Lau Back-arc Basin Research Articles

Arc–Backarc Exchange Along the Tonga–Lau System: Constraints From Volatile Elements

Abstract Analyses of olivine-, pyroxene- and plagioclase-hosted melt (now preserved as glass) inclusions (MI) in submarine glasses and subaerial scoria from seven volcanoes along the volcanic front of the Tonga arc provide the first comprehensive dataset including volatile contents (H2O, CO2, S, Cl and F) for this arc. Maximum water contents at each volcano are typical (3–5 wt %) of other arc volcanoes worldwide, and within each volcano, water generally correlates with sulfur, consistent with degassing but not diffusive re-equilibration. The Tonga arc is notable for the eruption of magmas sourced from strongly depleted upper mantle, including boninites in the northern half of the active arc. A key question has been whether such boninites are derived from high degrees of melting actively occurring under the arc, driven by high mantle temperatures and water contents, or from high integrated degrees of melting including melting of depleted mantle in the backarc. This study supports the latter view, based on sub-arc mantle melt fractions of 17–23%, calculated from primary melt water contents and mantle-melt thermobarometry. Such fractions are not high enough to generate boninites from a typical, fertile mantle source in a single melting stage, and melt fractions do not increase north of 22°S, where the boninitic magmas appear. Instead, the northern Tonga arc boninites reflect high cumulative degrees of melting (>30%, with respect to fertile mantle) and occur along the volcanic front where adjacent backarc spreading is fast, proximal and mature. Slab tracers such as the Ba/La ratio peak at around 20–21°S, in the central part of the Tonga arc. A Ba/La peak also appears at about the same latitude in the Lau backarc basin. This peak is consistent with the coolest slab temperatures beneath the arc and backarc at this latitude, as the slab warms to the north due to an increase in mantle potential temperature, and to the south due to less heat extraction from melting. A new concept developed here is cooling of the mantle wedge by melting, due to the supply of the heat of fusion and melt removal. Such a process can cool the mantle by more than 100 degrees, and thus affect slab heating as well. We find a critical distance of slab influence in the Lau spreading centers; back-arc magmas erupted >70 km from the arc are >210 km above the slab, beyond the point of slab dehydration, and show no enrichment in Ba/La. The coupled dynamics of the Tonga arc-Lau basin are rich in their connections, and demonstrate how melting processes beneath the back-arc and arc can affect slab processes, which in turn can affect the composition of fluids that feed the arc and backarc.

Back-Arc Spreading Centers and Superfast Subduction: The Case of the Northern Lau Basin (SW Pacific Ocean)

The Lau Basin is a back-arc region formed by the subduction of the Pacific plate below the Australian plate. We studied the regional morphology of the back-arc spreading centers of the Northern Lau basin, and we compared it to their relative spreading rates. We obtained a value of 60.2 mm/year along the Northwest Lau Spreading Centers based on magnetic data, improving on the spreading rate literature data. Furthermore, we carried out numerical models including visco-plastic rheologies and prescribed surface velocities, in an upper plate-fixed reference frame. Although our thermal model points to a high temperature only near the Tonga trench, the model of the second invariant of the strain rate shows active deformation in the mantle from the Tonga trench to ~800 km along the overriding plate. This explains the anomalous magmatic production along all the volcanic centers in the Northern Lau Back-Arc Basin.

Geologic and Structural Evolution of the NE Lau Basin, Tonga: Morphotectonic Analysis and Classification of Structures Using Shallow Seismicity

The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and strike-slip tectonics in the overriding plate. One prominent example is the northern Tonga subduction zone, where abundant strike-slip faulting in the NE Lau back-arc basin is associated with transform motion along the northern plate boundary and asymmetric slab rollback. Here, we address the fundamental question: how does this subduction-transform motion influence the structural and magmatic evolution of the back-arc region? To answer this, we undertake the first comprehensive study of the geology and geodynamics of this region through analyses of morphotectonics (remote-predictive geologic mapping) and fault kinematics interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data. Our results highlight two notable features of the NE Lau Basin: 1) the occurrence of widely distributed off-axis volcanism, in contrast to typical ridge-centered back-arc volcanism, and 2) fault kinematics dominated by shallow-crustal strike slip-faulting (rather than normal faulting) extending over ∼120 km from the transform boundary. The orientations of these strike-slip faults are consistent with reactivation of earlier-formed normal faults in a sinistral megashear zone. Notably, two distinct sets of Riedel megashears are identified, indicating a recent counter-clockwise rotation of part of the stress field in the back-arc region closest to the arc. Importantly, the Riedel structures identified in this study directly control the development of complex volcanic-compositional provinces, which are characterized by variably-oriented spreading centers, off-axis volcanic ridges, extensive lava flows, and point-source rear-arc volcanoes. This study adds to our understanding of the geologic and structural evolution of modern backarc systems, including the association between subduction-transform motions and the siting and style of seafloor volcanism.

Origin and Evolution of the Fatu Kapa Magmatic System (North-Western Lau Back-arc Basin): Insight on the Genesis of High-Silica Lavas

Abstract The South Eastern Futuna Volcanic Zone (SEFVZ; 50 km × 80 km) is located in the NW of the Lau back-arc basin at the northern boundary of the Futuna and YAO microplates. Fatu Kapa is an area (∼15 km × 10 km) of diffuse volcanic activity at the center of the SEFVZ and at the intersection of two volcanic axes with the Futuna fracture zone. At this place, the volcanic, tectonic, and hydrothermal activities are enhanced. The most differentiated volcanic series of the whole Futuna region are found in the Fatu Kapa area. Based on major and trace element data, four groups of volcanic rocks are distinguished in the Fatu Kapa area. The main groups form a well-defined transitional suite from basalts to andesitic and dacitic lavas that display enrichments in Zr, Cl and heavy rare earth elements. The most mafic lavas of this group are extracted from an Indian-type depleted mantle mixed with a component with trace element and isotopic characteristics similar to the nearby Samoa mantle plume. There is no evidence of subduction input. Petrological modelling shows that the mafic melts of the main group (Group 1A) evolve towards trachyandesites through fractional crystallization coupled with assimilation of 10 % of a Cl-rich and Th-poor melt generated by low-degree (≤2 %) partial melting of a hydrothermally altered basaltic crust. Rocks of Groups 1B(a) and 1B(b) were then generated by mixing of a mafic melt from Group 1A with a melt obtained by partial melting of andesitic and hydrothermally altered wallrock in shallow magma chambers. Extremely high Cl concentrations can be explained by assimilation of interstitial K- and Cl-rich brines in the pore water of the hydrothermally altered volcanic rocks before melting of these rocks. This is in good agreement with unusual extension of the Fatu Kapa hydrothermal area and indicates that, in the north of the Lau basin, the microplate boundaries are privileged zones to generate a complex felsic magmatism within areas of widespread hydrothermal alteration. Our study confirms the importance of assimilation and fractional crystallization and mixing processes in generating high-silica lavas in back-arc environments decoupled from subduction.

Nickel isotopes and rare earth elements systematics in marine hydrogenetic and hydrothermal ferromanganese deposits

Attention is now being given to Ni isotope systematics in hydrogenetic marine ferromanganese (Fe-Mn) crusts as paleoceanographic proxies. Previous work focused on identifying both mineralogy (post-depositional) and source effects (Gall et al. 2013; Gueguen et al. 2016), in particular regarding hydrothermal inputs in the oceans and the response of Ni isotope biogeochemical cycling through time. The most important sink for Ni in the oceans is the Fe-Mn oxides sink, but estimation of its Ni isotope composition is only based on hydrogenetic Fe-Mn crusts. In this study, we investigated a range of Fe-Mn deposits including Fe-Mn deposits variably affected by hydrothermal inputs, including hydrothermal deposits from the Lau back-arc basin (South West Pacific) and Lo'ihi seamount (Hawaii), hydrogenetic crust and nodules from the Bauer Basin (Pacific Ocean). Nickel isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using a double-spike (61Ni and 62Ni) correction method. The combination of Ni isotopes and rare earth element (REE) geochemistry show that Ni isotope fractionation in Fe-Mn deposits is essentially controlled by formation processes of the deposits (such as the rate of formation, the initial Mn-phase and sorption processes) which are also related to the depositional environment. Consistent with previous studies, pure hydrogenetic crusts are characterized by isotopically heavy Ni isotope signatures (δ60/58Ni values range from ‰ 0.9 and 2.5‰) and well-developed positive Ce anomalies. In contrast, mixed hydrothermal‑hydrogenetic crust and nodules from the Bauer Basin (East Pacific) display negative Ce anomaly and lighter δ60/58Ni values (0.3‰ to 0.4‰), which are interpreted as the result of far-field hydrothermal inputs of Fe-Mn precipitates from the East Pacific Rise.Nickel in hydrothermal deposits from the Lau Basin (0.5 and 1.1‰) and Lo'ihi seamount (−0.8 to −1.5‰) is isotopically lighter than in hydrogenetic Fe-Mn crusts. Light δ60/58Ni values in Lo'ihi deposits is due to the removal of Ni during Ni adsorption from seawater and from the hydrothermal fluid (between 0 and 1.4‰) on Fe-oxides followed by isotope fractionation between the fluid and the mineral phase. Results suggest that Ni isotopes in hydrothermal Fe-rich deposits are strongly fractionated relative to the seawater/fluid source due to partial removal of Ni on Fe-phases. Hydrothermal Mn-oxides deposits from the Lau Basin acquired their Ni isotope signature through Ni adsorption and continuous exchange of Ni with seawater. We propose that the systematic difference in Ni isotope signatures between hydrogeneous and hydrothermal Fe-Mn deposits is related to the mechanisms of Ni uptake into oxide minerals (e.g., birnessite vs. todorokite; Fe-oxides vs. Mn-oxides) which depend on the rate of formation and the source of Mn and Fe to marine ferromanganese deposits (i.e., depositional environment) rather than Ni sources.

Uppermost crustal structure across the eastern Lau spreading center from P-to-S converted waves

P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to investigate upper crustal structure in the Lau backarc basin. A combination of ray tracing and finite difference numerical wavefield simulation is used to identify P and P-to-S converted seismic phases. The phases primarily arise from two shallow interfaces, one at ~ 80 m depth or less, and the other at 500–650 m depth. The shallower interface is deeper than the sediment base, is observed across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a rapid change in crack density. The deeper interface is interpreted as the layer 2A–2B boundary, corresponding to a transition from lavas to sheeted dykes. Layer 2A, on average, is 150 m thicker in crust that formed at the spreading center when spreading was located near the arc ( 70 km away). Layer 2A thickness and Vp/Vs values indicate that a thicker and more porous lava layer, dominated by basalts to basalt-andesites, cap near-arc crust, while a thinner and less-porous, mostly basaltic, volcanic layer caps the far-arc crust. These results are consistent with the waning influence of slab-derived volatiles on crustal formation as seafloor spreading moves away from the active arc.

Hungry scale worms: Phylogenetics of Peinaleopolynoe (Polynoidae, Annelida), with four new species.

Polynoidae Kinberg, 1856 has five branchiate genera: Branchipolynoe Pettibone, 1984, Branchinotogluma Pettibone, 1985, Branchiplicatus Pettibone, 1985, Peinaleopolynoe Desbruyères & Laubier, 1988, and Thermopolynoe Miura, 1994, all native to deep-sea, chemosynthetic-based habitats. Of these, Peinaleopolynoe has two accepted species; Peinaleopolynoe sillardi Desbruyères & Laubier, 1988 (Atlantic Ocean) and Peinaleopolynoe santacatalina Pettibone, 1993 (East Pacific Ocean). The goal of this study was to assess the phylogenetic position of Peinaleopolynoe, utilizing DNA sequences from a broad sampling of deep-sea polynoids. Representatives from all five branchiate genera were included, several species of which were sampled from near the type localities; Branchinotogluma sandersi Pettibone, 1985 from the Galápagos Rift (E/V “Nautilus”); Peinaleopolynoe sillardi from organic remains in the Atlantic Ocean; Peinaleopolynoe santacatalina from a whalefall off southern California (R/V “Western Flyer”) and Thermopolynoe branchiata Miura, 1994 from Lau Back-Arc Basin in the western Pacific (R/V “Melville”). Phylogenetic analyses were conducted using mitochondrial (COI, 16S rRNA, and CytB) and nuclear (18S rRNA, 28S rRNA, and H3) genes. The analyses revealed four new Peinaleopolynoe species from the Pacific Ocean that are formally described here: Peinaleopolynoe orphanae Hatch & Rouse, sp. nov., type locality Pescadero Basin in the Gulf of California, Mexico (R/V “Western Flyer”); Peinaleopolynoe elvisi Hatch & Rouse, sp. nov. and Peinaleopolynoe goffrediae Hatch & Rouse, sp. nov., both with a type locality in Monterey Canyon off California (R/V “Western Flyer”) and Peinaleopolynoe mineoi Hatch & Rouse, sp. nov. from Costa Rica methane seeps (R/V “Falkor”). In addition to DNA sequence data, the monophyly of Peinaleopolynoe is supported by the presence of ventral papillae on segments 12–15. The results also demonstrated the paraphyly of Branchinotogluma and Lepidonotopodium Pettibone, 1983 and taxonomic revision of these genera is required. We apply the subfamily name Lepidonotopodinae Pettibone 1983, for the clade comprised of Branchipolynoe, Branchinotogluma, Bathykurila, Branchiplicatus, Lepidonotopodium, Levensteiniella Pettibone, 1985, Thermopolynoe, and Peinaleopolynoe.

Novel insights from Fe-isotopes into the lithological heterogeneity of Ocean Island Basalts and plume-influenced MORBs

The extent of lithological heterogeneity in the Earth's convecting mantle is highly debated. Whilst the presence of pyroxenite in the mantle source regions of Ocean Island Basalts (OIBs) has traditionally been constrained using the minor-element chemistry of olivine phenocrysts, recent studies have shown that the Ni and Mn contents of primitive olivines are influenced by the conditions of mantle melting, as well as magma chamber processes. Nevertheless, constraining the lithological properties of the mantle is important due to its influence on the P-T path followed by solid mantle material during adiabatic ascent, as well as the density of upwelling mantle plumes. We have therefore explored the use of Fe-isotopes as a novel method of tracing lithological heterogeneity in the mantle source regions beneath plume-influenced segments of the global Mid-Ocean Ridge system as well as OIBs.We present new Fe-isotope (δ56Fe) and trace-element data for 26 basaltic glasses from the plume-influenced Galápagos Spreading Centre to investigate the relative roles of pyroxenite and peridotite in the mantle source region of oceanic basalts. Our data reveals significant heterogeneity in the Fe-isotope composition of the Galápagos Spreading Centre basalts (+0.05 to +0.25‰ δ56Fe), which correlates with key major- and trace-element parameters (e.g. CaO(8)/Al2O3(8), [La/Sm]n). Application of new models developed to calculate Fe-isotope fractionation during mantle melting, alongside Monte Carlo simulations for melting of a 2-component peridotite mantle, show that this variation cannot be caused by changes in melting processes and/or oxygen fugacity of a peridotitic mantle. Instead, our new δ56Fe data is best explained by variations in the proportion of isotopically-heavy pyroxenite-derived melt that contributes to the GSC basalts, and conclusively shows that lithological heterogeneity exists in the Galápagos mantle plume. Our findings have implications for the moderately-heavy δ56Fe compositions measured in plume-influenced basalts from the Society Islands, Rochambeau Ridges of the Lau back-arc basin, and the FAMOUS segment of the Mid-Atlantic Ridge, which we suggest may also represent contribution from pyroxenite-derived melts.

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.

Tracing mantle sources in the northern Lau back-arc basin by independent component analysis of basalt isotopic compositions

ABSTRACT The northern Lau back-arc basin (NLB) lavas display a diverse geochemical nature caused by complex geological processes in this region. Independent component (IC) analysis was applied to investigate the nature of mantle sources in the NLB, based on a compiled geochemical data set from the NLB, central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoan islands. We identified three ICs in the five-dimensional space of Sr-Nd-Pb isotopic ratios, which can account for 96.5% of the isotopic variance. The correlations between the ICs and the incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) separates Samoan islands (IC1 < −1) from the other groups, and shows negative correlation with (La/Sm)N ratios. The second IC (IC2) discriminates mid-ocean ridge basalts (MORBs) from Pacific (IC2 > 0) and Indian (IC2 < 0) Ocean ridges as well as the C-SLB (IC2 > 0) and NLB (IC2 < 0) lavas. IC2 correlates positively with Ba/Th ratios. The third IC (IC3) distinguishes MORBs (IC3 > 0) and back-arc basin lavas (IC3 < 0), and displays a negative correlation with Th/Nb values. On a regional scale in the NLB, there is a broad increase in IC1, IC2, and IC3 from the north to the south. Only IC3 presents obvious decrease from the west to the east, i.e. with decreasing distance from the arc. The geochemical nature and the statistical properties of these ICs suggest that IC1 is related to an enriched mantle component most likely from the nearby Samoan plume, and the IC2 corresponds to a fluid-rich component from the subducting Pacific slab. The IC3 may represent partial melt of recycled subducted sediment or recycled continental crustal materials. The geographic distribution of three ICs supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized back-arc mantle may be responsible for the observed geochemical diversity in NLB lavas.

P-wave attenuation structure of the Lau back-arc basin and implications for mantle wedge processes

We image 3-D seismic P-wave attenuation structure of the Tonga subduction zone and the Lau back-arc basin using local earthquake waveforms recorded by land and ocean bottom seismographs of the 2009–2010 Lau Spreading Center Imaging project. The amplitude spectra of P waves from local earthquakes are inverted for the path-average attenuation operator (t⁎), with a variety of frequency-dependence exponent values (α). Analysis shows that the data are best fit by the assumption of a weak frequency dependence (α≈0.3), consistent with laboratory results modeled as the effects of grain boundary sliding. The t⁎ measurements are inverted for a 3-D tomographic QP−1 model. The results show high P-wave attenuation within the upper 100 km of the back-arc mantle, with a sharp boundary at the Tofua arc between high attenuation sub-arc mantle and the low attenuation fore-arc. Perhaps the highest P-wave attenuation (QP−1≈0.036 or QP≈28) known in the mantle occurs at 20–60 km depth beneath the back-arc spreading centers. High attenuation anomalies form an inclined zone dipping from the spreading centers to the west away from the slab, indicating abnormally high temperature and the existence of hydrous partial melt. These observations suggest that hot materials supplied from the Australian mantle upwell along with the mantle wedge flow pattern, triggering extensive decompression melting near the back-arc spreading centers. The along-strike variation of P-wave attenuation is different from that of SV-velocity, possibly implying the different effects of partial melt on seismic attenuation and velocity.

Iron isotope variability in ocean floor lavas and mantle sources in the Lau back-arc basin

Iron isotopes in ocean floor basalts (OFB) away from convergent margins comprising mid-ocean-ridge and ocean island lavas show significant variation of >0.4‰ (expressed in the delta notation δ57Fe relative to IRMM-014), but processes responsible for this variation remain elusive. Bond-valence theory predicts that valence states (Fe3+ vs. Fe2+) control Fe isotopes during partial melting and crystal fractionation along the liquid line of descent and thus contribute substantially to this variation. Memory of past melt extraction or metasomatic re-enrichment in the source of OFB may further add to the observed variability, but systematic investigations to elucidate the respective contributions of these effects have been lacking. Submarine ridges and rifts in the Lau back-arc basin offer a unique opportunity to compare Fe isotopes in OFB from different melting regimes and variably depleted mantle sources. New Fe isotope data is presented for submarine lavas from the Rochambeau Ridges (RR) and the Northwest Lau Spreading Centre (NWLSC), and is compared with published data from the Central Lau Spreading Centre (CLSC). In line with first principle calculations and observations from a range of natural systems, crystal fractionation is identified as the dominant, controlling process for elevating δ57Fe in the lavas with olivine tentatively identified as the key driver. To compensate for the effect of crystal fractionation, olivine is mathematically added towards calculated primitive melt compositions (δ57Feprim). For this, we used a constant Ol-melt isotope fractionation factor based on published equilibrium partition functions adapted to decreasing temperature in a cooling melt. The degree of calculated Fe isotope fractionation through olivine crystal fractionation (monitored as Δ57Fe = δ57Femeasured − δ57Feprim) is positively correlated with increasing S and decreasing Ni content in the cooling lavas, fortifying the validity of the approach. Primitive lavas from individual Lau spreading centres and ridges vary to 0.1‰ in δ57Feprim, similar to primitive open-ocean MORB. However, the entire spread in Fe isotope variability in the primitive melts remains at 0.3‰, which we propose to be the extent of isotope heterogeneity in Earth’s upper mantle, with few extreme exceptions. The largest variability in δ57Feprim is observed for RR intra-plate lavas, which have been associated with the Samoan mantle plume and melting in an edge-driven convection scenario. Low, mid-ocean ridge-like 87Sr/86Sr in RR lavas excludes significant influence of isotopically heavy Samoan EM2-type components. However, co-variations with rare earth element pattern in some RR intra-plate lavas indicate garnet plays a role in elevating δ57Feprim during deeper melting. Excluding these deep-seated melts uncovers systematically decreasing δ57Feprim coupled to the degree of mantle source depletion, as recorded in Lu/Hf and Sm/Nd, in the back-arc basin basalts. This, however, holds only true for a comparison between sources of individual ridges, whereas no co-variation is observed within ridge segment data. This suggests that a process other than source depletion and crystal fractionation further adds to Fe isotope variability in the order of 0.1‰ on scales of individual ridge segments. This either marks the degree of Fe isotope variability below ridge segments, or is caused by secondary processes, such as melt-wallrock interaction or RTX (recharge and crystal fractionation) magma chambers.

Cover: Journal of Basic Microbiology. 5/2018

Journal of Basic MicrobiologyVolume 58, Issue 5 COVER PICTUREFree Access Cover: Journal of Basic Microbiology. 5/2018 First published: 03 May 2018 https://doi.org/10.1002/jobm.201870018AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Cover illustration: Scanning electron microscopic image of intermediate portion of Fe-Mn crust recovered from the ‘Christmas Tree’ hydrothermal field, northern Lau back-arc basin reveals microbial aggregation leading to the formation of filament-like structures. The filaments are encrusted in exopolymeric material secreted by autochthonous microbes which acts as a nucleation site for deposition of metal oxides. In hydrothermal systems, microbes could thus be playing an important role in controlling the chemistry of metal ions in the aquatic system and aiding in the formation of metal oxides. (Photo: V. D. Khedekar, CSIR-National Institute of Oceanography, Goa, India) Volume58, Issue5May 2018 RelatedInformation

Trade-offs in a high CO2 habitat on a subsea volcano: condition and reproductive features of a bathymodioline mussel

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 574:49-64 (2017) - DOI: https://doi.org/10.3354/meps12196 Trade-offs in a high CO2 habitat on a subsea volcano: condition and reproductive features of a bathymodioline mussel Giulia S. Rossi1,3,*, Verena Tunnicliffe1,2 1Department of Biology, University of Victoria, Victoria, BC V8W 2Y2, Canada 2School of Earth & Ocean Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada 3Present address: Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada *Corresponding author: grossinetwork@gmail.com ABSTRACT: Northwest Eifuku submarine volcano (Mariana Volcanic Arc) emits very high concentrations of CO2 at a vent where the mussel Bathymodiolus septemdierum experiences pH as low as 5.2. We examined how this natural setting of high pCO2 influences shell, body, and reproductive condition. Calcification is highly compromised: at a given shell volume, shells from NW Eifuku weigh about half those from reference sites in the south Pacific, and dissolution of the inner shell is evident. However, the condition indices of some NW Eifuku mussels were equal to or higher than those from Lau back-arc basin and the New Hebrides Island Arc. NW Eifuku mussels in pH 5.2 fluids had the highest symbiont abundances in gill bacteriocytes, probably due to greater dissolved sulphide access. Excess energy demands imposed by high pCO2 conditions appears moderated by adequate food availability through symbiont chemosynthesis. In the sample with the lowest body condition, gametogenesis was lagging, although all mussels in high pCO2 had developing gonads and the complete gametogenic cycle was present in our samples. Gamete development is synchronous between sexes and is possibly periodic. While mussels are functionally dioecious, protogynous hermaphroditism can occur—a first record for the genus—which may be an adaptation to resource availability. B. septemdierum likely makes energy allocation trade-offs among calcification, body mass maintenance, reproduction and other processes to maximize fitness. We suggest that flexibility to divert energy from shell formation, combined with good food supply, can mitigate the manifestation of high CO2 stress on B. septemdierum. KEY WORDS: High pCO2 · Hydrothermal vent · Calcification · Condition · Gametogenesis · Protogyny · Acidification Full text in pdf format Supplementary material PreviousNextCite this article as: Rossi GS, Tunnicliffe V (2017) Trade-offs in a high CO2 habitat on a subsea volcano: condition and reproductive features of a bathymodioline mussel. Mar Ecol Prog Ser 574:49-64. https://doi.org/10.3354/meps12196 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 574. Online publication date: July 04, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Quantifying He fluxes from the mantle using multi-tracer data assimilation.

A global, coarse-resolution ocean model previously fitted to geostrophic shear estimates and to data of 10 hydrographic parameters and tracers has been used to simulate the 3He and 4He distributions resulting from the release of mantle helium from mid-ocean ridges. The model is in very good agreement with 14C and chlorofluorocarbon data and has realistic global ocean overturning strength as well as water mass formation and transport rates. It is found that previously published global mantle 3He fluxes are too high by a factor of about 2. In the model, optimal agreement of modelled δ3He with data is achieved for a global flux of 450 ± 50 mol 3He yr−1. The formulation of He source strengths proportional to ridge spreading rates appears compatible with data. A model/data misfit analysis shows significant and large-scale δ3He underestimation in the southwestern Pacific centred over the Lau Backarc Basin (approx. 179° W/20° S). These misfits disappear in a set-up with 30 of the 450 mol yr−1 global total 3He flux released in the Lau Basin over a depth range between 1250 and 2500 m. Such He flux contributions are missing in present mantle He source compilations. Hydrothermal fluxes of other trace elements and isotopes (TEI) can be calculated from He fluxes on the basis of TEI : He ratios measured close to the sources.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Sourcing of Miocene accretionary lapilli on ‘Eua, Tonga; atypical dispersal distances and tectonic implications for the central Tonga Ridge

ABSTRACTVolcaniclastics hosting accretionary lapilli on the Tonga Ridge were sourced from the remnant Lau Ridge, prior to Lau back-arc basin opening. For the ‘Eua occurrences, an atypical dispersal distance of not less than 70 km is estimated, partly arising from the anomalous easterly position of ‘Eua. Dispersal within ocean-surface pyroclastic density currents is supported but strike-slip movement in a fault zone south of ‘Eua, post-middle Miocene but pre-ridge-splitting, can account for part of the dispersal distance by vertical axis block rotation, a tectonic process common on the southern Tonga–Kermadec–Hikurangi trend. In this model, the volcano which sourced the ‘Eua tephra was on a subjacent block, rather than the block which hosts ‘Eua. After deposition but prior to the opening of the Lau Basin, the accretionary lapilli on ‘Eua became displaced by block rotation c. 40 km towards the Tonga trench and away from source.

Ca. 830 Ma back-arc type volcanic rocks in the eastern part of the Jiangnan orogen: Implications for the Neoproterozoic tectonic evolution of South China Block

The volcanic rocks in the eastern part of Jiangnan orogen provide an important suite to investigate magma petrogenesis and tectonic evolution of South China Block. We carried out a combined study of geochronology and geochemistry for the bimodal association of basalts and rhyolitic tuffs from the Shuangqiaoshan Group in south Anhui Province. Zircons from a basalt and two rhyolitic tuff samples yield crystallization ages of 829±12Ma, 832±9Ma and 832±10Ma, respectively. The basalts are characterized by left-sloping REE distribution patterns, enriched large iron lithosphere elements (LILEs, e.g. Rb, Ba, K) and depleted high field strength elements (HFSEs, e.g. Nb, Ta) relative to those in N-MORB, resembling those of modern Central Lau back-arc basin basalts (BABB). In addition, they have positive ɛNd(t) values (+4.6 to +5.7) with initial 87Sr/86Sr ratios in the range of 0.70706–0.70719. These geochemical and isotopic signatures indicate that the basalts were derived from an N-MORB-like source modified by slab-derived fluids. The rhyolitic tuffs have high SiO2 (72.9–78.3wt %) contents and high 10,000*Ga/Al ratios (mostly >2.6). Taking into account their aluminous nature (A/CNK>1.1), they are affinitive to aluminous A-type granites. They invariably show a relative enrichment in light-REE, and strong negative Eu anomalies (Eu/Eu*=0.16–0.31) and negative anomalies in HFSE, Sr and P compared with the neighboring elements in their trace element spidergrams. Their ɛNd(t) values range from −2.1 to +0.1. Detailed elemental and isotopic data suggest that the rhyolitic tuffs formed by partial melting of mafic lower crust triggered by asthenosphere upwelling. The occurrence of bimodal volcanic rocks with the basalts showing geochemical affinity to intra-oceanic BABB appears to support the development of a Neoproterozoic back-arc basin in the eastern part of Jiangnan orogen at ca. 830Ma. Combined with the published data from central and western part of Jiangnan orogen, it is inferred that the back-arc basin developed in the entire Jiangnan orogen during 850–810Ma. Considering the age of the strata overlying the folded basement sequence, we suggest that the final collision of the Yangtze and Cathaysia blocks happened during 820–810Ma along the Jiangnan orogen.

Petrogenesis and structure of oceanic crust in the Lau back-arc basin

Oceanic crust formed along spreading centers in the Lau back-arc basin exhibits a dramatic change in structure and composition with proximity to the nearby Tofua Arc. Results from recent seismic studies in the basin indicate that crust formed near the Tofua Arc is abnormally thick (8–9 km) and compositionally stratified, with a thick low-velocity (3.4–4.5 km/s) upper crust and an abnormally high-velocity (7.2–7.4+ km/s) lower crust (Arai and Dunn, 2014). Lava samples from this area show arc-like compositional enrichments and tend to be more vesicular and differentiated than typical mid-ocean ridge basalts, with an average MgO of ∼3.8 wt.%. We propose that slab-derived water entrained in the near-arc ridge system not only enhances mantle melting, as commonly proposed to explain high crustal production in back-arc environments, but also affects magmatic differentiation and crustal accretion processes. We present a petrologic model of Lau back-arc crustal formation that successfully predicts the unusual crustal stratification imaged in the near-arc regions of the Lau basin, as well as the highly fractionated basaltic andesites and andesites that erupt there. Results from phase equilibria modeling using MELTS indicate that the high water contents found in near-arc parental melts can lead to crystallization of an unusually mafic, high velocity cumulate layer. Best-fit model runs contain initial water contents of ∼0.5–1.0 wt.% H2O in the parental melts, and successfully reproduce geochemical trends of the erupted lavas while crystallizing a cumulate assemblage with calculated seismic velocities consistent with those observed in the near-arc lower crust. Modeled residual melts are also lower density than their dry equivalents, which aids in melt segregation from the cumulate layer. Low-density, water-rich residual melts can lead to the eruption of vesicular lavas that are unusually evolved for an oceanic spreading center.

Selective ingress of a Samoan plume component into the northern Lau backarc basin.

Intra-plate basalt isotopic trends require mixing between enriched mantle components (EM1, EM2, HIMU) and a primordial component with high (3)He/(4)He termed FOZO. However, proportions of components, geometric distributions within individual plumes, relative proportions of melting components and loci of mixing of melts and residues remain poorly understood. Here we present new Hf-Nd isotopic data of dredged sea floor basalts from the northern Lau backarc basin, ~250 km south of the subaerial and submerged Samoan chain, with high (3)He/(4)He, (20)Ne/(22)Ne and primordial (129)Xe/(130)Xe, characteristic of the FOZO component. Combined Hf-Nd-noble gas isotope systematics require mixing of refractory, sub-northwestern Lau backarc mantle only with a spatially restricted FOZO component, most plausibly sourced from part of the Samoan plume. Other geographically restricted and possibly volumetrically minor enriched Samoan plume components are not detectable in northern Lau backarc samples, consistent with selective plume ingress of the FOZO component beneath the basin.

Characteristics of Cu isotopes from chalcopyrite-rich black smoker chimneys at Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau basin

We analysed primary chalcopyrite from modern seafloor ‘black smoker’ chimneys to investigate high-temperature hydrothermal Cu isotope fractionation unaffected by metamorphism. Samples came from nine chimneys collected from Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau backarc basin. This is the first known study of Cu isotopes from submarine intraoceanic arc/backarc volcanoes, with both volcanoes discharging significant amounts of magmatic volatiles. Our results (n = 22) range from δ65Cu = −0.03 to 1.44 ± 0.18 ‰ (2 sd), with the majority of samples between ∼0.00 and 0.50 ‰. We interpret this cluster (n = 17) of lower δ65Cu values as representing a mantle source for the chimney Cu, in agreement with δ65Cu values for mantle rocks. The few higher δ65Cu values (>0.90 ‰) occur (1) within the same chimneys as lower values, (2) randomly distributed within the chimneys (i.e. near the top and bottom, interior and exterior), and (3) within chalcopyrite of approximately the same age (<1 year). This suggests the higher δ65Cu values are not related to oxidation by mixing with ambient seawater, but to isotopic variation within the vent fluids over a relatively short time. Theoretical studies demonstrate significant isotopic fractionation can occur between aqueous and vapourous complexing species. When combined with evidence for periodic release of magmatic volatiles at Brothers, we believe vapour transport of Cu is responsible for the observed isotopic fractionation. When compared to global δ65Cu data for primary chalcopyrite, volcanic arc chimneys are most similar to porphyry copper deposits that also form from magmatic-hydrothermal processes in convergent tectonic settings.