Kermadec Arc Research Articles

Ancient Tsunami Records in the Viscous Remanent Magnetization of Reworked Boulders in the Kingdom of Tonga

AbstractThe movement history of boulders is crucial for the reconstruction of paleo‐tsunamis. We report findings from viscous remanent magnetization studies of the boulders on Tongatapu Island, aiming to reconstruct their reworkings. Two boulders exhibited viscous remanence, whereas two larger boulders lacked viscous components but exhibited stable remanence. Both the viscous and stable components deviated from the geomagnetic field direction. These observations indicate that: (a) the boulders with a viscous component were reworked before the latest event, which could have reworked all boulders, and (b) the magnitude of the latest event was larger than that of an earlier event. The reworked timing indicated that the event occurred between 3,000 years ago and the fifteenth century. The difference in the wave height required to move boulders on the eastern and western coasts suggests that the source of the earlier tsunami was likely an eruption due to volcanoes along the Tonga Ridge.

The 2019 pumice raft forming eruption of Volcano-F (Volcano 0403–091) and implications for hazards posed by submerged calderas

Low volcanic explosivity index (VEI) eruptions are common occurrences in the Southwest Pacific but, as demonstrated by the 2021/2022 eruption of Hunga Volcano, submerged calderas in the region are also capable of producing much larger and more hazardous eruptions. As such, characterising smaller events from potentially hazardous systems is essential. The 2019 eruption of Volcano-F, a submerged caldera, would likely have gone totally undetected had it not produced a pumice raft that inundated beaches in Fiji and eventually washed up in Australia. New data, acquired 5 months after the eruption, reveal the development of a new vent and the accumulation of at least 3.1*107 m3 bulk volume (dense rock equivalent of 5.6*106 m3) of material on the seafloor. Between 30 and 70% of erupted material entered the raft, while the rest remained near to or was dispersed down-current of the vent. This previously unaccounted for material increases the volume estimate for the eruption, confirming it as a VEI 3 event and highlights the importance of considering not just the floating component of a pumice raft forming eruption for VEI estimation. Geochemical analysis reveals the eruption comprised a homogenous batch of dacitic magma, with compositional characteristics similar to that erupted from the same volcano in 2001, and an until-now-unidentified pumice raft in the Coral Sea in 1964. Volcano-F therefore appears to have had at least three explosive eruptions in the last 60 years, indicating it is significantly at unrest. Repeated eruptions of similar composition and low crystal content magma over decadal to centennial scales indicate the existence of a melt-dominant magma body beneath the volcano. Submerged calderas, like Volcano-F, are common in the wider Southwest Pacific region, with many such calderas producing regular eruptions, implicating active magmatic recharge. Our findings motivate a need to carefully monitor and characterise even apparently small eruptions at this volcano, and others along the Tonga-Kermadec Arc. This is because such eruptions have the potential to subsequently prime or trigger more explosive eruptions and provide critical geochemical evidence about the plumbing system and evolution of the volcano, essential for understanding the diverse hazards they pose.

Why are oceanic arc basalts Ca-rich and Ni-poor? Insights from olivine-hosted melt inclusions from Kibblewhite Volcano in the Kermadec arc

Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# >65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (>1.0) ratios and olivine crystals with anomalously low nickel contents (<0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed.Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.

A new species of Eustomias (Nominostomias) (Stomiiformes: Stomiidae) from the Kermadec Ridge—Tonga Trench region, western South Pacific Ocean, with notes on the barbel morphology of E. trewavasae

A new species of dragonfish Eustomias (Nominostomias) robertsi is described from the western South Pacific Ocean. The new species, the first record of a member of Eustomias (Nominostomias) Group III (sensu Gibbs et al. 1983) from the region, it is most similar to Eustomias suluensis in barbel morphology. However, it has a smaller proximal and distal bulb distance, the smallest distal inter–bulb value in the group, and much longer terminal filaments. Comments are also provided on the morphology of the diagnostic features on the barbel of Eustomias trewavasae, including an important character overlooked in the original description of the species. The barbel morphology is redescribed taking this into account. A brief observation is made recording the first record of the subgenus Triclonostomias from the western South Pacific Ocean.

Conversion from seismic to underwater sound waves along the Louisville Seamount Chain.

The conversion from seismic to ocean-acoustic waves occurs in different places on the bottom of the ocean, often hundreds to thousands of kilometers away from the epicenter. Here, we investigate this conversion process by studying 15 large-magnitude earthquakes that occurred between 2014 and 2022 along the Kermadec Arc in the southwestern Pacific Ocean. To pinpoint the location where seismic-to-acoustic conversion takes places, we analyze hydroacoustic signals recorded by a hydrophone triplet station of the International Monitoring System in the Juan Fernández archipelago. Results from direction-of-arrival and travel-time calculations indicate that the location of the conversion zone largely matches segments of the Louisville Seamount Chain, its lateral extent ranging from approximately 300 to 1800 km, and its location depending on the geometry between earthquake epicenter and the seamounts.

Highest resolution high signal-to-noise ratio sonic slowness and application to Brothers Volcano, Kermadec Arc, New Zealand

High resolution and accurate acquisition of underground sonic properties is essential for energy detection and earth exploration. An effective method is developed to accurately calculate sonic slowness at the highest resolution (receiver spacing of 0.5 ft) with a high signal-to-noise ratio. The method uses supervirtual interferometry to reconstruct a large number of waveforms for slowness extraction using redundant information from overlapping interreceiver spacing. Synthetic calculations are used to verify the validity of the method, especially in the limited case of strong noise interference and the presence of damaged channels in the array receivers, to accurately calculate the formation of sonic slowness. The theory is applied to process and interpret low data-quality field measurements acquired at Brothers Volcano from the International Ocean Discovery Program. The processing analysis indicates that in the case of very weak formation signals, the residual tool waves from the wireline sonic logging can still interfere with the high-resolution processing, whereas this method can effectively suppress this interference and accurately obtain the formation sonic velocity.

Significant Impact of Hydrothermalism on the Biogeochemical Signature of Sinking and Sedimented Particles in the Lau Basin

AbstractIron (Fe) is an essential micronutrient for diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly Fe, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the spatio‐temporal impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material (up to 88 mg d−1), indicative of the high productivity of the region. A significant portion of this material (∼21 ± 12 wt.%) was lithogenic of hydrothermal origin, as revealed through Al‐Fe‐Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates (&gt;47 cm kyr−1) and the presence of large, heterogeneous, metal‐rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size (1–976 μm) and the significant excess of Fe and Mn (2–20 wt.%). Overall, this study revealed that hydrothermal sources have a significant influence on the biogeochemical signature of particles in the region.

Hydrothermal vent fauna of the western Pacific Ocean: Distribution patterns and biogeographic networks

AbstractAimDeep‐sea hydrothermal vent habitats support a low‐diversity fauna in which most species are unique to the ecosystem. To inform conservation planning around this vulnerable marine ecosystem, we examine species distributions over a wide area to assess the underlying beta‐diversity components and to examine biogeographic patterns. We assess the concept of a highly connected fauna that would repopulate areas of local extinction from distal locations.LocationWestern Pacific Ocean from Japan to New Zealand.MethodsWe assemble a database of 295 confirmed species records for 11 western Pacific vent systems. The SET beta‐diversity framework supports query of the distribution of pairwise pattern components in comparisons among vent systems. We build a network based in graph theory to examine connectivity among vent systems based on shared species similarity. A bipartite network revealed the relative role of each species in linkages among vent system nodes. We assess the importance of sampling bias and distance between systems.ResultsOverall, two‐thirds of the taxa are restricted to a single basin or arc. The Mariana Trough system has the highest corrected weighted endemism for vent‐specific species, followed by that of the Okinawa Trough. Species replacement is the dominant feature of beta‐diversity. Eleven vent systems form seven network modules with stronger connectivity in the Southwest than Northwest Pacific. The Manus Basin vent system emerges as a network ‘hub’ reflecting its central geographic near the equator.Main ConclusionTwo western Pacific biogeographic provinces arise, north and south of the equator that few species transcend. Local and regional conservation plans should consider the low network connectivity and high system endemism in management of hydrothermal vent ecosystems in the event of seabed mining. Species recruitment is unlikely to transcend vent system boundaries. We identify Okinawa Trough, Mariana Trough, Manus Basin, Feni‐Tabar Arc and Kermadec Arc for development of conservation plans that initiate or expand protection.

Reconstructing the timing and conditions of fragmentation from water speciation in ash during a deep submarine eruption: 2012 eruption of Havre, Kermadec Arc

The lack of visual observations and difficulty recreating ambient conditions in the laboratory mean deep subaqueous volcanic eruptions (>500 m water depth) remain enigmatic. However, the pressure and temperature dependency of water speciation and solubility in volcanic melts and glasses, and water's high diffusion rate, offers a potential window into the processes and conditions during deep subaqueous volcanism.The 2012 submarine eruption of Havre Volcano is the ideal laboratory in which to use water species distributions to understand submarine eruption processes. Post-eruption AUV mapping, and ROV observation and sampling generated an unprecedented amount of contextual information for a deep submarine eruption including on the magma ascent conditions, eruption sequence, and magma fragmentation. Here we present high spatial resolution synchrotron-FTIR measurements of water speciation in glassy ash (125–500 μm) from several subunits (1, 2 and 3) of a widespread ash with lapilli deposit produced in the 2012 eruption.Measurements record OH depletion profiles around vesicles, along with H2Omol enrichment profiles. In several thick vesicle walls far-field regions unaffected by processes at the bubble margins have also been identified. By focusing on OH concentrations, which is effectively non-diffusing below the glass transition, we can remove the effects of secondary rehydration and calculate the pressure and water depth at which ash grain quenched.Far-field regions record quench depths within the 650–900 m water depth range for pre- and post- eruption vent depth, suggesting ash grains equilibrated at vent depth. OH depletion profiles produce quench pressures on vesicle margins typically corresponding to 200–400 m water depth shallower, suggesting a 2–4 MPa decompression before ash quench. Diffusion modelling suggests that at eruption temperature the observed OH depletion profiles formed through exsolution in <2 s. The difference in quench pressure within the ash is not reflective of a difference in the depth of quenching. Rather it is reflective of the rapidity of the quench process relative to the diffusion rate.The results imply transient low-pressure conditions formed during ash generation in the 2012 Havre eruption. We discuss several physical processes in the context of submarine volcanism that may explain the formation of transient low-pressure regions. The results have implications for how we understand submarine eruption processes and highlight the potential of high resolution FTIR for studying submarine volcanism.

Magnesian Andesites from Kibblewhite Volcano in the Kermadec Arc, New Zealand

Abstract Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (&amp;lt;15 km in thickness); however, there are no studies that corroborate the formation of primary andesitic magmas in the arc. The aim of this study is to develop a better understanding of primary andesites in oceanic arcs through the petrology of the Kermadec arc. Here, we present the petrology of volcanic rocks dredged from the Kibblewhite Volcano in the Kermadec arc during the R/V SONNE SO-255 expedition in 2017. Magma types range from andesite to rhyolite at the Kibblewhite Volcano, but basalts dominate at the neighboring cones. This study focuses on magnesian andesites from the northeastern flank of this volcano. The magnesian andesites are nearly aphyric and plagioclase free but contain microphenocrysts of olivine (Fo84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt % SiO2 and 10–12 wt % MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (&amp;gt;300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Constraining temporal variations in metal and sulfur sources using high-resolution mineral-scale analysis of pyrite: evidence from the Brothers volcano, Kermadec arc, New Zealand

Constraining temporal variations in metal and sulfur sources using high-resolution mineral-scale analysis of pyrite: evidence from the Brothers volcano, Kermadec arc, New Zealand

Developing a South Pacific tephra framework: Initial results from a Samoan Holocene sequence

ABSTRACTTephra preserved in sediments form useful isochronous marker layers, linking disparate geological, palaeoenvironmental and archaeological records. The application of tephrochronology is greatly enhanced through the detection of macroscopically invisible tephra (cryptotephra). Here, we identify two discrete cryptotephra in Samoan lake sediments, the first identification of cryptotephra in the region outside of New Zealand. Geochemical data suggest one ash layer is from a local Samoan source, providing the first data on an eruption of this age, adding to knowledge of the local volcanic record. The second has a distinctive rhyolitic glass composition, which matches either that of Raoul Island in the Kermadec Arc (1800 km south of Samoa), or two currently submarine volcanoes in the Tongan Arc, ‘Volcano F’ and Lateiki/Metis Shoal (550 and 700 km south of Samoa, respectively). In all possible source cases, this points to a regionally significant eruption of a Kermadec–Tongan volcano at ca. 10 000 a bp. The study marks the first step in the establishment of a South Pacific tephra framework that can be used to answer questions about the synchronicity of changes in hydroclimate, vegetation and early Polynesian migration patterns, as well as providing more information on the volcanic history of Pacific island volcanoes.

Two Volcanic Tsunami Events Caused by Trapdoor Faulting at a Submerged Caldera Near Curtis and Cheeseman Islands in the Kermadec Arc

AbstractTwo submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra‐caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault‐reservoir interaction in calderas.

High abundance of hydrocarbon-degrading Alcanivorax in plumes of hydrothermally active volcanoes in the South Pacific Ocean

Species within the genus Alcanivorax are well known hydrocarbon-degraders that propagate quickly in oil spills and natural oil seepage. They are also inhabitants of the deep-sea and have been found in several hydrothermal plumes. However, an in-depth analysis of deep-sea Alcanivorax is currently lacking. In this study, we used multiple culture-independent techniques to analyze the microbial community composition of hydrothermal plumes in the Northern Tonga arc and Northeastern Lau Basin focusing on the autecology of Alcanivorax. The hydrothermal vents feeding the plumes are hosted in an arc volcano (Niua), a rear-arc caldera (Niuatahi) and the Northeast Lau Spreading Centre (Maka). Fluorescence in situ hybridization revealed that Alcanivorax dominated the community at two sites (1210–1565 mbsl), reaching up to 48% relative abundance (3.5 × 104 cells/ml). Through 16S rRNA gene and metagenome analyses, we identified that this pattern was driven by two Alcanivorax species in the plumes of Niuatahi and Maka. Despite no indication for hydrocarbon presence in the plumes of these areas, a high expression of genes involved in hydrocarbon-degradation was observed. We hypothesize that the high abundance and gene expression of Alcanivorax is likely due to yet undiscovered hydrocarbon seepage from the seafloor, potentially resulting from recent volcanic activity in the area. Chain-length and complexity of hydrocarbons, and water depth could be driving niche partitioning in Alcanivorax.

Morphological diversity and a ribosomal phylogeny of Rhabdopleura (Hemichordata: Graptolithina) from the Western Pacific (Singapore and New Zealand), with implications for a re-evaluation of rhabdopleurid species diversity

The recent discovery of Rhabdopleura in Singapore and the chance collecting of fresh material from northern New Zealand (Three Kings Shelf) provided an opportunity to sequence the specimens with an aim to determine their species identity. Phylogeny reconstructions of two new Rhabdopleura taxa based on ribosomal and mitochondrial genes suggest a different identity from known samples, including putative Rhabdopleura annulata, first described from the Three Kings Shelf but sequenced from the Great Barrier Reef. Pairwise distances between rhabdopleurids for the 16S rRNA locus were several magnitudes larger than that of 18S rRNA, and might potentially be a suitable barcoding gene once sufficient samples of conspecifics are collected to determine the barcoding gaps. Type material of R. annulata was re-examined, as well as Rhabdopleura material from eight other New Zealand localities from north of subtropical Raoul Island (Kermadec Ridge) at ~29° S to the subantarctic Campbell Plateau at ~49° S. Six morphological characters, four of them new, were applied to all samples. The findings from morphology suggest (1) the holotype and cotype of R. annulata might not be conspecific; (2) there are 3–4 variants (species?) on the Three Kings Shelf; and (3) there are 2–3 additional variants (species?) elsewhere in the New Zealand region.

Chromium stable isotope distributions in the southwest Pacific Ocean and constraints on hydrothermal input from the Kermadec Arc

Special attention has been given to chromium (Cr) as a paleoproxy tracing redox cycling throughout Earth’s history, due to differences in the solubility of its primary redox species at Earth’s surface (Cr(III) and Cr(VI)) and isotope fractionation associated with their interconversion. In turn, chromium’s paleoproxy potential has motivated studies of the modern ocean to better understand which processes drive its cycling and to constrain their impact on the Cr isotope composition (δ53Cr) of seawater. Here, we present total dissolved seawater Cr concentrations and δ53Cr along the GEOTRACES GP13 section. This section is a zonal transect extending from Australia in the subtropical southwest Pacific Ocean. Surface signals of local biological Cr cycling are minimal, in agreement with distributions of dissolved major nutrients as well as biologically-controlled trace metals in this low productivity, oligotrophic environment. Depth profiles have Cr concentration minima in surface waters and maxima at depth, and are largely shaped by the advection of nutrient- and Cr-rich subsurface waters rather than vertically-driven processes. Samples close to the sediment–water interface indicate important benthic Cr fluxes across the section.The GP13 transect crosses the hydrothermally-active Kermadec Arc. Hydrothermal fluids (consisting of <15% background seawater) were collected from three venting sites at the Brothers Volcano (along the Kermadec Arc). These fluids yielded near-crustal δ53Cr values (−0.17 to +0.08‰) and elevated [Cr] (7.5–23 nmol kg−1, hydrothermal endmember [Cr] ≈ 8–27 nmol kg−1), indicating that the Kermadec Arc may be an isotopically light Cr source. Dissolved [Fe] enrichments have been reported previously in deep waters (∼1600–3000 m) along the GP13 transect, east of the Kermadec Arc. These same waters show elevated [Cr] compared to Circumpolar Deep Water ([Cr] = 3.88 ± 0.11, δ53Cr = 0.89 ± 0.08, n = 32), with an average [Cr] accumulation of 0.71 ± 0.11 nmol kg−1 (1 SD), and an estimated δ53Cr of +0.46 ± 0.30‰ (2 SD, n = 9) for the accumulated Cr. Comparing high-temperature vent and neutrally buoyant plume data, hydrothermal-sourced Cr is likely negligable compared to Cr contributions from other processes (benthic fluxes, release from particles), and the advection of more Cr-rich Pacific Deep Water. It is unlikely that hydrothermal vents would be a major contributor within the regional or global biogeochemical Cr cycle, even if hydrothermal fluxes change by orders of magnitude, and therefore δ53Cr trends in the paleorecord may be attributable, at least in part, to major changes in other controls on Cr (e.g. widespread anoxia).

Effects of variable slab components and tectonics on magma composition in the intra-oceanic Kermadec Arc-Backarc system

The Kermadec Arc-Backarc system (25–39°S) in the southwest Pacific is an important natural laboratory to examine the controls on arc magma compositions, because the physical parameters that are thought to influence subduction-related magmatism (the thickness and composition of the upper plate, mantle depletion and partial melting, and the slab components) change along the arc. Furthermore, the presence of back-arc and rear-arc volcanism allow insight into how mantle composition varies with distance to the trench. We present new major and trace element and Pb–Sr–Nd–Hf isotope data for 101 lava samples from 10 volcanoes along the Kermadec Arc, and combine these with published data for volcanic arc front, rear-arc, and back-arc lavas. The Kermadec Arc-Backarc magmas experienced similar fractional crystallization paths, and those erupted onto oceanic crust are not affected by crustal assimilation. The degree of partial melting varies along and across the arc, partly due to variations in the thickness of the lithosphere of the upper plate, but also due to variations in slab fluid input. We distinguish four different arc segments varying from 100 to 600 km wide, which erupt lavas with distinct compositions due to differences in along-arc slab input. All Kermadec arc and rear-arc lavas were derived from mantle sources to which the sediment melt and a fluid from subducted Pacific oceanic crust or the Hikurangi Seamounts had been added. Increasing sediment melt input into the mantle wedge southwards is reflected in increasing 87Sr/86Sr and Th/Nd, and decreasing 143Nd/144Nd southwards towards New Zealand. Some volcanoes from the central Kermadec Arc show with unusually radiogenic Pb isotope compositions that are probably derived from subducted alkaline rocks from seamounts built on the Hikurangi Plateau. The geochemical segmentation of the Kermadec Arc indicates that slab-derived components can be relatively homogeneous on large scales, but small features like seamounts cause complex additions to the slab component.

Megameter propagation and correlation of T-waves from Kermadec Trench and Islands

On 18 June 2020 and 4 March 2021, very energetic low-frequency underwater T-wave signals (2 to 25 Hz) were recorded at the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydrophone stations in the Pacific Ocean (Stations HA11 and HA03) and the South Atlantic Ocean (Station HA10). This work investigates the long-range (megameters) propagation of these T-waves. Their sources were three powerful submarine earthquakes in the Kermadec Trench and Islands, located at approximately 6000, 8800, and 15100 km from Stations HA11, HA03, and HA10, respectively. Arrival time and back azimuth of the recorded T-waves were estimated using the Progressive Multi-Channel Correlation algorithm installed on the CTBT Organization (CTBTO) virtual Data Exploitation Centre (vDEC). Different arrivals within the duration of the earthquake signals were identified, and their correlations were also analyzed. The data analysis at HA03 and HA10 revealed intriguing T-wave propagation paths reflecting, refracting, or even transmitting through continents, as well as T-wave excitation along a chain of seamounts. The analysis also showed much higher transmission loss (TL) in the propagation paths to HA11 than to HA03 and HA10. Moreover, strong discrepancies between expected and measured back azimuths were observed for HA11, and a three-dimensional (3D) parabolic equation model was utilized to identify the cause of these differences. Numerical results revealed the importance of 3D effects induced by the Kermadec Ridge, Fiji archipelago, and Marshall Islands on T-wave propagation to HA11. This analysis can guide future improvements in underwater event localization using the CTBT-IMS hydroacoustic sensor network.

Sources of Mg Enrichments in Vent Fluids From the Kermadec Arc Recorded by Li, B, and Mg Isotopes

AbstractMagnesium concentrations in acid‐sulfate fluids from Brothers and Macauley volcano, Kermadec arc, Western Pacific, exceed those of seawater and differ from previously known acid‐sulfate fluids with seawater‐like Mg concentrations. Earlier studies explained these Mg enrichments as due to phase separation, caminite (Mg1.25SO4(OH)0.5 × 0.5H2O) dissolution, water‐rock interaction and/or “mining” of deep seated magmatic brines. Since these different and partly contradictory explanations could neither be confirmed nor refuted, we investigated B, Li, and Mg isotope compositions in acid‐sulfate and black smoker fluids from both Brothers and Macauley to understand the origin of the high Mg concentrations and investigate a possible connection with the alteration of basement rocks. In addition, we performed seawater heating experiments to investigate Mg isotope fractionation during caminite precipitation. Lithium and B isotope signatures in all studied acid‐sulfate fluids indicate water‐rock interaction. Magnesium isotopes in fluids from Macauley correspond to seawater, implying Mg enrichment through phase separation in the fluids. In contrast, Mg‐enriched acid‐sulfate fluids from Brothers volcano have higher δ26Mg values (up to −0.62‰) than seawater. Considering that our seawater heating experiment shows a preferential incorporation of 24Mg into caminite, caminite dissolution would lead to lower δ26Mg values and thus cannot solely account for the enriched Mg concentrations seen at Brothers. Additionally, we thus suppose leaching of Mg from the basement during the interaction of acidic fluids with a relatively unaltered basement.