imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

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

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

ocean current and temperature time series at brothers volcano 1 time series of currents and temperatures were acquired near surface and at depth on three moorings above brothers volcano 34 52 25 s 179 04 e 25 km west of the kermadec ridge and 300 km northeast of the north island new zealand from september 2004 to may 2005 ocean current time series were previously unavailable within 300 km of this site and few such measurements exist anywhere near magmatically or hydrothermally active volcanoes such as brothers energy in near surface currents 163 527 m depth was primarily subtidal 64 subtidal motions to 527 m depth are shown to be reasonably well correlated with geostrophic currents derived from satellite sea level anomalies although the moorings were separated laterally by only 11 km the upper current meters also recorded significant differences in m2 tidal currents indicating the presence of significant internal tides at this site below volcano summit depth 1220 m the kinetic energy of currents was primarily tidal 54 longer period currents at depth evidenced the influence of volcano topography temperature time series showed a 3 month period of sharply decreased temperature that extended in depth from 163 m to at least 1260 m advanced very high resolution radiometer avhrr sea surface temperature data show colder surface water immediately above and north of brothers over the same time period these data together with the velocity records suggest an anomalous deep cold water mass arrived and lingered over brothers for 3 months in the austral late spring and early summer of 2004 2005

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

high resolution water column survey to identify active sublacustrine hydrothermal discharge zones within lake rotomahana north island new zealand abstract autonomous underwater vehicles were used to conduct a high resolution water column survey of lake rotomahana using temperature ph turbidity and oxidation reduction potential orp to identify active hydrothermal discharge zones within the lake five areas with active sublacustrine venting were identified 1 the area of the historic pink terraces 2 adjacent to the western shoreline subaerial steaming cliffs boiling springs and geyser 3 along the northern shoreline to the east of the pink terrace site 4 the newly discovered patiti hydrothermal system along the south margin of the 1886 tarawera eruption rift zone and 5 a location in the east basin northeast of patiti island the pink terrace hydrothermal system was active prior to the 1886 eruption of mount tarawera but venting along the western shoreline in the east basin and the patiti hydrothermal system appear to have been initiated in the aftermath of the eruption similar to waimangu valley to the southwest different combinations of turbidity ph anomalies both positive and negative and orp responses suggest vent fluid compositions vary over short distances within the lake the seasonal period of stratification limits vertical transport of heat to the surface layer and the hypolimnion temperature of lake rotomahana consequently increases with an average warming rate of 0 010 c day due to both convective hydrothermal discharge and conductive geothermal heating a sudden temperature increase occurred during our 2011 survey and was likely the response to an earthquake swarm just 11 days prior

hydrothermal venting at hinepuia submarine volcano kermadec arc understanding magmatic hydrothermal fluid chemistry the hinepuia volcanic center is made up of two distinct edifices aligned northwest to southeast with an active cone complex in the se hinepuia is one of several active volcanoes in the northern segment of the kermadec arc regional magnetic data shows no evidence for large scale hydrothermal alteration at hinepuia yet plume data confirm present day hydrothermal discharge suggesting that the hydrothermal system may be too young to have altered the host rocks with respect to measurable changes in magnetic signal gravity data are consistent with crustal thinning and shallow mantle under the volcanic center following the discovery of hydrothermal plumes over hinepuia the submersible shinkai 6500 was used to explore the se cone and sample hydrothermal fluids the chemistry of hydrothermal fluids from submarine arc and backarc volcanoes are typically dominated by water rock interactions and or magmatic degassing chemical analyses of vent fluids show that hinepuia does not quite fit either traditional model moreover the hinepuia samples fall between those typically ascribed to both end member fluid types when plotted on a k mg so4 ternary diagram due to evidence of strong degassing abundant native sulfur deposition and h2s presence the vent sampled at hinepuia is ultimately classified as a magmatic hydrothermal system with a water rock influence this vent is releasing water vapor and magmatic volatiles with a notable lack of salinity due to subcritical boiling and phase separation magmatic hydrothermal fluid chemistry appears to be controlled by a combination of gas flux phase separation processes and volcano evolution and or distance from the magma source

backarc rifting constructional volcanism and nascent disorganised spreading in the southern havre trough backarc rifts sw pacific high resolution multibeam em300 and seabeam data of the southern havre trough sht combined with observations and sample collections from the submersible shinkai6500 and deep tow camera are used to develop a model for the evolution and magmatism of this backarc system the havre trough and the associated kermadec arc are the product of westward subduction at the pacific australian plate boundary detailed studies focus on newly discovered features including a seamount saito seamount and a deep graben ngatoroirangi rift 4000 m water depth floored with a constructional axial volcanic ridge 5 km in length and in excess of 200 m high both of which are characterised by pillow and lobate flows estimated at elongate volcanic ridges at 35 15 s and 34 30 s and backarc seamounts 35 30 s 178 30 e occur at the eastern margin of the sht similar seafloor morphology is observed in the central and western portions of the basin suggesting that recent volcanism may be broadly distributed across the backarc mass balance modelling indicates a maximum crustal thickness of 11 km to 4000 m spreading systems these discontinuous spreading systems are characterised by failed rifts rift segmentation and propagation successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion cross arc volcanic chains isolated volcanoes and underlying basement plateaus are interpreted to represent a cap of recent extrusives however they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives

atmospheric co2 content in the last 120 000 years the phosphate extraction model broecker 1982 has proposed that during the retreat of the wisconsin ice sheets atmospheric co2 increased because of the extraction of phosphorus from the ocean as sea level rose using a time dependent box model we examine the consequences of the phosphate extraction hypothesis over the last 120 000 years assuming that 18o change in core v28 238 is analogous to sea level variation the model takes into account the total co2 and alkalinity balance in the deep sea and in an upper reservoir consisting of the surface ocean and atmosphere which are assumed to be in equilibrium dissolution of deep sea calcium carbonate sediment is assumed to respond to the supply of particulate carbonate from the upper box and to the dissolved carbonate ion concentration of the deep sea assuming 1015 mole of phosphorus and 1017 mole of carbon were extracted during deglaciation the predicted increase in pco2 is 54 ppm variation of pco2 follows the ice volume forcing function with a 1000 yr lag which is the input residence time of water in the deep sea the accumulation supply minus dissolution of caco3 and the percent preservation accumulation over supply follow the derivative of the ice volume function both are similar to the solution index and percent fragments of core v28 238 in addition unusually good preservation is predicted during deglaciation due to carbon extraction if phosphorus but not carbon is assumed to be extracted the accumulation of caco3 varies little remaining near a value equivalent to the river input

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

ocean chemistry during glacial time abstract measurements of co2 to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the co2 content of the atmosphere was considerably lower during peak glacial time than during holocene time the purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water two possible scenarios are presented one involves alternate storage and erosion of phosphorous leaving residues from shelf sediments the other involves changes in the c p ratio in the organic debris falling to the deep sea means of verifying the nutrient cycle hypothesis are also given it is shown that the 13c record as we know it in planktonic and benthic foraminifera the oxygen record as inferred from benthic foraminifera species distributions and the early post glacial caco3 preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented only if an early post glacial spike in the 13c record for planktonic shells could be found would it be possible to eliminate one of these hypotheses i e that involving shelf storage the implications of these nutrient hypotheses to climate theory are as follows if shelf storage is responsible for the glacial to interglacial co2 increase then the co2 change must be considered an amplifier of some primary cause the reason is that sea level changes are needed to drive deposition on and erosion from the shelves on the other hand if changes in the c p ratio for falling debris are responsible then the co2 change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle the latter is possible as self sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry

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

forearc structures and tectonic regimes at the oblique subduction zone between the hikurangi plateau and the southern kermadec margin the convergent southern kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic hikurangi plateau swath bathymetric data have been combined with other geophysical data to reveal that the south kermadec inner trench wall is segmented into distinct upper middle and lower margins the lower margin is a nongrowing compressionally unstable accretionary wedge that is dissected by transcurrent faults the wedge has been thinned and narrowed as a result of tectonic erosion the middle margin is stable by comparison and separated from the lower margin by the 220 km long awanui fault this fault is a landward dipping strike slip reverse fault the middle upper margin including east cape ridge ecr forms a deforming backstop with a sharp along strike change in tectonic regime south of 36 40 s transpression characterizes the ecr shear zone north of 36 40 s the ecr is marked by uplift by extension and farther north by subsidence it is suggested that subsidence and subsequent tectonic erosion of the margin north of 35 50 s in the wake of hikurangi plateau subduction have broken the link between once continuous segments of the ecr tonga platform the unstable nature of the southern kermadec wedge under compression together with a comparison of its taper characteristics with those of modern convergent margins implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime strike slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the hikurangi plateau

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

metallogenesis and mineralization of intraoceanic arcs i kermadec arc introduction the kermadec intraoceanic arc marks the southernmost section of the pacific ring of fire in the western pacific region a chain of mostly underwater volcanoes that stretches from north of japan southward to new zealand the kermadec arc striking for 1 200 km from monowai volcano to the north island of new zealand represents the southern portion of the contiguous 2 500 km kermadec tonga arc fig 1 de ronde et al 2003 the kermadec arc is populated by almost 30 volcanic centers comprising single large caldera or cone volcanoes or less commonly groups of relatively smaller cones de ronde et al 2001 2007 graham et al 2008 all of these volcanic centers lie to the west of the kermadec ridge in the southern and mid parts of the arc by up to 70 km in the case of rumble ii west then merge with the kermadec ridge near the kermadec islands around 30 s the volcanoes then diverge away westward from the kermadec ridge in the northern reaches of the arc fig 1 fig 1 location map for the kermadec arc and the various volcanoes referred to in the papers of this special volume the kermadec arc havre trough system is made up from east to west of a number of tectonic elements including the kermadec trench the kermadec ridge the havre trough or kermadec back arc and the colville ridge the volcanoes of the active arc front lie immediately west of the kermadec ridge in the southern and mid parts of the arc then merge with the kermadec ridge to the north the underlying bathymetry in shaded relief is derived from the satellite altimetry grid of smith and sandwell 1997 with more up to date bathymetry over top especially in the southern

hydrothermal venting and mineralization in the crater of kick em jenny submarine volcano grenada lesser antilles kick em jenny is a frequently erupting shallow submarine volcano located 7 5 km off the northern coast of grenada in the lesser antilles subduction zone focused and diffuse hydrothermal venting is taking place mainly within a small 70 110 m depression within the 300 m diameter crater of the volcano at depths of about 265 m much of the crater is blanketed with a layer of fine grained tephra that has undergone hydrothermal alteration clear fluids and gas are being discharged near the center of the depression from mound like vents at a maximum temperature of 180 c the gas consists of 93 96 co2 with trace amounts of methane and hydrogen gas flux measurements of individual bubble streams range from 10 to 100 kg of co2 per day diffuse venting with temperatures 5 35 c above ambient occurs throughout the depression and over large areas of the main crater these zones are colonized by reddish yellow bacteria with the production of fe oxyhydroxides as surface coatings fragile spires up to several meters in height and elongated mounds up to tens of centimeters thick a high resolution photomosaic of the inner crater depression shows fluid flow patterns descending the sides of the depression toward the crater floor we suggest that the negatively buoyant fluid flow is the result of phase separation of hydrothermal fluids at kick em jenny generating a dense saline component that does not rise despite its elevated temperature

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

3 d focused inversion of near seafloor magnetic data with application to the brothers volcano hydrothermal system southern pacific ocean new zealand 1 we describe and apply a new inversion method for 3 d modeling of magnetic anomalies designed for general application but which is particularly useful for the interpretation of near seafloor magnetic anomalies the crust subsurface is modeled by a set of prismatic cells each with uniform magnetization that together reproduce the observed magnetic field this problem is linear with respect to the magnetization and the number of cells is normally greater than the amount of available data thus the solution is obtained by solving an under determined linear problem a focused solution exhibiting sharp boundaries between different magnetization domains is obtained by allowing the amplitudes of magnetization to vary between a pre determined range and by minimizing the region of the 3 d space where the source shows large variations i e large gradients a regularization functional based on a depth weighting function is also introduced in order to counter act the natural decay of the magnetic field intensity with depth the inversion method has been used to explore the characteristics of the submarine hydrothermal system of brothers volcano in the kermadec arc by inverting near bottom magnetic data acquired by autonomous underwater vehicles auvs different surface expressions of the hydrothermal vent fields show specific vertical structures in their underlying demagnetization regions that we interpret to represent hydrothermal upflow zones for example at focused vent sites the demagnetized conduits are vertical pipe like structures extending to depths of 1000 m below the seafloor whereas at diffuse vent sites the demagnetization regions are characterized by thin and inclined conduits

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

geochemical characterization of highly diverse hydrothermal fluids from volcanic vent systems of the kermadec intraoceanic arc abstract during the r v sonne cruise so253 in 2016 2017 hydrothermal vent sites along the kermadec intraoceanic arc were sampled for hydrothermal fluids at four active volcanoes macauley haungaroa brothers and rumble iii respectively water depths ranged between 290 m and 1700 m a new vent field was discovered at haungaroa the samples were taken from diffuse flow sites as well as from white and black smokers rich in metals and gases with discharge temperatures as high as 311 c their fluid composition is very variable but basically divides into two types one that indicates distinct magmatic input and another that shows evidence for intense water rock interaction under hot acidic conditions fluid samples from macauley the shallowest sampling site 300 m had fe concentrations as high as 1 7 mm al concentrations up to 122 m and h2s up to 10 mm at a ph of only 1 2 at brothers the deepest sampling site down to 1600 m we identified two different fluid types 1 a magmatically influenced type at the upper and lower cone with highest temperatures of 115 c up to 95 6 mm mg the highest mg concentration measured in fluids from intraoceanic arc systems so far elevated so42 76 9 mm high h2s 5 0 mm but fe concentrations of only 15 m and 2 a fluid with low mg 5 4 mm low h2s 1 1 mm temperatures reaching 311 c and high fe contents 12 4 mm at the upper caldera and nw caldera wall typical of a black smoker fluid chloride concentrations in all fluids were similar or highly enriched when compared to seawater e g up to 787 mm brine fluids with also one low chlorinity vapor phase fluid sample recovered indicating that phase separation is occurring at brothers unusual highly elevated mg concentrations in fluids from the brothers lower cone 95 6 mm compared to 53 2 mm in ambient seawater combined with highly elevated concentrations of so42 76 9 mm compared to 29 0 mm in ambient seawater indicate dissolution of mg and so42 bearing minerals in the subsurface such as caminite our data show how highly diverse and variable island arc systems can be with respect to their fluid chemistry both spatially and temporally it adds to the still limited data set of arc systems compared to mid ocean ridges and supplies an important contribution towards a better understanding of geochemical processes along arc volcanoes the higher range in fluid chemistry together with shallower water depth implies that the fluids from intraoceanic arcs may contribute a significant fraction of dissolved metals not only to the global oceanic biogeochemical cycle but also into the photic zone the area of highest bioproductivity

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

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

on the coherency of ocean and land surface specular scattering for gnss r and signals of opportunity systems an analysis of the coherency properties of specular scattering from ocean and land surfaces as observed in global navigation satellite system reflectometry gnss r and signals of opportunity systems is presented this analysis applies existing approximate models for the coherent and incoherent contributions approximate expressions are developed for when one component of the return is likely to dominate as a function of surface and observing system properties the model developed is then applied for sea surface returns and the relative contribution of the coherent term is expressed as a function of the receiver height frequency incidence angle and wind speed for l band spaceborne measurements it is shown that coherence is expected only for wind speeds less than 2 3 m s while for p band spaceborne measurements coherence can dominate returns for wind speeds up to 5 7 m s for land surface measurements from space it is shown that the surface rms height needs to be sufficiently low for coherent components to dominate returns coherence dominates for roughness values not exceeding a range of 5 7 cm for the l band and 15 30 cm for the p band for the l band these conditions over land are likely to be created primarily by inland water bodies a model for the specular scattering from a water body including earth curvature effects is then developed to highlight the strong dependence of the resulting coherent field on the shape of the water body and any offset in its location from the specular point these results further clarify the significant variability that should be expected in coherent scattering from inland water bodies

helium isotope studies of geothermal fields in the taupo volcanic zone new zealand abstract we report new measurements of 3he 4he and ne on 69 samples of gas from geothermal fields in the central section of the ne sw trending taupo volcanic zone tvz we find 3 he 4 he ratios in the range 5 9 7 3 times the atmospheric ratio 5 9 7 3ra typical of subduction zone systems in some areas near the boundaries of the volcanic zone the ratio is much lower 2 4ra the values in the wairakei field are extremely uniform and vary by only a few per cent from 7 2 ra in contrast the wells in the ohaaki broadlands field show a striking factor of two variation with the west bank section characterised by values near 6 ra while the east bank section has values as low as 3 0 ra this suggests that mixing of two types of geothermal source fluids may be occurring at ohaaki broadlands an inverse relationship has been found between the 3 he 4 he ratio of the gases and the b cl ratios in the associated fluids thus the helium isotope ratios appear to be affected both by the flux of mantle helium associated with mantle subduction and also by the presence of sedimentary rocks beneath the geothermal field leaching by the hot ascending fluid of radiogenic 4he together with boron from the underlying high boron low chloride mesozoic greywacke basement is thought to be the cause of this inverse relationship the co 2 3 he ratio in the tvz exceeds the morb ratio by a factor of up to 100 with higher values corresponding to higher b cl fluid ratios suggesting that the greywacke may also be the major source of the co2 we have also measured argon concentrations and isotopes on 19 samples in the ohaaki broadlands area in order to further elucidate the nature of the striking helium isotope variations the argon isotope ratios are relatively close to atmospheric indicating that the majority of the argon comes from recirculated groundwater the simplified boiling model of mazor et al 1990 has been used to calculate the 3 he heat ratio in the ohaaki broadlands and wairakei fields the values obtained fall within the range previously reported for mid ocean ridge systems this model has been extended and has led us to the conclusion that compared to the 3 he 4 he ratio the 3 he 36 ar asw ratio is a much better indicator of the potential geothermal heat the 4 he 40 ar na where na non atmospheric ratio of both the high mantle derived and low crustal derived 3 he 4 he end members of the analyses reported here is about 3 6 but for samples with low 3 he 36 ar ratio this ratio falls to values below 0 5 the value of 3 6 is in the range which is expected for the radiogenic component and although considerably lower than that often found in subduction type mantle gases it is still within the accepted range of values

submarine magmatic hydrothermal systems at the monowai volcanic center kermadec arc the monowai volcanic center is located at the midpoint along the 2 530 km long tonga kermadec arc system the monowai volcanic center is comprised of a large elongate caldera monowai caldera area 35 km2 depth to caldera floor 1 590 m which has formed within an older caldera some 84 km2 in area to the south of this nested caldera system is a large composite volcano monowai cone which rises to within 100 m of the sea surface and which has been volcanically active for the past several decades mafic volcanic rocks dominate the monowai volcanic center basalts are the most common rock type recovered from the cone whereas basaltic andesites are common within the caldera hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone 1 the summit of the cone 2 low temperature venting 60 c mussel ridge on the southwestern wall of the caldera and 3 a deeper caldera source with higher temperature venting that has yet to be observed the cone summit plume shows large anomalies in ph a shift of 2 00 ph units and 3he 358 and noticeable h2s up to 32 m and ch4 up to 900 nm the summit plume is also metal rich with elevated total dissolvable fe tdfe up to 4 200 nm tdmn up to 412 nm and tdfe tdmn up to 20 4 particulate samples have elevated fe si al and ti consistent with addition to the hydrothermal fluid from acidic water rock reaction plumes extending from 1 000 to 1 400 m depth provide evidence for a major hydrothermal vent system in the caldera the caldera plume has lower values for tdfe and tdmn although some samples show higher tdmn concentrations than the cone summit plume caldera plume samples are also relatively gas poor i e no h2s detected ph shift of 0 06 ph units ch4 concentrations up to 26 nm the composition of the hydrothermal plumes in the caldera have higher metal contents than the sampled vent fluids along mussel ridge requiring that the source of the caldera plumes is at greater depth and likely of higher temperature minor plumes detected as light scattering anomalies but with no 3he anomalies down the northern flank of the monowai caldera most likely represent remobilization of volcanic debris from the volcano flanks we believe the monowai volcanic center is host to a robust magmatic hydrothermal system with significant differences in the style and composition of venting at the cone and caldera sites at the cone the large shifts in ph very high 3he values elevated tdfe and tdfe tdmn and the h2s and ch4 rich nature of the plume fluids together with elevated ti p v s and al in hydrothermal particulates indicates significant magmatic volatile metal contributions in the hydrothermal system coupled with aggressive acidic water rock interaction by contrast the caldera has low tdfe tdmn in hydrothermal plumes however elevated al and ti contents in caldera particulate samples combined with the presence of alunite pyrophyllite sulfide minerals and native sulfur in samples from mussel ridge suggest past and perhaps recent acid volatile rich venting and active fe sulfide formation in the subsurface

sea ice detection using u k tds 1 gnss r data a sea ice detection algorithm developed using the u k techdemosat 1 u k tds 1 global navigation satellite systems gnsss reflectometry data over the arctic and antarctic regions is presented it is based on measuring the similarity of the received gnss reflected waveform or delay doppler map ddm to the coherent reflection model waveform over the open ocean the scattered signal has a diffusive incoherent nature it is described by the rough surface scattering model based on the geometric optics and the gaussian statistics for the ocean surface slopes over sea ice and in particular newly formed sea ice the scattered signal acquires a coherence which is characteristic for a surface with large flat areas in order to measure the similarity of the received waveform or ddm to the coherent reflection model three different estimators are presented the normalized ddm average the trailing edge slope tes and the matched filter approach here a probabilistic study is presented based on a bayesian approach using two different and independent ground truth data sets this approach allows one to thoroughly assess the performance of the estimators the best results are achieved for both the tes and the matched filter approach with a probability of detection of 98 5 a probability of false alarm of 3 6 and a probability of error of 2 5 however the matched filter approach is preferred due to its simplicity data from amsr2 processed using the arctic radiation and turbulence interaction study sea ice algorithm and from an special sensor microwave imager sounder radiometer processed by ocean and sea ice saf have been used as ground truth a pixel has been classified as a sea ice pixel if the sea ice concentration sic in it was larger than 15 the measurement of the sic is also assessed in this paper but the nature of the u k tds 1 data lack of calibrated data does not allow to make any specific conclusions about the sic

crustal magnetization of brothers volcano new zealand measured by autonomous underwater vehicles geophysical expression of a submarine hydrothermal system near bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at brothers volcano southern kermadec arc new zealand crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields one of which was unknown prior to our surveys the magnetization pattern is consistent with a model of discrete individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields

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

detailed morphology and structure of an active submarine arc caldera brothers volcano kermadec arc a survey of the brothers caldera volcano kermadec arc with the autonomous underwater vehicle abe has revealed new details of the morphology and structure of this submarine frontal arc caldera and the geologic setting of its hydrothermal activity brothers volcano has formed between major sw netrending faults within the extensional field of the havre trough brothers may be unique among known submarine calderas in that it has four active hydrothermal systems two high temperature sulfide depositing sites associated with faulting on the northwestern and western walls i e the nw caldera and w caldera hydrothermal sites respectively and gas rich sites on the summits of the constructional cones that fill most of the southern part of the caldera i e the upper and lower cone sites the 3 0 3 4 km caldera is well defined by a topographic rim encompassing 320 of its circumference and which lies between the bounds of two outer half grabenshaped faults in the northwest and southeast sectors there is not a morphologically well defined continuous ring fault at the map resolution although near vertical scarps are present discontinuously at the base of sections of the wall the width of the wall varies from 200 m at its southwest portion to 750 m on its northern section the widest part of the wall is its northwest sector which also has the largest documented area of hydrothermal alteration and where sea floor magnetization is lowest in addition to primary northwest southeast elongation and southwest northeast structures caused by faulting within the regional back arc strain field there are also less well developed west southwestnorth northeast regional structures intersecting the volcano that is appar ent on the abe bathymetry and at outcrop scale from submersible observations asymmetrical trap doorstyle caldera collapse is considered a possible mechanism for the formation of the brothers caldera

on the calculation of total heat salt and tracer fluxes from ocean hydrothermal events abstract total heat salt and other tracer masses released during a hydrothermal event are shown to be proportional to but not necessarily equal to volume integrals of resulting water column anomalies proportionality coefficients depend on anomaly definition on background hydrographic and tracer profiles on expansion coefficients of the equation of state at an appropriate pressure and on tracer to heat anomaly ratios at the venting source for gorda ridge event plumes which are described in other papers of this issue volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2 4 if the discharge is not anomalously saline under certain combinations of hydrographic and source anomaly conditions not unlike those found on the east pacific rise at 10 n the apparent total heat released during an event can be deceivingly zero this analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source one consequence is that the ratio of anomalous 3 he to heat in gorda ridge event plumes is approximately 2 4 larger in the water column than it is at the source results are independent of the entrainment process involved in event plume formation and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth

bulk properties of hot smoker plumes the hot buoyant fluid from a submarine vent rises as a plume entraining fluid from the stratified ocean until a height is reached where the driving buoyant force vanishes and shortly thereafter the diluted plume fluid intrudes into the ocean interior at its equilibrium depth while a large positive heat flux issues from the vent the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height depends on the oceanic stratification of both salinity and potential temperature and on the salinity of the source fluid at the vent for example in the atlantic ocean the oceanic stratification causes the spreading plume fluid to be cooler and fresher than the surrounding seawater of the same density while in the pacific the spreading fluid is relatively warm here the well known solution for a turbulent plume rising in a linearly stratified environment is used to obtain simple formulae for 1 the potential temperature anomaly of the horizontally spreading fluid 2 the heat flux associated with this injection of diluted vent water at this density level and 3 the nondimensional solution for the evolution of tracer properties e g salinity and potential temperature in the rising plume

formation of laminite deposits in anaerobic dysaerobic marine environments abstract vraconian sediments from the bandol area var southeastern france are characterized by alternations of thin dark laminae in which organic matter is concentrated and light calcareous silt grained laminae the dark laminae black shales are regarded as the result of autochthonous i e in place hemipelagic sedimentation the light laminae calcareous silt result from the partial reworking of turbiditic influxes by deep marine currents comparison with modern marine environments suggests that the vraconian laminite deposits from the bandol area were formed by sedimentation from deep marine circulations the laminated structures formed in a deep marine environment subjected to periodic variations in the physicochemical conditions organic productivity and thus the nature of organic matter the dark laminae were deposited in an anaerobic marine environment in which the organic matter was preserved during the deposition of the light laminae the depositional environment changed to dysaerobic conditions allowing the development of a peculiar benthic fauna responsible for the decreasing organic matter content source rock characteristics of these deposits are inferred from their laminated structure and fine texture coupled with the occurrence of organic rich layers

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

the rapuhia scarp northern hikurangi plateau its nature and subduction effects on the kermadec trench abstract multibeam bathymetric data seismic reflection and magnetic data are used to examine the nature of the 1 km high rapuhia scarp and the tectonic effects of its subduction on the kermadec margin the rapuhia scarp is the boundary between the hikurangi plateau and 85 ma oceanic crust its intersection with the kermadec trench moves south at 4 cm yr magnetic modelling indicates that the northern hikurangi plateau may be formed of 5 km of relatively strongly magnetised extrusive and intrusive volcanics rocks uplift of the rapuhia scarp near the trench is consistent with vertical motion on a tear fault in the subducting plate the coincident intersection of the subducting rapuhia scarp with a nnw trending volcanic ridge intersection ridge and the kermadec trench results in an abrupt 1 5 km increase in the trench depth and inner margin topography the rapuhia scarp boundary sweeps south along the margin slumping and subsidence of the inner margin resulting from this transition occur on a variety of scales from 5 km wide 1 km high arcuate frontal erosion slumps to regional subsidence of up to 4 km across the 50 100 km wide mid upper inner trench margin as it adjusts to the new subduction geometry the pattern of inner margin subsidence involving the entire forearc is broadly similar to that observed 1200 km further north in the wake of subduction of the louisville ridge seamount chain subsidence of the inner trench margin results in the loss of approximately 400 km3 per kilometre of margin as the subducting hikurangi plateau support is removed we estimate however only one fifth of the above subsidence is frontal or basal erosion highlighting the risks of estimating tectonic erosion from margin subsidence if the cause of subsidence is mis identified the remaining 320 km2 of subsidence is due to the steeper dip of the subducting plate associated with subduction of the comparatively dense 85 ma oceanic crust

hydrothermal helium plumes in the pacific ocean hydrothermal activity along the global mid ocean ridge system and at active seamounts introduces a 3 he rich signal into the deep ocean basins which can be used to trace patterns of ocean circulation and mixing this hydrothermal helium signal is especially strong in the pacific ocean where the spreading rate of the ridges is the greatest in several areas of the pacific the hydrothermal activity is of sufficient strength to produce intense helium plumes which clearly define the regional circulation among these are 1 a pair of helium plumes at 2500 m depth which extend westward from the east pacific rise at 10 n and 15 s into the interior of the pacific basin 2 a helium plume at 2000 m depth extending southwest from sources on the juan de fuca ridge in the northeast pacific and 3 a plume at 1100 m depth emanating from loihi seamount at 20 n which extends eastward from hawaii to the coast of mexico the flow field implied by the helium distribution appears to be in reasonable agreement with reid s 1997 steric height analysis for the pacific

influence of sea level changes on the deposition of carbonaceous sediments abstract the mid cretaceous deposits aptian lower cenomanian of the provence basin se france show a predominance of interbedding of limestone and marl which indicates a deep marine environment the stratigraphic intervals of this series have been dated with ammonites foraminifera dinocysts and calcareous nannofossils the sedimentological paleobiological and paleoecological data indicate the occurrence during the aptian lower cenomanian period in basse provence of an open marine environment or at least in communication with open sea subjected to some land influence fairly distant from the site of deposition the sediments studied have a high carbonate content and include several levels rich in organic matter 0 5 11 of toc the nature of this organic material makes it possible to distinguish three different origins for the organic matter 1 1 a marine origin represented by phytoplankton and zooplankton 2 2 a continental origin with plant cuticles tracheophytes ligneous tissues spores and pollen 3 3 a bacterial origin with amorphous material strontium and manganese content fluctuations in the carbonated phase of the facies studied reflect variations in the chemical composition of the sea water function of the geological time these fluctuations coincide globally with the sea level variation curves sedimentological paleontological and geochemical analyses of the mid cretaceous series in provence have made it possible to detect positive and negative fluctuations in the sea level within this series and to correlate deposition and nature of the facies rich in organic matter with high and low sea level periods the results of this study indicate that the sedimentation sources of facies rich in organic matter are many and independant of sea level variations as this type of deposit appears equally at high and low sea levels

trace element mapping of copper and zinc rich black smoker chimneys from brothers volcano kermadec arc using synchrotron radiation xfm and la icp ms

hikurangi plateau a cretaceous large igneous province in the southwest pacific ocean the first dredge haul of basement rocks obtained from the hikurangi plateau in the southwest pacific ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre late cretaceous age all samples have undergone extensive seafloor weathering to phyllosilicate and zeolite bearing assemblages petrography mineral chemistry and whole rock element concentrations and ratios of the least altered lavas e g tio2 1 3 1 6 wt cr 132 224 ppm zr y 2 9 3 6 zr nb 19 29 ti v 22 29 suggest a restricted igneous compositional range broadly comparable with normal to enriched mid ocean ridge basalt however isotopic compositions of leached samples 87sr 86sri 0 70361 0 70374 endi 5 7 6 2 are comparable with oceanic island basalt suites collectively these petrological characteristics are similar to rocks from large igneous provinces of cretaceous age in the western pacific ocean e g manihiki and ontong java plateaus consequently we interpret the hikurangi plateau as another of these early cretaceous basaltic oceanic plateaus speculatively parts of the obducted tangihua and matakaoa igneous complexes of the north island may have been derived from the hikurangi plateau as the plateau collided with the new zealand continent in the early miocene

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

analysis of scattering characteristics from inland bodies of water observed by cygnss abstract measurements made by spaceborne global navigation satellite system reflectometry gnss r instruments have shown strong reflected power over inland waters that has been attributed to coherent scattering coming from the first fresnel zone previous work in the field has shown the ability of gnss r to observe the global surface water distribution by generating dynamic maps of wetlands and other inundated areas these maps can be generated by leveraging the large difference in received power of the gnss signals as they reflect from water surfaces compared to land in this paper we utilize a full forward scattering model approach to evaluate the accuracy of these maps the cygnss end to end simulator e2es was extended to include the contributions from coherent surface scattering in heterogeneous regions where the area around the specular point contains both water and land in complex geometries the simulator is then used to analyze the accuracy of a current approach to estimate the surface water content in the first fresnel zone from a single measurement called a fractional water in footprint approach we find that contributions to the total received power by the scattering from outside the first fresnel zone as well as cygnss instrument effects impact the accuracy of this approach furthermore the measured signal from calibration scattering targets is compared to the results of simulation to validate the scattering model the work shows that the variability of the peak reflected power over inland bodies of water due to the scene heterogeneities should be accounted for in designing retrieval algorithms to map wetland extent

effect of sea roughness on bistatically scattered range coded signals from the global positioning system a series of aircraft experiments was performed using a specialized gps receiver and a nadir oriented left hand circularly polarized antenna this apparatus received reflections of the gps signals from water surfaces under a variety of sea states the cross correlation between the reflected signal and a reference pseudo random noise code was recorded as a function of the relative time delay the shape of this function showed a dependence on the roughness of the reflecting surface this dependence generally followed that predicted by theory for bistatic scattering of range coded signals use of this information as a remote sensing technique for the determination of sea state is discussed

helium isotopic composition of the tabar lihir tanga feni island arc papua new guinea helium abundances and isotopic ratios have been measured in samples of geothermal gases submarine ultramafic xenoliths and subaerial mafic phenocrysts and xenoliths from the tabar lihirtanga feni tltf arc in the bismarck archipelago of papua new guinea these unusual volcanoes are produced by adiabatic decompression melting of subduction modified upper mantle and apparently carry an exceptionally large slab derived component they therefore provide an ideal setting for studying the role of subducted helium in arc volcanoes helium isotopic ratios in the geothermal fluids submarine xenoliths and most helium rich subaerial samples indicate the mantle source regions beneath the tabar island group and lihir island have 3he 4he ratios of about 7 2 times the atmospheric ratio r a whereas the mantle source for ambitle island feni group in the more southern tltf arc has a lower ratio of about 6 6 r a these ratios are only slightly lower than typical depleted upper mantle values of about 8 5 r a indicating that even severely slab modified mantle wedge carries a relatively minor slab derived helium component the systematically lower 3he 4he ratios of the ambitle samples are interpreted to reflect a slightly greater slab derived helium component helium isotopic ratios as low as 2 r a which are correlated with low total helium abundances were observed in subaerial augitic clinopyroxene phenocrysts coexisting olivine and sodium rich acmitic clinopyroxene have higher helium abundances and isotopic ratios this isotopic disequilibrium between coexisting mineral phases is interpreted as crustal level addition of a low 3he 4he component to an actively outgassing magma from which different mineral phases trap helium sequentially

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

propagation linkage and interaction of caldera ring faults comparison between analogue experiments and caldera collapse at miyakejima japan in 2000 the formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash flow eruptions caldera collapse occurred in 2000 at miyakejima island japan in response to a lateral intrusion based on geophysical data it is inferred that a set of caldera ring faults was propagating upward to understand the kinematics of ring fault propagation linkage and interac tion we describe new laboratory sand box experiments that were analyzed through digital image correlation dic and post processed using 2d strain analysis the results help us gain a better understanding of the processes occurring during caldera subsidence at miyakejima we show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards then a set of normal faults initiates from tension fractures at the surface propagating downwards to link with the reverse faults at depth with increasing amounts of subsidence interaction between the reverse and normal fault segments results in a deactivation of the reverse faults while displacement becomes focused on the outer normal faults modeling results show that the area of faulting and collapse migrates successively outward as peak displace ment transfers from the inner ring faults to later developed outer ring faults the final structural architecture of the faults bounding the subsiding piston like block is hence a consequence of the amount of subsidence in agreement with other caldera structures observed in nature the experimental simulations provide an analogy to the obser vations and seismic records of caldera collapse at miyake jima volcano but are also applicable to caldera collapse in general

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

new multibeam mapping and geochemistry of the 30 35 s sector and overview of southern kermadec arc volcanism new multibeam mapping and whole rock geochemistry establish the first order definition of the modem submarine kermadec arc between 30 degrees and 35 degrees s twenty two volcanoes with basal diameters 5 km are newly discovered or fully mapped for the first time giggenbacb macauley havre haungaroa kuiwai ngatoroirangi sonne kibblewhite and yokosuka for each large volcano edifice morphology and structure surficial deposits lava fields distribution of sector collapses and lava compositions are determined macauley and havre are large silicic intra oceanic caldera complexes for both concentric ridges on the outer flanks are interpreted as recording mega bedforms associated with pyroclastic density flows and edifice foundering other stratovolcanoes reveal complex histories with repeated cycles of tectonically controlled construction and sector collapse extensive basaltic flow fields and the development of summit craters and or small nested calderas combined with existing data for the southernmost arc segment we provide an overview of the spatial distribution and magmatic heterogeneity along similar to 780 km of the kermadec arc at 30 degrees 36 degrees 30 s coincident changes in arc elevation and lava composition define three volcano tectonic segments a central deeper segment at 32 degrees 20 34 degrees 10 s has basement elevations of 3200 m water depth and relatively simple stratovolcanoes dominated by low k series basalt basaltic andesite in contrast the adjoining arc segments have higher basement elevations typically 3 km coincide with the shallower arc segments the dominant mode of large caldera formation is interpreted as mass discharge pyroclastic eruption with syn eruptive collapse hence the shallower arc segments are characterized by both the generation of volatile enriched magmas from crustal melting and a reduced hydrostatic load allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions proposed initiation parameters for submarine pyroclastic eruptions are water depths 70 wt sio2 and a high discharge rate c 2005 elsevier b v all rights reserved

near field hydrothermal plume dynamics at brothers volcano kermadec arc a short lived radium isotope study abstract the naturally occurring radium ra isotope quartet was measured for the first time at an intraoceanic arc hydrothermal setting along the kermadec island arc sw pacific 224ra 3 7 d half life and 223ra 11 4 d half life are assumed to be tracers solely originating from hydrothermal discharge and have been used to resolve near field hydrothermal plume dynamics by applying both advection and eddy diffusion based dispersion models assuming dominant advective processes we identified different horizontal plume age structures at brothers volcano with a slow moving plume inside the caldera 0 060 cm s and a faster moving plume outside the caldera 0 323 cm s indicating a longer plume residence time within the caldera structure the decreasing trend in ra activity also allows the calculation of horizontal eddy diffusion coefficients inside and outside of the caldera with 0 164 m2 s and 3 545 m2 s for 224ra respectively these trends are in accordance with the radial horizontal eddy diffusion model our results reveal that net advection might not necessarily play a major role at brothers as radial horizontal eddy diffusion reproduces the observed ra activity distribution to the same degree additional model simulations based on a combination of advection and diffusion favor slow net advection to explain the ra activity distribution within the rising non buoyant plume ra isotopes suggested constant dilution along the first 170 m of the vertical water column and a vertical transport velocity on the order of 0 156 cm s for altitudes up to 150 m above the vent and around 0 004 cm s for altitudes above 150 m when a common source of diffuse and focused venting is assumed and only small amounts of ra are added to the ascending plume due to recoil originating from high water to rock ratios fluids from diffuse vents show a slower migration rate through the subsurface than fluids from focused vents our data imply that the fluids from diffuse vents needed approximately five days longer to percolate through the underlying subsurface to reach the water column plume compared to the fluids from direct vents which emanate through chimney structures this study demonstrates the potential of short lived ra isotopes as tracers of near field plume dynamics which will improve our understanding of hydrothermal vents as sources for various trace elements to the global ocean

scattering of gps signals from the ocean with wind remote sensing application a theoretical model that describes the power of a scattered global positioning system gps signal as a function of geometrical and environmental parameters has been developed this model is based on a bistatic radar equation derived using the geometric optics limit of the kirchhoff approximation the waveform i e the time delayed power obtained in the delay mapping technique depends on a wave slope probability density function which in turn depends on wind waveforms obtained for aircraft altitudes and velocities indicate that altitudes within the interval 5 15 km are the best for inferring wind speed in some regimes an analytical solution for the bistatic radar equation is possible this solution allows converting trailing edges of waveforms into a set of straight lines which could be convenient for wind retrieval a transition to satellite altitudes together with satellite velocities makes the peak power reduction and the doppler spreading effect a significant problem for wind retrieval based on the delay mapping technique at the same time different time delays and different doppler shifts of the scattered gps signal could form relatively small spatial cells on sea surface suggesting mapping of the wave slope probability distribution in a synthetic aperture radar sar fashion this may allow more accurate measurements of wind velocity and wind direction

microbial mineral floc associated with nascent hydrothermal activity on coaxial segment juan de fuca ridge following a june 1993 seafloor eruption at coaxial segment juan de fuca ridge extensive accumulation of microbial mineral floc was observed in association with abundant diffuse venting on new lava flows and around less extensive new venting on older basalts microscopic and sem examination revealed a diverse assemblage of microbial forms contained within a poorly organized organo mineral matrix samples more closely resembled microbial floc described from other hydrothermal areas than the layered bacterial mats observed after the eruption at 9 n on the east pacific rise iron and silica were the major compositional elements in all samples with the latter likely important in preserving microbial remains organic carbon content was on the order of 3 4 enzyme assays indicated the presence of autotrophic co2 fixation and potential for sulfide oxidation floc covered 7 14 of the seafloor on the hydrothermally active part of the new lava flow we discuss the possibility that the coaxial floc originated as ejecta from the sub seafloor hydrothermal system rather than from in situ production around vent openings

microthermometric evidence for the formation of permian vhms deposits in tasik chini area central belt of peninsular malaysia abstract the vhms deposits of tasik chini area are located in the pahang state of central belt peninsular malaysia the deposits occur in a package of permian volcanics within mixed volcano sedimentary sequence and have been exploited from the bukit botol and bukit ketaya deposits they have many similarities with kuroko style massive sulfide deposits the host rocks at both deposits are felsic volcanic rocks of rhyolitic to rhyodacitic composition mineralization occurs as distinct ore zonation forming a stringer to massive sulfide zone at the footwall followed by barite lenses and exhalite layers fe mn si ore at the stratigraphic top the sulfide phases are characterized by pyrite as the major sulfide mineral with subordinate chalcopyrite sphalerite and rare galena traces of gold silver and sn bearing minerals occur in the massive sulfide and barite ores primary fluid inclusions within quartz and barite occur as two types 1 type i two phase liquid vapour inclusions without daughter minerals and 2 type ii three phase liquid co2 bearing carbonic inclusions these three phase inclusions include liquid h2o liquid co2 and vapour co2 with variable co2 content and often decrepitate before complete homogenization microthermometric measurements of the type i inclusions show that the temperature of homogenization th varies between 180 c and 310 c with salinities ranging from 1 0 to 15 0 wt nacl equivalent in the quartz samples homogenization temperatures th of 190 c to 300 c and salinities between 1 0 and 13 0 wt nacl equivalent are recorded from the barite samples type ii inclusions have an initial melting temperature range of 57 1 c to 57 5 c clathrate melting temperature tmclathrate of 7 4 c to 8 0 c and co2 homogenization temperature thco2 between 30 1 c and 30 3 c and salinity ranging from 4 0 to 5 1 wt nacl equivalent densities pressure and depth of ore forming fluids range from 0 711 to 0 970 g cm3 12 to 93 bars and 1500 m depth of seawater laser raman spectroscopic analysis shows the presence of co2 in the carbonic inclusions the fluid inclusion results suggest that the main ore fluid during the formation of the tasik chini vhms deposit is a seawater dominated fluid but contribution from a modified seawater or possibly magmatic origin is also indicated by higher salinities relative to seawater 3 2 wt nacl and the presence of co2 taking into account the regional geodynamic evolution it is also concluded that the formation of vhms deposits in the tasik chini area occurred in a back arc geotectonic setting

vailulu u seamount samoa life and death on an active submarine volcano submersible exploration of the samoan hotspot revealed a new 300 m tall volcanic cone named nafanua in the summit crater of vailulu u seamount nafanua grew from the 1 000 m deep crater floor in 4 years and could reach the sea surface within decades vents fill vailulu u crater with a thick suspension of particulates and apparently toxic fluids that mix with seawater entering from the crater breaches low temperature vents form fe oxide chimneys in many locations and up to 1 m thick layers of hydrothermal fe floc on nafanua high temperature 81 c hydrothermal vents in the northern moat 945 m water depth produce acidic fluids ph 2 7 with rising droplets of probably liquid co2 the nafanua summit vent area is inhabited by a thriving population of eels dysommina rugosa that feed on midwater shrimp probably concentrated by anticyclonic currents at the volcano summit and rim the moat and crater floor around the new volcano are littered with dead metazoans that apparently died from exposure to hydrothermal emissions acid tolerant polychaetes polynoidae live in this environment apparently feeding on bacteria from decaying fish carcasses vailulu u is an unpredictable and very active underwater volcano presenting a potential long term volcanic hazard although eels thrive in hydrothermal vents at the summit of nafanua venting elsewhere in the crater causes mass mortality paradoxically the same anticyclonic currents that deliver food to the eels may also concentrate a wide variety of nektonic animals in a death trap of toxic hydrothermal fluids

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

evidence for iron and sulfur enrichments in hydrothermal plumes at axial volcano following the january february 1998 eruption in response to 12 days of seismic activity at axial volcano juan de fuca ridge in january february 1998 the nsf noaa sponsored axial response team conducted three response cruises in february august and august september to map and sample hydrothermal plumes over the region vertical profiles of particulate fe and s over the eruption site show high concentrations from about 1400 m to the bottom chemical and scanning electron microscope analysis of the february plume samples revealed anhydrite fe ferrihydrites elemental sulfur and angular glassy basalt shards up to 190 m in the longest dimension many of these shards had halite coatings which is consistent with subseafloor basalt seawater reactions at temperatures 450 c at 1500 m depth in august and september cruises the basalt shards were no longer present in the hydrothermal plumes instead the plumes were predominantly composed of fe ferrihydrites elemental sulfur and sulfur filaments a unique feature of this data set is the high concentrations of elemental sulfur in the lower 60 m of the water column the sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time within the axial caldera approximately 10 20 of the total sulfur is present as sulfur filaments these post eruption particle compositional changes have strong similarities to the results from the 1993 coaxial eruption on the juan de fuca ridge

rapid rates of growth and collapse of monowai submarine volcano in the kermadec arc most of earth s volcanoes are under water as a result of their relative inaccessibility little is known of the structure and evolution of submarine volcanoes advances in navigation and sonar imaging techniques have made it possible to map submarine volcanoes in detail and repeat surveys allow the identification of regions where the depth of the sea floor is actively changing here we report the results of a bathymetric survey of monowai submarine volcano in the tonga kermadec arc which we mapped twice within 14 days we found marked differences in bathymetry between the two surveys including an increase in seafloor depth up to 18 8 m and a decrease in depth up to 71 9 m we attribute the depth increase to collapse of the volcano summit region and the decrease to growth of new lava cones and debris flows hydroacoustic t wave data reveal a 5 day long swarm of seismic events with unusually high amplitude between the surveys which directly link the depth changes to explosive activity at the volcano the collapse and growth rates implied by our data are extremely high compared with measured long term growth rates of the volcano demonstrating the pulsating nature of submarine volcanism and highlighting the dynamic nature of the sea floor

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

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

characteristics of cu isotopes from chalcopyrite rich black smoker chimneys at brothers volcano kermadec arc and niuatahi volcano lau basin we analysed primary chalcopyrite from modern seafloor black smoker chimneys to investigate high temperature hydrothermal cu isotope fractionation unaffected by metamorphism samples came from nine chimneys collected from brothers volcano kermadec arc and niuatahi volcano lau backarc basin this is the first known study of cu isotopes from submarine intraoceanic arc backarc volcanoes with both volcanoes discharging significant amounts of magmatic volatiles our results n 22 range from 65cu 0 03 to 1 44 0 18 2 sd with the majority of samples between 0 00 and 0 50 we interpret this cluster n 17 of lower 65cu values as representing a mantle source for the chimney cu in agreement with 65cu values for mantle rocks the few higher 65cu values 0 90 occur 1 within the same chimneys as lower values 2 randomly distributed within the chimneys i e near the top and bottom interior and exterior and 3 within chalcopyrite of approximately the same age 1 year this suggests the higher 65cu values are not related to oxidation by mixing with ambient seawater but to isotopic variation within the vent fluids over a relatively short time theoretical studies demonstrate significant isotopic fractionation can occur between aqueous and vapourous complexing species when combined with evidence for periodic release of magmatic volatiles at brothers we believe vapour transport of cu is responsible for the observed isotopic fractionation when compared to global 65cu data for primary chalcopyrite volcanic arc chimneys are most similar to porphyry copper deposits that also form from magmatic hydrothermal processes in convergent tectonic settings

iron ii distribution and oxidation kinetics in hydrothermal plumes at the kairei and edmond vent sites indian ocean deep sea hydrothermal activity cycles the entire volume of the global ocean through deep sea hydrothermal plumes at least every 4 8 103 a a rapid timescale that is comparable to global deep ocean mixing an important process within hydrothermal plumes is the oxidation of dissolved iron discharged with vent fluids leading to the co precipitation of many other vent sourced metals thus modifying gross hydrothermal fluxes to the deep ocean and acting as a net sink in the ocean chemical budgets of certain key tracers here we report direct measurements of dissolved fe ii oxidation rates from two recently discovered indian ocean vent sites that are intermediate between pacific and atlantic values thus demonstrating that the rate of iron oxidation and formation of scavenging fe iii solid phases in hydrothermal plumes varies systematically along the global deep ocean conveyor the average pseudo first order rate constant for the oxidation reaction is 0 303 h 1 and the corresponding half life 2 31 h the rate plots show some curvature however indicating additional processes possibly interactions with organic matter may also influence the net oxidation removal rate different rates of oxidation of iron in hydrothermal plumes in different ocean basins have important implications for the dispersion of produced oxide phases which in turn may impact on uptake of other metals from seawater and deposition into underlying ridge crest and ridge flank sediments

discovery of hydrothermal sulfide mineralization from southern kermadec arc volcanoes sw pacific abstract we report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern kermadec frontal arc volcanoes brothers and rumble ii west of the 1200 km long kermadec havre arc back arc system the brothers and rumble ii west calderas both with re surgent domes and comprising mostly dacite and basalt andesite host rocks respectively rise to water depths of 1500 m these calderas comprise mostly effusive lavas and volcaniclastic deposits including talus breccias along their inner caldera walls these two southern kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites two principle ore assemblages are identified comprising i chalcopyrite pyrite barite and ii sphalerite marcasite barite pyrite the most prevalent ore texture consists of massive chalcopyrite pyrite sphalerite commonly overprinting barite early marcasite cores within large euhedral pyrites and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization the ore mineralogy is consistent with postulated venting temperatures of 300 c sulfide geochemistry is consistent with the ore mineralogy with concentrations of cu fe and zn up to 15 3 19 1 and 18 8 wt respectively the sulfide geochemistry is distinct however from comparable western pacific vent sites and may reflect source heterogeneity due to proximity to continental new zealand and sediment recycling along the southern kermadec trench arc system the recovery of two partial caridean vent shrimps suggests that present day hydrothermal venting is occurring within the brothers caldera

hydrothermal exploration of mid ocean ridges where might the largest sulfide deposits be forming highlights black smokers on slow spreading ridges can host larger seafloor massive sulfide sms deposits than on fast ridges the largest slow ridge sms deposits are typically associated with long lived tectonic fractures sustained fluid flow ultramafic tectonic hosted sites exhibit high average cu 10wt and au 3ppm contents in their surface sms samples hydrothermal plume surveys are consistent with equal abundances of magmatic vs tectonic vent control along ultraslow ridges the first black smokers on any ultraslow ridges exhibit high cu au sms contents at both magmatic and tectonic vent sites abstract here we review the relationship between the distribution of modern day seafloor hydrothermal activity along the global mid ocean ridge crest and the nature of the mineral deposits being formed at those sites since the first discovery of seafloor venting a sustained body of exploration has now prospected for one form of hydrothermal activity in particular high temperature black smoker venting along 30 of the global mid ocean ridge crest while that still leaves most of that 60 000 km continuous network to be explored some important trends have already emerged first it is now known that submarine venting can occur along all mid ocean ridges regardless of spreading rate and in all ocean basins further to a first approximation the abundance of currently active venting as deduced from water column plume signals can be scaled linearly with seafloor spreading rate a simple proxy for magmatic heat flux what can also be recognized however is that there is an excess of high temperature venting along slow and ultra slow spreading ridges when compared to what was originally predicted from seafloor spreading magmatic heat budget models an examination of hydrothermal systems tracked to source on the slow spreading mid atlantic ridge reveals that no more than half of the sites responsible for the black smoker plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast spreading ridges the other half of all currently known active high temperature submarine systems on the mid atlantic ridge are hosted under tectonic control these systems appear both to be longer lived than and to give rise to much larger sulfide deposits than their magmatic counterparts presumably as a result of sustained fluid flow a majority of these tectonic hosted systems also involve water rock interaction with ultramafic sources importantly from a mineral resource perspective this subset of tectonic hosted vent sites also represents the only actively forming seafloor massive sulfide deposits on mid ocean ridges that exhibit high concentrations of cu and au in their surface samples 10 wt average cu content and 3 ppm average au along ultraslow spreading ridges first detailed examinations of hydrothermally active sites suggest that sulfide deposit formation at those sites may depart even further from the spreading rate model than slow spreading ridges do hydrothermal plume distributions along ultraslow ridges follow the same 50 50 distribution of black smoker plume signals between magmatic and tectonic settings as the slow spreading mar however the first three black smoker sites tracked to source on any ultra slow ridges have all revealed high temperature vent sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings further deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of cu and au respectively an important implication is that ultra slow ridges may represent the strongest mineral resource potential for the global ridge crest despite being host to the lowest magmatic heat budget

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