intra oceanic subduction related hydrothermal venting kermadec volcanic arc new zealand intra oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front more than half seven of 13 of the volcanoes surveyed along 260 km of the southern kermadec arc ne of new zealand are hydrothermally active our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow 2 km water depth vent fields expelling fluids of a unique and heterogeneous composition into the oceans

a decade of exploring a submarine intraplate volcano hydrothermal manganese and iron at lō ihi volcano hawai i 1 decadal time series observations of hydrothermal fluid emissions from l ihi volcano were initiated in 1992 using a combination of submersible and shipboard sampling strategies magmatic tectonic processes associated with a spectacular seismic event in july august 1996 led to the collapse of pele s vents 31 c near the volcano summit into a new pit crater pele s pit inundated by high temperature tmax 198 c focused and lower temperature 30 to 200 fold 1 5 months postcollapse and during the following year decreased a further twofold to threefold at pele s pit and 30 fold at east pit while a steady concentration of 400 nmol l tdfe prevailed throughout the remaining years of this study at pele s pit a gradual and threefold decrease in the concentration of dmn to about 15 nmol l was observed high temperature fluids 128 198 c 1997 1999 90 c 2001 venting simultaneously from different orifices within pele s pit had distinguishable fe mn ratios that can be attributed to different subseafloor origins fe mn ratios characteristic of fluids moderated by high temperature water rock reaction had low values in 1997 1998 1 6 0 7 increasing to about 7 in 1999 fluids moderated by magmatic degassing of co2 had much higher fe mn ratios increasing from 24 15 in 1997 1998 to 50 in 1999 and 63 87 in 2001 fe mn values of dispersing plumes at l ihi reflect an admixture of these sources and a relative fe abundance that is consistently high compared to mid ocean ridge systems the pulsed injection of mn and fe into the surrounding ocean associated with the 1996 tectonic magmatic event at l ihi was massive our decadal observations confirm that mn and fe are useful markers of the magnitude and evolution of the effects of magmatic perturbation on hydrothermal systems influenced by chronic magmatic degassing

dissolved and particulate fe in a hydrothermal plume at 9 45 n east pacific rise slow fe ii oxidation kinetics in pacific plumes production of fe iii particles in hydrothermal plumes is of fundamental importance to the long term effect of hydrothermal circulation on seawater composition to elucidate the fundamental controls on fe redox kinetics and solution particle partitioning in neutrally buoyant plumes we sampled near field 3 km plume particles at 9 45 n on the east pacific rise in 1996 returning in 1997 to sample both particulate and dissolved phases 0 40 m filter concentrations of dissolved fe varied from 320 to 20 nm in proximal 0 3 km from vent site to distal samples 1 3 km downfield constituting 85 50 of total fe respectively based on vent fluid dilution factors calculated from dissolved mn a mass balance for vent fluid fe at this site indicates that 65 of fe is lost to particulate sulfide settling in the buoyant plume and that particulate fe in distal 1 3 km samples is twice as concentrated as predicted from dilution of particles in proximal plume water these observations are consistent with a calculated fe ii oxidation half time of 3 3 h long enough that fe iii colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions preventing generation of high particulate fe concentrations 11 56 nm observed a general investigation of fe ii oxidation rates in plumes worldwide gives fe ii oxidation half lives as short as 17 min at some atlantic sites and as long as 6 h at some pacific sites the calculations indicate that the distribution of fe particles in plumes depends chiefly on inter basin differences in ambient deep water chemistry primarily ph and dissolved o2 and on local currents driving plume dilution and to a much lesser extent on variations in primary vent fluid composition long term changes in thermohaline circulation or ocean biogeochemistry may therefore alter fe dynamics and minor element fluxes associated with global hydrothermal activity independent of variations in crustal production rates

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

hydrothermal activity on near arc sections of back arc ridges results from the mariana trough and lau basin 1 the spatial density of hydrothermal venting is strongly correlated with spreading rate on mid ocean ridges with the interesting exception of hot spot affected ridges evidently because spreading rate is a reliable proxy for the magma budget this correlation remains untested on spreading ridges in back arc basins where the magma budget may be complicated by subduction induced variations of the melt supply to address this uncertainty we conducted hydrothermal plume surveys along slow spreading 40 60 mm yr and arc proximal 10 60 km distant sections of the southern mariana trough and the valu fa ridge lau basin on both sections we found multiple plumes overlying 15 20 of the total length of each section a coverage comparable to mid ocean ridges spreading at similar rates these conditions contrast with earlier reported results from the two nearest arc segments of a faster spreading 60 70 mm yr back arc ridge the east scotia ridge which approaches no closer than 100 km to its arc there hydrothermal venting is relatively scarce 5 plume coverage and the ridge characteristics are distinctly slow spreading small central volcanic highs bookended by deep median valleys and axial melt lenses restricted to the volcanic highs two factors may contribute to an unexpectedly low hydrothermal budget on these east scotia ridge segments they may lie too far from the adjacent arc to benefit from near arc sources of melt supply and subduction aided migration of mantle from the bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening analogous to the reykjanes ridge thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid ocean ridges a well defined trend of spreading rate versus hydrothermal activity on most ridge sections plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

water column hydrothermal plumes on the juan de fuca ridge hydrographic surveys on the juan de fuca ridge jdfr carried out from 1980 to 1987 show a complex pattern of sup 3 he and mn rich water column plumes produced by venting from several submarine hot spring areas in the vicinity of axial volcano at latitude 46 degree n distinct plumes were detected in 1980 1982 and 1983 with sup 3 he signatures up to delta sup 3 he 64 at approximately 1 500 m depth at distances of approximately 10 km from the seamount summit however the same plumes had no detectable thermal signature a paradox which is attributed to the high sup 3 he heat ratios and low salinities of the fluids venting within the caldera of axial volcano profiles directly over the seamount show hydrothermal sup 3 he in the water column up to 300 m above the caldera floor with the sup 3 he signal increasing with depth to very high and uniform ratios of delta sup 3 he 108 150 below the approximately 1 500 m caldera sill depth another apparent locus of hydrothermal input is helium basin a depression on the northeast flank of axial volcano which had delta sup 3 he 51 when first sampled in 1980 however subsequent hydrocasts into helium basin in 1982 and 1983 yielded lowermore helium enrichments suggesting either a decrease in hydrothermal input or flushing of the basin via a mixing event to the south of axial volcano high delta sup 3 he values of approximately 40 observed over the ridge axis at 45 degree 18 prime n and 45 degree 39 prime n indicate venting on this previously unexplored section of the ridge the water column plumes over the us geological survey vent site at approximately 44 degree 40 prime n on the southern jdfr have very high mn sup 3 he ratios of 4 600 mol cm sup 3 an apparently unique characteristic which can be used to distinguish these plumes from those originating at other jdfr vent fields less

tracking submarine volcanic activity at monowai constraints from long range hydroacoustic measurements monowai is a submarine volcanic center in the kermadec arc southwest pacific ocean in the past activity at the volcano had been intermittently observed in the form of fallout at the sea surface discolored water changes in seafloor topography and t phase seismicity but there is no continuous record for more recent years in this study we investigated 3 5 years of recordings at a hydrophone array of the international monitoring system ims located near juan fernandez islands for long range underwater sound waves from monowai results from direction of arrival calculations and density based spatial clustering indicate that 82 discrete episodes of activity occurred between july 2003 and march 2004 and from april 2014 to january 2017 volcanic episodes are typically spaced days to weeks apart range from hours to days in length and amount to a cumulative sum of 137 days of arrivals in total making monowai one of the most active submarine arc volcanoes on earth the resolution of the hydrophone recordings surpasses broadband network data by at least one order of magnitude identifying seismic events as low as 2 2 mb in the kermadec arc region further observations suggest volcanic activity at a location approximately 400 km north of monowai in the tonga arc and at healy or brothers volcano in the southern kermadec arc our findings are consistent with previous studies and highlight the exceptional capabilities of the ims network for the scientific study of active volcanism in the global ocean supporting information

iron manganese and lead at hawaii ocean time series station aloha temporal variability and an intermediate water hydrothermal plume trace metal clean techniques were used to sample hawaii ocean time series hot station aloha on seven occasions between november 1998 and october 2002 on three occasions full water column profile samples were obtained on the other four occasions surface and near surface euphotic zone profiles were obtained together with three other published samplings this site may have been monitored for dissolved 0 4 or 0 2 m fe more frequently than any other open ocean site in the world low fe concentrations 2500 m the concentrations we observe 0 4 0 5 nmol kg 1 are significantly lower than some other deep north pacific stations but are similar to values that have been reported for a station 350 miles to the northeast we attribute these low deepwater values to transport of low fe antarctic bottom water into the basin and a balance between fe regeneration and scavenging in the deep water near surface waters have higher fe levels than observed in the lower euphotic zone significant temporal variability is seen in near surface fe concentrations ranging from 0 2 0 7 nmol kg 1 we attribute these surface fe fluctuations to variable dust deposition biological uptake and changes in the mixed layer depth this variability could occur only if the surface layer fe residence time is less than a few years and based on that constraint it appears that a higher percentage of the total fe must be released from north pacific aerosols compared to north atlantic aerosols surprisingly significant temporal variability and high particulate fe concentrations are observed for intermediate waters 1000 1500 m these features are seen in the depth interval where high 3he from the nearby loihi seamount hydrothermal fields has been observed the total fe 3he ratio implies that the hydrothermal vents are the source of the high and variable fe the vertical profile of mn at aloha qualitatively resembles other north pacific mn profiles with surface and intermediate water maxima but there are some significant quantitative differences from other reported profiles the 0 4 m mn concentration is highest near the surface decreases sharply in the upper 500 m then shows an intermediate water maximum at 800 m and then decreases in the deepest waters these concentrations are higher than observed at a station 350 miles to the northeast that shows similar vertical variations it appears that there is a significant mn gradient throughout the water column from hot towards the northeast compared to the first valid oceanic pb data for samples collected in 1976 pb at aloha in 1997 1999 shows decreases in surface waters and waters shallower than 200 m pb concentrations in central north pacific surface waters have decreased by a factor of 2 during the past 25 yr from 65 to 30 pmol kg 1 surface water pb concentrations in the central north atlantic and central north pacific are now comparable we attribute the surface water pb decrease to the elimination of leaded gasoline in japan and to some extent by the u s and canada we attribute most of the remaining pb in pacific surface waters to asian emissions more likely due to high temperature industrial activities such as coal burning rather than to leaded gasoline consumption a 3 year mixed layer time series from the nearby hale aloha mooring site 1997 1999 shows that there is an annual cycle in pb with concentrations 20 higher in winter months this rise may be created by downward mixing of the winter mixed layer into the steep gradient of higher pb in the upper thermocline pb concentrations double between the surface and 200 m from 200 m to the bottom pb concentrations decrease to levels of 5 9 pmol kg 1 near the bottom for most of the water column thermocline and deepwater pb concentrations do not appear to have changed significantly during the 23 yr interval

manganese and iron in hydrothermal plumes resulting from the 1996 gorda ridge event abstract we sampled hydrothermal plumes over the n gorda ridge four times between march and august 1996 to document mn and fe discharge resulting from a magmatic intrusion seafloor eruption two separate event plumes ep96a and b and chronic hydrothermal emissions lasting 6 months were characterized shipboard time series measurements of an event plume sample were used to calibrate an fe phase clock useful for estimating sample age up to 6 days after fluid discharge samples collected from ep96a and b had mn heat 2 mol mol 1 ratios similar to historical event plume observations we suggest these signature ratio values are generally characteristic of event plumes and hypothesize that mn and fe may be supplied to event plumes by different processes mn by entrainment of fluids from an extant shallow subseafloor reservoir and fe by short lived high temperature water rock reaction coincident with dike emplacement calculations based on the fe phase clock indicate that the two event plumes were released more than a month apart the largest event plume ep96a 2 3 106 m mn and 13 106 m fe formed 7 march soon after seismic activity began the smaller ep96b 0 49 106 m mn and 3 5 106 m fe was not discharged until 11 april 3 weeks after the cessation of seismic activity detectable by sosus t phase monitoring we hypothesize that the subseafloor disturbance that triggered ep96b also resulted in the episodic flushing of a reservoir of chronic plume like fluids total event plume inventories of mn and fe at n gorda ridge are much smaller than those associated with the 1986 event at n cleft segment of the juan de fuca ridge but comparable to event plume inventories at n cleft segment in 1987 and coaxial segment in 1993 mn heat values for chronic plumes over the eruption site underlying ep96a evolved from moderate 0 25 oonmol j 1 reflecting probable admixture with event plume formation fluids to high 0 7 nmol j 1 typical of chronic plumes to low 0 1 nmol j 1 similar to diffuse vent fluid values marking a complete episode of intrusion eruption induced hydrothermal discharge

the importance of shallow hydrothermal island arc systems in ocean biogeochemistry hydrothermal venting often occurs at submarine volcanic calderas on island arc chains typically at shallower depths than mid ocean ridges the effect of these systems on ocean biogeochemistry has been under investigated to date here we show that hydrothermal effluent from an island arc caldera was rich in fe iii colloids 0 02 0 2 m 46 of total fe contributing to a fraction of hydrothermal fe that was stable in ocean water iron iii colloids from island arc calderas may be transferred into surrounding waters generally 0 1500 m depth by ocean currents thereby potentially stimulating surface ocean primary productivity hydrothermal fe oxyhydroxide particles 0 2 m were also pervasive in the studied caldera and contained high concentrations of oxyanions of phosphorus p vanadium v arsenic as and manganese mn hydrothermal island arcs may be responsible for 50 of global hydrothermal p scavenging and 40 v scavenging despite representing 10 of global hydrothermal fluid flow

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

the valu fa ridge the pattern of volcanic activity at a back arc spreading centre abstract the central valu fa ridge an active back arc spreading segment bounded by small overlapping spreading centres can be divided into three regions each 8 12 km long with distinct bathymetric morphological and magnetic characteristics these observations suggest that each region has a different volcanic history driven by variations in melt supply along axis two of the three regions are centred on small deviations of the ridge axis which appear to be the local foci of extrusion there appears to have been a recent re organisation of the magmatic supply to the ridge in the past the highest magmatic budget was at the southern end of the ridge but it has now switched to the northern end where it coincides with a small overlapping spreading centre this re organization may be associated with island are activity beneath the ridge a bright reflector which is interpreted as being from the roof of a magma chamber is laterally more continuous that the inferred volcanic segmentation this implies that mixing of magma within the chamber must be limited along strike the magmatic budget at various parts of the ridge inferred from axial morphology can be correlated with variations in the width and brightness of the reflector this correlation suggests that the size of the chamber is dynamic and changes in response to variations in supply from below or delivery upwards

morphology of the valu fa spreading ridge in the southern lau basin multibeam echosounder surveys and seismic reflection profiling were conducted in the southern lau back arc basin sw pacific the bathymetric survey covered a 130 km long section of the active spreading ridge 3 cm yr half spreading rate the n s trending valu fa ridge vfr while seismic profiles concentrated on a 35 km long ridge section analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber the ridge shows similarities but also differences to the segmentation model developed for mid ocean ridges analogous to this model the section of the vfr studied displays third and fourth order segmentation the amplitudes of the along axis undulations however are a factor of 5 to 10 greater than at mid ocean ridges additionally the vfr has numerous small volcanic cones along the axis which have previously only been observed at much slower spreading ridges in cross section the vfr has a very steep narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas the axial magma chamber appears to vary in width 1 4 km and location relative to the axis beneath different morphological segments perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity a small overlapping spreading center osc this is counter to observations from mid ocean ridges off axis traces of ridge segments allow us to reconstruct the evolution of this osc for the past 220 kyr we conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid ocean ridges but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

submarine volcanism deeply explosive volcanic activity is much more common beneath the oceans than on land yet has been observed only rarely direct measurements of an eruption in the southwest pacific ocean reveal unexpectedly explosive activity at great depths

potential field and bathymetric investigation of the monowai volcanic centre kermadec arc implications for caldera formation and volcanic evolution the dynamics of caldera formation are affected by internal factors such as magma chamber structure and properties and regional tectonic stresses understanding their interrelationship is important for assessing volcanic hazard and reconstructing the evolution of a volcano the monowai volcanic centre located at the tonga kermadec arc is a basaltic andesitic submarine stratovolcano and caldera recent surveys have shown that it is active but little is known about its structure and evolution here we present a combined analysis of swath bathymetry and potential field data from monowai acquired during cruise so215 on the r v sonne in 2011 april june the monowai caldera is associated with a 20 25 mgal bouguer gravity anomaly high and a broad positive magnetic anomaly short wavelength magnetic anomalies of up to 1400 and 800 nt are observed along the caldera rim and on the summit of monowai cone inversion of the bouguer gravity anomaly data shows that the caldera high is caused by a buried dense body with density contrast of up to 450 kg m 3 with respect to the surrounding arc crust the body has a volume of 250 km3 and is divided into a main unit extending from 3 to 6 km depth and a shallower ring structure that underlies the rim of monowai caldera we estimate the density of the body to be 2650 2850 kg m 3 suggesting a mafic composition the ring structure is interpreted as set of ring dykes and the main unit as a solidified or partly solidified magma chamber system the monowai pluton the observed magnetic anomalies are consistent with pluton intrusion within the bruhnes magnetic polarity analysis of the swath bathymetry data shows that monowai is located in a 20 km wide graben part of an en echelon left stepping horst and graben system that spans the length of the arc and backarc the monowai caldera is elongated perpendicular to the direction of rifting suggesting that it is affected by the regional stress regime geomorphological analysis suggests that the caldera was created by repeated collapse of a single caldera structure and that its complex shape and multiple ring faults can be attributed to prolonged activity and multiple cycles of inflation and collapse we propose a multiphase evolution of the monowai volcanic centre comprising initial magma emplacement and crustal inflation magma chamber evacuation and caldera collapse caldera resurgence and mass waisting southward migration of the eruptive vent and formation of monowai cone it remains unclear whether the current eruption at the monowai cone is fed directly from a small underlying magma chamber or laterally from the monowai pluton observation of radial fissure ridges on its flanks suggests that a shallow magma body may have been recently emplaced beneath the summit causing inflation and the formation of extensional cracks the summit appears unstable and may be prone to lateral or vertical collapse therefore the region remains a potential source of regional volcanic and tsunami hazard

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

multiple hydrothermal sources along the south tonga arc and valu fa ridge 1 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 he 3 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 3 he 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 water column chemical signature after the 1998 eruption of axial volcano the eruption of axial volcano in january 1998 produced extensive plumes in the overlying water column with large anomalies in fe mn ph light attenuation and temperature a strong correlation between total iron and light attenuation dc suggests a low abundance of particulate sulfur ps in the plumes because total carbon dioxide co2 samples were not collected high precision ph measurements were used to estimate maximum co2 anomalies co2 which when compared to the other physical and chemical data suggest that the fluids being vented 3 weeks after eruption were formed by the interaction between an intruded dike and a mixture of interstitial seawater and mature hydrothermal fluids

venting of a separate co2 rich gas phase from submarine arc volcanoes examples from the mariana and tonga kermadec arcs submersible dives on 22 active submarine volcanoes on the mariana and tonga kermadec arcs have discovered systems on six of these volcanoes that in addition to discharging hot vent fluid are also venting a separate co2 rich phase either in the form of gas bubbles or liquid co2 droplets one of the most impressive is the champagne vent site on nw eifuku in the northern mariana arc which is discharging cold droplets of liquid co2 at an estimated rate of 23 mol co2 s about 0 1 of the global mid ocean ridge mor carbon flux three other mariana arc submarine volcanoes nw rota 1 nikko and daikoku and two volcanoes on the tonga kermadec arc giggenbach and volcano 1 also have vent fields discharging co2 rich gas bubbles the vent fluids at these volcanoes have very high co2 concentrations and elevated c 3he and 13c co2 ratios compared to mor systems indicating a contribution to the carbon flux from subducted marine carbonates and organic material analysis of the co2 concentrations shows that most of the fluids are undersaturated with co2 this deviation from equilibrium would not be expected for pressure release degassing of an ascending fluid saturated with co2 mechanisms to produce a separate co2 rich gas phase at the seafloor require direct injection of magmatic co2 rich gas the ascending co2 rich gas could then partially dissolve into seawater circulating within the volcano edifice without reaching equilibrium alternatively an ascending high temperature co2 rich aqueous fluid could boil to produce a co2 rich gas phase and a co2 depleted liquid these findings indicate that carbon fluxes from submarine arcs may be higher than previously estimated and that experiments to estimate carbon fluxes at submarine arc volcanoes are merited hydrothermal sites such as these with a separate gas phase are valuable natural laboratories for studying the effects of high co2 concentrations on marine ecosystems

chemical and physical diversity of hydrothermal plumes along the east pacific rise 8 45 n to 11 50 n we conducted a survey of water column hydrothermal plumes along a 350 km long section of the east pacific rise epr axis stretching from 8 45 n to 11 50 n including the region at 9 45 54 n where 1991 alvin dives found evidence for recent seafloor volcanic eruptions our survey included measurements of temperature anomaly light attenuation suspended particles methane hydrogen iron manganese helium and aluminum we detected strong light attenuation plumes in two main regions south of the clipperton transform fault ctf from 9 27 n to 9 57 n and north of the ctf from 11 05 n to 11 35 n however the plumes at these two regions had very different physical and chemical characteristics south of the ctf where the ridge is thought to be magmatically robust the plumes had very high ratios of he heat methane mn and s fe plumes north of the ctf had much lower ratios of he heat methane mn and s fe these striking differences in volatile heat and volatile metal ratios suggest that the plumes in the vicinity of 9 50 n were derived from a young evolving hydrothermal system while the activity at 11 10 n is from an older stable system this survey demonstrated that water column plume measurements can be used not only to accurately locate regions of hydrothermal activity but also to determine the first order chemical and physical characteristics of the hydrothermal fluids exiting from the seafloor

manganese and methane in hydrothermal plumes along the east pacific rise 8 40 to 11 50 n abstract in november 1991 we surveyed the water column for hydrothermal plumes along 350 km of the east pacific rise axis from 8 40 to 11 50 n using a combination of physical and chemical measurements our survey included the two major ridge segments north and south of the clipperton transform fault at about 10 10 n both limbs of the overlapping spreading centers osc s at 9 03 n and 11 45 n and a 30 km section of the next ridge segment to the south we found vigorous plumes along most of this ridge axis in keeping with its magmatically robust cross section axial summit caldera and shallow magma related seismic reflector these plumes were detectable by both physical temperature and light attenuation and chemical dissolved mn and ch4 measurements although the chemical measurements were more sensitive the least active sections were the southern third of the northern segment from 10 20 to 52 n and the oscs especially the osc at 11 45 n plumes there had weak mn and ch4 signals and were barely detectable by physical methods these axial sections were the only ones surveyed that lie deeper than 2600 m and appear to be magma starved the most active sections on the northern segment gave stronger signals for mn and temperature than for ch4 and light attenuation whereas the opposite was true on the southern segment which was the site of a volcanic eruption at 9 45 52 n only seven months prior to our cruise on the northern segment the four physical and chemical plume tracers correlated positively and linearly with one another suggesting that the segment was fed by relatively uniform end member fluids with a mean ch 4 mn molar ratio of 0 075 the southernmost section surveyed from 8 42 to 9 08 n closely resembled the northern segment the rest of the southern segment fell into three sections with different ch 4 mn ratios 9 39 to 53 n with ch 4 mn as high as 10 9 08 to 39 n with ch 4 mn of 0 51 and 9 53 to 10 07 n with ch 4 mn of 0 85 the section with the highest ch 4 mn was the site of the volcanic eruption which produced high temperature low salinity gas rich vent fluids carrying abundant bacterial particles the high ch4 concentrations are clearly associated with the volcanic eruption but the origin of the ch4 is unclear

three dimensional crustal structure and magma chamber geometry at the intermediate spreading back arc valu fa ridge lau basin results of a wide angle seismic tomographic inversion in this paper the results of a tomographic analysis of a 3 d wide angle seismic refraction data set acquired at the valu fa ridge vfr in 1995 are presented the vfr is an intermediate spreading ridge located in the southern lau back arc basin in the southwest pacific the ridge comprises three morphological segments the southern central and northern valu fa ridges separated by overlapping spreading centres oscs previous seismic experiments have identified a robust axial magmatic system beneath the central segment cvfr and the osc with the northern segment nvfr the experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post rift crust a regularized inversion scheme that minimizes model roughness was applied to the first arrival traveltime picks made from the wide angle data a quantitative approach for determining data uncertainties is described based on the signal to noise ratio of the arrivals several initial model assumptions were tested including one with a thin melt lens representing a seismic reflector identified in previous studies explicitly included in the initial model the inversion results suggest that crustal layer 2 exhibits northward thickening which mirrors a similar northward thickening of the whole crust in addition local thinning of layer 2 is identified in the vicinity of the boundary between pre and post rift crust which is thought to represent thinning of the crust prior to the onset of rifting axial low velocity anomalies are identified in layer 2b c and layer 3 the models are consistent with a continuous 6 km wide negative velocity anomaly in layer 3 with an amplitude of 0 7 0 9 km s 1 relative to off axis post rift crust this anomaly is consistent with the presence of an axial mush zone comprising a small percentage 1 per cent of partial melt the negative velocity anomaly in layer 2b c is modelled with its largest amplitude 0 5 km s 1 beneath the northern osc possible origins for this anomaly include locally thicker crust or locally higher porosity near the osc or a high temperature anomaly associated with the axial magmatic system

high resolution hydrothermal mapping of brothers caldera kermadec arc submarine edifices with caldera summits are common along volcanic arcs and much more likely than simple cones to host hydrothermal venting compared with cones however locating all vent field locations on a caldera s complex bathymetry is a daunting logistical challenge here we describe the first use of an autonomous underwater vehicle abe to fully map the distribution of near bottom hydrothermal tracers over the caldera walls and cone complex of brothers volcano the most active hydrothermal source on the southern kermadec arc sensors on abe simultaneously measured hydrothermal plume anomalies in temperature light backscattering particle concentration and oxidation reduction potential dissolved reduced species every 2 to 3 m along track local maxima in these tracers confirmed known sites on the northwestern wall and the cone summits and more precisely mapped their extents we discovered evidence for new sites throughout the entire northwestern half of the caldera wall at the base of the southeastern wall and on the flanks of both cones systematic variations in the backscattering temperature ratio identified different types of fluid discharge plumes from high temperature metal rich sources dominated the caldera wall while on top of the caldera wall and on the cones we found only plumes from low temperature diffuse flow including occasional s rich plumes source distribution on the walls appears fault controlled but we detected no sources along the deepest fault defining the rim of the downdropped caldera floor advanced deep sea survey techniques using autonomous vehicles is an indispensible and cost effective tool for acquiring a comprehensive understanding of the relationship between hydrothermal discharge and geology on individual volcanoes

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

geology hydrothermal activity and sea floor massive sulfide mineralization at the rumble ii west mafic caldera abstract sea floor imagery volcanic rock massive sulfide and hydrothermal plume samples 3 he ph dissolved fe and mn and particulate chemistry have been collected from the rumble ii west volcano southern kermadec arc new zealand rumble ii west is a caldera volcano with an 3 km diameter summit depression bounded by ring faults with a resurgent central cone rocks recovered to date are predominantly mafic in composition i e basalt to basaltic andesite with volumetrically lesser intermediate rocks i e andesite on the basis of its size geometry volcanic products and composition rumble ii west can be classified as a mafic caldera volcano rumble ii west has a weak hydrothermal plume signature characterized by a small but detectable 3 he anomaly 25 time series light scattering data though obtained from vertical casts and tow yos do show that hydrothermal activity has increased in intensity between 1999 and 2011 massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite with lesser sphalerite pyrite and traces of galena the weak hydrothermal plume signal indicates that the volcano is in a volcanic hydrothermal quiescent stage compared to other volcanoes along the southern kermadec arc although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past of the volcanoes along the kermadec tonga arc known to host massive sulfides i e clark rumble ii west brothers monowai volcano 19 and volcano 1 the majority five out of six are dominantly mafic in composition and all but one of these mafic volcanoes form moderate size to large calderas to date mafic calderas have been largely ignored as hosts to sea floor massive sulfide deposits that 75 of the presently known massive sulfide bearing calderas along the arc are mafic in composition the dacitic brothers volcano is the exception has important implications for sea floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land

hydrothermal plumes along the east pacific rise 8 40 to 11 50 n particle distribution and composition the discovery of a volcanic eruption at the 9 45 52 n site on the east pacific rise epr led us to investigate how the composition of suspended matter in hydrothermal plumes may be used to provide a better understanding of the relative age and vent fluid characteristics of a ridge crest system during surface ship surveys during the troughs tropical ridge observations of underwater geochemical hydrothermal signals expedition we collected hydrothermal plume particles along a 350 km section extending from approximately 8 40 n to 11 50 n of the epr north and south of the clipperton transform fault over the segment south of the clipperton transform fault the sfe ratio in the particles ranged from about 0 9 to 10 with the highest values immediately over the new vent field at the 9 45 52 n site these enrichments exactly coincide with the elevated 3he heat and ch4mn ratios in the plumes over the same vent field the s rich particles are primarily organic and may be related to the bacteria blizzard associated with the eruption in addition very high cufe and znfe ratios were observed in the hydrothermal plumes south of the clipperton transform fault these high ratios are indicative of the predominance of high temperature h2s rich vents between 9 35 n and 9 50 n in contrast the samples from the segment north of clipperton transform fault had sfe ratios which ranged from 0 04 to 0 82 typical of mature fe rich hydrothermal plumes these results suggest that the gas rich vent fluids at the 9 45 52 n site produced the sulfur enriched particles in the water column gas rich plumes and sulfur enriched plume particles were also observed underneath the 1986 megaplume on the juan de fuca ridge jdfr suggesting a similar mode of formation north of the clipperton transform fault the hydrothermal particles are dominated by fe rich oxyhydroxide particles characteristic of mature high temperature vent fields the plume maximum is centered at 11 08 n the fe oxyhydroxide particles scavenge ca si p v cr and as from seawater for p and v the mean element fe ratios in the epr hydrothermal particles are intermediate between jdfr and tag hydrothermal systems for as the mean element fe ratios of all three systems are about the same new evidence for si scavenging by hydrothermal fe oxyhydroxide particles indicates that this sink for dissolved si in the oceans is small

hydrothermal activity and volcano distribution along the mariana arc 1 submarine volcanic arcs extend for fully a third of the length of the mid ocean ridge system but systematic investigation of hydrothermal venting on those arcs is sparse here we report on the distribution of hydrothermal activity along the intraoceanic mariana arc we identify 76 volcanic edifices along 1370 km of arc grouped into 60 volcanic centers of which at least 26 20 submarine are hydrothermally or volcanically active the overall volcanic center density is 4 4 100 km of arc and that of active centers is 1 9 100 km an equal length of the tonga kermadec arc holds fewer centers 2 9 100 km but a similar density of active centers 1 8 100 km the lower percentage of active centers on the mariana arc results from its having both fewer caldera volcanoes since on both arcs calderas are twice as likely as cones to be active and a lower percentage of active calderas and cones active centers are found from 80 to 230 km above the subducting pacific slab and a quarter lie behind the arc front the frequency distribution of center spacing along the arc front peaks between 20 and 30 km and shows the asymmetric long tail shape typical for many other arcs especially those with volcano populations 20 there is no evidence for a regular spacing of volcanic centers using the new mariana data and recent data from the tonga kermadec arc we estimate that all intraoceanic arcs combined may contribute hydrothermal emissions equal to 10 of that from the global mid ocean ridge

dust and loessic alluvial deposits in northwestern namibia damaraland kaokoveld sedimentology and palaeoclimatic evidence based on luminescence data abstract in northwestern namibia silty deposits are widespread the silts consist of two major components locally produced weathering detritus and allochthonous dust blown in from the western kalahari to the eastern margin of the namib desert sedimentological analysis and luminescence age estimations allow identification of distinct sedimentological and geomorphological processes associated with two different climatic periods between about 30 and 8 ka loessic alluvial deposits several metres in thickness accumulated in valleys and basins the deposition was generated by weak slope wash and or endoreic drainage of the ephemeral rivers the dust input and the accumulation of silt in the valleys indicate semi arid climatic conditions with weak summer rains and long dry seasons since about 9 8 ka silt terraces caused by fluvial incision and erosion have been formed these indicate more humid conditions than had occurred before the change from a dry to a moister semi arid climate at about 9 8 ka is confirmed by proxy data from drill cores off the namibian coast and by dune studies in the south western kalahari

tectonic processes in papua new guinea and past productivity in the eastern equatorial pacific ocean phytoplankton growth in the eastern equatorial pacific ocean today accounts for about half of the new production the fraction of primary production fuelled by externally supplied nutrients in the global ocean the recent demonstration that an inadequate supply of iron limits primary production in this region1 supports earlier speculation that in the past fluctuations in the atmospheric deposition of iron bearing dust may have driven large changes in productivity2 but we argue here that only small 2 nm increases in the iron concentration in source waters of the upwelling equatorial undercurrent are needed to fuel intense diatom production across the entire eastern equatorial pacific ocean episodic increases in iron concentrations of this magnitude or larger were probably frequent in the past because a large component of the undercurrent originates in the convergent island arc region of papua new guinea which has experienced intensive volcanic erosional and seismic activity over the past 16 million years cycles of plankton productivity recorded in eastern equatorial pacific sediments may therefore reflect the influence of tectonic processes in the papua new guinea region superimposed on the effects of global climate forcing

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

submarine volcanoes and high temperature hydrothermal venting on the tonga arc southwest pacific submarine hydrothermal vents and associated seafloor 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 c occur at water depths of 385 540 m near the summit of one volcano at 24 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

morphology of the faial island shelf azores the interplay between volcanic erosional depositional tectonic and mass wasting processes 1 the extents of volcanic island shelves result from surf erosion which enlarges them and volcanic progradation which reduces them however mass wasting tectonics and sediment deposition also contribute to their morphology in order to assess the relative significance of these various processes we have mapped in detail faial island s shelf in the azores archipelago based on interpretation of geophysical and geological data the nearshore substrates of the island down to 30 50 m depth are rocky and covered by volcaniclastic boulder deposits formed by surf action on now submerged lava flows below those depths sandy and gravel volcaniclastic beds dominate building clinoforms up to the shelf edge in some sectors of the coast prograding lava has narrowed the shelf but in contrast to nearby pico island we find fewer submarine emplaced lavas on the shelf in this island we interpret the distance between the coastline and the shelf edge as almost entirely a result of a straightforward competition between surf erosion and lava progradation in which erosion dominates therefore shelf width can be used as a proxy for coastline age as well as for wave energy exposure the stratigraphy of shelf deposits in boomer seismic data is examined in detail to assess the roles of different sediment sources accommodation space and wave exposure in creating these deposits we also show evidence of mass wasting at the shelf edge and discuss the possible origins of slope instability finally we discuss the contributing role of tectonics for the development of the shelf

hydrothermal venting along earth s fastest spreading center east pacific rise 27 5 32 3 s 1 during march april 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of earth s fasting spreading mid ocean ridge 27 5 32 3 s on the east pacific rise we compared the distribution and chemistry of hydrothermal plumes to geological indicators of long term spreading rate and moderate term ridge inflation variations in magmatic budget in this large offset propagating rift setting these geological indices span virtually the entire range found along fast spreading ridges worldwide hydrothermal plumes overlaid 60 of the length of superfast 130 km myr spreading axis surveyed and defined at least 14 separate vent fields we observed no plumes over the slower spreading propagating segments finer scale variations in the magmatic budget also correlated with hydrothermal activity as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation along the entire ridge crest the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years plume samples with high volatile metal ratios generally indicating recent seafloor volcanism were scarce along axis trends in both volatile 3he ch4 ph a proxy for co2 and particulate s and nonvolatile fe mn species showed a first order agreement with the trend of ridge inflation nevertheless a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

hydrothermal sedimentation adjacent to the propagating valu fa ridge lau basin sw pacific abstract the partition geochemistry and hydrothermal metal accumulation rates ar during the late quaternary 3 km from hydrothermally active areas still shows an enrichment of mn ag ba and zn when compared to regional 500 m depth lau basin sea water the elements ni zn cu co and mo are associated with mn oxides in the sediment and are thought to have been scavenged by hydrothermal oxides during the transport of hydrothermal oxides away from the spreading center only a low proportion of fe is bound to the hydrothermal oxide phase in the sediments and this element is mainly associated with silicate phases principal components analysis pca of geochemical data in the detrital free caco 3 free fraction of the sediments supports a geochemical hydrothermal oxide association between mn ni mo cu zn pb p be ba and la with fe associated with the silicate weathered detrital fraction iron manganese ratios in the oxide phase range between 1 2 and 2 8 these values are below those found in sediments at mid ocean ridges peaks in the downcore distribution in mn appear to be contemporaneous in the cores studied and are probably the result of hydrothermal discharges on the vfr hemipelagic sedimentation rates throughout the area average 2 3 cm ka 1 hydrothermal mn ar range between 3 and 82 mg cm 2 ka 1 during the 0 30 ka b p period and between 5 and 65 mg cm 2 ka 1 during the 30 120 ka b p period maximum mn ar adjacent to the vfr are comparable to the highest values previously reported for recent sediments proximal to the east pacific rise epr during the entire period 0 120 ka b p mn ar are highest in core so48 101 located about 90 km behind the propagating tip of the vfr recent hydrothermal input has also occurred about 33 km behind the propagator this is in accord with the suggested location of fully evolved steady state magma chambers occurring some distance tens of kilometers behind propagating rift tips

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

geological chemical and biological evidence for recent volcanism at 17 5 s east pacific rise the superfast spreading portion of the east pacific rise at 17 5 s is a magmatically robust section of mid ocean ridge plumes characterized by high volatile metal ratios which has been hypothesized to be indicative of recent magmatism were found between 17 22 s and 17 35 s in november december 1993 dives with the french submersible nautile in december 1993 observed young sheet flows and widespread venting centered on the segment s shoalest point at 17 26 s eight submersible dives made one year later with the japanese submersible shinkai 6500 in september november 1994 found glassy unsedimented lavas a range of diffuse and high temperature vents and several types of biologic communities on the ridge crest beneath the volatile rich 1993 plumes the diffuse vents range from small areas in the youngest lavas characterized by very low temperature flow 10 c and a relatively low macrofaunal diversity to larger areas where the apparent older surface was covered with a dense and diverse biotope including anemones tubeworms mussels and serpulids several inactive vents marked by dead mussel clam beds are present in the southernmost portion of the site in the older lavas submersible mapping indicates that the youngest lava l0 unit erupted for at least 4 km along the strike of the ridge at the southern site and for 5 10 km at the northern site the flow is up to 400 m in width at the southern site and may be as much as 1 2 km in width at the northern site comparison of the observed 1993 fe s h2s heat and 3he heat plume values with time series measurements at sites on the northern east pacific rise and the juan de fuca ridge where the onset of the magmatic event is known implies that the last magmatic event at 17 5 s occurred within several years prior to 1993 continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives the type and diversity of the vent biota found on the l0 vents also suggests a young system the spatial and temporal scales of the accretion event at the 17 5 s epr site are similar in scale to eruptions on the intermediate rate spreading north cleft segment juan de fuca ridge but the 17 5 s event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system

geological evolution of the volcanic island la gomera canary islands from analysis of its geomorphology abstract erosion and landslide processes played an important role in shaping the geomorphology of the canary islands but for the heavily eroded la gomera island those processes have been unclear we conducted an integrated study of its offshore extension onshore geology and links between them in order to determine the geomorphological evolution of the island relative to the rest of the archipelago the characteristics of the island s barrancos and its interfluves have led us to differentiate four types of morphological areas whose fluvial networks are in different stages of evolution those barrancos offshore are interrupted by la gomera s shelf and its erosion has been much greater than for the rest of the canary islands such erosion is both a function of time and varied swell action la gomera s insular slope is carved by a system of submarine canyons and channels that have morphology indicative of turbidity current erosion many of the barrancos onshore and the canyons offshore have steps or knickpoints some are the result of varied resistance to erosion and others are explained by other mechanisms a ridge whose crest is covered by cone like shape structures is interpreted as of volcanic origin with the particularity that it lacks onshore continuation the opposite of what has been observed elsewhere in the canary islands also in contrast with the other islands of the archipelago la gomera s margin does not seem to have resulted from destructive mass wasting and the only offshore sign of catastrophic events are the presence of two embayments along la gomera s northern shelf edge and a lobe structure on the insular apron at 3000 m depth these features may be the remains of the catastrophic failures that took place on the island 9 4 8 7 ma ago alternatively they could be due to recent catastrophic failures on the upper slope due to sediment steeping since the conclusion of the main volcanic activity 4 0 ma ago la gomera has been intensively eroded by gradual fluvial denudation and secondary failures la gomera s insular margin reveals much regarding its evolution but more comprehensive studies including seismic and coring are needed to understand its whole geological history

hydrothermal methane and manganese variation in the plume over the superfast spreading southern east pacific rise abstract onboard analyses of dissolved ch4 and mn were conducted on the water column along the southern east pacific rise between 13 50 and 18 40 s during the 1993 ridge flux cruise many hydrothermal plumes were identified by chemical anomalies which overlie more than half of the surveyed ridge crest the ch 4 mn ratio in the plumes shows wide variability ranges from about 0 05 to 3 9 due to wide range of ch 4 concentration the ch 4 rich plumes were detected in the region of 16 00 18 40 s where vigorous magmatic budget was confirmed by previous studies moreover the ch 4 mn ratio of the plumes is highly correlated with other volatile metal parameters such as the s fe ratio in plume particulate these results strongly suggest that volatile input caused by magmatic perturbation to hydrothermal systems is responsible for the ch 4 mn variability in the southern epr plumes based on analogy of the case at the epr 9 50 n site the detection of several volatile rich plumes and the extensive hydrothermal flux of volatile species are attributed to frequent magmatic events in the superfast spreading southern epr

hydrothermal activity and magma genesis along a propagating back arc basin valu fa ridge southern lau basin valu fa ridge is an intraoceanic back arc spreading center located at the southern prolongation of the lau basin bathymetric observations as well as detailed sampling have been carried out along the spreading axis in order to trace hydrothermal and volcanic activity and to study magma generation processes the survey shows that widespread lava flows from recent volcanic eruptions covered most of the vai lili hydrothermal vent field only diffuse low temperature discharge and the formation of thin layers of siliceous precipitates have been observed evidence of present day hydrothermal activity at the hine hina site is indicated by a thermal anomaly in the overlying water column our studies did not reveal any signs of hydrothermal activity either above the seismically imaged magma chamber at 22 25 s or across the southern rift fault zone 22 51 s lavas recovered along the valu fa ridge range from basaltic andesites to rhyolites with sio2 contents higher than reported from any other intraoceanic back arc basin on the basis of the highly variable degrees of crystal fractionation along axis the development of small disconnected magma bodies is suggested in addition the geochemical character of the volcanic rocks shows that the transition zone from oceanic spreading to propagating rifting is located south of the hine hina vent field in the vicinity of 22 35 s

submarine hydrothermal activity and gold rich mineralization at brothers volcano kermadec arc new zealand brothers volcano of the kermadec intraoceanic arc is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world it has two distinct vent fields known as the nw caldera and cone sites whose geology permeability vent fluid compositions mineralogy and ore forming conditions are in stark contrast to each other the nw caldera site strikes for 600 m in a sw ne direction with chimneys occurring over a 145 m depth interval between 1 690 and 1 545 m at least 100 dead and active sulfide chimney spires occur in this field and are typically 2 3 m in height with some reaching 6 7 m their ages at time of sampling fall broadly into three groups 4 23 and 35 years old the chimneys typically occur near the base of individual fault controlled benches on the caldera wall striking in lines orthogonal to the slopes rarer are massive sulfide crusts 2 3 m thick two main types of chimney predominate cu rich up to 28 5 wt cu and more commonly zn rich up to 43 8 wt zn geochemical results show that mo bi co se sn and au up to 91 ppm are correlated with the cu mineralization whereas cd hg sb ag and as are associated with the dominant zn rich mineralization the cone site comprises the upper cone site atop the summit of the recent main dacite cone and the lower cone site that straddles the summit of an older smaller more degraded dacite cone on the ne flank of the main cone huge volumes of diffuse venting are seen at the lower cone site in contrast to venting at both the upper cone and nw caldera sites individual vents are marked by low relief 0 5 m mounds comprising predominately native sulfur with bacterial mats vent fluids of the nw caldera field are focused hot 300 c acidic ph 2 8 metal rich and gas poor calculated end member fluids from nw caldera vents indicate that phase separation has occurred with cl values ranging from 93 to 137 of seawater values by contrast vent fluids at the cone site are diffuse noticeably cooler 122 c more acidic ph 1 9 metal poor and gas rich higher than seawater values of so4 and mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water rock interactions iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from fe rich brines evidence for magmatic contributions to the hydrothermal system at brothers includes high concentrations of dissolved co2 e g 206 mm kg at the cone site high co2 3he negative d and 18oh2o for vent fluids negative 34s for sulfides to 4 6 sulfur to 10 2 and 15n2 to 3 5 vent fluid ph values to 1 9 and mineral assemblages common to high sulfidation systems changing physicochemical conditions at the brothers hydrothermal system and especially the cone site occur over periods of months to hundreds of years as shown by interlayered cu au and zn rich zones in chimneys variable fluid and isotopic compositions similar shifts in 3he 4he values for both cone and nw caldera sites and overprinting of magmatic mineral assemblages by water rock dominated assemblages metals especially cu and possibly au may be entering the hydrothermal system via the dissolution of metal rich glasses they are then transported rapidly up into the system via magmatic volatiles utilizing vertical 2 5 km long narrow 300 m diameter pipes consistent with evidence of vent fluids forming at relatively shallow depths the nw caldera and cone sites are considered to represent stages along a continuum between water rock and magmatic hydrothermal dominated end members

huge erratic boulders in tonga deposited by a prehistoric tsunami along some coastlines there are erratic boulders apparently emplaced by tsunamis or cyclonic storms evaluating their origin and time of emplacement places constraints on the frequency severity and location of coastal hazards seven such large coral limestone boulders are present near fahefa village on tongatapu island southwest pacific apparently emplaced by a prehistoric tsunami these boulders are 10 20 m above sea level and above any possible source and all are 100 400 m from the present shoreline coral 230th ages indicate that the limestone formed during the last interglacial sea level highstand ca 120 130 ka the largest boulder is 20 times more massive than any reported boulders emplaced by historically documented storms and may be the largest known tsunami or storm erratic worldwide situated above its source we performed computer simulations to assess whether tsunamis produced by earthquakes undersea landslides or volcanoes could emplace the boulders the simulations indicate that either volcanic flank collapse along the tofua arc 30 40 km to the southwest or undersea landslides on the submarine slopes of tongatapu could be responsible either could explain why these boulders are not widespread on tongatapu and instead occur in a localized group along the western coast this study demonstrates that small 1 km3 submarine slope failures sometimes generate locally large tsunamis the fahefa boulders are in a well studied and well populated area yet were unknown to the scientific community until recently this suggests that systematic searches elsewhere for erratic boulders and other tsunami deposits might provide new information for assessing the size and extent of prehistoric tsunamis

abundant hydrothermal venting along melt rich and melt free ridge segments in the lau back arc basin 1 the relationship between the spatial density of hydrothermal venting ph and spreading rate us is undefined along back arc ridges here we report a continuous survey of hydrothermal plumes along the 400 km long eastern lau spreading center the mean ph us ratio for each of three tectonic segments followed the global midocean ridge trend with ph doubling as us increased northward from 39 to 96 mm yr however we found little correlation between ph and the previously imaged distribution of axial melt lenses which are near continuous along the southern two segments but wholly absent along the northernmost segment we infer that venting on the southern segments is melt driven whereas venting on the northern elsc is entirely powered with heat extracted by deep circulation through hot rock this segment may be a rare example of an entire fast spreading segment where hydrothermal convection cells are powered by downward migrating cracking fronts

evidence for an extensive hydrothermal plume in the tonga fiji region of the south pacific 1 several hydrographic stations in the vicinity of the samoa islands have 3he 4he above the regional background in the depth range of 1500 1800 m indicating injection of mantle helium from a local hydrothermal source the highest 3he 43 4 was detected at 1726 m depth at 15 0 s 173 1 w in the bathymetric gap between the samoa islands and the northern end of the tonga kermadec arc the 3he profile at this station decreases to 3he 26 at 2500 m depth the relatively shallow depth of the maximum hydrothermal signal suggests a source different from the conventional pacific basin helium plume centered at 2500 m that is carried westward from the east pacific rise stations to the west of this locality show a progressive decrease in the maximum 3he values in the depth range of 1480 1790 m out to 169 e stations east of the tonga fiji region show lower 3he values 26 at 1700 m and the profiles are dominated by a deeper maximum at 2500 m presumably the distal traces of hydrothermal input from east pacific rise this pattern in the 3he distribution suggests that the 1700 m deep helium plume is carried in a northwesterly direction some 2000 km from its source near the northern end of the tonga kermadec arc at this time very little is known about the source of this hydrothermal plume or the details of its areal extent numerous seamounts and rift zones in the region are possible hydrothermal sources for the plume the summit crater of vailulu u a young seamount at the eastern end of the samoa chain was recently discovered to be hydrothermally active at 600 m depth hart et al 2000 however this shallow hydrothermal field on vailulu u is an unlikely source for the deeper 1700 m signal the most likely source would appear to be the extensional zones of the northern lau basin system such as the mangatolo triple junction just as the helium plume emanating from lo ihi has helped our understanding of the circulation near the hawaiian islands lupton 1996 this helium plume in the tonga fiji region has great potential for delineating circulation in this area of the south pacific

comment on reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores by hildenbrand et al 2012 j volcanol geotherm res 241 242 2012 39 48 abstract hildenbrand et al 2012 have presented a model for the geological evolution of faial island in their interpretation the oldest volcanic system extended much further than the present day eastern part of the subaerial island with a ridge like morphology into the faial s jorge channel furthermore the western side of the system was suggested to have been later destroyed by mass wasting based on now extensive multibeam bathymetry collected surrounding faial island we find no evidence supporting such interpretations the geometry of the shelf edge suggests that a near circular edifice occupied the eastern part of the present day subaerial island in addition the submarine morphology does not show massive mass wasting deposits around the base of the edifice rather bathymetric gradients and morphology suggest that the submarine slopes were formed from volcanic emplacements or from material displaced in only small scale mass movements or sedimentary flows and have not failed catastrophically evidence that a ridge like feature extended into the faial s jorge channel is also lacking these observations highlight the potentially valuable information that sonar data can provide to evaluating ocean island evolution from the more limited extend of outcrops on land

hydrothermal activity in the lau back arc basin sulfides and water chemistry the submersible nautile completed 22 dives during the nautilau cruise r v nadir april 17 may 10 1989 for a detailed investigation of the southern lau basin near tonga the objective of the scientific team from france germany and tonga was to understand the process of sea floor ore formation associated with hydrothermal circulation along the valu fa back arc ridge behind the tonga kermadec trench the four diving areas between lat 21 25 s and 22 40 s in water 2000 m deep were selected on the basis of results from cruises of the r v jeancharcot and r v sonne the nadir cruise provided proof of hydrothermal activity in all four areas over more than 100 km as indicated by the widespread occurrence of hydrothermal deposits and by heat flow conductivity and temperature measurements near the sea bottom the most spectacular findings were high temperature white and black smokers and associated fauna and ore deposits hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges this difference is seen in the water chemistry of the hydrothermal fluid ph 2 and high metal content and the chemical composition of sulfides enrichment in ba as and pb

exploring the ocean for hydrothermal venting new techniques new discoveries new insights abstract enumerating active hydrothermal fields on the seafloor has been a challenge since their discovery almost 40 years ago high temperature hydrothermal fields are readily discoverable primarily by detecting mineral laden plumes but low temperature particle poor vent fields resist discovery decades of exploration for vent fields have covered though often cursorily about one third of the global lengths of both oceanic spreading ridges osrs and volcanic arcs identifying some 630 active vent fields about 80 of these fields are on osrs and the spatial frequency of those fields is currently estimated as 0 5 5 100 km generally increasing with spreading rate over the last decade however a few detailed surveys have added sensors capable of detecting ephemeral chemical tracers oxidation reduction potential in addition to standard sensors that detect quasi conservative optical tracers such as light backscattering this approach has revealed a new view of the distribution of venting fields along fast spreading 55 mm yr osrs studies of four such ridge sections totaling 1470 km length suggest that the present inventory of vent fields may underestimate the true global population of vent fields on fast spreading osrs by a factor of 3 6 this increase implies that ridge axes are unexpectedly leaky reservoirs from which hydrothermal fluids escape at far more sites than presently assumed that the supply of dissolved hydrothermal iron which may be fertilizing the primary production of the southern ocean is higher than now calculated and that present estimates of recoverable sulfide tonnage from ridge axes may be too low along slow spreading ridges which account for 60 of the global osr length and 86 of known sulfide tonnage expansive axial valleys present special exploration challenges that will not be easily overcome

seismic imaging of the axial region of the valu fa ridge lau basin the accretionary processes of an intermediate back arc spreading ridge in 1995 a multidisciplinary geophysical experiment targetted the intermediate spreading valu fa ridge full rate 60 mm yr 1 which is centred on 22 20 s 176 40 w in the lau basin as part of this experiment wide angle and normal incidence seismic profiles were collected both along and across axis to determine the crustal structure of the central valu fa ridge cvfr and its overlap with the northern valu fa ridge nvfr controlled source electromagnetic profiles and underway gravity magnetic and bathymetry data were also collected in this paper we describe the results of forward modelling of the along and across axis wide angle and normal incidence seismic data an axial low velocity block and its underlying slightly broader zone of depressed seismic velocities low velocity zone have been identified and these features are interpreted as corresponding to a melt lens and underlying magma chamber the low velocity block is 1 2 km wide and has a first order upper boundary from which large amplitude reflections are observed amplitude analysis of these indicates an interconnected melt fraction the nature of the lower boundary is more poorly constrained as no reflection event corresponding to the base of the low velocity block is observed modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region 5 5 km s 1 are achieved by 250 m below the upper boundary possibly suggesting a gradational lower boundary with high velocity gradient the low velocity zone lvz is interpreted as an 4 km wide magma chamber delineated by a seismic velocity anomaly of 0 2 km s 1 extending down through layer 3 to within 1 5 2 km of the moho the velocity anomaly and dimensions of the lvz are generally smaller than those observed at the east pacific rise epr and mid atlantic ridge mar the observed along axis continuity of the low velocity block is remarkable extending from the southern tip of the cvfr to the overlapping spreading centre osc with the nvfr a low velocity block is modelled beneath the inside flanks i e the slopes that dip into the overlap basin of both ridges at the osc although the existence of a single low velocity block beneath the overlap basin itself cannot be ruled out the identification of a single lvz centred on the overlap region rather than two merged lvzs beneath each segment implies that the material in each low velocity block originates from the same crustal magma source a reflection event from the moho is observed from directly beneath the axis on both across axis profiles which indicates that a distinct crust mantle boundary may be formed within the axial region many of the observations at the valu fa ridge are consistent with those at the epr and the reykjanes ridge mar which implies that regardless of spreading rate crustal accretionary processes at mid ocean ridges with similar magmatic budgets are also broadly similar

the morphology of the submarine flanks of volcanic ocean islands a comparative study of the canary and hawaiian hotspot islands the submarine flanks of volcanic islands are shaped by volcanic constructional processes landslides erosion sediment deposition and tectonic movements we use a newly acquired multibeam sonar dataset from the westerly canary islands el hierro la palma and tenerife to develop a comparison with the hawaiian islands which suggests differences in the processes constructing and modifying their flanks landslides affect the flanks of both island groups debris avalanches fast moving shallow landslides have left smooth chutes and blocky deposits in both cases but blocks within some hawaiian avalanche deposits are markedly larger we attribute the larger block sizes in the hawaiian islands to the fact that their avalanches were relatively unconfined whereas many canary and hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes forcing interactions between blocks within the flows and encouraging disintegration furthermore the hawaiian avalanches with the largest blocks initiated near sea level whereas many of the canary avalanches initiated above sea level so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow slow moving deep seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several hawaiian rift zones but are not well developed in the canaries although volcanic morphology is partly obscured by sedimentation in the canaries we are able to interpret lava terraces around the deep flanks of el hierro which are similar to those found in the hawaiian islands however cones rather than terraces are the most common volcanic forms in the canary islands implying that flank eruptions have involved magma with significant volatile contents assuming that volatile contents dictate whether cones or terraces are formed these differences may ultimately originate from the different building rates of the two island groups for example the lack of evidence for high level magma chambers in the canaries associated with their lower outputs implies that there is less possibility for degassing of magma below the summit before lateral intrusion down rift zones hence cones rather than lava terraces are more commonly observed the apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high level magma chambers in the canaries

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

geochemistry of hydrothermal sediments from odp sites 834 and 835 in the lau basin southwest pacific abstract geochemical investigations have been carried out on hydrothermally enriched metalliferous sediments collected at odp sites 834 and 835 in the lau basin enhanced hydrothermal inputs of both mn and fe occur in an acidreducible oxide phase unlike mid ocean ridge sediments where fe is mainly present in more resistant hcl soluble phases barium v co ni cu zn pb and rees mainly follow mn and fe in their behaviour and associations and are thought to be largely scavenged from seawater by mn and fe rich plume particulates although there is a significant hcl soluble non hydrothermal component of ba present compositional differences between eastern lau spreading centre and central lau spreading centre hydrothermal sediments and between them and hydrothermal sediments on mid ocean ridges are thought to be due to a combination of factors including temporal and areal variation in the composition of hydrothermal solutions variation in hydrothermal flux to the sediments related to their proximity to propagator or spreading centre and variable containment of the lau basin hydrothermal plume consequent on changing basin morphology through time our results are consistent with published models for the evolution of the lau basin and confirm that a time lag occurs between propagator passage and maximum hydrothermal flux to associated sediments

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