fryer Site was drilled by the Ocean Drilling Program in 2001 resulting in six holes, the deepest being 266 m (Hole 1200C). Cuttings ~50 cm deep still cover an approximately 60 m radius around Hole 1200C even 22 years later, and there is no sign of recovery. Low geofluid supply in serpentinite‐hosted seamounts may not allow decadal recovery of animal colonies, unlike a previous drilling site in an Okinawa Trough vent.

Active near-surface mobilisation of slab-derived geochemical signatures by hyperalkaline waters in brecciated serpentinites

Unusual hyperalkaline meteoric groundwaters on Troodos massif, Cyprus, issue from highly deformed and completely serpentinized ultramafic brecciated rock masses of the Artemis Diapir and have high salinity (25–30% seawater total dissolved solids) and some of the highest recorded pH values (11−13) for natural waters. These waters have elevated dissolved Na, K, Li, B, Ba, Rb, Cs, Cl and SO4 with ion/chloride substantially above seawater ratios, overprinting minor contributions from marine aerosols. For example, K concentrations are similar to seawater values. Water stable isotope ratios imply extensive water-rock interaction and 87Sr/86Sr is rock-dominated (0.705) and significantly lower than seawater. These rare fluids contrast with surface and shallow ground waters draining the ultramafic Troodos Mantle Sequence that have pH of 8.5–9, meteoric stable isotope ratios, 87Sr/86Sr similar to early Miocene seawater (0.7085) and low dissolved salts albeit with ion/chloride ratios also greater than seawater. The combination of high pH and salinity is unusual in ophiolites and these hyperalkaline waters are most similar to end-member fluids emanating from serpentine mud volcanoes in the Mariana forearc. Rainwater rapidly transiting terraces of asbestos mine tailings on Troodos mostly resemble the surface waters but show slight contamination by a saline component. We propose that inclusions within the serpentinite that contain highly soluble salts have been made accessible by tectonic deformation during uplift and diapirism, or comminution during mining activity. A multistage process is proposed whereby alkali and other signatures have been released from the stalled subducting slab beneath the Troodos massif and have infused the mantle wedge. Some of these rocks have been carried to the surface by diapiric uplift and erosion, where tectonic deformation or mining activities have enabled the near-surface mobilisation of slab-derived geochemical signatures by modern meteoric waters.

Fluid‐Mantle Interaction Along the Mariana Convergent Margin

AbstractActive serpentinite mud volcanoes in the forearc region of the Izu‐Bonin‐Mariana system represent an excellent natural laboratory for studying the geochemical processes along convergent plate margins and the associated forearc. During IODP Expedition 366, serpentinite mud with lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific Plate was recovered. Ultramafic clasts from Fantangisña Seamount reveal very high degrees of serpentinization with mesh and bastite textures as well as development of late lizardite and chrysotile veins, which suggests serpentinization temperatures below 200°C. On the other hand, recovered harzburgites and, on occasion, dunites from Asùt Tesoru Seamount show a well‐preserved primary assemblage with low degrees of serpentinization and forearc peridotite characteristics. Fine‐grained antigorite associating with lizardite has been identified throughout the serpentine mud matrix, suggesting an alteration temperature of c. 340°C. Furthermore, alteration conditions during rodingitization point to temperatures of at least 228°C, estimated via chlorite geothermometry. Additionally, a rare ophicarbonate clast containing andraditic as well as Cr‐rich hydrogarnets from Asút Tesoru Seamount indicates crystallization temperatures of at least 230°C. Hence, a trend of lower temperature of serpentinization and higher degree of alteration closer to the trench. The detailed characterization of the fluid‐rock alteration conditions as well as fluids composition and transport permits a better constraining of the fluid–rock interactions and related mass transfers within subduction zones and during ascent of serpentinite fault gouge within mud volcano conduits and in mudflows after their emplacement on the flanks of the edifices.

Foraminifera assemblages from Fantangisña serpentinite mud seamount in the NW Pacific Ocean during the Pleistocene (IODP Expedition 366).

The Mariana forearc system, in the northwestern Pacific, is known as the only convergent margin setting with currently active serpentine mud volcanism. The Fantangisña serpentinite mud volcano lies 62 km west of the Mariana trench, within the influence of the North Equatorial Current (NEC). Cores recovered by International Ocean Discovery Program (IODP) Expedition 366 contain pelagic sediments overlying layered serpentinite mud deposits. At the bottom of the sequence, nannofossil-rich forearc deposits were recovered from under the seamount edifice. In this study, we investigated 47 samples from Site U1498A on the southern flank of the seamount for benthic and planktonic foraminifera assemblages. Statistical analyses on planktonic assemblages differentiated two sample groups related to the ratio between thermocline/mixed layer taxa, which indicate fluctuations in the depth of the thermocline (DOT) during the Pleistocene. Variations in the DOT reflect changes in the intensity of the NEC associated with El Niño/La Niña conditions. Mudflows do not influence the ecology of planktonic foraminifera but possibly enhance their preservation against dissolution, which was instead detected in the pelagic deposit as suggested by common Globigerinoides conglobatus. Benthic foraminifers were rare in serpentinite mud deposits as they are severely affected by mudflows. Conversely, they showed high diversity pre-/post-mud-volcanism, and indicate oligotrophic bottom-water conditions.

Geophysical evidence for a serpentine mud volcano in the relict slow-spreading center of the South China Sea

The seamounts formed in spreading centers are of basaltic with prominent gravity peaks and high density. But Longmen Seamount, 1.5 km high and ∼ 20 km wide, located in the relict slow-spreading center of the South China Sea, shows a weak gravity peak with a low-amplitude gravity variation (∼19 mGal). To reveal the geological origin of the unusual seamount, we conducted pre-stack depth-migrated image and 3D forward gravity modeling for Longmen Seamount based on the constraint from reflection and refraction seismic data. Our results show that bulk density and seismic velocity of Longmen Seamount is as low as ∼2.1 g/cm3, and 1.6–3.2 km/s, respectively, equivalent to that of poorly consolidated sediments. Furthermore, typical structural elements of the mud volcano system, such as a reflection blank zone, up-bent and downward bending reflections, are observed beneath LM Seamount. For the crust beneath LM Seamount is thin (1.6 km) and highly faulted revealed by numerous clear discontinuous reflections, it is easy for seawater to infiltrate through the thin crust and leads to a high degree of serpentinization of the upper mantle rocks. Combined with serpentine mud volcanoes featuring low bulk density and low seismic velocity, Longmen Seamount is most probably a serpentine mud volcano in a slow-spreading mid-ocean center. For the discovered serpentine mud volcanoes are all restricted to the convergent plate margins, this study will lead to a new understanding of the mechanism of serpentine mud volcanism.

Episodicity of structural flow in an active subduction system, new insights from mud volcano's carbonate veins – Scientific Ocean drilling expedition IODP 366

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 mélange 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 Fantangisña 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, 87Sr/86Sr ratios ranging from 0.704977 to 0.705798 and δ18O 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.

Rheology and Biostratigraphy of the Mariana Serpentine Muds Unravel Mud Volcano Evolution

AbstractIn late 2016, the IODP Expedition 366 drilled 21 holes on summits and flanks of three serpentinite mud volcanoes (SMVs) of the Mariana Forearc: Yinazao (Blue Moon), Fantangisña (Celestial), and Asùt Tesoru (Big Blue). The drilling reached the forearc sediments underneath the SMVs only at Site U1498 where, according to data from this study, the biostratigraphic age of Fantangisña appears to be well constrained, with a pelagic cover on top of the serpentine mud flows of 0.44 Ma and an age of the forearc sediments of 11.21 Ma. With an estimated age of at least 10.77 Myr this is, to our knowledge, the oldest mud volcano ever dated. To link the age of the edifice with its eruptive dynamics, we performed rheological tests of natural samples of serpentine mud from the Fantangisña SMV for the first time. These experiments not only confirm the highly viscous characteristic of the mud breccia but also identify different muds' mineralogical compositions (i.e., serpentine‐rich vs. serpentine‐poor sediments) and water contents as being responsible for the main rheological variations. The viscosity from the rheological data, together with the physical properties of the mud breccia and the clasts measured onboard IODP366, under several assumptions, allow us to estimate of flow velocities and a depth of the mud source at ∼8.4 km below seafloor. Moreover, comparison of erupted mud breccia volumes from bathymetry, seismic data, and calculations shows how the SMV evolution is linked to episodic events, highlighting that Fantangisña has actively been expelling sediments during 2.41% of its life span.

Blueschist from the Mariana forearc records long-lived residence of material in the subduction channel

From ca. 50 Ma to present, the western Pacific plate has been subducting under the Philippine Sea plate, forming the oceanic Izu-Bonin-Mariana (IBM) subduction system. It is the only known location where subduction zone products are presently being transported to the surface by serpentinite-mud volcanoes. A large serpentine mud “volcano” forms the South Chamorro Seamount and was successfully drilled by ODP during Leg 195. This returned mostly partially serpentinized harzburgites enclosed in serpentinite muds. In addition, limited numbers of small (1 mm–1 cm) fragments of rare blueschists were also discovered. U–Pb dating of zircon and rutile from one of these blueschist clasts give ages of 51.1 ± 1.2 Ma and 47.5 ± 2.0 Ma, respectively. These are interpreted to date prograde high-pressure metamorphism. Mineral equilibria modelling of the blueschist clast suggests the mineral assemblage formed at conditions of ∼1.6 GPa and ∼590°C. We interpret that this high-pressure assemblage formed at a depth of ∼50 km within the subduction channel and was subsequently exhumed and entrained into the South Chamorro serpentinite volcano system at depths of ∼27 km. Consequently, we propose that the material erupted from the South Chamarro Seamount may be sampling far greater depths within the Mariana subduction system than previously thought. The apparent thermal gradient implied by the pressure–temperature modelling (∼370°C/GPa) is slightly warmer than that predicted by typical subduction channel numerical models and other blueschists worldwide. The age of the blueschist suggests it formed during the arc initiation stages of the proto-Izu-Bonin-Mariana arc, with the P–T conditions recording thermally elevated conditions during initial stages of western Pacific plate subduction. This indicates the blueschist had prolonged residence time in the stable forearc as the system underwent east-directed rollback. The Mariana blueschist shows that subduction products can remain entrained in subduction channels for many millions of years prior to exhumation.

First seep-dwelling Desbruyeresia (Gastropoda: Abyssochrysoidea) species discovered from a serpentinite-hosted seep in the Southeastern Mariana Forearc

ABSTRACTThe South Chamorro Seamount (13°47'N, 146°00'E) on the southeastern Mariana Forearc is one of a series of serpentine mud volcanoes in the area, and hosts an active fluid seepage area on its summit. During a recent research cruise three specimens of a small rissoid-like gastropod were collected with chemoautotrophic bivalves. Subsequent morphological and molecular investigations revealed it to be a novel species of Desbruyeresia (Abyssochrysoidea: Provannidae), described herein as Desbruyeresia chamorrensis n. sp. Although all known specimens had a heavily corroded apex, so the protoconch could not be observed, its radular characteristics are very similar to those of other Desbruyeresia species. Phylogenetic reconstruction using the cytochrome c oxidase subunit I (COI) gene also supports its placement within Desbruyeresia. The new species is distinguished from all other described congeners by having more numerous (17–20) axial ribs on the teleoconch, and a broad shell for the genus (shell width to height ratio 0.6–0.65). Living Desbruyeresia were previously only known from hydrothermal vents and this is the first species known to inhabit seeps or serpentinite-hosted chemosynthetic habitats.http://zoobank.org/urn:lsid:zoobank.org:act:3BDA021E-45ED-439A-9D5D-402BBB29293E

Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle

Serpentine seamounts located on the outer half of the pervasively fractured Mariana forearc provide an excellent window into the forearc devolatilization processes, which can strongly influence the cycling of volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the Mariana forearc reveal microstructures, mineral assemblages and compositions that are indicative of a complex polyphase alteration history. Petrologic phase relations and oxygen isotopes suggest that ultramafic clasts were serpentinized at temperatures below 200°C. Several successive serpentinization events represented by different vein generations with distinct trace element contents can be recognized. Measured in situ Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is consistent with Cs mobilization from sediments at lower temperatures and lends further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element (FME) concentrations than early veins, suggesting a decreasing influence of fluid discharge from the subducting slab on the composition of the serpentinizing fluids. The continuous microfabric and mineral chemical evolution observed in the ultramafic clasts may have implications as to the origin and nature of the serpentinizing fluids. We hypothesize that opal and smectite dehydration produce quartz-saturated fluids with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive serpentinization. The partially serpentinized material may then be eroded from the basal plane of the suprasubduction mantle wedge. Serpentinization continued but the interacting fluids did not carry a pronounced sedimentary signature, either because FMEs were no longer released from the slab, or due to an en route loss of FMEs. Late chrysotile veins that document the increased access of fluids in a now fluid-dominated regime are characterized by reduced trace element contents with a slightly increased Rb/Cs ratio near 10. This lack of sediment-dominated geochemical signatures consistently displayed in all late serpentinization stages may indicate that the sediment-derived fluids have been completely reset (i.e. the FME excesses were removed) by continued water–rock reaction within the subduction channel. The final stage of buoyant rise of matrix and clasts in the conduits is characterized by brucite-dominated alteration of the clasts from the clast rim inward (independent of the intra-clast fabric relations), which corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.

Improved Precision of Radiocarbon Measurements for CH4 and CO2 Using GC and Continuous-Flow AMS Achieved by Summation of Repeated Injections

Compound specific radiocarbon measurements can be made instantaneously using a gas chromatograph (GC) combustion system coupled to a 14C AMS system fitted with a gas ion source. Samples below 10 μg C can be analyzed but the precision is reduced to 5–10% because of lower source efficiency. We modified our GC for CH4 and CO2 analysis and injected samples multiple times to sum data and increase precision. We attained a maximum precision of 0.6% for modern CO2 from 25 injections of 27 μg C and a background of ≃0.5% (40 kyr) for ancient methane. The 14C content of dissolved CO2 and CH4 in water samples collected at a deep-sea hydrothermal vent and a serpentine mud volcano was measured and the results for the vent sample are consistent with previously published data. Further experiments are required to determine a calibration and correction procedure to maximize accuracy.

Early Archean serpentine mud volcanoes at Isua, Greenland, as a niche for early life

The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate-rich, high-pH hydrothermal solutions at medium temperature (100-300 °C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.

Structure and elasticity of serpentine at high-pressure

Serpentines occur in the subduction zone settings, both along the slab and within the mantle wedge, they are candidates for transporting water in to the deep earth. Their presence is manifested by serpentine mud volcanoes, high electrical conductivities, magnetic and seismic anomalies. Using theoretical methods, we predict a pressure induced structural transformations in serpentine. The transformations are related to the behavior of the silicate framework and misfit between octahedral and tetrahedral layers. As the structure is compressed, the octahedral layer and tetrahedral layers are compressed at different rates. At 7 GPa, the misfit between the layers vanishes. This causes non-linear pressure dependence of tetrahedral rotational angle. This is also manifested by the onset of anomalous pressure dependence of the elastic constants c 11, c 33, c 12, c 13. Beyond 7 GPa, the misfit between the layers grows again reaching extremum at 22 GPa. This is also manifested by discontinuity in average Si–O bond length, volume of tetrahedron and re-orientation of hydroxyl vector. The symmetry of the crystal-structure however, remains unaffected. Evidence of pressure-induced hydrogen bonding is absent in serpentine, as evident from reduction of O–H bond length upon compression. Results of compression for the low-pressure regime ( P < 7 GPa) is well represented by a fourth order Birch–Murnaghan finite strain expression with K 0 = 79 GPa, K 0′ = 12 and K 0″ = − 2, where K is the bulk modulus, prime indicates pressure derivatives, and O refers to zero pressure. Our best estimates of K 0, K 0′ and the Grüneisen parameter, γ at 300 K and zero pressure based on our results are: 61 GPa, 17, and 0.77, respectively. At low pressures, serpentine structure is anisotropic with c 11 ~ 2.4 × c 33. The pressure derivative of elastic constants ( ∂c ij / ∂P) are such, that around 22 GPa c 11~ c 33. An elastic instability ( c 66 < 0) at somewhat higher pressures (> 50 GPa) is also noted. The elastic constant tensor reveals large acoustic anisotropy (41% in V P ) and seismic wave velocities that are significantly higher than those inferred from experiments on serpentinites.

Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean

Chlorine stable isotopes ( δ 37Cl) and halogen concentrations ( e.g. Br/Cl) in 168 pore Fluids and 23 serpentines and other solids from three subduction zones, the Nankai Trough, Costa Rica, and Mariana Forearc, provide critical information on fluid sources, flow paths, and reaction conditions. The δ 37Cl values of pore fluids at the Nankai and Costa Rica subduction zones, are significantly more negative (minimum − 7.8‰, 2σ ± 0.3‰) than seawater value (0‰). At Nankai Trough, the minimum δ 37Cl value is situated below the décollement and evolves laterally from − 7.8‰ at the most arcward ODP Site 808, to − 7.1‰ at Site 1174, ∼ 2 km seaward from Site 808, and to − 5.8‰ at the reference Site 1173. At Costa Rica, along the décollement the minimum δ 37Cl value evolves from − 5.5‰ at the most arcward ODP Site 1040/1254, to − 3.2‰ at Site 1043/1255, ∼ 1 km seaward, and to 0‰ at the reference Site 1039/1253. At both subduction zones, the Br/Cl ratios are higher than the seawater value (1.5 × 10 − 3 ) and also show seaward evolutions. These pore fluids originate from greater depth arcward, at ≥ 250 °C, from hydrous mineral formation that preferentially incorporates 37Cl and excludes Br. In contrast, the δ 37Cl values in the pore fluids at the Mariana serpentine mud volcanoes are higher than the seawater value (+ 0.3‰ to + 1.8‰); and the Br/Cl ratios are lower. These pore fluid values and the high Cl concentrations with positive δ 37Cl values (+ 1.2 to + 6.0‰) in the serpentines, support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in 37Cl, into the fluid phase. The constancy of the ocean δ 37Cl over the past 200 Ma suggests that the isotopically fractionated chlorine in serpentinites and the Cl exchanged in subduction zones are efficiently recycled back into seawater. If the efficiency is < 100%, the residual would be transferred to the mantle, with a maximum Cl flux between 2 to 3 × 10 17 moles/Ma that would lead to an isotopic difference between the mantle and seawater over the age of the earth on the order of a few per mil.

Formation and emplacement of the Dolodrook serpentinite body, Lachlan Orogen, Victoria

Serpentinised peridotite and ultramafic breccia make up an approximately 5 km‐long, 1 km‐wide fault slice within turbidites in the Dolodrook River region of the central Lachlan Orogen. The serpentinite body is surrounded by juvenile, mafic‐ultramafic sedimentary rocks with Cambrian limestone olistoliths representative of locally derived debris flows, and Middle to Upper Ordovician black shale, chert, sandstone and mudstone. The antiformal geometry and nature of the ultramafic breccia and mafic‐ultramafic sedimentary rocks (Garvey Gully Formation) indicate that the serpentinite body may have been either a former oceanic transform fault zone, a Marianas‐style serpentine seamount or a combination of these. Observations of modern‐day forearc regions show that faulting processes have led to the exposure of serpentinised peridotite horst blocks and serpentine mud volcanoes that have intruded along fault conduits (e.g. Marianas and Izu‐Bonin forearcs). At Dolodrook, the structural and metamorphic relationships with the surrounding rocks, and the lithological associations, have much in common with these observations and indicate that Dolodrook may be an ancient, on‐land example of an accreted seamount or oceanic topographic high. Structural relationships, the very low metamorphic grade of all rocks at Dolodrook, and the presence of broken formation developed in not‐fully lithified Middle to Upper Ordovician sandstone and mudstone indicate that the serpentinite body was emplaced at shallow crustal levels within the turbidite wedge (Tabberabbera Zone), possibly as an offscraped topographic high during marginal basin closure. The Dolodrook serpentinite has previously been inferred as part of the Cambrian igneous sequence (‘greenstones’) exposed in the Governor, Mt Wellington and Heathcote Fault Zones, but structural and metamorphic relationships with surrounding rocks, and the Cambrian tectonic setting in which it formed, have remained speculative.

Serpentinization of the forearc mantle

A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge. However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous. Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic velocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity, an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength. Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin–Mariana, and central Japan. Serpentinization may explain why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the low density, weak serpentinized forearc mantle. Large volumes of H 2O may be released from serpentinized forearc mantle by heating during ridge subduction or continent collision.

Mariana blueschist mud volcanism: Implications for conditions within the subduction zone

Several recently discovered active mud volcanoes on the nonaccretionary Mariana convergent plate margin are erupting slab-derived fluids, serpentine mud, and metamorphosed rocks from depths of as great as 25 km. Blueschist materials from the metamorphosed subducted plate are contained in the muds. Pore fluids indicate a depth dependence for decarbonation. The mud volcanoes record in situ conditions along the decollement, including pressure and temperature conditions and physical properties within the subduction zone. Similar mud-flow material occurs worldwide as “sedimentary serpentinite” deposits in accreted fragments of former convergent margins, making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized.

Blueschist metamorphism in an active subduction zone

THE high-pressure, low-temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones, where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures. Until now, however, the link between blueschist-facies rocks and subduction zones has been indirect, relying on a spatial association of blueschists with old subduction complexes, and estimates of the geothermal gradients likely to exist in subduction zones. Here we strengthen this link, by reporting the discovery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxene and blue amphibole) in clasts from a serpentinite seamount in the forearc of the active Mariana subduction zone. The metamorphic conditions estimated from the mineral compositions are 150–250 °C and 5–6 kbar (16–20 km depth). The rocks must have been entrained in rising serpentine mud diapirs, and extruded from mud volcanoes onto the sea floor. Further study of these rocks may provide new insight into the tectonics of trench-forearc systems, and in particular, the processes by which blueschist-facies clasts come to be associated with forearc sediments in ancient subduction complexes.