Central Lau Basin Research Articles

The NE Lau Basin: Widespread and Abundant Hydrothermal Venting in the Back-Arc Region Behind a Superfast Subduction Zone

The distribution of hydrothermal venting reveals important clues about the presence of magma in submarine settings. The NE Lau Basin in the southwest Pacific Ocean is a complex back-arc region of widespread hydrothermal activity. It includes spreading ridges, arc volcanoes, and intra-plate volcanoes that provide a perhaps unique laboratory for studying interactions between hydrothermal activity and magma sources. Since 2004, multiple cruises have explored the water column of the NE Lau Basin. Here we use these data to identify and characterize 43 active hydrothermal sites by means of optical, temperature, and chemical tracers in plumes discharged by each site. Seventeen of 20 prominent volcanic edifices dispersed among the Tofua arc, spreading ridges, and plate interiors host active hydrothermal sites. Fourteen apparently discharge high-temperature fluids, including a multi-year submarine eruption at the intra-plate volcano W Mata. The 430 km of spreading ridges host 31 active sites, one an eruption event in 2008. Our data show that the relationship between site spatial density (sites/100 km of ridge crest) and ridge spreading rate (8-42 mm/yr) in the NE Lau Basin follows the same linear trend as previously established for the faster-spreading (40-90 mm/yr) ridges in the central Lau Basin. The lower site density in the NE Lau Basin compared to the central Lau is consistent with recent plate reconstructions that more than halved earlier estimates of ~50-100 mm/yr spreading rates in the NE Lau Basin. Combined data from the spreading ridges throughout the entire Lau back-arc basin demonstrates that hydrothermal sites, normalized to spreading rate, are ~10× more common than expected based on existing mid-ocean ridge data. This increase documents the ability of meticulous exploration, using both turbidity and chemical sensors, to more fully describe the true hydrothermal population of a spreading ridge, compared to conventional techniques. It further reveals that the Lau back-arc basin, benefiting from both ridge and arc magma sources, supports an exceptionally high population of ridge and intra-plate hydrothermal sites.

Shallow seismicity and tectonics of the central and northern Lau Basin

The Lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time. As such, crustal earthquakes within the Lau Basin derive from intrabasin tectonic, volcanic, and hydrothermal sources. Because of high mantle attenuation, small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity. However, the 1994 LABATTS ocean bottom seismograph experiment in the Lau basin recorded more than 100,000 local T- and body phases allowing for a detailed examination of basin seismotectonics. Nearly 1000 events are located within the basin proper, with another 2000 associated with the forearc and the aseismic front. We identify two previously undocumented tectonic features, a triple junction in the northwestern basin and a nanoplate in the central Lau basin. The single most seismically active feature within the Lau basin is the Lau extensional transform zone (LETZ) just north of the Central Lau Spreading Center. Significant seismicity also occurs near the northern extension of the Eastern Lau Spreading Center and along a southeast extension of the LETZ delineating the boundary of a previously unknown nanoplate. Swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics. We see no evidence of deformation along the southern boundary of the Niuafo'ou microplate, where 4.5 cm/yr of strain is predicted. It is possible that the plate boundary extends eastward from the Fonualei rift (FR) tip towards the trench creating a Niuatoputapu–Tonga plate division rather than westward implied by the currently accepted Niuafo'ou–Tonga plate system, but more data are necessary from the FR to test that hypothesis. Instead, we suggest that the Niuafo'ou–Tonga pole is ~ 1.5° further north than previously proposed, reducing the predicted strain in this region to < 2 cm/yr.

Major and trace element and Sr–Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle

New major and trace element and Sr–Nd isotope data are presented for basaltic glasses from active spreading centers (Central Lau Spreading Center (CLSC), Relay Zone (RZ) and Eastern Lau Spreading Center (ELSC)) in the Central Lau Basin, SW Pacific. Basaltic lavas from the Central Lau Basin are mainly tholeiitic and are broadly similar in composition to mid-ocean ridge basalts (MORB). Their generally high 87Sr/ 86Sr ratios, combined with relatively low 143Nd/ 144Nd ratios are more akin to MORB from the Indian rather than Pacific Ocean. In detail, the CLSC, RZ and ELSC lavas are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the Central Lau Basin was modified by subducted slab-derived components. Fluid mobile/immobile trace element and Sr – Nd isotope ratios suggest that the subduction components were essentially transferred into the mantle via hydrous fluids derived from the subducted oceanic crust; contributions coming from the subducted sediments are minor. Compared to CLSC lavas, ELSC and RZ lavas show greater enrichment in fluid mobile elements and depletion in high field strength elements, especially Nb. Thus, with increasing distance away from the arc, the influence of subduction components in the mantle source of Lau Basin lavas diminishes. The amount of hydrous fluids also influences the degree of partial melting of the mantle beneath the Central Lau Basin, and hence the degree of melting also decreases with increasing distance from the arc.

Mantle dynamics and mantle melting beneath Niuafo’ou Island and the northern Lau back-arc basin

A suite of young volcanic basaltic lavas erupted on the intra-plate island of Niuafo’ou and at active rifts and spreading centres (the King’s Triple Junction and the Northeastern Lau Spreading Centre) in the northern Lau Basin is used to examine the pattern of mantle flow and the dynamics of melting beneath this complex back-arc system. All lavas contain variable amounts of a subduction related component inherited from the Tonga subduction zone to the east. All lavas have higher 87Sr/86Sr, lower 143Nd/144Nd and more radiogenic Pb isotope compositions than basalts erupted at the Central Lau Spreading Centre in the central Lau Basin, and are interpreted as variable mixtures of subduction-modified, depleted upper mantle, and mantle residues derived from melting beneath the Samoan Islands which has leaked through a tear in the subducting Pacific Plate beneath the Vitiaz Lineament at the northern edge of the Lau Basin. Our data can be used to map out the present-day distribution of Samoan mantle in this region, and show that it influences the compositions of lavas erupted as far as 400 km from the Samoan Islands. The distribution of Samoan-influenced lavas implies south- and southwest-wards mantle flow rates of >4 cm/year. U-series disequilibria in historic Niuafo’ou lavas have average (230Th/238U) = 1.13, (231Pa/235U) = 2.17, (226Ra/230Th) = 2.11, and together with major and trace element data require ∼5% partial melting of mantle at between 2 and 3 GPa, with a residual porosity of 0.002 and an upwelling rate of 1 cm year−1. We suggest that intraplate magmatism in the northern Lau Basin results from decompression melting during southward flow of mantle from beneath old (110–120 Ma), relatively thick Pacific oceanic lithosphere to beneath young (<5 Ma), thinner oceanic lithosphere beneath the northern Lau Basin.

U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins

New thermal ionization mass spectrometry U-series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back-arc Lau Basin (SW Pacific), together with additional inductively coupled plasma mass spectrometry trace element analyses and Sr–Nd–Pb isotope data. Valu Fa Ridge samples, adjacent to the arc front, have high U/Th and (230Th/238U) <1, implying a recent (≪350 ka) addition of a U-rich slab-derived fluid. The Valu Fa data can be combined with existing 230Th–238U data for the Central Tonga arc to infer a fluid addition event at ∼50 ka. The similar sources and time scales for fluid transfer beneath the Valu Fa Ridge and beneath the arc itself suggest that the Valu Fa Ridge is propagating into the arc-front region. Central Lau Basin samples, further behind the arc, have lower U/Th and (230Th/238U) ≥1, similar to typical mid-ocean ridge basalts (MORB). Within the Central Lau Basin, a water-rich subduction component is seen only in samples closest to the arc, and this fluid does not have the high-U/Th composition of the fluid at Valu Fa. Melt generation in the Central Lau Basin appears to be dominated by normal ridge-type processes, but the relatively low (230Th/238U) for these shallow ridges compared with global MORB could be a consequence of increased melt productivity as a result of the elevated water contents. The transition from 238U to 230Th excesses within the back-arc basin is not a smooth function of decreasing addition of a U-rich fluid moving away from the arc front, but also reflects the complex dynamics between two major mantle domains within the mantle wedge (‘Pacific’ beneath Valu Fa Ridge, ‘Indian’ beneath the Central Lau Basin).

Refractory Magmas in Back-Arc Basin Settings--Experimental Constraints on the Petrogenesis of a Lau Basin Example

We present the results of an experimental study on a refractory back-arc basin glass composition 123 95-1 recovered close to the intersection of the North Western Lau Spreading Centre and the Peggy Ridge in the Central Lau Basin. We used both the inverse and forward experimental approaches to determine that a picrite composition of ~16 wt %MgO was parental to the refractory back-arc basalt composition and that this picrite was in equilibrium with a residual lherzolite assemblage at ~2.1 GPa, ~1460°C. Our experimental results suggest that this primary picrite represents neither an aggregate of small melt fractions collected over a depth interval in a melting column nor a small melt fraction. Instead, the primary picrite composition represents a significant melt fraction (15% melting) of a lherzolite source, and its composition is closely modelled by equilibrium batch melting. Our preferred model of magma genesis involves three-dimensional diapiric flow during which significant solid-melt re-equilibrium occurs. Melts are trapped within an ascending diapir until melting reaches a moderate to high fraction (15-25%); at which point the primary picrite magmas segregate from the diapir.

Oxygen isotope variations in Lau Basin lavas

New laser-fluorination oxygen isotope data are presented for volcanic glasses and phenocrysts from Lau and North Fiji Basin lavas. The low oxygen blank of the technique allows accurate analysis of these often scarce igneous phases. δ 18O values of volcanic glass range from 5.53‰ to 6.06‰. Oxygen isotope ratios of ferromagnesian and plagioclase phenocrysts are lower and higher than their host glasses, respectively, and coexisting phases appear to represent oxygen isotope equilibrium at magmatic temperatures. Evolved lavas from the propagating and dying rift tips in the Central Lau Basin display significant 18O-enrichment with magmatic evolution due to FeTi-oxide crystallisation and require that precursor melts had relatively low δ 18O values immediately prior to oxide saturation. Development of these precursor melts from basaltic parents involved assimilation of hydrothermally altered sheeted dykes or gabbros in recently accreted oceanic crust accompanied by crystallisation of a plagioclase-rich assemblage. The stable isotope geochemistry and petrology of Central Lau Basin lavas constrain magmatic evolution to depths between 2.5 and 5 km beneath the seafloor. A dacite from Valu Fa Ridge is slightly 18O-enriched relative to Valu Fa basalts, consistent with crystallisation of a mineral assemblage similar to mid-ocean ridge basalts. 18O/ 16O ratios of Lau Basin basaltic glasses decrease with Mg Number; this observation could be reconciled with the assimilation and fractional crystallisation model proposed for initial differentiation of the evolved lavas. However, oxygen isotope data for Central Lau Basin basalts correlate positively with Ba/La while basaltic glasses from throughout the region display a negative correlation between δ 18O and Na 8.0 suggesting that recycling of subducted oxygen and/or mantle fertility may also exert an influence on 18O/ 16O of primary melts. Presently, the lack of quantitative control over oxygen isotope fractionation in basaltic systems prohibits resolution of these effects. Detailed investigation of less complex suites of lavas, employing the precision offered by laser fluorination, may provide new insights into the behaviour of oxygen at magmatic temperatures.

Carbon isotope variations of CO 2 in Central Lau Basin basalts and ferrobasalts

CO 2 abundance and carbon isotope data are reported for basalt glasses from three spreading centres in the Central Lau Basin. Abundances of CO 2 dissolved in glasses vary from 9 ppm to 217 ppm with δ 13C ranging from −16.4‰ to −8.8‰. Evolved ferrobasalts from the propagating rift tip of the Central Lau Spreading Centre have both the lowest CO 2 concentrations and δ 13C values, while more magnesian basalts from the Intermediate and Eastern Lau Spreading Centres have higher CO 2 abundances with higher and less variable δ 13C values. Variations in abundance and carbon isotope ratios of dissolved CO 2 can be modelled by degassing during eruption of melts originally containing 410–440 ppm CO 2, with δ 13C of −7.7‰, using a vapour-melt isotope fractionation of 2.3‰. The type of degassing experienced by melts is related to their style of eruption, which is, in turn, governed by their eruption site. Central Lau Basin glasses, along with glasses from other marginal basins, display carbon isotope ratios which are lower than MORB. While the variations within the suite are adequately described by various degrees of degassing, carbon isotope heterogeneity between sources of oceanic basalts can not be discounted.

Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity

Abstract Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (&gt;90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H 2 O) all present close to the arc, only Ba, Rb and H 2 O present at intermediate distances and just H 2 O perceptible at the furthest distances.

Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: Evidence of crust/mantle interactions in a back-arc basin

We report helium and argon isotope analyses of fresh Lau Basin volcanic glasses from six sites within the central basin (~18°S), six localities along the Valu Fa Ridge (21°–22.4°S), and a single site from the northeastern basin (~ 15°S). Central basin basalts have 3He 4He ratios ( R) between 8.2–8.5 R A (R A = air 3He 4He ), 40Ar 36Ar ratios significantly greater than atmosphere (up to 4900), and 4He contents from 3.5–9.4 × 10 −6 cm 3 STP/g, similar to N-MORBs worldwide. These results are consistent with trace element and other (radiogenic) isotope data on these samples which indicate derivation from a depleted mantle source region. In contrast, evolved lavas from the Valu Fa Ridge all have R R A < MORB and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the 3He 4He ratios of these samples and their chemistry: basaltic andesites have 6.02 < R c R a < 7.65 , andesites have lower ratios (2.37−2.65 R A ), and a dacite has the lowest value of the entire sample suite (1.19 R A ). All lavas have 40Ar 36Ar ratios similar to the atmospheric value and low helium concentrations, from 3–11 × 10 −8 cm 3 STP/g, or between 30 and 300 times less than the central basin basalts. Although the helium isotope results of the Valu Fa lavas mirror the shift to more radiogenic values of other isotope systems (e.g., 206Pb 204Pb , 87Sr 86Sr ) which indicate addition of subducted sediment to these magma sources, we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers. Instead, the most plausible mechanism to explain its incorporation into the Valu Fa lavas is by assimilation of old Lau crust in the near-surface environment by previously degassed magma. We argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back-arc and ocean ridge lavas such as their high volatile and LIL element contents with low rare gas concentrations, and their mantle 3He 4He ratios with (hydrated oceanic) crustal D/H values. The realisation that the helium and argon systematics of the Valu Fa lavas are controlled by crust/mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas, and we show that fractional crystallisation processes can most readily account for the low concentrations of, and systematic trends in, the mantle-derived helium and argon component of these lavas. In addition, because pre-existing crust in the Lau Basin must be old and/or altered enough to supply the radiogenic helium and atmospheric-like 40Ar 36Ar component to the Valu Fa lavas, the occurrence of crust/mantle interactions implies that old (forearc) crust may have been trapped within the Lau Basin: such a scenario has a clear bearing on ideas of the tectonic development of the basin. Finally, because of the potential of crust/mantle interactions to modify 3He 4He and 40Ar 36Ar ratios of mantle-derived melts, we assess the implications for using He and Ar tracers to characterise mantle sources in arcs, back-arcs, and spreading ridges, and consider the consequences for the combined use of rare gases with other (radiogenic) isotopic tracers of magma provenance at such settings. The basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the Valu Fa lavas as it falls within the 3He 4He range (6.9 R A ) of the other basaltic andesites. Interestingly, other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3He 4He ratios, from MORB values up to 22 R A . The low 3He 4He ratio of the basaltic andesite, therefore, serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas: an observation which is important not only for this part of the Lau Basin but for other localities worldwide.

He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components

Helium isotope ratios in Lau Basin back-arc basalts range from 7 to 22 times the atmospheric value ( R A ), i.e. from ratios typical of MORB (Depleted Mantle) helium (R/R A = 8 ± 1) to ratios similar to ‘high- 3He’ hotspots as observed in the Hawaiian, Icelandic, and nearby Samoan plume ( R/R A = 24 ). Along the Central Lau Basin spreading axis and its northward extension in the region around Niuafo'ou Volcano, 3He/ 4He ratios have typical MORB values (range = 7.5–8.6), but on Rochambeau Bank, the southern flank of a large seamount, ratios up to 22 R A occur. These high 3He/ 4He ratios are extrema of linear arrays (11–22 R A ) of He vs. Sr, Nd and Pb isotope ratios, between a Depleted Mantle (MORB) end-member and a Primitive Helium Mantle component (PHEM). PHEM is the Enriched Mantle end-member for the ‘depleted’ array formed with the DM component, and at the same time the Depleted end-member for the ‘enriched’ array formed with ‘EM’ the EM2-type end-member for Masefau Bay, Samoan basalts, as these two binary arrays intersect at its composition. Sr and Nd isotopic arrays vs. each other and vs. 3He are consistent with these binary ‘mirror arrays’ for Lau and Masefau basalts. The 3He data show unequivocally that deep-mantle plume material is present at Rochambeau Bank, and to some extent in the leaky transform/spreading axis along Peggy Ridge. We suppose that the Samoan plume component regards itself as an ‘off-ridge’ hotspot relative to the nearby Lau spreading axis, and that some of its material is channeled toward Peggy Ridge in a manner similar to the channeling we observe at the Galapagos and Pascua (Easter Island) hotspots.

Role of ridge jumps and ridge propagation in the tectonic evolution of the Lau back-arc basin, southwest Pacific

GLORIA sidescan imagery of the central Lau Basin reveals a complex neotectonic framework in which the central Lau spreading center is actively propagating southward at the expense of the eastern Lau spreading center. The latter is located about 75 km closer to the active Tofua arc, and the transfer of spreading motion across this distance is achieved via a relay spreading-ridge segment. New magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern Lau spreading center between 5 Ma and the present day. The processes of arcward ridge migration, ridge propagation, and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back-arc spreading, and may be a common feature of this tectonic environment.

Geochemistry of marine sediments from the Lau Basin, Havre Trough, and Tonga—Kermadec Ridge

Abstract A geochemical survey of sediments from Lau Basin, Havre Trough, and Tonga–Kermadec Ridge has been undertaken in order to study the general pattern of sedimentation and to locate areas of possible metalliferous sediment occurrence. The study was based on the analysis of 206 surface sediment samples for 19 elements. The data were subject to detailed statistical (factor and cluster) analysis. Selected sediment samples from each cluster were also subjected to partition (differential leaching) studies. The bulk sediment composition appears to be dependent on the distribution of four components: 1. Biogenic carbonate occurs mainly in the west of the region. Ca and Sr are dominantly associated with this phase. 2. Volcaniclastic sediments are found on the western flanks of the Tonga–Kermadec Ridge and adjacent to White Island, New Zealand. Al, Mg, Ti, V, Fe, and Co are strongly associated in the Tonga–Kermadec Ridge sediments. The sediments around White Island are much higher in Cr and Ni but lower in Al, Mg, Ti, and Co and much lower in Fe and V than the Tonga-Kermadec Ridge sediments. This reflects the more andesitic volcanism around White Island. 3. Terrigenous sediment is found immediately north of New Zealand and is enriched in Li, Be, and La. 4. Semiopaque Fe-Mn oxides are found only in central Lau Basin sediments where they occur in concentrations of up to 15%. These sediments are enriched in Mn, Cu, Zn, Pb, and P, and, to a lesser extent, Fe, Co, and V. The data indicate the possible importance of the central Lau Basin as a zone of metalliferous sediments of exhalative origin but do not support a similar origin for Havre Trough sediments.