The Hawaiian–Emperor volcanic chain has a distinctive bend. Geochemical analyses show that lavas erupted on the ocean floor close to the bend formed during deformation of the Pacific Plate, implying the bend was caused by changes in plate motion. Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes. Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor1 and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions1,2,3. A distinctive bend in the Hawaiian–Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate4,5, movement of the Hawaiian plume6,7,8, or a combination of both9. However, these links are uncertain because there is no independent record that precisely dates tectonic events that affected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian–Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. 40Ar/39Ar dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian–Emperor bend, 53–52 and 48–47 million years ago. We conclude that the Hawaiian–Emperor bend was formed by plate–mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins.

Estimates of the relative motion between the Hawaiian and Louisville hot spots have consequences for understanding the role and character of deep Pacific‐mantle return flow. The relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails. We report 40Ar/39Ar ages for 18 lavas from 10 seamounts along the Hawaiian‐Emperor Seamount Chain (HESC), showing that volcanism started in the sharp portion of the Hawaiian‐Emperor Bend (HEB) at ≥47.5 Ma and continued for ≥5 Myr. The slope of the along‐track distance from the currently active Hawaiian hot spot plotted versus age is constant (57 ± 2 km/Myr) between ∼57 and 25 Ma in the central ∼1900 km of the seamount chain, including the HEB. This model predicts an age for the oldest Emperor Seamounts that matches published ages, implying that a linear age‐distance relationship might extend back to at least 82 Ma. In contrast, Hawaiian age progression was much faster since at least ∼15 Ma and possibly as early as ∼27 Ma. Linear age‐distance relations for the Hawaii‐Emperor and Louisville seamount chains predict ∼300 km overall hot spot relative motion between 80 and 47.5 Ma, in broad agreement with numerical models of plumes in a convecting mantle, and paleomagnetic data. We show that a change in hot spot relative motion may also have occurred between ∼55 Ma and ∼50 Ma. We interpret this change in hot spot motion as evidence that the HEB reflects a combination of hot spot and plate motion changes driven by the same plate/mantle reorganization.

The Calypso Hydrothermal Vent Field (CHVF) is located along an offshore extension of the Taupo Volcanic Zone (TVZ), an area of abundant volcanism and geothermal activity on the North Island of New Zealand. The field occurs within a northeast-trending submarine depression on the continental shelf approximately 10–15km southwest of the White Island volcano in the Bay of Plenty. The graben has been partially filled by tephra from regional subaerial volcanic eruptions, and active hydrothermal venting occurs at several locations along its length. The vents occur at water depths of 160 to 190m and have temperatures up to 201°C. Recovered samples from the vent field include variably cemented and veined volcaniclastic sediments containing an assemblage of clay minerals, amorphous silica, barite, As–Sb–Hg sulfides, and abundant native sulfur. The volcanic glass has been altered primarily to montmorillonite and mixed-layer illite–montmorillonite; illite, and possibly minor talc and mixed-layer chlorite–smectite or chlorite–vermiculite are also present. A hydrothermal versus diagenetic origin for the smectite is indicated by the presence of both illite and mixed-layer clays and by the correlation between the abundance of clay minerals and the abundance of native sulfur in the samples. The mineralization and alteration of the volcanic host rocks are similar to that observed in near-neutral pH geothermal systems on land in the TVZ (e.g., Broadlands–Ohaaki). However, the clay minerals in the CHVF have a higher concentration of Mg in the dioctahedral layer and a higher interlayer Na content than clay minerals from Broadlands–Ohaaki, reflecting the higher concentrations of Mg and Na in seawater compared to meteoric water. Minerals formed at very low pH (e.g., kaolinite and alunite), typical of steam-heated acid-sulfate type alteration in the TVZ geothermal environment, were not found. Mixing with seawater likely prevented the formation of such low-pH mineral assemblages. The occurrence of illite and mixed-layer illite–smectite close to the seafloor in the CHVF, rather than at depth as in the Broadlands system, is interpreted to reflect the higher pressures associated with submarine venting. This allows hotter fluids to be discharged before they boil, and thus minerals that are encountered mainly at depth in subaerial geothermal systems can form close to the seafloor.

New geochemical data are presented for fresh lavas from off-axis seamounts east and west of the Valu Fa Ridge and from seamounts south of the propagating rift tip of the Valu Fa Ridge. Most seamount lavas are basaltic and thus more primitive than the mostly andesitic to dacitic lavas from the adjacent Valu Fa Ridge. Slight differences in fluid-mobile element ratios and in Sr isotope ratios suggest small-scale heterogeneities of the slab component in this region with an apparently higher sedimentary input into the off-axis seamount lavas than into the island arc magmas. Off-axis volcanoes near back-arc basin spreading centres thus resemble off-axis seamounts adjacent to mid-oceanic ridges which also show more extreme compositions than the well-homogenized mid-ocean ridge basalts. The geochemical variations support a diapiric ascent of the hydrated or partially molten mantle leading to separate magma supply systems for the different volcanic structures. The mantle wedge between the Valu Fa Ridge and the island arc appears to be uniformly depleted in fluid-immobile incompatible elements compared to average depleted mid-ocean ridge basalts whereas lavas from seamounts west of the Valu Fa Ridge show less depletion in agreement with a depletion event of the whole mantle wedge in the backarc melting zone. Seamount lavas south of the Valu Fa Ridge propagating rift resemble the Valu Fa lavas indicating an increased magma production caused by the influx of water into the mantle.

An active seafloor hydrothermal system subjects the background sediments of the Grimsey Graben (Tjörnes Fracture Zone) to alteration that produces dissolution of the primary volcaniclastic matrix and replacement/precipitation of sulfides, sulfates, oxides, oxyhydroxides, carbonates and phyllosilicates. Three types of hydrothermal alteration of the sediment are defined on the basis of the dominant hydrothermal phyllosilicate formed: smectite, kaolinite, chlorite. The most common alteration is near‐total conversion of the volcaniclastic material to smectite (95–116°C). The dominant smectite in the deepest sediments sampled is beidellite, which is replaced by montmorillonite and an intimate mixture of di‐ and tri‐octahedral smectite up core. This gradual vertical change in smectite composition suggests an increase in the Mg supply upward, the result of sediment alteration by the ascending hydrothermal fluids mixing with descending seawater. The vertical sequence kaolinite → kaolinite‐smectite mixed‐layer → smectite from bottom to top of a core, as well as the distinct zonation across the veins (kaolinite in the central zone → kaolinite‐smectite in the rim), suggests hydrothermal transformation of the initially formed smectite to kaolinite through kaolinite‐smectite mixed‐layer (150–160°C). The cause of this transformation might have been an evolution of the fluids toward a slightly acidic pH or a relative increase in the Al concentration. Minor amounts of chamosite fill thin veins in the deepest sections of some cores. The gradual change from background clinochlore to chamosite across the veins suggests that chamosite replaces clinochlore as Fe is made available from hydrothermal dissolution of detrital Fe‐containing minerals. The internal textures, REE distribution patterns and the mode of occurrence of another magnesian phyllosilicate, kerolite, suggest that this mineral is the primary precipitate in the hydrothermal chimneys rather than an alteration product in the sediment. Kerolite precipitated after and grew on anhydrite in the chimneys. Oxygen isotope ratios are interpreted to reflect precipitation of kerolite at temperatures of 302° to 336°C. It accumulated in the hydrothermal mounds following the collapse of the chimneys and subsequent dissolution of anhydrite, thereby forming highly permeable aquifer layers underlying the vent field. Some kerolite was redeposited in the near vent field sediments by turbidity flows. The altered sediments are depleted in Mn, Rb and Sr, and enriched in U, Mo, Pb, Ba, As, Bi, Sb, Ag, Tl and Ga, as a result of leaching and precipitation, respectively. Conservative elements (Ti, Zr, Hf, Sc, Cr, Nb and Sn) are depleted or enriched in the altered sediments because of passive (precipitation or leaching of other phases) rather than active (because of their mobility) processes.

Monowai submarine volcano is one of the three most historically active volcanoes of the Kermadec arc. Repeat multibeam surveys of Monowai Cone from September 1998 and September 2004 and T wave data recorded by the Réseau Sismique Polynésien network for the same period document the collapse and subsequent regrowth of the cone within this 6‐a period. Grid differencing of the two bathymetric data sets, acquired 6 a apart, reveals that a landslide ∼2230 m long occurred between the surveys, within which a postcollapse cone and talus ridge (∼0.023 km3 in volume) subsequently formed. The volume of this collapse, minus postcollapse construction, is ∼0.085 km3. We interpret an unusual, strong‐amplitude T wave event on 24 May 2002 as recording “hot landsliding”, where the 100‐ to 160‐m‐thick collapse has “unroofed” the uppermost parts of the vent conduit, with the subsequent explosive interaction, and cooling, of hot magma and volcaniclastic rubble with ambient seawater. This interpretation is consistent with the lack of emergent events, sharp onset, and large amplitude of the 24 May 2002 T waves. The subsequent >2500 T wave events, between November 2002 and September 2004, occurred in swarms with emerging and waning activity and with typical explosive volcanic acoustic signatures, which are interpreted as recording the regrowth of an ∼90‐m‐high cone back to a near‐1998 elevation, at an average rate of 47 m a−1. This study provides (1) a lower bound for frequency‐magnitude relationships of landsliding for submarine arc volcanoes and (2) estimates of 0.013 km3 a−1 of submarine cone growth during eruptive cycles.

A submersible dive conducted on Volcano #1 located near 21 degrees 09'S-175 degrees 45'W on the Tonga Arc showed that the volcanic edifice with a caldera floor area of 30 km(2) located at and 450 m deep (b.s.l.=below sea level) was constructed recently during episodic volcanism. The sequential volcanic events are recorded along a faulted terrain formed in response to the collapse of the caldera wall. The post-caldera events are marked by occasional eruptions that have built scoriaceous cones associated with low-temperature hydrothermal venting and localized small-scale collapse features. The stratigraphy of the caldera wall indicates that the volcano was built by explosive volcanism alternating with quieter eruptive events. The repeated, violent explosive events formed <= 20 m thick sequences composed of alternating fine-grained ash beds and sand- to boulder-sized pyroclastic layers. During quieter volcanic events. dykes and massive flows intruded and/or accompanied the eruption of the volcaniclastic deposits throughout the sections of the wall explored. Massive columnar-jointed flows consist of viscous, silica-rich lavas forming tabular and giant radial-jointed (GRJ) flows formed in large (>8 m in diameter) conduits and extruded onto the sea floor. In addition, massive lava flows forming sill-like complexes were observed underneath and near the giant radial-jointed columnar flows. Also, an intermittent quiet type of eruption produced vesicular lava flows, which are interbedded within the pyroclastic layered deposits. The massive and vesicular lavas consist of andesites and dacites with Ca-depleted (pigeonite) and Ca-enriched (salite) pyroxene, and intermediate (andesine-labradorite) to calcic (bytownite) plagioclase. They are depleted in total alkalis (Na2O+K2O <3%), K2O (<1%), Zr/Y (<1.8), Nb/Zr (<0.01) and light Rare Earth Elements. We interpret that these andesite-dacite series were erupted after undergoing crystal-liquid fractionation in a magma chamber located underneath the caldera floor. (C) 2008 Elsevier B.V. All rights reserved.

The element compositions Si, Ca and Al of up to 2 1.1 ka old sediments in about 10 in long cores from the southern basin of the Shaban and Kebrit deeps in the northern Red Sea allowed a classification of major sediment types in carbonate sands and -muds and siliceous oozes. A FeOOH-enriched sediment horizon and a few samples with high Zn values in the Kebrit core indicate a hydrothermal origin probably near the brine-sea water interface with subsequent transport of hydrothermal compounds into the deep sediments.High organic carbon contents up to 8.4% are positively correlated with the Ba concentrations, which suggests that high bioproductivity, and rapid deposition (C-14 dating suggests a sedimentation rate near 70 cm/ka) led to the formation of sapropelic sediments between 11.8 and 13.6 ka (Younger Dryas). Organic petrological observations showed that the sediment organic material largely consists of <20 gm-sized roundish fecal pellets (intimate mixtures of organic matter and inorganic constituents) and bituminite. Terrestrial organic matter (pollens of land plants, fusinite etc.) is very rare in the sediment cores from both deeps.Organic-geochemical investigations of kerogens and organic extracts show that a significant (hydrothermal) hydrocarbon production did not occur in near-surface sediments of the Shaban and Kebrit deeps. Rock Eval pyrolysis of kerogens characterised the organic matter to be of type II quality. The delta C-13 values of the kerogens from the most prominent sapropel in the Shaban deep indicate an enrichment of(C-12-rich) nutrients in the water column during postglacial sapropel formation in the Younger Dryas. The n-alkane spectra are dominated by short chain lengths between n-C-15 and n-C-25 Prevailing n-C-15 to n-C-25 alkanes in low mature sediments are indicative of algal and microbial source. Pristane/phytane ratios are generally low (< I to similar to 1) which suggests that anoxic conditions prevailed within the anaerobic brine-filled deeps for the whole time covered by the sediments. This again indicates that sapropel formation was caused by high bioproductivity in the northern Red Sea rather than episodic stagnation with better preservation of the organic matter.Long-chain alkenones and sterols are the dominating compounds of the lipid fraction. Cholesterol contents in the sediment cores reflect phases of eukaryotes production in the water column, whereas the positive correlations of dinosterol with TOC and the amounts of total extract suggests that the major organic carbon source in the northern Red Sea during postglacial high-productivity stages were dinoflagellates. Another important carbon source, however, is indicated by the occurrence of 22,29,30-trisnorhopan-21 -one (TNH). Although the formation of TNH from its precursors is not fully understood, this compound probably results from microbial. degradation of intact bacteriohopanepolyols (BHP), which can be used as indicators for bacterial abundances and phyla. TNH is most likely produced at the brine-sea water interface where sedimenting organic matter accumulates and, if the redoxcline corresponds to the density gradient, the organic matter is subjected to efficient aerobic bacterial degradation processes. However, during high bioproductivity stage (Younger Dryas) the redoxcline was probably higher in the water column and thus, a significant TNH production at the brine-sea water interface did not occur at times of sapropel formation in the northern Red Sea deeps. (C) 2007 Elsevier B.V All rights reserved.

Quantifying hydrothermal venting at the boundaries of tectonic plates is an outstanding geoscience problem. Considerable progress has been made by detailed surveys along mid‐ocean ridges (MORs), but until recently little was known about fluid venting along volcanic arcs. We present the first systematic survey for hydrothermal venting along the 425‐km‐long south Tonga arc and new chemistry data for particle and thermal plumes previously reported along an adjacent 88‐km‐long section of the back‐arc Valu Fa Ridge (VFR). Eleven hydrothermal plumes, recognized by their anomalous light backscattering, Eh, temperature, pH, dissolved 3He, CH4, and total dissolvable Fe and Mn, were identified arising from seven volcanic centers along the arc. Five plumes on the VFR were characterized chemically. Vent field density for the south Tonga arc was 2.6 sites/100 km of arc front, comparable to that found by surveys of the Kermadec arc (1.9 to 3.8 sites/100 km) and to MORs in the eastern Pacific (average value for 2280 km of surveyed ridgecrest: 3.2 sites/100 km). A “vent gap” occurs along a 190 km section of the arc closest to the VFR, and a site density twice the average for MORs on the eastern edge of the Pacific plate was found on this part of the VFR (6.6 sites/100 km). We suggest magmas ascending under the adjacent south Tonga arc have been captured by the VFR. While chemical enrichments of plumes on the south Tonga arc were, in general, slightly less than those on the Kermadec arc, several instances of excessive anomalies in pH suggest a similar presence of fluids enriched in magmatic volatiles (CO2‐SO2‐H2S). Locally, venting on the VFR has contributed to accumulations of 3He, Fe, and Mn within the southern Lau basin. On a broader scale, our results provide considerable support for the notion that venting from intraoceanic arcs on the convergent margin of the Pacific plate adds significantly to the total hydrothermal input into the Pacific Ocean.

The well-studied Galápagos Archipelago is a small part of the much larger Galápagos Volcanic Province (GVP) consisting of the Cocos, Carnegie, Coiba and Malpelo aseismic ridges and related seamount provinces. Although these aseismic ridges and seamounts dominate the morphology of the region, little is known about their origin due to a lack of direct age and geochemical information. In order to establish how well the GVP fits with the predictions of the 'standard' fixed hotspot and mantle plume hypotheses we conducted a first reconnaissance dredge/grab sampling of submerged regions of the GVP. We present here 40Ar/39Ar ages for many of these new basement samples and evaluate their implications for the various models put forward to explain the origin of the GVP. Correlating new and published sample-site ages with distance from the western side of the Galápagos Islands show that volcanism has not progressed in narrow, time-progressive lines of seamounts and ridges as predicted by the conventional fixed hotspot and mantle plume hypothesis. Rather, volcanism apparently migrated time-progressively across the GVP in broad regions of long-lived and possibly concurrent volcanism. We propose that the most viable explanation for these observations is that the GVP is the product of Cocos and Nazca plate motions across a broad hotspot melt anomaly. The complex spreading history of the Cocos–Nazca spreading centre likely controlled the relative distribution of GVP volcanism between the Cocos and Nazca plates while creating lithosphere of variable age/thickness across the region. While the notion of a broad Galápagos hotspot melting anomaly linked to a complex regional tectonic history requires significant testing it nevertheless highlights the need to test alternative mantle upwelling shapes and sizes compared to the widely accepted notion of a narrow continuous long-lived Galápagos mantle plume conduit defined by the size and location of a Galápagos island.

西太平洋中大多数已知的高温热液喷发都出现在洋内弧后环境中,如劳海盆、北斐济海盆和马里亚纳海沟.在弧前的海底火山上已经发现了热液喷发和相应的矿床,其中许多火山还在排气,并有释放到海底的岩浆挥发物的证据,但是只在水深超过1 km的最深的塌陷火山口发现了黑烟喷口、典型深洋中脊和弧后扩张中心.本文描述了汤加岛弧浅海底火山上首次发现的高温热液喷发和大面积的硫化物-硫酸盐烟道.

Submarine hydrothermal vents and associated sealfoor mineralization on the Tonga arc have been found for the first time, in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents (245-265 degrees C) occur at water depths of 385-540 m near the summit of one volcano at 24 degrees S. The vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls. Clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) at the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges.