Lau Basin Research Articles

Lineament Extraction from SPOT 5 and NigeriaSat-X Imagery of the Upper Benue Trough, Nigeria

Abstract. The Upper Benue Trough is part of the Benue Trough of Nigeria and is comprised of three basins: the east–west trending Yola Basin (Yola Arm), the north–south trending Gongola Basin (Gongola Arm) and the northeast–southwest trending Lau Basin (Main Arm). This research is an ongoing research at understanding the structural framework of the Upper Benue Trough using several techniques including the use of Remote Sensing and GIS. Several digital image enhancement techniques such as general contrast stretching and edge enhancement were applied to the NigeriaSat-X and SPOT 5 image in ERDAS IMAGINE 9.2 after which structures were mapped out on-screen using ArcMap 10. The Digital Elevation Model (DEM) of the Trough was also used to enhance geomorphic features. The analysis carried out on the images revealed that lineaments are abundant in the Upper Benue Trough and they can be subdivided into four major trends, NE–SW, NW–SE, W–E and N–S in order of abundance and range in length from about 300 m to 26 km. Several faults were also mapped out within the Basin such as a sinistral fault around Bakoreji village in Bauchi, a dextral fault close to Kalmai town in Gombe and a dextral fault close to Wong in Taraba. It was discovered that some of the sites where minerals such as lead and zinc ores are being mined occur in the zones of high lineament density. This study shows the capability of the DEM, SPOT 5 and NigeriaSat-X images for lineament/structural interpretations.

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

Eruptive modes and hiatus of volcanism at West Mata seamount, NE Lau basin: 1996-2012

We present multiple lines of evidence for years to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau basin over a 16 year period, and a hiatus in summit eruptions from early 2011 to at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009, and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996–2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ∼2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996–2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Reconciling mantle attenuation-temperature relationships from seismology, petrology, and laboratory measurements

Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt-free rocks, but require ∼102−103 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation ( QS−1), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (QS ∼ 25–80) at 1 Hz and depths of 50–100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300–1450°C. The QS measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict QS too high by a factor of 1.5–5. Modifying models to include effects of H2O and rheology-dependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H2O effects are modest. Therefore, high H2O in the arc source region cannot explain the low QS, nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave QS can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.

남서태평양 라우분지 TA19-1 해산과 TA19-2 해산의 지자기 특성 연구

We conducted the geophysical survey of possible hydrothermal vent sites at 2009, in the Lau Basin, the south western Pacific and analyzed the magnetic characteristics of TA19-1 and TA19-2 seamounts. TA19-1 is a cone-shaped seamount with a caldera summit. TA19-2 seamount is bigger and shows more complicated topography than TA19-1 seamount. TA19-2 has a large caldera, a summit in the west side of the caldera and several crests. Simple dipole anomalies with a high over the north and a low over the south occur in TA19-1 seamount. High magnetic anomalies are located in the northern flank and the summit of TA19-2 seamount and low anomalies around the summit and the caldera. The results of bathymetry and magnetic data suggest that TA19-2 seamount might have more complicated magmatic process than TA19-1. Low magnetization zones are located over the summit, the calderas and the caldera rims. The magnetization lows indicate that submarine hydrothermal vents, along faults and fracture zones, could have caused an alteration of magnetic minerals. The magnetization highs over the summit and the calderas might have been related with later magmatisms like volcanic sills, intrusions.

Hydroacoustic investigation of submarine landslides at West Mata volcano, Lau Basin

AbstractSubmarine landslides are an important process in volcano growth yet are rarely observed and poorly understood. We show that landslides occur frequently in association with the eruption of West Mata volcano in the NE Lau Basin. These events are identifiable in hydroacoustic data recorded between ~5 and 20 km from the volcano and may be recognized in spectrograms by the weak and strong powers at specific frequencies generated by multipathing of sound waves. The summation of direct and surface‐reflected arrivals causes interference patterns in the spectrum that change with time as the landslide propagates. Observed frequencies are consistent with propagation down the volcano's north flank in an area known to have experienced mass wasting in the past. These data allow us to estimate the distance traveled by West Mata landslides and show that they travel at average speeds of ~10–25 m/s.

Community succession in hydrothermal vent habitats of the Eastern Lau Spreading Center and Valu Fa Ridge, Tonga

Patterns of succession in Lau Basin hydrothermal vent communities determined with high‐resolution imagery and in situ physico—chemical data collected over 4 yr and analyzed within a Geographic Information System show that Alviniconcha snails are a pioneering group, the snail Ifremeria nautilei is a mid‐successional species, and the heat‐intolerant mussel Bathymodiolus brevior dominates when venting declines. The associated fauna also changes as communities progress through the successional stages, and eventually non‐vent—endemic deep‐sea species appear when venting has mostly subsided. This is a unique example of primary succession in which the primary producers form symbiotic associations with mobile animals, resulting in successional patterns not observed in other systems. I. nautilei dominates newly formed substrates or venting sources where both I. nautilei and Alviniconcha spp. are already established (e.g., by migration), while Alviniconcha spp. seem to be better at colonizing newly active vents (e.g., by settlement) that are remote from colonized vents. Thus, on the scale of a 5–39 m2 diffuse flow area or a single edifice, the mid‐successional species dominates new substrates instead of the pioneering group. These communities are remarkably stable over long time periods relative to other hydrothermal vent regions. In addition to the sequential replacements of species as sites age and overall conditions change, Lau vent animals track changes in vent fluids and relocate themselves when local hydrothermal plumbing changes over small spatial scales.

Comparative population structure of two deep-sea hydrothermal-vent-associated decapods (Chorocaris sp. 2 and Munidopsis lauensis) from southwestern Pacific back-arc basins.

Studies of genetic connectivity and population structure in deep-sea chemosynthetic ecosystems often focus on endosymbiont-hosting species that are directly dependent on chemical energy extracted from vent effluent for survival. Relatively little attention has been paid to vent-associated species that are not exclusively dependent on chemosynthetic ecosystems. Here we assess connectivity and population structure of two vent-associated invertebrates—the shrimp Chorocaris sp. 2 and the squat lobster Munidopsis lauensis—that are common at deep-sea hydrothermal vents in the western Pacific. While Chorocaris sp. 2 has only been observed at hydrothermal vent sites, M. lauensis can be found throughout the deep sea but occurs in higher abundance around the periphery of active vents We sequenced mitochondrial COI genes and deployed nuclear microsatellite markers for both species at three sites in Manus Basin and either North Fiji Basin (Chorocaris sp. 2) or Lau Basin (Munidopsis lauensis). We assessed genetic differentiation across a range of spatial scales, from approximately 2.5 km to more than 3000 km. Population structure for Chorocaris sp. 2 was comparable to that of the vent-associated snail Ifremeria nautilei, with a single seemingly well-mixed population within Manus Basin that is genetically differentiated from conspecifics in North Fiji Basin. Population structure for Munidopsis lauensis was more complex, with two genetically differentiated populations in Manus Basin and a third well-differentiated population in Lau Basin. The unexpectedly high level of genetic differentiation between M. lauensis populations in Manus Basin deserves further study since it has implications for conservation and management of diversity in deep-sea hydrothermal vent ecosystems.

Seismic anisotropy predicted for 2-D plate-driven flow in the Lau back–arc basin

The Lau back–arc basin and spreading centers exhibit significant along-strike variations in plate motion rates and distance between the spreading centers and the Tonga trench. The mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint. Linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy. Three cross-basin transects allow us to isolate the effects of trench-spreading center distance as well as rates of convergence and spreading. Lattice preferred orientations (LPO) corresponding to each transect’s 2-D flow field are calculated for peridotite polycrystals. Predicted P-wave anisotropy of several percent and shear-wave split times up to 3.5s are determined, with measurable differences predicted between transects. Variation in the 2-D estimates of flow driven by each transect’s plate kinematics will induce along-strike flow in the mantle wedge. We find that the rate of such flow would likely not alter the subduction/spreading induced LPO sufficiently to explain the reported trench/arc parallel fast seismic directions in the Lau Basin; additional geodynamic factors must play a role.

Subduction-related halogens (Cl, Br and I) and H2O in magmatic glasses from Southwest Pacific Backarc Basins

Submarine magmatic glasses from the Manus, Woodlark, North Fiji and Lau backarc basins in the Southwest Pacific, and Volcano A on the volcanic front of the Tonga Arc adjacent to the Lau Basin, were investigated to characterise the Cl, Br and I elemental budgets in subduction systems. In particular we seek to determine the extent of variability in the Br/Cl and I/Cl ratios of backarc basin basalts (BABB) and evaluate if these ratios could improve constraints on the source of subducted volatile components in backarc basins worldwide. The selected glasses represent variably evolved melts of boninite, basalt, basaltic-andesite, dacite and rhyolite composition and were selected from spreading centres and seamounts located at varying distances from the associated arcs. In general the strongest subduction signatures (e.g. Ba/Nb of 100–370) occur in the samples closest to the arcs and lower more MORB-like Ba/Nb of <16 are found in the more distal samples. The glasses investigated have extremely variable halogen concentrations (e.g. 3–4200 ppm Cl), with the highest concentrations in enriched glasses with the most evolved compositions. As observed in previous studies, the K/Cl, Br/Cl and I/Cl ratios of glasses from individual settings do not vary as a function of MgO and are considered representative of the magma sources because these ratios are not easily altered by partial melting or fractional crystallisation. Systematic variations in these ratios between basins can therefore be related to mixing of halogens from different sources including: (i) the mantle wedge which has MORB-like Br/Cl and I/Cl; (ii) a subduction-derived slab fluid with estimated salinity of ∼4–10 wt% salts and variable I/Cl; and (iii) brines characterised by salinities of 55±15 wt% salts and Br/Cl slightly higher than seawater, that are sometimes assimilated in crustal magma chambers. The slab fluids enriching the Woodlark Basin, North Fiji Basin and the Fonualei Spreading Centre of the Lau Basin have MORB-like I/Cl and Br/Cl overlapping the lower end of the MORB range, indicating a probable source from dehydration of altered ocean crust (AOC). In contrast, slab fluids with I/Cl ratios of up to 10 times the MORB value were detected in BABB from Manus Basin, the Valu Fa Ridge and the Tonga Arc, and in these cases the elevated I/Cl ratios are most easily explained by the involvement of fluids released by breakdown of I-rich serpentinites. The data show that slab fluids vary in composition across the Tonga Arc and from north to south in the Lau Basin. However, the compositional range of subducted halogens overlaps with that of MORB indicating subduction could be a major source of halogens in the Earth's mantle.

Methane sources and sinks in the subtropical South Pacific along 17°S as traced by stable isotope ratios

We analyzed the concentration and stable carbon isotopic ratio (δ13C-CH4) of methane in the atmosphere and in dissolved methane in water column along 17°S in the subtropical South Pacific. Additionally, the hydrogen isotopic ratios (δD-CH4) of some water samples were analyzed. The sea–air CH4 flux is high in the eastern region and off the west coast of Australia, which is related to the high concentrations of dissolved CH4 and high wind speeds. Moreover, there is a positive correlation between the CH4 and chlorophyll a concentrations at the surface. This consistency suggested that active CH4 productions related to the primary production cause surface CH4 accumulation. CH4 shows a decrease in concentration and an increase in δ13C-CH4 and δD-CH4 values from the surface to the depth of about 1000m. The relationship between δ13C-CH4 values and CH4 concentration indicates that the isotopic enrichment of CH4 reflects microbial oxidation of CH4 with isotopic fractionation during vertical transport via vertical sinking and/or zooplankton migration. East of 120°W, δ13C-CH4 values at around 1000m exceed −30.0‰. The relationships among the δ13C-CH4 values, CH4 concentrations, and oxygen concentrations indicate that the 13C-enriched CH4 originates not only from in situ CH4 production and oxidation but also from CH4 transported from the eastern margin off Peru. Furthermore, at a site near the Central Lau Spreading Centers in the Lau Basin, high δ13C-CH4 values (up to −21.4‰) are observed in the benthic water, suggesting a hydrothermal field source.

Evidence for a broadly distributed Samoan-plume signature in the northern Lau and North Fiji Basins

Geochemical enrichment of lavas in the northern Lau Basin may reflect the influx of Samoan-plume mantle into the region. We report major and trace element abundances and He-Sr-Nd-Hf-Pb-isotopic measurements for 23 submarine volcanic glasses covering 10 locations in the northern Lau and North Fiji Basins, and for three samples from Wallis Island, which lies between Samoa and the Lau Basin. These data extend the western limit of geochemical observations in the Basins and improve the resolution of North-South variations in isotopic ratios. The Samoan hot spot track runs along the length of the northern trace of the Lau and North Fiji Basins. We find evidence for a Samoan-plume component in lavas as far West as South Pandora Ridge (SPR), North Fiji Basin. Isotopic signatures in SPR samples are similar to those found in Samoan Upolu shield lavas, but show a slight shift toward MORB-like compositions. We explain the origin of the enriched signatures by a model in which Samoan-plume material and ambient depleted mantle undergo decompression melting during upwelling after transiting from beneath the thick Pacific lithosphere to beneath the thin lithosphere in the northern Lau and North Fiji Basins. Other lavas found in the region with highly depleted isotopic signatures may represent isolated pockets of depleted mantle in the basins that evaded this enrichment process. We further find that mixing between the two components in our model, a variably degassed high-3He/4He Samoan component and depleted MORB, can explain the diversity among geochemical data from the northern Lau Basin.

Petrologic perspectives on tectonic evolution of a nascent basin (Okinawa Trough) behind Ryukyu Arc: A review

Okinawa Trough is a back-arc, initial marginal sea basin, located behind the Ryukyu Arc–Trench System. The formation and evolution of the Okinawa Trough is intimately related to the subduction process of the Philippine Sea Plate beneath the Eurasian Plate since the late Miocene. The tectonic evolution of the trough is similar to other active back-arcs, such as the Mariana Trough and southern Lau Basin, all of which are experiencing the initial rifting and subsequent spreading process. This study reviews all petrologic and geochemical data of mafic volcanic lavas from the Okinawa Trough, Ryukyu Arc, and Philippine Sea Plate, combined with geophysical data to indicate the relationship between the subduction sources (input) and arc or back-arc magmas (output) in the Philippine Sea Plate–Ryukyu Arc–Okinawa Trough system (PROS). The results obtained showed that several components were variably involved in the petrogenesis of the Okinawa Trough lavas: sub-continental lithospheric mantle underlying the Eurasian Plate, Indian mid-oceanic ridge basalt (MORB)-type mantle, and Pacific MORB-type mantle. The addition of shallow aqueous fluids and deep hydrous melts from subducted components with the characteristics of Indian MORB-type mantle into the mantle source of lavas variably modifies the primitive mantle wedge beneath the Ryukyu and subcontinental lithospheric mantle (SCLM) beneath the Okinawa Trough. In the northeastern end of the trough and arc, instead of Indian MORB-type mantle, Pacific MORB-type mantle dominates the magma source. Along the strike of the Ryukyu Arc and Okinawa Trough, the systematic variations in trace element ratios and isotopic compositions reflect the first-order effect of variable subduction input on the magma source. In general, petrologic data, combined with geophysical data, imply that the Okinawa Trough is experiencing the “seafloor spreading” process in the southwest segment, “rift propagation” process in the middle segment, and “crustal extension” process in the northeast segment, and a nascent ocean basin occurs in the southwest segment.

Seismological study of Lau back arc crust: Mantle water, magmatic differentiation, and a compositionally zoned basin

On the basis of seismic tomography analyses, a structural model of crust formed along the Eastern Lau Spreading Center (ELSC) is presented as evidence for a transition from a “hydrous” type of oceanic crust to a more typical oceanic crust. The seismic data indicate that as the spreading center moved away from the active arc, the crust thinned from 8–9 km to ∼7 km, the lower crust changed from high P wave velocity values (7.2–7.4 km/s) to typical values for oceanic crust (7.0–7.2 km/s), and the upper-crustal volcanic layer changed from a thick low-velocity layer to a thinner layer with more typical wave speeds. The seismic results, in combination with other geophysical and geochemical data, suggest that crustal formation along the ELSC is strongly controlled by the influence of slab water. When a spreading center is near the active arc, water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting. Thereafter, the water enhances crustal differentiation within sub-ridge magma chambers. This creates an anomalous “hydrous” form of oceanic crust with a thick felsic volcanic layer and a mafic/ultramafic lower crust – features that are not typically observed in crust formed at mid-ocean ridges. The Lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust, which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc. The abundance and high rate of production of the “hydrous” crust suggests that such crust may make up a significant proportion of the arc-like crust that forms continents.

Nanoparticulate pyrite and other nanoparticles are a widespread component of hydrothermal vent black smoker emissions

The presence of nanoparticulate pyrite is reported in hydrothermal emissions from Rainbow, TAG and Snakepit on the Mid-Atlantic Ridge (MAR). When coupled with previously collected data from East Pacific Rise 9°N (EPR) and Lau Basin, these data demonstrate that pyrite nanoparticles are a widespread component of black smoker emissions from hydrothermal vents and are found in significant concentrations at a fast spreading mid-ocean Ridge (EPR), a back-arc basin (Lau Basin), and a slow spreading mid-ocean Ridge (MAR). The maximum percentage of filtered iron emitted as nanoparticulate pyrite was found to be as high as 25%, 10%, and 5%, respectively. As a widespread component of hydrothermal vent emissions, these nanoparticles may be an important source of iron to the world's oceans. Metals such as Cu and Zn are detected in pyrite-containing aggregates at all sites, and chalcopyrite was a component of nanoparticle aggregates at MAR. Iron containing silicate nanoparticles are also identified, and indicate that nanoparticles other than sulfides should also be considered when determining transport implications of hydrothermal vent emissions. The varied morphologies and the presence of different minerals within these nanoparticles provide insight into their formation and stability.

Acoustic response of submarine volcanoes in the Tofua Arc and northern Lau Basin to two great earthquakes

Acoustic response of submarine volcanoes in the Tofua Arc and northern Lau Basin to two great earthquakes

Vulcanolepas scotiaensis sp. nov., a new deep-sea scalpelliform barnacle (Eolepadidae: Neolepadinae) from hydrothermal vents in the Scotia Sea, Antarctica.

A new deep-sea stalked barnacle, Vulcanolepas scotiaensis sp. nov. is described from hydrothermal vents at depths of 2400-2600 metres along segments of the East Scotia Ridge and from 1400 metres in the Kemp Caldera. Both locations are areas of volcanic activity that lie on the Antarctic-South American Ocean Ridge complex near the South Sandwich Islands. This discovery confirms a wide distribution in southern seas for Vulcanolepas, complementing the previous records from deep-sea vents in the Lau Basin and Kermadec Ridge in the southwest Pacific, and the Pacific Antarctic Ridge in the southeast Pacific. V. scotiaensis sp. nov., the third described species of Vulcanolepas shows an extraordinary range in morphology, requiring a reassessment of the original diagnosis for Vulcanolepas. Although the morphological envelope of V. scotiaensis sp. nov. includes representatives with a peduncle to capitulum ratio similar to that observed in most neolepadines, the peduncle generally shows greater proportional length than in species in any neolepadine genus except Leucolepas; it is distinguished from other species of Vulcanolepas by a broader capitulum, much smaller imbricating scales on the peduncle and more ornamented capitulum plates. The morphological diversity of V. scotiaensis sp. nov. is interpreted as having arisen due to abrupt changes in water temperature.LSID: urn:lsid:zoobank.org:act:AA2AFDA5-0B08-466A-A584-D3FDBDE9DA61.

Geoid low and highs of the Indian Ocean and Western Pacific: Implications to mantle convection

As geoid anomalies have capacity to unravel deep-seated density structures, it has acquired special significance in recent years for this purpose. Geoid anomalies of India and SE Asia are characterized by the Indian Ocean geoid low and geoid highs of the Western Pacific. The spectral analysis of the Indian Ocean geoid low (IOGL) suggests depths of ∼1300, ∼700 and ∼340km for relatively large wavelength components. They are related to middle mantle and transition zones in the upper mantle, respectively where olivine changes to spinel with sharp increase in the velocity and density. This geoid low is caused by the subducted Indian lithosphere that rolled back after subduction and got accumulated in this section. In this process, the subducting slab would acquire same temperature as surroundings and would be characterized by low velocity and density as it primarily consists of the low density rocks of continental lithosphere. The dehydration of subducting slab of different temperatures at different levels would set in convection cells. In this case, mid-to-upper mantle circulation (convection) is envisaged that has caused Central Indian Ocean Deformation Zone with large scale faults, folds, high heat flow and seismic activity. Presence of such convection at the tail end of the Indian plate drives it relatively faster (∼5cm/year). The IOGL may mark the site of first collision between the Indian and the Asian plates.The geoid highs of the Western Pacific coincide with the subduction zones in this sector. Its spectral analysis provided average depths to the large wavelength sources as ∼2335, ∼845 and ∼523km. They are attributed to sources above D″ layer, and the transition zones in the upper-middle mantle within the error limit of 10–15% depending on the wavelengths. Deeper level of sources for the geoid highs of Pacific Ocean may be attributed to high angle of subduction in this sector where cold subducting slab of high velocity and density can get easily detached and sink into the lower mantle. In this case due to high angle of subduction of oceanic lithosphere, the subducting slab would be at lower temperature compared to surroundings maintaining it brittle even in the transition zone that has given rise to deep focus earthquakes almost up to 700km. The high angle subduction along the subduction zones in the Western Pacific causes faster subduction that drives the Pacific plate faster (∼4–10cm/year). In this case, the dehydration of the subducting slab would set in whole mantle convection between the lower and the upper mantle in the back arc regions of the mantle wedge. It has plausibly given rise to extensional regime forming back arc basins on large scale such as Japan Sea, Philippine Sea, Lau basin and others.

Quantifying brine assimilation by submarine magmas: Examples from the Galápagos Spreading Centre and Lau Basin

Volatiles are critically important in controlling the chemical and physical properties of the mantle. However, determining mantle volatile abundances via the preferred proxy of submarine volcanic glass can be hampered by seawater assimilation. This study shows how combined Cl, Br, I, K and H2O abundances can be used to unambiguously constrain the dominant mechanism by which melts assimilate seawater-derived components, and provide an improved method for determining mantle H2O and Cl abundances. We demonstrate that melts from the northwest part of the Lau Basin, the Galápagos Spreading Centre and melts from other locations previously shown to have anomalously high Cl contents, all assimilated excess Cl and H2O from ultra-saline brines with estimated salinities of 55±15wt.% salts. Assimilation probably occurs at depths of ∼3–6km in the crust when seawater-derived fluids come into direct contact with deep magmas. In addition to their ultra-high salinity, the brines are characterised by K/Cl of <0.2, I/Cl of close to the seawater value (∼3×10−6) and distinctive Br/Cl ratios of 3.7–3.9×10−3, that are higher than both the seawater value of 3.5×10−3 and the range of Br/Cl in 43 pristine E-MORB and OIB glasses that are considered representative of diverse mantle reservoirs [Br/Clmantle=(2.8±0.6)×10−3 and I/Clmantle=(60±30)×10−6 (2σ)]. The ultra-saline brines, with characteristically elevated Br/Cl ratios, are produced by a combination of fluid-rock reactions during crustal hydration and hydrothermal boiling. The relative importance of these processes is unknown; however, it is envisaged that a vapour phase will be boiled off when crustal fluids are heated to magmatic temperatures during assimilation. Furthermore, the ultra-high salinity of the residual brine that is assimilated may be partly determined by the relative solubilities of H2O and Cl in basaltic melts. The most contaminated glasses from the Galápagos Spreading Centre and Lau Basin have assimilated ∼95% of their total Cl and up to 35–40% of their total H2O, equivalent to the melts assimilating 1000–2000ppm brine at an early stage of their evolution. Dacite glasses from Galapagos contain even higher concentrations of brine components (e.g. 12,000ppm), but the H2O and Cl in these melts was probably concentrated by fractional crystallisation after assimilation. The Cl, Br, I and K data presented here confirm the proportion of seawater-derived volatiles assimilated by submarine magmas can vary from zero to nearly 100%, and that assimilation is closely related to hydrothermal activity. Assimilation of seawater components has previously been recognised as a possible source of atmospheric noble gases in basalt glasses. However, hydrothermal brines have metal and helium concentrations up to hundreds of times greater than seawater, and brine assimilation could also influence the helium isotope systematics of some submarine glasses.

Comparison of pyrite (FeS2) synthesis mechanisms to reproduce natural FeS2 nanoparticles found at hydrothermal vents

Here we report the synthesis of pyrite in micrometer and nanometer sizes and with varied morphology. The purpose of these syntheses was to mimic natural pyrite nanoparticles recently found at hydrothermal vents, and to better understand the formation processes of these nano and micrometer sized particles. The duration of synthesis, surfactant used, and chemical mechanism of pyrite formation were all found to have an effect on resultant size and morphology. Yields of pyrite were sometimes low as our goal was to stop growth quickly after nucleation, in order to mimic hot vent fluids (350°C) emanating to cold ocean bottom waters (2°C). Natural nanoparticles forming under these conditions have currently been identified at diverse hydrothermal vents including Lau Basin, the Mid Atlantic Ridge (MAR), and the East Pacific Rise (EPR). These natural nanoparticles were variable in size and morphology, with aggregates of 50–350nm diameter being the most prevalent. Syntheses occurring via the H2S pathway mimicked natural nanoparticles more closely than syntheses via the polysulfide pathway. One synthesis reported is suggestive of a pyrite/pyrrhotite equilibrium, indicating that we have succeeded in mimicking redox conditions similar to those that occur at certain hydrothermal vents.