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

short paper seismic reflection evidence for the location of the iapetus suture west of ireland a new deep seismic profile recorded west of ireland by the british institutions reflection profiling syndicate birps images prominent north dipping reflectors west of the shannon estuary qualitatively similar north dipping reflectors have been imaged on previous birps profiles recorded across the solway line both east and west of britain and have there been interpreted as structures related to the iapetus suture by analogy with these other north dipping reflectors and because the newly discovered reflectors lie close to previously hypothesized locations of the iapetus suture the structures seen on the new birps profile are here interpreted as representing part of the iapetus suture zone

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

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

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

three dimensional gravity and magnetic modelling of the irish sector of the ne atlantic margin abstract a new 3d lithospheric model has been constructed using high resolution gravity data from the irish national seabed survey the sedimentary component of the model incorporated density variations due to laterally varying overcompaction associated with cenozoic denudation after optimisation based on gravity inversion regional crustal thickness variations were defined which are in reasonable agreement with the results of wide angle seismic experiments high crustal extension factors 5 characterise the deeper parts of the rockall and porcupine basins and in places the model indicates extreme stretching 10 beneath these basins this could be because of instability in the gravity inversion although other recent investigations have independently suggested similarly high extension factors in contrast the hatton basin is characterised by an apparent extension factor of about 2 the modelling resolves a pattern of ne to nne trending local mesozoic basins on the margins of the rockall trough helping to delineate structures that were previously only sparsely sampled by seismic surveys it appears possible that rifts with similar trends underlie the volcanic rocks which obscure the deeper parts of the hatton basin the linear trends of the basins to the south and east of ireland are interpreted to have been inherited from a basement fabric that was initially established during the late precambrian assembly of this basement and subsequently subjected to caledonian and variscan reactivation magnetic modelling indicates that the variations in the thickness of the crystalline crust predicted by the gravity models can explain the regional magnetic anomaly patterns over the rockall and porcupine basins but that significant additional magnetic material probably igneous rocks of both palaeogene and cretaceous ages is required to explain the anomalies in the hatton basin region the magnetic signature of the rockall basin is distinctly different to that over the basement of similar apparent thickness formed during mid cretaceous c34n opening of the ocean basin to the south this is an impediment to hypotheses that invoke mid cretaceous sea floor spreading rather than intracontinental rifting to explain the development of the basin the exception is in the extreme south of the basin where the volcanism associated with the barra volcanic ridges combined with indications of relatively strong lithosphere could be evidence of incipient ocean opening

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

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

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

organic maturation levels thermal history and hydrocarbon source rock potential of the namurian rocks of the clare basin ireland abstract vitrinite reflectance data from two inland cored boreholes confirm high maturation levels throughout the onshore part of the irish clare basin and suggest erosion of 2 to 4 km of late carboniferous cover and elevated palaeogeothermal gradients in the carboniferous section the observed maturation gradients are fully consistent with the published hypothesis of a late carboniferous permian superplume beneath pangaea but local vertical reversals in gradients also suggest a complex thermal regime probably involving advective heating the uppermost visean lower namurian clare shale is laterally extensive and up to 300 m thick although this unit is post mature toc values of up to 15 suggest that it could have considerable hydrocarbon source rock potential in any less mature offshore parts of the basin

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

gas hydrate concentration and characteristics within hydrate ridge inferred from multicomponent seismic reflection data 1 a seismic experiment composed of streamer and ocean bottom seismometer obs surveys was conducted in the summer of 2002 at southern hydrate ridge offshore oregon to map the gas hydrate distribution within the hydrate stability zone gas hydrate concentrations within the reservoir can be estimated with p wave velocity vp however we can further constrain gas hydrate concentrations using s wave velocity vs and use vs through its relationship to vp vp vs to reveal additional details such as gas hydrate form within the matrix i e hydrate cements the grains becomes part of the matrix frame or floats in pore space both vp and vs can be derived simultaneously by inverting multicomponent seismic data in this study we use obs data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments once vp and vs are estimated they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration we model vp using an equation based on a modification of wood s equation that incorporates an appropriate rock physics model and vs using an empirical relation the gas hydrate concentration is estimated to be up to 7 of the rock volume or 12 of the pore space however vp and vs do not always fit the model simultaneously vp can vary substantially more than vs thus we conclude that a model in which higher concentrations of hydrate do not affect shear stiffness is more appropriate results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area

from orogenic collapse to rifting a case study of the northern porcupine basin offshore ireland abstract offshore ireland the north atlantic opening is generally interpreted as successive 10 to 15 myr long rifting events during the mesozoic however their interaction is poorly documented in terms of structural inheritance and fault reactivation from extensive seismic and well data in the northern porcupine basin we show that extension actually evolved over an abnormal period of ca 220 myrs with overlapping tectonic stages that all together reflect the irish atlantic margin construction from the palaeozoic during the carboniferous extension initiated throughout the variscan and caledonian thickened crusts and controlled the deposition of generally slightly tilted continental clastics two subsequent rifting events during the triassic lower jurassic and upper jurassic lower cretaceous controlled shallow to deep marine sedimentation in separated sub basins that progressively widen and connect once extension localised along bounding faults we propose that extensional tectonics first controlled by a general orogenic collapse reflecting an early stage of stretching is then followed by several rifting periods that ultimately evolved toward the hyper thinning of the margin and shaped an aborted rift propagator this evolution is due to pre existing orogenic related structures and boundary conditions variations such a continuum of deformation although pulsed implies varying structural interactions of crustal scale that are often recorded in syn tectonic sediments

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

varnet 96 influence of the variscan and caledonian orogenies on crustal structure in sw ireland summary varnet an international multidisciplinary project was designed to examine the variscan front in the southwest of ireland as part of the seismic experiment a 200 km long wide angle seismic pro le line a was recorded from the old head of kinsale in the south of ireland to galway bay on the west coast along line a 170 seismic stations were deployed at approximately 1 km intervals results from ray trace modelling indicate a multilayered crust the upper crust extending to a depth of about 14 km is laterally variable south of the shannon estuary sedimentary basins alternate with uplifted basement 5 8 6 0 km s 1 this correlates well with the surface geology the larger sedimentary basin in the south reveals a shallowly south dipping structural feature that coincides with the killarney mallow fault zone north of the shannon estuary surprisingly high velocities 6 4 km s 1 at shallow depths were observed between the upper and middle crust there is a transition layer about 2 4 km thick the middle crust thickens towards the shannon estuary these structural changes coincide with the proposed location of the iapetus suture zone the middle crust is underlain by a lower crust with a variable velocity structure and the total crustal thickness varies from 29 32 km detailed traveltime investigations have also revealed a re ector in the upper mantle at depths between 39 and 44 km the seismic model shows that there are no deep crustal variations beneath the killarney mallow fault zone indicating a thin skinned mechanism of deformation

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

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

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

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

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

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 clare basin a geological and geophysical outline the clare basin fig 1 is a poorly documented carboniferous basin remnant situated offshore west of ireland which is considered to be an extension of the onshore west clare namurian basin wcnb it has in the past been variously described as or as part of the clare shelf and shannon basin e g cope et al 1992 the shannon trough e g sevastopulo 1981 and the dingle shannon basin e g gardiner maccarthy 1981 it lies immediately north of the iapetus suture which is assumed to have closed during the devonian figure 1 shows the general location of the clare basin the outline of the younger porcupine basin and slyne trough and the onshore geological outcrop map also shown are the locations of boreholes and the line of cross section figured in the paper

the role of pressure solution seam and joint assemblages in the formation of strike slip and thrust faults in a compressive tectonic setting the variscan of south western ireland abstract the ross sandstone in county clare ireland was deformed by an approximately north south compression during the end carboniferous variscan orogeny the initial assemblage consists of mutually abutting orthogonal arrays of 170 oriented set 1 joints veins jvs and approximately 75 oriented set 1 pressure solution seams psss formed under the same stress conditions orientations of splay jvs and psss set 2 suggest a clockwise remote stress rotation of about 35 responsible for the contemporaneous shearing of the set 1 arrays among these nearly orthogonal strike slip faults the prominent set is sub parallel to set 1 jvs these faults are formed by the linkage of en echelon segments with broad damage zones responsible for right lateral offsets of hundreds of meters thrust faults with up to 30 m of offset initiate within shale horizons and follow either the psss in the sandstones or high angle shales within tilted sequences within the large thrust fault zones compartmentalised blocks of rocks are bounded by thrust faults segments with various dip angles strike slip and thrust faults are contemporaneous and owe their existence to initial weaknesses in the form of jvs and psss rather than by switching relative stress magnitudes and orientations associated with andersonian models of faults and related stress orientations

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

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

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

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

deep structure of southern ireland a new geological synthesis using birps deep reflection profiling the birps deep seismic reflection data of wire west of ireland 1 1b from galway bay to the celtic sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern ireland the iapetus suture forms a zone of north dipping reflectors which projects up between the shannon estuary and the dingle peninsula and is overlain by a late caledonian reflector with a shallow south dip across the iapetus suture zone the crust thickens southward and the lower crust becomes less reflective it is proposed that the lower palaeozoic volcano sedimentary rocks which form the leinster massif extend southwestward beneath the variscan province and at least 30 km offshore to the wire lines we argue that the weak south dipping reflectors in the upper and middle crust south of the iapetus suture represent an early caledonian fault system linked to a mid crustal detachment which formed during the south dipping subduction event recorded onshore by the ordovician volcano sedimentary successions these faults were selectively reactivated causing the predominant ene trend of younger regional tectonic features in and around southern ireland the three onshore upper palaeozoic basins close east of the wire lines we point out that the variscan front is only a map line across the british isles with no consistent tectonic character thus the south dipping reflector below the north margin of the north celtic sea basin on swat 4 generally referred to as the variscan front is correlated by us with the northern boundary of the rosslare complex thus making it the northern tectonic boundary of the caledonian basement block known as the rosslare monian terrane furthermore the north dipping south irish sea lineament on winch 4 can be correlated with the menai straits lineament and could thus be the southern boundary of this terrane the south dipping reflector on swat 2 also previously called the variscan front is correlated with the johnston benton fault system in southwest wales

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

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

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

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