initial chronology of a recently discovered hydrothermal field at 14 45 n mid atlantic ridge two expeditions of the sevmorgeologija association 1991 1994 led to the discovery of two new hydrothermal sites on the mid atlantic ridge mar south of the 15 20 north fracture zone one around 14 45 n and the other around 14 42 n the northern one between 14 45 and 14 45 3 n has been studied in detail about 12 mounds have been mapped and 3 of them have been sampled using a large hydraulic grab sampler the largest one is about 200 m long and 200 m wide when progressively moving up on the slope of an uplifted block of the rift valley floor the sulphide samples have revealed ages ranging from about 10 ka to 60 ka the ages were obtained using the 230th 234u dating method used for chronological studies of diverse hydrothermal fields the general picture of this lateral location of the samples of different ages provides evidence of a shift in the focus of hydrothermal activity with time moreover there were rejuvenation stages of hydrothermal activity including black and white smokers

middle pleistocene climate and hydrological environment at the boxgrove hominin site west sussex uk from ostracod records the sediments of the slindon formation at the junction of the chalk south downs and the west sussex coastal plain as revealed and studied at boxgrove contain evidence for early middle pleistocene environments in southern england around half a million years ago the archaeological importance of the deposits is attested to by the recovery of stone and organic tools butchered fauna and fossil hominin remains we combine palaeoecological and geochemical analyses of ostracods from the coastal plain deposits to reconstruct the climate and environment at one period of the hominin occupation the stratigraphy of our study area is indicative of a terrestrial environment surrounding a small lake or pond ostracod assemblages from the sediments indicate that the ponds were shallow but permanent and fed by groundwater and springwater however the species alone cannot confirm that these ponds were completely fresh application of mutual climatic range mcr techniques shows that summer temperatures were similar to present day values and winter temperatures were probably colder whereas mean annual air temperature was similar to present or perhaps slightly cooler trace element and strontium isotope analyses of ostracods from the sediments support the conclusion that the ponds were fed largely by fresh groundwater derived from the chalk although there may have been a minor seawater input the carbon isotope ratios of the ostracod shells are consistent with a groundwater source although they also indicate that the dissolved inorganic carbon most likely did not reach equilibrium with atmospheric carbon dioxide suggesting that the ponds had short residence time the combination of palaeotemperature estimates with oxygen isotope values from ostracod shells allowed us to reconstruct the oxygen isotope ratio of the pond water which was close to the value of early middle pleistocene precipitation at this site values were similar to present day suggesting that there may have been a change in seasonality or precipitation source since the mean air temperatures might have been lower

the role of magma injection in localizing black smoker activity the mechanisms for localization of black smoker systems at mid ocean ridges remain to be fully understood seismic data for a segment of the juan de fuca ridge with long lived black smoker vents reveal ongoing magma recharge into the crustal magma chamber thereby providing an explanation for the localization

growth history of a diffusely venting sulfide structure from the juan de fuca ridge a petrological and geochemical study centimeter scale mineralogical and chemical analyses of a diffusely venting 280 c sulfide structure called roane from the mothra hydrothermal field on the juan de fuca ridge reveal a complex growth history these analyses document four well defined zones which from the exterior to the interior of the structure include a barite silica zone fossil worm tube zone a silica sulfide zone silica zone a sulfide silica zone outer sulfide zone and a sulfide gordaite silica zone inner sulfide zone these features are a product of a myriad of processes that include extensive mineral replacement ingress of seawater fracturing and breakout of hot hydrothermal fluids significant conductive cooling and the sustained broad scale outflow of warm fluids from the walls through a porous and permeable matrix roane lacks an open throughgoing chalcopyrite lined central conduit instead it hosts an anastomosing discontinuous network of tortuous channels within the interior of a sponge like matrix of amorphous silica sulfide sulfate and clay minerals megafaunal communities at the summit and sides of this mature diffusely venting chimney provide a constructional framework for new growth isolation of hydrothermal fluid from seawater during progressive fossilization of megafauna and the early formation of barite silica assemblages promote flow of higher temperature fluids within the interior continued isolation of interior higher temperature fluids through increased mineral precipitation and expansion of the structure leads to a zn sulfide pyrite dominated permeable matrix and a network of tortuous channels that form a central porous conduit this conduit is shielded by a weakly silicified metal sulfide inner wall and a strongly silicified outer wall the most outer portions of the walls are highly porous and sites of significant advection and mixing between hydrothermal fluids and seawater the mineralogy and chemistry of roane are strongly affected by ammonia ammonium buffering of the ph during cooling of the hydrothermal fluids the two major results of the buffering are 1 precipitation of zn sulfide at higher temperatures than are typical for bare rock systems leading to the correlation of cu zn cd and se and 2 increased deposition of amorphous silica and clay minerals

detailed geomorphology and neotectonics of the endeavour segment juan de fuca ridge new results from seabeam swath mapping a 36 km wide corridor of seabeam bathymetry was collected on the juan de fuca ridge north of 48 00 n in may 1983 these data merged with previous seabeam bathymetric compilations for the cobb offset as well as with sea marc i and sea marc ii side scan sonar seismic reflection magnetometer and deep tow photography data have been used to identify morphological variations along the spreading center of the endeavour segment of the juan de fuca ridge between the cobb offset and sovanco fracture zone the endeavour segment consists of two ridge sections separated by the 13 km wide endeavour offset which formed when spreading jumped from middle valley to west valley 200 000 yr ago the locus of spreading occurs along the entire endeavour segment as a narrow 1 2 km wide shallow 10 30 m deep inner rift which is superimposed upon larger structures that range from broad 5 10 km wide deep as much as 3 000 m median valleys to narrow 5 km wide shallow 2 100 m volcanic ridges an abrupt discontinuity in the tectonic fabric occurs at about 48 05 n south of this latitude spreading occurs within an axial high endeavour ridge which gradually deepens to the south south endeavour valley as it approaches the zone of overlap with the propagating rift at the cobb offset north of 48 05 n the axis is characterized by broad deep median valleys north endeavour valley and west valley these large scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism waning magma supply at the distal ends of ridge axes thermal contrasts across ridge offsets propagating rifts and an unstable triple junction the pronounced axial deep of west valley is interpreted as reflecting the youth of the structure 200 000 yr and may be primarily due to collapse with little extension and magmatism the rift axis jump to west valley fortuitously isolated endeavour seamount the youngest edifice in the heck seamount chain from the pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed endeavour offset development of a remarkably simple overlapping conjugate rift pair at endeavour offset has resulted in spite of the complex and highly variable crustal thickness magma supply and pre existing structural grain rifting in south west valley has caused pre existing topography generated at the endeavour ridge to be destroyed by subsidence and burial thereby creating the apparent discontinuity in morphology north of 48 05 n

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

constraints on the energy and chemical balances of the modern tag and ancient cyprus seafloor sulfide deposits the size chemical composition energy flux and fluid composition of the tag hydrothermal sulfide deposit at the mid atlantic ridge and the size chemical composition and reaction zone characteristics of the skouriotissa volcanogenic massive sulfide deposit of the troodos ophiolite in cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit we conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years with episodes of activity as short as a few tens of years separated by thousands of years of inactivity about 2 1019 j of energy supplied at high temperature is required to form a deposit the size of the tag mound metals up to 4 times the mass of any element present in the sulfide deposit are leached out of relatively small reaction zones most likely at the base of the sheeted dikes chemical balance can be struck for all elements except sulfur with a reaction zone 1 2 km3 in volume from which a small proportion of iron and large proportions of copper zinc and manganese are removed a sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate as suggested by stable isotope analyses we argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section furthermore we speculate that activity of the tag hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in hawaii and iceland

estimations of heat transfer from grotto s north tower a neptune observatory case study abstract the overall heat transfer through an active hydrothermal sulfide edifice in particular the north tower of grotto has been estimated at 80 mw or greater based on the following in situ measurements and assumptions 1 the heat transfer by diffuse flow is estimated at 33 380 mw based on extrapolating the acoustically mapped area to all sides of the north tower visible area 30 m 2 extrapolated area 100 m 2 and using the range of available spot measurements of temperature 6 23 c and vertical velocity 0 07 0 28 m s the lower number 33 mw is more likely but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain 2 the heat transfer by focused flow is estimated at 30 70 mw based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges 3 conductive heat transfer out of the mound is unknown but is likely to be much less than the advective heat transfer additionally the plume transport of heat is estimated at 20 40 mw based on the direct measurement of temperature within the plume at 5 25 m above the top of the edifice despite uncertainties the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow furthermore not all plumes from individual smokers may merge even for so small an area as the north tower of grotto

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

discovery of hydrothermal vent fields on alarcon rise and in southern pescadero basin gulf of california author s paduan jb zierenberg ra clague da spelz rm caress dw troni g thomas h glessner j lilley md lorenson t lupton j neumann f santa rosa del rio ma wheat cg abstract 2018 the authors hydrothermal vent fields located in the gap between known sites in guaymas basin and 21 n on the east pacific rise were discovered on the alarcon rise and in southern pescadero basin the alarcon rise spreading segment was mapped at 1 m resolution by an autonomous underwater vehicle individual chimneys were identified using the bathymetric data vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives the ja sit pericu and meyibo active fields are near the eruptive fissure of an extensive young lava flow vent fluids up to 360 c from meyibo have compositions similar to northern east pacific rise vents the tzab ek field is 850 m west of the volcanic axis and active chimneys rise up to 33 m above a broad sulfide mound the inactive field is 10 km north northeast along the rift axis and most sulfide chimneys are enriched in zn and associated elements that are transported at lower temperature compared to the more cu rich active fields in southern pescadero basin the auka field is on the margin of a sediment filled graben at 3 670 m depth discharging fluids are clear contain hydrocarbons and have neutral ph elevated salinity and temperatures up to 291 c they have deposited massive mounds of calcite with minor sulfide the fluids are compositionally similar to those in guaymas basin produced by high temperature basalt seawater interaction followed by reaction with sediment the paucity of sulfide minerals suggests subsurface deposition of metals

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

near vent chemical processes in a hydrothermal plume insights from an integrated study of the endeavour segment support from onc for sample collection is gratefully acknowledge as is the analytical expertise of jody spence bruno keiffer and maureen soon who helped generate the data set reported here funding for sample analysis came from an nserc discovery grant to l a c ocean networks canada is supported through the canadian foundation for innovation all data are provided in the supporting information or through onc at http dmas uvic ca datasearch

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

fault geometry and permeability contrast control vent temperatures at the logatchev 1 hydrothermal field mid atlantic ridge high temperature 300 c off axis hydrothermal systems found along the slow spreading mid atlantic ridge are apparently consistently located at outcropping fault zones while preferential flow of hot fluids along highly permeable fractured rocks seems intuitive such efficient flow inevitably leads to the entrainment of cold ambient seawater the temperature drop this should cause is difficult to reconcile with the observed high temperature black smoker activity and formation of associated massive sulfide ore deposits here we combine newly acquired seismological data from the high temperature off axis logatchev 1 hydrothermal field lhf1 with numerical modeling of hydrothermal flow to solve this apparent contradiction the data show intense off axis seismicity with focal mechanisms suggesting a fault zone dipping from lhf1 toward the ridge axis our simulations predict high temperature venting at lhf1 only for a limited range of fault widths and permeability contrasts expressed as the fault s relative transmissibility the product of the two parameters the relative transmissibility must be sufficient to capture a rising hydrothermal plume and redirect it toward lhf1 but low enough to prevent extensive mixing with ambient cold fluids furthermore the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow spreading ridges occurs at low temperatures

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

radiochronological investigation of hydrothermal deposits from the meso zone central indian ridge abstract hydrothermal activity in the indian ocean has hitherto only been poorly documented first indications of hydrothermal mineralization were observed in 1987 during the gemino cruise over the 4th ridge segment of the central indian ridge 270 km north of the rodriguez triple junction subsequent cruises in 1993 hydrotrunc and in 1995 hydrock were carried out in order to obtain detailed information about the geological setting and the extent and mineral distribution of the field but primarily to sample the first massive sulfides of the indian ocean floor this hydrothermal field which was called the meso zone was studied by detailed mapping and by ocean bottom photography hydrothermal precipitates were recovered with a tv grab sampler at four sampling locations two in the northern part of the mineralized field called the talus tips site and two in the central part called the sonne field the sulfide samples were age dated using the 230 th 234 u method they show different periods of activity at each site 52 5 22 7 3 and 16 2 ka for the main events at the talus tips site and 18 2 and 12 5 1 5 ka for the sonne field we conclude from these data that the two sites were not active at the same time if it is assumed that there is a complete cessation of activity between 52 and 23 ka for more recent periods from 23 to 10 ka it may be postulated that the deep seated hydrothermal circulation is continuous but that the discharge of hot fluid occurs alternately at the sonne field or at the talus tips site comparing our results with those obtained previously at the different hydrothermal sites we favor the conclusion of episodic activity moreover in the sonne field jasper formation resulting from a low temperature hydrothermal event dated at around 11 ka is presently the youngest hydrothermal formation in the entire meso zone

geochemical time series of hydrothermal fluids from the slow spreading mid atlantic ridge implications of medium term stability abstract time series investigations of hydrothermal fluids from different geological settings at the slow spreading mid atlantic ridge mar indicate a surprising medium term stability of their chemical composition over up to 20 years this does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited such as the logatchev 1 hydrothermal field at 14 45 n we also found the 407 c hot and extremely metal rich fluids at the 5 s site turtle pits 2986 m water depth to discharge at constant composition indicating supercritical phase separation for at least 4 years which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast spreading east pacific rise epr possibly fluid pathways and pressure temperature conditions are more stable in tectonically controlled settings at mar than at the epr where frequent volcanic activity affects permeability within the hydrothermal systems despite some spatial variability hydrothermal fluids discharging at the shallower ca 810 870 m menez gwen hydrothermal field at 37 50 n seem to represent a stable discharge of phase separated vapor type fluids over 16 years this demonstrates that medium term stability of hydrothermal fluid composition seems to be a general feature at slow spreading ridges as a consequence hydrothermal vent organisms at the mar may live in a more stable chemical environment compared to vent systems at fast spreading ridges considering the fact that slow spreading ridges account for more than half of the submarine ridge system such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium term stable source of fe and other micronutrients into the ocean

an estimate of hydrothermal fluid residence times and vent chimney growth rates based on210pb pb ratios and mineralogic studies of sulfides dredged from the juan de fuca ridge abstract the210pbpb ratios across two sulfide samples dredged from the juan de fuca ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration 210pb is added to the hydrothermal fluid by two processes 1 high temperature alteration of basalt and 2 if the residence time of the fluid is on the order of the 22 3 year half life of210pb by in situ growth from222rn krishnaswami and turekian 1982 stable lead is derived only from the alteration of basalt the210pb pb ratio across one sample was 0 5 dpm 10 6 g pb and across the other it was 0 4 dpm 10 6 g pb these values are quite close to the238up b ratios of basalts from the area suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of210pb i e the time required for ingrowth from the radon an apparent growth rate of 1 2 cm yr is derived from the slope of the210pb pb curve for one of the samples this is consistent with its mineralogy and texture which suggest an accretionary pattern of development there is no obvious sequential growth pattern and virtually no gradient in210pb pb across the second sample this is consistent with alteration of the original210pb pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample

gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss hydrothermal fluid samples collected in 1984 1987 and 1988 from a large vent field near 47 57 n on the endeavour segment of the juan de fuca ridge jfr have been analyzed for major and minor elements and gases there are of the order of 100 individual smoker vents on 10 large sulfide structures which are localized along faults and fault intersections across the vent field each sulfide structure has a characteristic fluid composition which varies very little from one vent orifice to the next or from year to year on a given structure however there are large gradients in fluid composition across the vent field with endmember chlorinity increasing from 255 mmol kg in the sw to 505 mmol kg in the ne end member concentrations of major elements are well correlated with chlorinity and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids the gradients in composition across the vent field and measured vent fluid temperatures 400 c are consistent with supercritical phase separation and loss of brine phase below the seafloor the factor of 2 variation in co2 and h2s is larger than expected for loss of a very high chlorinity brine concentrations of iron and manganese are not positively correlated with chlorinity suggesting that temperature and ph are more important in controlling metal solubility elevated ammonia and bromide chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter and high boron concentrations point to a sedimentary source

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

a model for growth of steep sided vent structures on the endeavour segment of the juan de fuca ridge results of a petrologic and geochemical study petrologic and geochemical studies of vent solids from the main endeavour field mef and the high rise field hrf juan de fuca ridge demonstrate that the steep sided vent structures characteristic of these sites form dominantly by flange growth combined with diffuse flow through sealed portions of structures and incorporation of flanges into structures geochemical calculations suggest that the prevalence of amorphous silica and flanges in endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia at endeavour as the temperature of vent fluid decreases ammonia ammonium equilibrium buffers ph and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher ph than conductively cooled ammonia poor fluids present at most other unsedimented mid ocean ridge vent sites deposition of silica stabilizes flanges and allows structures to attain large size it also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones thus decreasing entrainment of seawater most inactive vent samples recovered from areas peripheral to the hrf and mef are similar to barite silica rich samples from the explorer ridge and axial seamount and likely formed from precipitation of silica and barite on a biological substrate active white smoker chimneys from the clam bed field located south of the hrf are pyrrhotite rich and likely formed from vent fluids that are depleted in zn and cd and enriched in pb and ba relative to fluids exiting trans atlantic geotraverse tag and cleft segment white smoker chimneys

linkages between mineralogy fluid chemistry and microbial communities within hydrothermal chimneys from the endeavour segment juan de fuca ridge rock and fluid samples were collected from three hydrothermal chimneys at the endeavour segment juan de fuca ridge to evaluate linkages among mineralogy fluid chemistry and microbial community composition within the chimneys mossbauer midinfrared thermal emission and visible near infrared spectroscopies were utilized for the first time to characterize vent mineralogy in addition to thin section petrography x ray diffraction and elemental analyses a 282 c venting chimney from the bastille edifice was composed primarily of sulfide minerals such as chalcopyrite marcasite and sphalerite in contrast samples from a 300 c venting chimney from the dante edifice and a 321 c venting chimney from the hot harold edifice contained a high abundance of the sulfate mineral anhydrite geochemical modeling of mixed vent fluids suggested the oxic anoxic transition zone was above 100 c at all three vents and that the thermodynamic energy available for autotrophic microbial redox reactions favored aerobic sulfide and methane oxidation as predicted microbes within the dante and hot harold chimneys were most closely related to mesophilic and thermophilic aerobes of the betaproteobacteria and gammaproteobacteria and sulfide oxidizing autotrophic epsilonproteobacteria however most of the microbes within the bastille chimney were most closely related to mesophilic and thermophilic anaerobes of the deltaproteobacteria especially sulfate reducers and anaerobic hyperthermophilic archaea the predominance of anaerobes in the bastille chimney indicated that other environmental factors promote anoxic conditions possibilities include the maturity or fluid flow characteristics of the chimney abiotic fe2 and s2 oxidation in the vent fluids or o2 depletion by aerobic respiration on the chimney outer wall

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

trace metal distributions in sulfide scales of the seawater dominated reykjanes geothermal system constraints on sub seafloor hydrothermal mineralizing processes and metal fluxes abstract mineral precipitation in the seawater dominated reykjanes geothermal system on the mid atlantic ridge iceland is caused by abrupt artificially induced pressure and temperature changes as deep high temperature liquids are drawn from reservoir rocks up through the geothermal wells sulfide scales within these wells represent a complete profile of mineral precipitation through a seafloor hydrothermal system from the deep reservoir to the low temperature silica rich surface discharge mineral scales have formed under a range of conditions from high pressures and temperatures at depth 2 km to boiling conditions in the upflow zone and at the surface consistent trace element enrichments similar to those in black smoker chimneys are documented cu zn cd co te v ni mo w sn fe and s are enriched at higher pressures and temperatures in the deepest scales zn and cu bi pb ag as sb ga hg tl u and th are enriched at lower temperatures and pressures nearer to the surface a number of elements e g co se cd zn cu and au are deposited in both high and low pressure scales but are hosted by distinctly different minerals other trace elements such as pb ag and ga are strongly partitioned into low temperature minerals such as galena pb ag and clays ga boiling and destabilization of metal bearing aqueous complexes are the dominant control on the deposition of most metals particularly au other metals e g cu and se may also have been transported in the vapor phase very large enrichments of au ag and pb in the scales e g 948 ppm au 23 200 ppm ag and 18 8 wt pb versus average concentrations in black smoker chimneys likely reflect that some elements are preferentially deposited in boiling systems a mass accumulation of 5 7 t yr of massive sulfide was calculated for one high temperature production well equating to metal fluxes of 1 7 t yr zn 0 3 t yr cu 23 kg yr pb 4 1 kg yr ag and 0 5 kg yr au at least three quarters of the major and trace element load is precipitated within the well before reaching the surface we suggest that a similar proportion of metals may be deposited below the seafloor in submarine hydrothermal systems where significant boiling has occurred mass accumulation estimations over the lifetime of the reykjanes system may indicate significant enrichment of zn pb au and ag relative to both modern and ancient mafic dominated seafloor massive sulfide deposits and highlights the potential for metal enrichment and accumulation in the deep parts of geothermal systems

the influence of hydrothermal fluid composition and advection rates on black smoker chimney mineralogy insights from modeling transport and reaction abstract a detailed study of black smoker chimneys was carried out by drawing together mineralogical observations and measured fluid compositions the modeling technique which establishes a link between vent fluid chemistry and vent deposit mineralogy is used to examine the influence of the physical environment on transport across short length scales centimeters which are subject to steep thermal 350 c and chemical gradients and to predict saturation states of minerals as functions of position within chimney walls owing to short length scales steep gradients and constancy of bounding fluid compositions concentration profiles are controlled by transport the effect of reaction rates on profiles is assumed to be negligible precipitation at points within the chimney wall is thus kinetically controlled local equilibrium does not apply and precipitation must be suppressed when calculating distributions of species and saturation states of minerals model calculations are performed in a series of four steps the first is speciation of measured fluid compositions at in situ temperature and pressure conditions results of calculations are plotted on stability diagrams of log fo2 log f s2 and log fo2 log a fe 2 a cu 2 to examine how closely the predicted saturation states fit with observations of minerals coexisting with the hydrothermal fluid at the vent sites the second step is calculation of heat and mass transport across the chimney wall the third is speciation of the pore fluid compositions calculated to exist in the wall subsequent to transport and the fourth is comparison of calculated mineral saturation states to observations of naturally occurring chimney samples application of the modeling technique to five vent sites which exhibit distinct fluid compositions allows reproduction of observed mineralogy model results demonstrate the sensitivity of geochemical processes to the physical environment porosity tortuosity permeability mineral composition to rates of advection of seawater inward or hydrothermal fluid outward across chimney walls and to endmember fluid composition

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

structural control evolution and accumulation rates of massive sulfides in the tag hydrothermal field the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge is one of the best studied hydrothermal systems to date however high resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle auv reveal new information about the distribution of active and inactive hydrothermal deposits and their relation to structural features the discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow spreading ridges the interpretation of ship based and high resolution auv based data sets allowed for the determination of the main tectonic stress regimes that have a first order control on the location and distribution of past and present hydrothermal activity the data reveal the importance of cross cutting lineament populations and temporal variations in the prevalent stress regime a dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material 29 mt the accumulation rate of 1 500 t yr is comparable to those of other modern seafloor vent fields however our observations suggest that the tag segment is different from many other slow spreading ridge segments in its tectonic complexity which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation the inactive and weakly active mounds contain almost 10 times the amount of material as the active high temperature mound providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

growth of large sulfide structures on the endeavour segment of the juan de fuca ridge abstract mapping and sampling with dsrv alvin has established that sulfide blocks 0 5 m across dredged from the axial valley of the endeavour segment at 47 57 n are samples of unusually large sulfide structures the steep sided structures up to 30 m in length 20 m in height and 10 15 m across are localized by venting along normal faults at the base of the western axial valley wall and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest high temperature fluids 350 to more than 400 c pass through the massive sulfide structures and enter seawater through small concentric nozzle like features projecting from the top or the sides of the larger vent structures diffuse low temperature flow is pervasive in the vicinity of the active sulfide structures exiting from basalt and sulfide surfaces alike evidence of recent volcanic activity is sparse the two largest samples taken with the dredge would not have been recoverable using the submersible these samples represent massive complex portions of the sulfide structures which were not closely associated with rapid high temperature fluid flow at the time of sampling they contain textural evidence of sealed hydrothermal fluid exit channels mineralogy is dominated by fe sulfides nnd amorphous silica pyrite marcasite wurtzite chalcopyrite and iss are the most common sulfide phases pyrrhotite galena and sphalerite are present in trace amounts barite amorphous silica and chalcedony are the only non sulfide phases anhydrite is not observed in any of the dredge samples although it is common in the chimney like samples recovered by alvin specific mineralogical textural zones within the dredge samples are anaoogous to individual layers in east pacific rise at 21 n and southern juan de fuca ridge samples with two exceptions a coarse grained highly porous fe sulfide rich interior containing sulfidized tubeworm casts and a 2 5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica the porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber the amorphous silica deposited from a seawater hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate these silica rich zones are the densest most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features observations in these samples are consistent with two distinct phases of development phase 1 is analogous to chimney growth and construction at 21 n and ends when flow channels become sealed to rapid flow of through going fluid the flow is evidently redirected within the structure phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater hydrothermal fluid mixtures evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures

hydrothermal circulation within the endeavour segment juan de fuca ridge areas of the seafloor at mid ocean ridges where hydrothermal vents discharge are easily recognized by the dramatic biological physical and chemical processes that characterize such sites locations where seawater flows into the seafloor to recharge hydrothermal cells within the crustal reservoir are by contrast almost invisible but can be indirectly identified by a systematic grid of conductive heat flow measurements an array of conductive heat flow stations in the endeavour axial valley of the juan de fuca ridge has identified recharge zones that appear to represent a nested system of fluid circulation paths at the scale of an axial rift valley conductive heat flow data indicate a general cross valley fluid flow where seawater enters the shallow subsurface crustal reservoir at the eastern wall of the endeavour axial valley and undergoes a kilometer of horizontal transit beneath the valley floor finally exiting as warm hydrothermal fluid discharge on the western valley bounding wall recharge zones also have been identified as located within an annular ring of very cold seafloor around the large main endeavour hydrothermal field with seawater inflow occurring within faults that surround the fluid discharge sites these conductive heat flow data are consistent with previous models where high temperature fluid circulation cells beneath large hydrothermal vent fields may be composed of narrow vertical cylinders subsurface fluid circulation on the endeavour segment occurs at various crustal depths in three distinct modes 1 general east to west flow across the entire valley floor 2 in narrow cylinders that penetrate deeply to high temperature heat sources and 3 supplying low temperature diffuse vents where seawater is entrained into the shallow uppermost crust by the adjacent high temperature cylindrical systems the systematic array of conductive heat flow measurements over the axial valley floor averaged 150 mw m2 suggesting that only about 3 of the total energy flux of ocean crustal formation is removed by conductive heat transfer with the remainder being dissipated to overlying seawater by fluid advection

a seismic swarm and regional hydrothermal and hydrologic perturbations the northern endeavour segment february 2005 the february 2005 swarm at the overlapping spreading center osc on the northern end of the endeavour segment is the first swarm on the juan de fuca ridge recorded on a local seafloor seismic network the swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the endeavour vent fields as well as regional scale hydrologic pressure perturbations the spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex small magnitude events dominate and seismicity rates wax and wane indicating a magmatic process the main swarm initiates at the northern end of the endeavour ridge however most of the moment release including six strike slip events occurs in the southwest endeavour valley where the swarm epicenters generally migrate south the main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes 25 105 km away we infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern endeavour axis resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the endeavour osc in the southwest endeavour valley consistent with the eventual decapitation of the endeavour by the west valley segment furthermore 2 5 days after the start of the main swarm seismicity is triggered beneath the endeavour vent fields and temperature increases at a diffuse vent in the mothra field we infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion

new age data for mid atlantic ridge hydrothermal sites tag and snakepit chronology revisited the chronologies of tag and snakepit hydrothermal fields have been established using 210pb pb 230th 234u and 14c dating at the tag field a mn oxide record indicative of low temperature events began at least 125 000 years and possibly 140 000 years ago with maximum intensities at 15 000 7000 and 4000 years before present high temperature events giving rise to sulfide deposits began about 100 000 years ago and have been intermittent to the present day a presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20 000 years decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000 7000 and 9000 years ago after a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago the snakepit field is much younger and no sulfides older than 4000 years have been recovered relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at tag reactivation of snakepit took place about 80 years ago and is presently concurrent with that of tag comparison with hydrothermal sites on the east pacific rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits

evolution of hydrothermal activity on the juan de fuca ridge observations mineral ages and ra isotope ratios we discuss observations of hydrothermal activity from sites along the juan de fuca ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 alvin dives in september 1988 this was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting the ages are based on the ingrowth of 228th from 228ra in both active and inactive chimneys and chimney talus composed of barite anhydrite chalcopyrite wurtzite and pyrite samples were obtained from from north to south the fairycastle site northern endeavour ridge the endeavour hydrothermal site and the southern juan de fuca sites named vent 3 vent 1 and plume areas samples from active chimneys 20 yielded 228th 228ra ages in the range 0 2 to 1 2 years we recorded no ages in the range 1 2 to 12 years for inactive chimneys or rubble our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structures are 2 years old in the fairycastle site there is at least one episode of hydrothermal activity recorded in the rubble which occurred at least 20 years earlier large sulfide edifices in the endeavour hydrothermal site probably record a minimum of 20 years of discharge for each particular structure chimney structures in the southern juan de fuca ridge may be older they show no evidence of significant recent growth except for an outer barite rich layer the results of this study suggest that hydrothermal chimneys of diverse mineralogy can be dated by measuring the ingrowth of 228th from 228ra decay the technique is most accurate for chimneys that are less than 10 years old the technique in conjunction with detailed petrological analyses could be used to assess the growth rates of individual chimney walls variations in the 228ra 226ra activity ratios among the three areas reflect either differences in fluid residence time depth of generation of hydrothermal fluids or interaction with basaltic rocks previously altered at low temperatures

228th 228ra ages of a barite rich chimney from the endeavour segment of the juan de fuca ridge ages in the range 1 15 yr for chimneys deposited from hydrothermal vents from the endeavour segment of the juan de fuca ridge are estimated based on the ingrowth of228th from228ra cleaning samples by digestion in an acid bath confirms that the mineral barite is the phase being dated a detailed examination of sample alv 2463 r7 collected from the clam bed site yielded a vertical growth rate of 5 8 cm yr and a radial growth rate of 2 6 cm yr this investigation revealed that this chimney did not grow in a regular layer by layer accretion therefore an average of ages obtained from several barite crystals a few centimeters apart 10 cm was used to define the age of each segment ages obtained from the sulfide rich main field site sample alv 2466 r5 suggest that the barite in this sample is from late stage precipitation in contrast to the clam bed sample where barite formed the primary matrix these samples conform to different models of barite incorporation into hydrothermal chimneys

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

geology and hydrothermal evolution of the mothra hydrothermal field endeavour segment juan de fuca ridge detailed characterization of the mothra hydrothermal field the most southern and spatially extensive field on the endeavour segment of the juan de fuca ridge provides new insights into its geologic and hydrothermal development meter scale bathymetry side scan sonar imagery and direct dive observations show that mothra is composed of six actively venting sulfide clusters spaced 40 200 m apart chimneys within each cluster have similar morphology and venting characteristics and all clusters host a combination of active and extinct sulfide structures black smoker chimneys venting fluids above 300 c are rare while more common lower temperature diffusely venting chimneys support dense colonies of macrofauna and bacterial mat hydrothermal sediment and extinct sulfide debris cover 10 15 m of the seafloor surrounding each vent cluster obscuring the underlying basaltic substrate of light to moderately sedimented pillow lobate sheet and chaotic flows basalt talus and collapse terrain extinct sulfide chimneys and debris between the clusters indicate that hydrothermal flow was once more widespread and that it has shifted spatially over time the most prominent structural features in the axial valley at mothra are regional 020 trending faults and fissures and north south trending collapse basins the location of actively venting clusters within the field is controlled by 1 localization of fluid upflow along the western boundary fault zone and diversion of these fluids by antithetic faults to feed vent clusters near the western valley wall and 2 tapping of residual magmatic heat in the central part of the axial valley which drives flow beneath vent clusters directly adjacent to the collapse basins 70 90 m east of the western valley wall these processes form the basis for a model of axial valley and hydrothermal system development at mothra in which the field is initiated by an eruptive diking episode and sustained through intense microseismicity and non eruptive diking events

geology of the alarcon rise southern gulf of california author s clague da caress dw dreyer bm lundsten l paduan jb portner ra spelz madero r bowles ja castillo pr guardado france r le saout m martin jf santa rosa del rio ma zierenberg ra abstract 2018 the authors meter scale auv bathymetric mapping and rov sampling of the entire 47 km long alarcon rise between the pescadero and tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest hottest and highest effusion rate lava flows this shallowest inflated part is located 1 3 of the way between the tamayo and pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the east pacific rise or on the gorda and juan de fuca ridges starting 5 km south of the pescadero transform 6 5 km of the alarcon rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77 3 wt sio2 these are the first known rhyolites from the submarine global mid ocean ridge system silicic lavas range from g11 7 ka to as young as 1 1 ka a basalt to basaltic andesite sequence and an andesite to dacite to rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite magmatism occurred along the bounding tamayo and pescadero transforms as extensive channelized flows the flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills the transforms are transtensional to accommodate magma migration from the adjacent alarcon rise

marine mineral exploration with controlled source electromagnetics at the tag hydrothermal field 26 n mid atlantic ridge seafloor massive sulphide sms deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand the trans atlantic geotraverse tag hydrothermal field at 26 n on the mid atlantic ridge hosts several such deposits this study presents new controlled source electromagnetic csem bathymetric and magnetic results from the tag field potential sms targets were selected based on their surface expressions in high resolution bathymetric data high resolution reduced to the pole magnetic data show negative anomalies beneath and surrounding the sms deposits revealing large areas of hydrothermal alteration csem data sensitive to the electrical conductivity of sms mineralization further reveal a maximum thickness of up to 80 m and conductivities of up to 5 s m sms samples have conductivities of up to a few thousand s m suggesting that remotely inferred conductivities represent an average of metal sulphide ores combined with silicified and altered host basalt that likely dominates at greater depths

age dating of sulfide deposits from axial and off axial structures on the east pacific rise near 12 50 n abstract during several cruises clipperton 1981 cyatherm 1982 and geocyarise leg 3 1984 conducted between 12 40 n and 12 54 n on the east pacific rise massive sulfide deposits have been dredged and recovered by the diving saucer cyana these well localized and documented samples have been analyzed for their 210 pb pb ratio and uranium series disequilibrium in view of dating the hydrothermal phenomenon using samples which are presently forming the initial 210 pb pb ratio has been established as 0 60 0 06 ten sulfide samples from the central graben near 12 50 n have ages less than about 10 years and are younger than most of the deposits found at 21 n on the east pacific rise one sample from the eastern rim of the graben presents an age of about 150 years two off axis structures with hydrothermal sulfide deposits were studied the first is a semi circular volcanic structure where low temperature hydrothermal vents were observed at 500 m and 1000 m from the rise axis sulfide samples collected from these sites yielded ages of about 2000 years further east about 6 km from the rise axis the second off axis structure called south eastern se seamount presents thick inactive sulfide deposits as well as mn rich and nontronite deposits an age around 20 000 years has been found for these deposits these ages are discussed and taking into account the asymmetrical structure of the ridge near 12 43 n it is concluded that the formation of the se seamount occurred 20 000 years ago on a basement of about 100 000 years old accompanied by an extensive hydrothermal event this indicates that an important eastward shift 4 5 km from the graben of the volcano hydrothermal activity took place in this area 12 43 n

sulfide geochronology along the endeavour segment of the juan de fuca ridge 1 forty nine hydrothermal sulfide sulfate rock samples from the endeavour segment of the juan de fuca ridge northeastern pacific ocean were dated by measuring the decay of 226ra half life of 1600 years in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years this dating method is effective for samples ranging in age from 200 to 20 000 years old and effectively bridges an age gap between shorter and longer lived u series dating techniques for hydrothermal deposits results show that hydrothermal venting at the active high rise sasquatch and main endeavour fields began at least 850 1450 and 2300 years ago respectively barite ages of other inactive deposits on the axial valley floor are between 1200 and 2200 years old indicating past widespread hydrothermal venting outside of the currently active vent fields samples from the half graben on the eastern slope of the axial valley range in age from 1700 to 2925 years and a single sample from outside the axial valley near the westernmost valley fault scarp is 5850 205 years old the spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift these relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years

a semipermeable interface model for the genesis of subseafloor replacement type volcanogenic massive sulfide vms deposits subseafloor replacement style volcanogenic massive sulfide vms deposits are a subset of vms deposits where sulfides have replaced unconsolidated volcanic volcano sedimentary and sedimentary material these deposits are anomalously large and are important global sources of metals they have distinct textures at the sulfide ore interface including bed by bed replacement of sedimentary layers and typically fill void space between unconsolidated volcaniclastic detritus or fractures in flows or intrusions at the microscale metalbearing sulfides have partially to fully replaced framboidal bacteriogenic sulfides or the framboidal sulfides have acted as nuclei upon which additional metalliferous massive sulfide is deposited the textures presented are reconciled within a semipermeable interface model for replacement in this model unconsolidated sediment volcaniclastic rocks or fractured coherent volcanic rocks provide a permeable to semipermeable interface that allowed ingress of cold seawater into the pore spaces of the stratigraphic sequence prior to and during lulls in hydrothermal activity seawater sulfate in the pore water is partially reduced by bacteria to provide reduced sulfur h2s as well as framboidal pyrite in the host sequence s the reduced sulfur and framboidal pyrite as well as the cool pore water provided a thermal redox and chemical gradient in which upwelling hydrothermal fluids interact in such an environment rising hydrothermal fluids mix with cold water not only at the seawater interface leading to exhalative sulfide deposition but also in the subseafloor leading to sulfide precipitation via replacement the upwelling hydrothermal fluids can also interact with bacterial h2s in the pore spaces of the unconsolidated material resulting in additional subseafloor precipitation of metal sulfides the fluids also result in replacement of framboidal pyrite nuclei pseudomorphous after the original framboidal masses this semipermeable interface also favors enhanced zone refining assuming the hydrothermal system is sufficiently long lived leading to upgrading of the tenor of the sulfides with well developed metal zoning as observed in many ancient replacement type deposits furthermore the precipitation of a significant subseafloor sulfide mineralization results in greater trapping of metals from upwelling fluids and larger tonnage deposits with greater contained metal this model may also be applicable to other replacement type deposits in broadly similar geologic and hydrothermal environments e g sediment hosted and irish type zn pb deposits additional critical tests are required to validate and refute the model and potential tests are presented herein

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

hydrothermal activity along the slow spreading lucky strike ridge segment mid atlantic ridge distribution heatflux and geological controls we have reviewed available visual information from the seafloor and recently acquired microbathymetry for several traverses across the lucky strike segment to evaluate the distribution of hydrothermal activity we have identified a new on axis site with diffuse flow ewan and an active vent structure similar to 1 2 km from the axis capelinhos these sites are minor relative to the main field and our total heatflux estimate for all active sites 200 1200 mw is only slightly higher than previously published estimates we also identify fossil sites w of the main lucky strike field a circular feature similar to 200 m in diameter located on the flanks of a rifted off axis central volcano is likely a large and inactive hydrothermal edifice named grunnus we find no indicator of focused hydrothermal activity elsewhere along the segment suggesting that the enhanced melt supply and the associated melt lenses required to form central volcanoes also sustain hydrothermal circulation to form and maintain large and long lived hydrothermal fields hydrothermal discharge to the seafloor occurs along fault traces suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones

heat flux from black smokers on the endeavour and cleft segments juan de fuca ridge we have estimated the heat flux from black smoker vents on the juan de fuca ridge to evaluate their importance for heat transfer from young oceanic crust the velocity and temperature of smoker effluent were measured from the manned submersible alvin within a few centimeters of vent orifices using a turbine flowmeter with an attached temperature probe exit velocity was calculated from a simple plume model and vent orifices were measured in photographs and video records the estimated power output from smokers alone is 49 13 mw for the plume site vent 1 and vent 3 on the southern cleft segment near 45 n 364 73 mw for the main vent field on the endeavour segment near 48 n and 122 61 mw for the tubeworm field 2 km north the estimates for the cleft and tubeworm fields could be too low because of undiscovered vents these values constitute only 4 to 14 of the total advective heat flux estimated for these vent fields from measurements in the nonbuoyant plume and of diffuse flow at the seafloor indicating that most of the heat advected at these hydrothermal vent sites is carried by diffuse rather than focused flow values for individual smokers vary from 0 1 to 94 mw with an average of 6 2 mw at the endeavour field and 3 1 mw at the cleft field our estimates agree well at all scales with those of bemis et al 1993 based on measurements made during the same dives in some cases simultaneously up to 50 m high in the buoyant plume the good agreement between the two techniques implies that little diffuse flow at either high or low temperature is incorporated into the buoyant plumes generated by smokers at these sites velocity temperature measurements at vents excavated by alvin could not be modeled successfully suggesting that vent structures may grow in equilibrium with the force of the exiting water such that orifice size is determined by volume flux at the endeavour field the heat flux is focused by faults

sulfide geochronology along the northern equatorial mid atlantic ridge abstract hydrothermal processes and seafloor massive sulfide sms deposits have different characteristics at fast and slow spreading mid ocean ridges one such parameter is the age of a sms deposit which differs by 1 2 orders of magnitude between the fast spreading east pacific rise epr and the slow spreading mid atlantic ridge mar the large collection of sms samples dated from the 18 hydrothermal fields of the northern equatorial part of the mid atlantic ridge 194 samples demonstrates a relatively old average age of hydrothermal fields here 66 ka with the oldest one estimated as ca 223 ka peterburgskoye field based on geochronological data it was confirmed that hydrothermal discharge has an episodic character active and inactive periods of the sms formation alternate the distribution of events at all hydrothermal fields demonstrates that maximum activity occurred at 38 35 30 20 and 8 2 ka and increased with time based on statistical analyses dating variations can be explained as a superposition of several periods of activity with the duration of 15 10 and 5 ka relationship between the age and distance from the axial rift zone as well as between the age and aerial distribution is different for sms deposits hosted by basalts and by gabbro peridotites depending on their geological setting on the particular mar segment this difference can be explained by a variety of hydrothermal processes determined by tectonic or magmatic segment evolution and symmetrical or asymmetrical mode of accretion escartin et al 2008

submeter bathymetric mapping of volcanic and hydrothermal features on the east pacific rise crest at 9 50 n 1 recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid ocean ridge crests imagenex 881 675 khz scanning sonar data collected during low altitude 5 m surveys conducted with dsv alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the east pacific rise epr ridge2000 integrated study site 9 50 n bull s eye data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high resolution 0 5 m grid composite maps these are the first submeter bathymetric maps generated with a scanning sonar mounted on alvin the composite maps can be used to quantify the dimensions of meter scale volcanic and hydrothermal features within the epr axial summit trough ast including hydrothermal vent structures lava pillars collapse areas the trough walls and primary volcanic fissures existing autonomous benthic explorer abe bathymetry data 675 khz scanning sonar collected at this site provide the broader geologic context necessary to interpret the meter scale features resolved in the composite maps the grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high resolution bathymetry maps from data collected during multiple coordinated short duration surveys after primary dive objectives are met this method can also be used to colocate future near bottom sonar data sets within the high resolution composite maps enabling quantification of bathymetric changes associated with active volcanic hydrothermal and tectonic processes

eruptive and tectonic history of the endeavour segment juan de fuca ridge based on auv mapping data and lava flow ages high resolution bathymetric surveys from autonomous underwater vehicles abe and d allan b were merged to create a coregistered map of 71 7 km2 of the endeavour segment of the juan de fuca ridge radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle doc ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history the ages range from modern to 10 700 marine calibrated years before present yr bp during a robust magmatic phase from 10 700 yr bp to 4300 yr bp flows erupted from an axial high and many flowed 5 km down the flanks some partly buried adjacent valleys axial magma chambers amcs may have been wider than today to supply dike intrusions over a 2 km wide axial zone summit seamount formed by 4770 yr bp and was subsequently dismembered during a period of extension with little volcanism starting 4300 yr bp this tectonic phase with only rare volcanic eruptions lasted until 2300 yr bp and may have resulted in near solidification of the amcs the axial graben formed by crustal extension during this period of low magmatic activity infrequent eruptions occurred on the flanks between 2620 1760 yr bp and within the axial graben since 1750 yr bp this most recent phase of limited volcanic and intense hydrothermal activity that began 2300 yr bp defines a hydrothermal phase of ridge development that coincides with the present day 1 km wide amcs and overlying hydrothermal vent fields

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems

geothermal heat flux from hydrothermal plumes on the juan de fuca ridge thesis m s massachusetts institute of technology dept of ocean engineering and woods hole oceanographic institution 1991

geology and venting characteristics of the mothra hydrothermal field endeavour segment juan de fuca ridge the 500 m long mothra hydrothermal field is the largest venting site in areal extent on the endeavour segment of the juan de fuca ridge within this field there are at least five actively venting sulfide complexes spaced 40 to 200 m apart along a trend of 020 the clusters are composed of multiple steep sided pinnacles that rise up to 20 m above the seafloor most of the sulfide structures are awash in diffusely venting fluids 30 200 c that support rich and diverse macrofaunal and microbial communities isolated black smoker chimneys vent 302 c fluids the linear arrays of sulfide complexes delineate a zone of active faulting near the western wall of the axial valley this fault network serves as the conduit for the rising fluids

three dimensional hydrogeological reconstruction based on geological depositional model a case study from the coastal plain of arborea sardinia italy abstract this study presents a novel approach for the hydrogeological assessment of sedimentary coastal aquifers specifically the methodology is tailored for modeling groundwater flow and nitrates contamination in typical mediterranean coastal plains with high anthropogenic pressures as exemplified by the arborea plain central western sardinia italy the study started with development of an updated geological depositional model based on sequential stratigraphy geological and geophysical data processed in a geographic information system gis environment supported the definition of a 3d hydrogeological conceptual model and provided a solid basis for the interpretation of groundwater flow directions the 3d hydrogeological model allowed constraining groundwater circulation flow paths and distribution of nitrate concentrations in the aquifers the methodology appears as a valid tool applicable in other coastal areas to determine geological and hydrogeological settings the definition of a quantitative hydrogeological framework will support the effective management of local water resources