Lau Back-arc Basin Research Articles

Controls on Heat Flow, Fluid Migration, and Massive Sulfide Formation of an Off-axis Hydrothermal System-- the Lau Basin Perspective

A numerical model has been developed to investigate heat and fluid migration in a modern off-axis seafloor hydrothermal system, that is, hydrothermal activity and fluid discharge distal to an axial magma chamber emphasizing model geometry, rock/fault properties and fault distribution. The model is based on geophysical data and seafloor observations of the Lau back-arc basin and results suggest a different hydrothermal convection scheme than axial hydrothermal systems. Major hydrothermal activity is predicted to occur at topographic highs due to significant fluid migration along inferred basement topography off-axis with associated permeability differences. Major hydrothermal fluid discharge occurs at off-axis topographic elevated positions with temperatures (150°C - 450°C) and exit fluid velocities (∼4 m/s), in good agreement with seafloor observation and theoretical calculations. Heuristic mass calculations pertaining to the formation of massive sulfide deposits imply that a significant base metal sulfide deposit (5 Mt at 10% Cu + Zn) may form in less than 6,000 years, assuming a fluid containing a maximum of 10 ppm base metals and a deposition efficiency of 10 percent. The size and distribution patterns of massive sulfide deposits are determined primarily by fault distribution, provided that adequate fluid flow pathways and heat supply exist.

Formation of massive sulfide ore deposits on the seafloor —constraints from numerical heat and fluid flow modeling

Numerical simulations of seafloor hydrothermal systems based on the Lau back-arc basin were carried out to investigateconditions that lead to the formation of economic massive sulfide ore deposits in seafloor environments. Fluid discharge behavior, fluid exit temperatures and fluid velocities depend primarily on rock and fault properties and to a lesser degree on fault distribution and surface/basement topography. Ideal conditions promoting vigorous hydrothermal convection and effective leaching of base metals are found in highly permeable fresh rocks with an effective fault system. High-temperature (350—400 °C), high-velocity (>5 × 10 -7 m/s) hydrothermal fluids capable of forming significant massive sulfide ore bodies occur near topographic highs on the flanks of the basin structure and significant base metal ore bodies (>1 Mt ore-grade material) can be formed in 700–3000 years with fluids containing 10 ppm combined base metals and a 10% deposition efficiency.

Tectonic implications of boninite, arc tholeiite, and MORB magma types in the Josephine Ophiolite, California-Oregon

Abstract The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.

Mantle wedge control on back-arc crustal accretion.

At mid-ocean ridges, plate separation leads to upward advection and pressure-release partial melting of fertile mantle material; the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away. In back-arc basins, the subducting slab is an important control on the pattern of mantle advection and melt extraction, as well as on compositional and fluid gradients. Modelling studies predict significant mantle wedge effects on back-arc spreading processes. Here we show that various spreading centres in the Lau back-arc basin exhibit enhanced, diminished or normal magma supply, which correlates with distance from the arc volcanic front but not with spreading rate. To explain this correlation we propose that depleted upper-mantle material, generated by melt extraction in the mantle wedge, is overturned and re-introduced beneath the back-arc basin by subduction-induced corner flow. The spreading centres experience enhanced melt delivery near the volcanic front, diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away. Our model explains fundamental differences in crustal accretion variables between back-arc and mid-ocean settings.

Three-dimensional crustal structure and magma chamber geometry at the intermediate-spreading, back-arc Valu Fa Ridge, Lau Basin-results of a wide-angle seismic tomographic inversion

In this paper the results of a tomographic analysis of a 3-D wide-angle seismic refraction data set acquired at the Valu Fa Ridge (VFR) in 1995 are presented. The VFR is an intermediate-spreading ridge located in the southern Lau back-arc basin in the southwest Pacific. The ridge comprises three morphological segments, the Southern, Central and Northern Valu Fa Ridges, separated by overlapping spreading centres (OSCs). Previous seismic experiments have identified a robust axial magmatic system beneath the central segment (CVFR) and the OSC with the northern segment (NVFR). The experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post-rift crust. A regularized inversion scheme that minimizes model roughness was applied to the first-arrival traveltime picks made from the wide-angle data. A quantitative approach for determining data uncertainties is described based on the signal-to-noise ratio of the arrivals. Several initial model assumptions were tested, including one with a thin melt lens, representing a seismic reflector identified in previous studies, explicitly included in the initial model. The inversion results suggest that crustal layer 2 exhibits northward thickening, which mirrors a similar northward thickening of the whole crust. In addition, local thinning of layer 2 is identified in the vicinity of the boundary between pre- and post-rift crust, which is thought to represent thinning of the crust prior to the onset of rifting. Axial low-velocity anomalies are identified in layer 2B/C and layer 3. The models are consistent with a continuous ? 6 km wide negative velocity anomaly in layer 3 with an amplitude of ? 0.7-0.9 km s-1 relative to off-axis post-rift crust. This anomaly is consistent with the presence of an axial mush zone comprising a small percentage (< 1 per cent) of partial melt. The negative velocity anomaly in layer 2B/C is modelled with its largest amplitude (?0.5 km s-1) beneath the northern OSC. Possible origins for this anomaly include locally thicker crust or locally higher porosity near the OSC, or a high-temperature anomaly associated with the axial magmatic system.

A new genus and species of barnacle (Cirripedia, Verrucomorpha) associated with vents of the Lau Back-Arc Basin: its gross morphology, inferred first juvenile stage and affinities

A new genus and species of barnacle (Cirripedia, Verrucomorpha) associated with vents of the Lau Back-Arc Basin: its gross morphology, inferred first juvenile stage and affinities

Constraints on the origin of slab and mantle wedge anomalies in Tonga from the ratio of S to P velocities

We examine two prominent upper mantle velocity anomalies in the southwest Pacific, the Tonga slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land‐sea deployment of temporary seismometers. The linear geometry and small interstation spacing of the instruments yield high‐resolution data along a cross section of the Tonga subduction zone, including the actively spreading Lau back arc basin. We estimate the relative variation of P and S velocity, often described as ν = δlnVs/δlnVp, for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residuals, and detailed modeling of the velocity structure using a three‐dimensional finite difference travel time algorithm. The two methods yield similar results, with ν of the slab being 1.1–1.5 and ν of the mantle wedge being 1.2–1.3. These values are consistent with experimental data concerning the effect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presence of partial melt. These observations imply that either the theoretical estimates of ν for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly removed from the rock matrix, such that the velocity anomalies are due to increased temperature, and not melt. The bulk of the velocity anomaly in the mantle wedge can be explained by temperature anomalies of 400–600°C because of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back arc basin, can still leave the mantle near the solidus, even after accounting for the effect of increased volatile content in the mantle wedge. The lower‐amplitude velocity reductions in the deeper wedge are likely related to an increased concentration of volatiles from the subducting slab.

Modeling the Tonga slab: Can travel time data resolve a metastable olivine wedge?

We present the results of detailed modeling of the Tonga slab with the goals of determining whether high‐resolution travel time data (1) can be fit by simple thermal and petrological slab models and (2) can resolve a metastable olivine wedge at depths greater than the equilibrium olivine‐spinel phase boundary. We model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers (OBS) and island broadband seismic stations extending from the Lau backarc basin, across the Tonga trench and onto the Pacific plate. The data consist of 388 local, P wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3‐month OBS deployment in late 1994. We locate the events using both local and teleseismic arrival times, and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference Earth model. The modeling consists of grid searches using a three‐dimensional finite difference algorithm to compute local, first arriving travel times for equilibrium and metastable P wave velocity models constructed from thermal, mineralogical, and morphological constraints. The travel time anomalies are well fit by standard slab thermal models and P velocity temperature derivatives of −0.4 to −0.3 ms−1°C−1. Forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low‐velocity region. Wedge velocity models provide a slightly better fit to the data than equilibrium models, but F tests indicate the improvement is not significant at the 95% level. Our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary.

Attenuation of Broadband P and S Waves in Tonga: Observations of Frequency Dependent Q

Teleseismic broadband recordings of intermediate and deep focus earthquakes are used to quantify both compression (Q α) and shear (Q β) wave attenuation within the Lau backarc basin. A spectral-ratio method is employed to measure differential attenuation (δt*) between the depth phases sS, pP, and sP and the direct S and P phases over the frequency band 0.05 and 0.5 Hz. We use a stacking algorithm to combine the spectra of several phase pairs from a single event, having similar azimuth and range, to obtain more robust δt* measurements; these estimates are then used to compute the average Q above the focal depth. Q β and Qα are measured directly from the sS-S and pP-P phase pairs respectively, however, the interpretation of δt* measured from sP-P requires assumptions about the ratio Q α/Q β. We find an empirical ratio of Q α/Q β = 1.93 for this region and use it to compute Qα and Q β from the Q sP observations.

Sedimentary evidence relating to the tectonic evolution of the Lau Basin, SW Pacific, from ODP Sites 834–839 (ODP Leg 135)

Abstract Six sites were drilled during Ocean Drilling Program Leg 135 in the western Lau backarc basin in the southwest Pacific (Sites 834–839). These sites are all located in basins within a horst and graben terrain and form an approximate transect across the rifted arc basement onto crust considered to have been formed at the East Lau Spreading Centre. The sedimentary sequences recovered from these backarc sites range in age from the late Miocene to the Holocene; they consist primarily of a lower succession of volcaniclastic sediment gravity-flow deposits interbedded with hemipelagic clayey nannofossil oozes and nannofossil clays, overlain by a distinctive upper succession of hydrothermally stained hemipelagic, and locally redeposited, clayey nannofossil oozes. The volcaniclastic sediment gravity-flow deposits are predominantly massive, proximal, vitric gravels, sands and silts, that are mainly locally derived from adjacent basement ridges and intrabasin seamount volcanoes. At Site 835, which was drilled in a small extensional sub-basin in the oldest part of the Lau Basin, the upper clayey nannofossil ooze sequence is anomalously thick and rigorous sedimentological analysis shows that much of this sequence is redeposited. Thick clayey nannofossil ooze turbidite muds are identified that closely resemble the enclosing clayey nannofossil ooze hemipelagites. These thick turbidite muds are associated with mudclast conglomerates, interpreted as muddy debris-flow deposits, and a number of coherent rafted blocks of older hemipelagic sediment. These allochthonous deposits testify to several episodes of instability in the sub-basin that may be related to large-scale tectonic activity caused by the southward passage of ocean-ridge propagator tips past the latitude of the drillsite. Episodes of increased sediment deposition, due to increased frequency of turbidite emplacement, are evident at all of the Lau Basin backarc drillsites, and these correlate moderately well with the closest approach of the propagating ridge tip to each sub-basin. The research presented illustrates how turbidites, once identified, can be used to decipher complex tectonic histories, and demonstrates the importance of local tectonic controls on sediment redeposition in backarc basins.

Propagating rift tectonics of a Caledonian marginal basin: Multi-stage seafloor spreading history of the Solund-Stavfjord ophiolite in western Norway

The Late Ordovician Solund-Stavfjord ophiolite in western Norway represents a remnant of the Iapetus oceanic lithosphere that developed in a Caledonian marginal basin. The ophiolite contains three structural domains that display distinctively different crustal architecture that reflects the mode and nature of magmatic and tectonic processes operated during the multi-stage seafloor spreading evolution of this marginal basin. Domain I includes, from top to bottom, an extensive extrusive sequence, a transition zone consisting of dike swarms with screens of pillow breccias, a sheeted dike complex, and plutonic rocks composed mainly of isotropic gabbro and microgabbro. Extrusive rocks include pillow lavas, pillow breccias, and massive sheet flows and are locally sheared and mineralized, containing epidosites, sulfide-sulfate deposits, Fe-oxides, and anhydrite veins, reminiscent of hydrothermal alteration zones on the seafloor along modern mid-ocean ridges. A fossil lava lake in the northern part of the ophiolite consists of a >65-m-thick volcanic sequence composed of a number of separate massive lava units interlayered with pillow lavas and pillow breccia horizons. The NE-trending sheeted dike complex contains multiple intrusions of metabasaltic dikes with one- and two-sided chilled margins and displays a network of both dike-parallel normal and dike-perpendicular oblique-slip faults of oceanic origin. The dike-gabbro boundary is mutually intrusive and represents the root zone of the sheeted dike complex. The internal architecture and rock types of Domain I are analogous to those of intermediate-spreading oceanic crust at modern mid-ocean ridge environments. The ophiolitic units in Domain II include mainly sheeted dikes and plutonic rocks with a general NW structural grain and are commonly faulted against each other, although primary intrusive relations between the sheeted dikes and the gabbros are locally well preserved. The exposures of this domain occur only in the northern and southern parts of the ophiolite complex and are separated by the ENE-trending Domain III, in which isotropic to pegmatitic gabbros and dike swarms are plastically deformed along ENE-striking sinistral shear zones. These shear zones, which locally include fault slivers of serpentinite intrusions, are crosscut by N20°E-striking undeformed basaltic dike swarms that contain xenoliths of gabbroic material. The NW-trending sheeted dike complex in the northern part of Domain II curves into an ENE orientation approaching Domain III in the south. The anomalous nature of deformed crust in Domain III is interpreted to have developed within an oceanic fracture zone or transform fault boundary. REE chemistry of representative extrusive and dike rocks from all three domains indicates N- to E-MORB affinities of their magmas with high Th/Ta ratios that are characteristic of subduction zone environments. The magmatic evolution of Domain I encompasses closed-system fractional crystallization of high-Mg basaltic magmas in small ephemeral chambers, which gradually interconnected to form large chambers in which mixing of primary magmas with more evolved and fractionated magma caused resetting of magma compositions through time. The compositional range from high-Mg basalts to ferrobasalts within Domain I is reminiscent of modern propagating rift basalts. We interpret the NE-trending Domain I as a remnant of an intermediate-spread rift system that propagated northeastwards (in present coordinate system) into a pre-existing oceanic crust, which was developed along the NW-trending doomed rift (Domain II) in the marginal basin. The N20°E dikes laterally intruding into the anomalous oceanic crust in Domain III represent the tip of the rift propagator. The inferred propagating rift tectonics of the Solund-Stavfjord ophiolite is similar to the evolutionary history of the modern Lau back-arc basin in the SW Pacific and suggests a complex magmatic evolution of the Caledonian marginal basin via multi-stage seafloor spreading tectonics.

Sea-floor spreading in the Lau back-arc basin

We report the first 3-D magnetization inversion of bathymetry and magnetics data for the entire Lau Basin and confirm a history of southward-propagating sea-floor spreading during the past 4 Myr. Magnetic lineations, seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back-arc basin is fundamentally similar to that occurring on mid-ocean ridges. The Brunhes Chron spreading rates (65–100 mm/yr) are < 75% of the opening rates determined geodetically. In the absence of earthquake evidence for significant off-axis deformation between 18–21°S, we infer that opening rates have recently increased on the central and east Lau spreading centers.

Hydrothermal sedimentation at ODP Sites 834 and 835 in relation to crustal evolution of the Lau Backarc Basin

Abstract Hydrothermally metal-enriched sediments from ODP Leg 135 Sites 834 and 835 in the Lau Backarc Basin have been studied with respect to basin evolution. Multivariate statistics (factor analysis) indicate element associations in the non-detrital, carbonate-free sediment fraction characteristic of hydrothermal oxide plume fallout. Selective chemical leach, X-ray diffraction and lithological studies confirm that the hydrothermal enrichment in the sediments at both sites results from distal oxide plume fallout. No evidence of a sulphide/weathered sulphide component is seen, suggesting that the sites have never been proximal to a hydrothermal discharge source. Factor scores for the hydrothermal oxide phase of the sediments reflect the hydrothermal activity associated with the propagation of the Eastern Lau Spreading Centre (ELSC) into the basin. The subsequent propagation of the Central Lau Spreading Centre (CLSC) into the basin is not clearly recorded. Non-detrital element accumulation rates for Mn and Fe confirm this. However, the results suggest a ‘time lag’ between the closest passage of the ELSC past the latitude of Sites 834 and 835 and maximum hydrothermal plume fallout flux to the sediment section. This may be due to a delay between ridge formation and either the initiation of hydrothermal systems on the ridge and/or their full development. In addition, however, plume dispersion patterns may have been affected by palaeo-current directions and basin topography. Element accumulation rate values show that the hydrothermal flux from the ELSC was of much greater intensity than that from the CLSC. The latter was comparable with values in presently accumulating sediments proximal to the CLSC and within the range typical of East Pacific Rise (EPR) values. The ELSC accumulation rates are comparable with, or higher than, the highest reported values from the EPR. Compared with EPR sediments, the Lau Basin hydrothermal plume fallout is rich in Mn relative to Fe. In particular, plume fallout associated with the ELSC exhibits a two- to four-fold enrichment of Mn relative to Fe compared with the EPR.

A new sessile barnacle (Cirripedia, Brachylepadomorpha) from the Lau Back-Arc Basin, Tonga ; fïrst record of a living representative since the Miocene

Un representant actuel des Brachylepadomorpha, Neobrachylepas relica, n. gen., n. sp., a ete decouvert dans les sources hydrothermales du bassin de Lau. Le sous-ordre, comprenant les cirripedes les plus anciens, est apparu au Jurassique. Il subit ensuite une diversification modeste avant la fin du Cretace, a partir duquel il commenca a decliner en meme temps qu'apparaissaient et se diversifiaient les cirripedes modernes (Verrucomorpha et Balanomorpha), tout comme certains predateurs broyeurs de coquilles; il etait absent depuis cette mention fossile du Miocene. Ainsi, Neobrachylepas est unique parmi les cirripedes deja connus de sources hydrothermales: Neolepas (Scalpellomorpha), Neoverruca (Verrucomorpha) et Eochionelasmus (Balanomorpha), en etant le seul membre vivant connu plutot que simplement le plus primitif de ce sous-ordre. La faune du bassin de Lau inclut des representants des quatre sous-ordres et represente ainsi la faune de cirripedes la plus diversifiee de toutes les communautes connues de sources hydrothermales.

Extensional transform zones and oblique spreading centers

Extensional transform zones (ETZs) are plate boundary segments of order 100 km long that strike at angles between 15° and 45° to the extension direction. They are characterized by neovolcanic/tectonic zones comprising overlapping en echelon volcanic systems and/or faults that trend 30°–75° to the extension direction, sometimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate boundary is probably continuous. ETZs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years. ETZs link oblique spreading segments to transform faults in the Manus and probably the Lau backarc basins. The Reykjanes Peninsula and Tjornes Fracture Zone in Iceland and the Mak'Arrasou in Afar are ETZs that link subaerial to submarine spreading or rifting segments. The Brawley and Cerro‐Prieto seismic zones appear to be ETZs in the Imperial and Mexicali valleys that link the San Andreas, Imperial, and Cerro‐Prieto transform faults. Experimental and analytical models of transtensional deformation in brittle‐ductile systems match many of the observed characteristics of ETZs and oblique spreading centers, including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction. The contrasting fault patterns reflect the rheology of the models and lithosphere: they are more sigmoidal when the strain in the lower ductile layer is more focused, causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins. The angle (Ø) between the faults and the extension direction decreases with the angle (α) between the strike of the zone and the extension direction. ETZs occur in the range 15° ≤ α ≤ 45°, whereas oblique spreading centers have 45° &lt; α &lt; 90° and transform relay zones have 0° &lt; α &lt; 15°. Oblique fast spreading segments exhibit ridge‐parallel faults and volcanic systems (Ø = α), presumably reflecting locally rotated stress fields, whereas at oblique slow spreading centers, Ø is closer to orthogonal (α &lt; Ø &lt; 90°).

Volcanism and sedimentation in a rifting island-arc terrain: an example from Tonga, SW Pacific

Abstract Scientific drilling of narrow sub-basins within the Lau back-arc basin system of the SW Pacific has recovered uppermost Miocene to Recent volcaniclastic sediment and pelagic nannofossil oozes. Pliocene sediment gravity flow and turbidite sands from the western Lau Basin indicate a local source for the sediment, probably intrabasinal seamount volcanoes active during the initial stages of arc rifting. Derivation of abundant material from either the remnant volcanic arc (Lau Ridge) or the new Tofua arc is ruled out by the rugged topography of the basin and the proximal nature of the facies. Sediments from the Tonga platform, adjacent to the present day Tofua arc, indicate a peak in volcanic activity prior to and during the generation of the first back-arc basin crust at 5.25 Ma. A 2.0 Ma hiatus in arc volcanism on the trench side of the basin after rifting was brought to an end by the foundation of the Tofua arc at 3.0 Ma (Late Pliocene). On the basis of the sedimentary, geochemical and seismic data it is suggested that basin rifting involved an initial stage of extension of the original island arc, accompanied by volcanism in the form of major seamount volcanoes within the basin. These produced volcanic ash by submarine eruption, which was then reworked into adjacent sub-basins by slumping, gravity flow or turbidity current. Basin opening proceeded with a trenchward migration of extension and volcanism with time. This system was disrupted by the southward propagation of the Eastern Lau Spreading Centre into the southern Lau Basin at 1.5–1.0 Ma. This resulted in extension and volcanism being concentrated along the median valley of the spreading centre and a cessation in explosive volcanism of wide compositional range. Sedimentation in the Lau Basin since that time has been principally pelagic with minor ash layers mostly derived from the Tofua arc.

Noble gases in submarine pillow basalt glasses from the Lau Basin: Detection of a solar component in backarc basin basalts

Noble gas elemental and isotopic abundances have been analysed in eight samples of youthful basaltic glass dredged from three different locations within the Lau Backarc Basin: (1) the King's Triple Junction, (2) the Central Lau Spreading Centre at 18°S and (3) the Eastern Lau Spreading Centre at 19°S. Samples from the Lau central and eastern spreading centres have MORB-like helium isotopic ratios of approximately 1.2 × 10 −5 (8.5 R/R A). In contrast, the samples from the King's Triple Junction yield helium isotopic ratios averaging 9.4 (±0.8) × 10 −6 (6.7 ± 0.6 R/R A), systematically lower than the MORB-like value, which may be reflecting the addition of radiogenic 4He released from the descending slab. Neon isotopic ratios are enriched in 20Ne and 21Ne with respect to atmospheric ratios by as much as 23% and 62% respectively. These observations further confirm that non-atmospheric neon is a common characteristic of samples derived from the mantle. The helium and neon isotopic signatures in the samples can be explained by mixing of a primordial solar component, radiogenic and nucleogenic components produced by radioactive processes inside the Earth, and an atmospheric component. This reconnaissance survey of noble gases in a backarc basin indicates that current volcanism is dominated by magmas from the mantle wedge, a source similar to that from which MORBs are derived. The heavier noble gases (argon, krypton and xenon), however, show more atmosphere-like compositions, either indicating strong interaction of the magmas with the atmosphere or the presence of a recycled component derived from the underlying subducting slab.

Time-series of temperature from three deep-sea hydrothermal vent sites

Temperature time-series have been recorded with THYDRO, a new multi-probe temperature recorder, in three different hydrothermal vent sites: 13°N on the East Pacific Rise (EPR), the Lau Back-Arc Basin, and the North-Fiji Basin. The probes were placed on clumps of living animals (molluscs and polychaetes) for periods of up to 47 h, in zones where hydrothermal fluids mix with seawater. Spatial variability of temperature was often very important, thus demonstrating the necessity of a thorough study of the hydrothermal microenvironment. Temporal variability was analysed through standard spectral analysis methods. Several periodicities are superimposed, ranging from tens of seconds to ten of hours. The origin of these periods is discussed, and although the tidal cycle seems to be of greater importance in the driving forces of these variations, other periods exist that may be generated by the turbulent mixing occurring in this environment, and/or by variations in the hydrothermal fluid discharge. Aperiodic phenomena were also noted and, therefore, random processes should not be neglected. Consequences of the encountered variabilities on the vent fauna are also discussed, especially for Alvinella species that live in the studied zone and must have developed adaptive responses.

Three new species of Pogonophora, including two vestimentiferans, from hydrothermal sites in the Lau Back-arc Basin (Southwest Pacific Ocean)

Hydrothermal fields on the Valu Fa Ridge at about 22°32′S, 176°43′W, were explored by the submersible Nautile during the French BIOLAU expedition in 1989. Pogonophorans were found during six of the 12 dives, at depths of about 1900 m. They were exposed to the influence of hydrothermal fluids but probably did not experience elevated temperatures. One new species of sediment-living perviate pogonophore was collected. Two new obturate pogonophores (vestimentiferans), one belonging to a new family, lived attached to hard substrata. There is a complete faunistic separation of the Lau Basin vestimentiferans from the eastern Pacific vent faunas. Families as well as genera are different, and the Lau species appear to have more in common with cold seep faunas.

Hydrothermal activity and metallogenesis in the Lau back-arc basin

IN 1989, the submersible Nautile discovered one of the most active hydrothermal fields on the modern ocean floor, in the Lau back-arc basin (Fig. 1). The field contains high-temperature white and black smokers, and as we report here, its characteristics contrast strongly with those of the hydrothermal fields found at normal mid-ocean ridges. The main differences are the acidity (pH as low as 2), chemistry and temperature (up to 400 °C) of the hydrothermal fluids, the composition of the ore deposits, and the volcanic and tectonic environments. The fluids also have very high concentrations of trace metals, and primary gold is present in the accompanying mineral deposits. Our data show that these back-arc deposits in the Lau Basin are intermediate between typical mid-ocean-ridge mineralization and massive sulphide deposits of the Kuroko type.