petrogenesis of axial lavas from the southern chile ridge major element constraints we present major element glass data for 163 rock samples collected from four ridge segments of the southern chile ridge between the chiloe fracture zone and the chile margin triple junction including the segment currently being subducted at the chile trench segment 1 the subridge mantle is heterogeneous at small spatial scales normal mid ocean ridge basalts n morb recovered from all four ridge segments have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents f and initial pressures po of melting of a slightly heterogeneous depleted source type 1 e morb found only on segment 4 have trace element affinities to some ocean island basalts display a large range of major element variations at constant and high mgo and are spatially associated with n morb type 2 e morb have trace element affinities with suprasubduction zone settings they are found at two segment 1 sites and along most of segment 3 in order to minimize fractionation and source heterogeneity effects and assess melting conditions e morb compositions were double backtracked to 8 wt mgo and a k ti ratio of 0 1 although the magnitudes of f and po are model dependent we find that n morb and both types of e morb were generated under similar melting conditions these observations indicate that spreading rate and mantle temperature exert primary control on the southern chile ridge thermal regime we see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench

a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

hydrothermal alteration of basalts beneath the bent hill massive sulfide deposit middle valley juan de fuca ridge burial of midocean ridges by clastic sediments particularly at continental margins profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust middle valley the sediment covered northern extension of the bare rock endeavour segment of the juan de fuca ridge is host to the base metal rich cu zn bent hill massive sulfide deposit at a water depth of 2 400 m the 9 mt bent hill deposit is a steep sided body 200 m across and 100 m thick ocean drilling program odp hole 856h penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized quartz chlorite sediment pile and into the uppermost volcanic basement the basaltic rocks beneath the bent hill deposit include narrow sills intruded into indurated sediments a volcanic flow erupted on top of sediments and pillow lavas below the lowermost sediments recovered similar styles of alteration are present in both the sills and flows and alteration is dominated by the effects of large scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions the basalts are slightly to completely altered to greenschist facies secondary minerals principally quartz chlorite and titanite with subsidiary epidote cu fe sulfides and rare actinolite there are steep mineralogical chemical and isotopic alteration gradients from the highly altered basalt sediment interfaces down to the less altered flow interiors suggesting the channeling of hydrothermal fluids along the basalt sediment boundaries alteration is reflected in intense metasomatic changes in the basalts assuming immobile tio 2 the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite quartz rocks with depletions in silica alumina and alkali alkali earth and base metals chloritized pillow margins with strongly light rare earth element enriched chondrite normalized patterns la sm n 1 5 cf fresh basalts 0 7 that mimic profiles for midocean ridge hydrothermal fluids require fluid rock exchange with large quantities of hydrothermal fluid w r 27 000 oxygen isotope compositions of chlorite quartz rocks 18 o 1 8 2 4 suggest that alteration occurred between 320 and 370 c strontium isotope compositions of the altered basalts and the chlorite quartz rocks are not homogeneous and range from 87 sr 86 sr ratio 0 7037 to 0 7046 there is a strong mode in 87 sr 86 sr ratio at 0 7038 suggesting that much of the alteration occurred by isotopic exchange with a hydrothermal fluid of that composition this ratio is significantly lower than that measured for 265 c fluids venting from the nearby odp mound 87 sr 86 sr 0 7044 the occurrence of epidote and isocubanite in the chloritized glassy pillow margins suggests that these rocks may retain a record of the high temperature 350 c hydrothermal fluid responsible for the formation of the overlying bent hill massive sulfide deposit the strontium isotope composition of the chloritized glassy pillow margins and hence the mineralizing fluid is slightly more radiogenic 87 sr 86 sr 0 7046 this composition could result from the addition of 15 percent of a pelagic or sedimentary component to the 87 sr 86 sr ratio 0 7038 fluid responsible for most of the sr isotope exchange with the upper basement the sediments beneath the bent hill deposit are also strongly recrystallized to quartz and chlorite although their strontium isotope compositions are much lower than those in pelagic or terrigenous sediments in the region 87 sr 86 sr 0 709 0 720 the range of compositions 87 sr 86 sr 0 7046 0 7060 has little overlap with that of the altered basalts and chlorite quartz rocks from the sills and uppermost basement this lack of overlap suggests that the sediments either retain some of their original sedimentary strontium or that there is a range of fluid compositions in the sediment pile beneath the bent hill deposit sediments from the margins of middle valley far from zones of active black smoker venting odp site 855 have 87 sr 86 sr ratios with a significant hydrothermal component 0 7059 0 7086 these ratios indicate that the subsurface hydrology of middle valley is dominated by evolved fluids rather than seawater and that recharge into this system is not through boundary faults or through the sedimentary blanket as suggested by previous models rather exposed basement rocks that form the flanks of middle valley are the most likely zones of regional seawater recharge to the deep high temperature hydrothermal systems

upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

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

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

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

on the cause of the asymmetric distribution of seamounts about the juan de fuca ridge ridge crest migration over a heterogeneous asthenosphere abstract the distribution of non hotspot seamounts in the northeast pacific is highly asymmetric small seamount chains and isolated edifices are numerous on the pacific plate but nearly absent on the juan de fuca plate we propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference plate motion solutions indicate that the juan de fuca ridge is migrating to the west at a ridge perpendicular rate of 20 mm a 1 which is large compared to the half spreading rate of 30 mm a 1 migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre if slightly enriched lower melting temperature heterogeneities are present in the upper mantle they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite seamount volcanism will occur preferentially on the pacific plate because of two factors firstly more asthenosphere and hence more early melt heterogeneities must ascend to supply the pacific plate which is translating more rapidly than the relatively stationary juan de fuca plate secondly the asthenosphere that is required to ascend to supply the thickening of the juan de fuca plate will have been flushed of its significant shallow level heterogeneities by the previous advance and passing of the ridge few should remain to cause volcanism on the juan de fuca plate testable physical and petrologic predictions of this model can be identified for example within a small seamount chain the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis variation in lava chemistry along a small seamount chain which is interpreted to represent magmatism from a single large heterogeneity will provide the major discriminant of this hypothesis systematic variations should be observed with earlier formed edifices being constructed of more enriched lavas and later younger ones of more depleted lavas isotope ratios should be unaffected by varying degrees of partial melting but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant

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

geochemical evidence for low magma supply and inactive propagation at the galapagos 93 25 w overlapping spreading center 1 major and trace element concentrations sr nd and pb isotopic ratios and plagioclase and olivine compositions of samples from 92 35 w to 94 10 w on the galapagos spreading center gsc constrain magmatic processes associated with the 93 25 w overlapping spreading center osc significant variations in parental magma compositions and extent of low pressure fractionation over short along axis distances suggest small discontinuous or poorly mixed magma chambers mineralogical petrological and geochemical data indicate this osc is characterized by low overall melt supply regional along axis geochemical gradients along the western gsc e g in 206pb 204pb nb zr and la 8 sm 8 are interrupted by increased variability within the osc consistent with less efficient mixing of melts in this region despite a long term history of westward propagation bathymetric and geochemical data are inconsistent with recent propagation of this offset the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone the eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements one outside and one within the overlap zone the overlapping portion of the western limb also has low ratios of these elements these patterns are consistent with previously depleted mantle being preferentially melted beneath this offset

the concentration and isotopic composition of carbon in basaltic glasses from the juan de fuca ridge pacific ocean the abundance and 13c12c ratios of carbon were analyzed in basaltic glass from twenty locations along the juan de fuca ridge using a 3 step combustion extraction technique carbon released during the first two combustion steps at 400 500 c and 600 650 c is interpreted to be secondary and only the carbon recovered during a final combustion step at 1200 c is thought to be indigenous to the samples for carbon released at 1200 c glasses analyzed as 1 2 mm chips contained 23 146 ppm c with 13c values of 4 8 to 9 3 whereas samples crushed to 38 63 m or 63 90 m yielded 56 103 ppm c with 13c values of 6 1 to 9 2 the concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean ridge basalts the juan de fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor evidence of disequilibrium includes 1 1 a large range of carbon contents among glasses collected at similar depths 2 2 a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample and 3 3 a lack of correlation between vesicle abundance carbon concentration and depth of eruption variations in carbon concentration and 13c ratios along the ridge do not correlate with major element chemistry the observed relationship between carbon concentrations and 13c values may be explained by late stage variable degrees of open system rayleigh like degassing

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

evidence for remobilization of barite affecting radiometric dating using 228ra 228th and 226ra ba values implications for the evolution of sea floor volcanogenic massive sulfides samples of chimneys black smokers and other volcanogenic massive sulfide vms mineralization were collected from hydrothermal vent fields associated with submarine arc and back arc volcanoes in the western pacific each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site information that is vital when establishing the time elapsed to amass an ore deposit in each case the radiometric dating relied on the radium isotopes 228 ra and 226 ra with half lives of 5 75 and 1 600 years respectively which coprecipitate with barite and then decay we began dating vms mineralization 15 years ago knowing that the 226 ra ba decreases with time with the ba and ra assumed to be from a common source such that the initial 226 ra ba value i e the value at the onset of mineralization was constant thus the age of an old chimney up to 15 000 yr was calculated by comparing its 226 ra ba value with the initial value established from nearby young chimneys 228 ra since then we have discovered that chimneys and other forms of sea floor vms mineralization can contain barite from two or more hydrothermal events that is older barite can be dissolved and reprecipitated remobilized from earlier mineralization deposited subseafloor or earlier sea floor mineralization overprinted crosscut by new in many cases the 226 ra ba dating did not provide an absolute age for the older material but rather a difference in age between two samples that had incorporated varying proportions of older barite that was low or dead i e 15 000 yr with respect to 226 ra due to radioactive decay remobilizing of older barite is consistent with zone refining of metals in large vms deposits but we have only considered this recently however our investigations show that this can affect the initial 226 ra ba values for vms mineralization from arc and back arc volcanoes and also from mid ocean ridges where sufficient samples contained 228 ra we estimated an age for the remobilized barite by plotting a 226 ra ba versus 228 ra ba mixing line for the vms at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for 15 000 years such long histories of hydrothermal activity indicate the propensity for producing large metal rich deposits on the seafloor

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

geochemistry of axial seamount lavas magmatic relationship between the cobb hotspot and the juan de fuca ridge axial seamount located along the central portion of the juan de fuca ridge axis and at the eastern end of the cobb eickelberg seamount chain is the current center of the cobb hotspot the axial seamount lavas are transitional between n type and e type mid ocean ridge basalt morb characteristics that they share with lavas along the rest of the juan de fuca ridge there are however subtle but distinct differences between the seamount lavas and those of the adjoining ridge segments these include higher na2o cao and sr at a given mgo content and lower silica saturation in the seamount lavas as compared with the ridge lavas lava chemistry and bathymetry indicate that axial seamount is a discrete volcanic unit with a more productive shallow magmatic plumbing system separate from the adjacent ridge segments these high magma supply rates have sustained a continuously replenished steady state magma reservoir that has erupted remarkably homogeneous lavas over a long time period despite this classic association of spreading center and hotspot volcanic activity there is no evidence in the lavas for geochemical or isotopic enrichment typical of hotspot or mantle plume activity the differences in composition between the axial seamount lavas and the juan de fuca ridge lavas are attributed to melting processes rather than to any fundamental differences in their mantle source compositions the higher magma production rates higher sr and lower silica saturation in the seamount lavas relative to the ridge lavas are thought to be a consequence of melt initiation at greater depths the melting column producing the seamount lavas is thought to be initiated in the stability field of spinel peridotite whereas the ridge lavas are produced from a melting column initiated at shallower levels possibly within or close to the stability field of plagioclase peridotite implicit in this interpretation is the conclusion that the juan de fuca ridge lavas and by analogy most morb are generated at shallow mantle levels mostly within the stability field of plagioclase peridotite this interpretation also requires that for the upwelling mantle to intersect the solidus at different depths the mantle supplying axial seamount must be hotter than the rest of the juan de fuca ridge axial seamount therefore reflects a thermal anomaly in the mantle rather than a geochemically enriched ocean island basalt type mantle plume

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

segmentation of mid ocean ridges attributed to oblique mantle divergence mantle flow beneath mid ocean ridges was thought to respond passively to plate motions analysis of seismic data from ridges reveals a skew between the directions of plate motion and mantle flow implying mantle flow may stress the plates

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

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

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

steady and non steady state magma chambers below the east pacific rise volcanic rocks collected on flowline traverses on the flanks of the east pacific rise epr document changes in axial magma chemistry with time providing a record of the thermal history of axial magma chambers amc we present data for a sample set of closely spaced 1 2 km samples along epr flowlines at three localities out to 800 ka showing both steady constant average temperature and non steady state behavior of magma chambers on time scales of 200 500 ka though based on only three symmetrical traverses so far it appears that magmatically robust ridge locations 11 20 n and 9 30 n have steady state chambers whereas a magmatically starved axis 10 30 n shows large temperature changes with time these observations provide a new petrologic perspective to the ongoing debate regarding the significance and causes of morphologic variations along the axis of the east pacific rise

hydrothermal venting and the apparent magmatic budget of the juan de fuca ridge the fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large scale geological controls on the distribution of hydrothermal venting along the mid ocean ridge we undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the juan de fuca ridge a medium rate spreading center with segments of markedly differing morphology hydrothermal anomalies were detected by towing a high precision conductivity temperature depth transmissometer ctdt package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment these surveys conducted between 1985 and 1989 have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid ocean ridge hydrothermal discharge is strongest on those segments or portions of segments where the apparent magmatic budget is highest as indicated by the degree of along axis inflation and other morphological characteristics this correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber we propose that the probability of discharge occurring at any point on the mid ocean ridge is proportional to the spreading rate this probability scaled by the observation that significant plumes presently emanate from 20 of the juan de fuca ridge is expected to range from 5 on the slowest spreading ridge to 50 on the fastest spreading ridge

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

geochemical and age constraints on the formation of the gorda escarpment and mendocino ridge of the mendocino transform fault in the ne pacific the mendocino transform fault is an active dextral strike slip zone that separates the gorda plate from the pacifi c plate in the ne pacifi c ocean the compositions of the igneous rocks exposed along the southern margin of the mendocino transform fault include tholeiitic and alkaline basalts majorelement trace element and radiometric data suggest that the rocks were generated through fractionation of different parental melts derived by varying degrees of partial melting from different depths at or near the intersection of the mendocino transform fault with the gorda ridge there is evidence for extensive cooling and fractionation reminiscent of the transform fault effect of langmuir and bender 1984 alkaline and highal compositions also argue for melts from a deeper source than a normal mid ocean ridge environment the preferred geochemical analogue for the mendocino transform fault is a failed rift system where mid ocean ridge basalt morb compositions likely represent basalts created at a waning spreading center before its abandonment the morb compositions were subsequently buried by younger enriched e morb and alkaline basalts derived from deeper melting and or a more enriched source we suggest that a period of rift failure abandonment and continued alkaline volcanism occurred on the southernmost gorda ridge or on a series of short intratransform spreading center segments during plate reorganization thus the mendocino transform fault provides a record of ridge migration abandonment and residual volcanism of the southern gorda ridge spreading system from 23 to 11 ma

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

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

correlations of mid ocean ridge basalt chemistry with the geoid abstract the major element composition of mid ocean ridge basalts morb shows geochemical variations controlled by the temperature pressure and source composition produced by convection in the mantle beneath the spreading centers the long wavelength signal of the geoid reflects deep mantle processes in this paper we characterize the correlations between the major element chemistry of morb and the geoid anomalies at different wavelengths to determine the extent to which the major element composition of the morb reflects deep mantle circulation the chemical effects of the low pressure fractionation have been corrected using a new method based on the feot mgo ratio the fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the mgo denominator converge to a common point with 8 mgo the separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99 level with the axial depth for both the mid atlantic ridge mar and the east pacific ridge epr and with the geoid only for the mar the latter correlation suggests that the major element composition of the morb is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle the lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the k2o tio2 ratio and the geoid indicates that either source composition does not influence morb chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

distribution patterns of gypsum and kalistrontite in a dry lake basin of the southwestern kalahari omongwa pan namibia the deposits of the omongwa pan southwestern kalahari namibia are partly gypsiferous and locally contain small amounts of kalistrontite the distribution patterns of gypsum caso4 2h2o and kalistrontite k2sr so4 2 are mainly determined by groundwater depth and by the lithological composition of the deposits the latter determines the hydraulic conductivity for saturated and unsaturated flow and therefore controls the depth of the interval within the deposits where evaporation and mineral precipitation can take place other factors that affect the distribution of the evaporite minerals are the patterns of groundwater flow within the basin and the occurrence of a redistribution of salts during short flooding stages gypsum occurs as crystals of four distinct morphological types which each developed in different conditions prismatic crystals formed both as subaqueous precipitates and as crystals that developed within the sediment matrix of a brine saturated surface layer sub euhedral tabular crystals formed within brine filled macropores hemi bipyramidal crystals formed in a nearly permanently brine saturated part of a subsurface horizon phreatic zone as the product of recrystallization of an older gypsum occurrence and lenticular crystals formed by intrasediment growth in the vadose zone the development of these morphological types of gypsum crystals is marked by differences in the differential inhibition of growth of the various crystal forms the further growth of lenticular crystals which are most strongly affected by differential growth inhibition is still characterized by a difference in degree of inhibition between the 111 and 103 forms expressed as a change in orientation of the plane of flattening copyright 1999 john wiley sons ltd

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

along axis variations within the plate boundary zone of the southern segment of the endeavour ridge abstract the width of and along axis variations within the plate boundary zone of the southern segment of the endeavour ridge of the juan de fuca ridge system have been quantified using sea marc i sidelooking sonar imagery the sonar imagery was calibrated with video and photographic surveying to provide visual corroboration of the activity spacing and offset of faults and fissures this ridge segment contains an axial high bisected along its length by a summit depression whose width relief of its rims depth of its floor and spacing of faults and fissures vary systematically away from the midpoint of the ridge segment toward its tips the axial high and its summit depression are likened to an elongated shield volcano that is being disrupted by collapse of its crest into a widening linear caldera the observed distribution of faults and fissures could be produced by mechanisms which stretch young ocean crust which is characterized by rheological properties which vary with distance from the center of the elongated volcano reprints jhd

temporal and spatial history of the 1999 2000 endeavour segment seismic series juan de fuca ridge 1 two large clusters of earthquake activity in june of 1999 and january of 2000 have dominated recent seismicity along the endeavour segment of the juan de fuca ridge the impacts of the june 1999 sequence on the hydrologic system which include changes in vent temperature and chemistry within the main endeavour vent field have been well documented previously analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm like behavior characterized by the absence of a dominant main shock event the epicentral locations of events within the two swarms overlap spatially with centroid positions near 47 49 and 47 46 n latitude during the june 1999 swarm the initial activity spans the along axis region where a shallow axial magma chamber reflector was later imaged the epicenters then migrate 12 km to the south at a rate of 0 3 m s consistent with lateral dike propagation a distinct subcluster of events also occurred in the vicinity of surveyor volcano on the overlapping portion of the cobb segment given its distance from the main swarm this activity may represent a triggered response to dynamic shaking the january 2000 swarm has a more limited along axis extent relative to the june 1999 swarm with no indication of lateral migration much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999 although it contains fewer total events and is of shorter duration relative to the june 1999 swarm the january 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude as in situ temperature monitoring was not in place during january 2000 and vent fluids were not sampled until june 2000 the impacts of this swarm on the hydrothermal system are unknown the southernmost tip of the endeavour segment also is found to be a region of repeating swarm activity although morphologic evidence indicates the cobb segment has been propagating northward recently this seismic activity suggests that the western limb of the endeavour cobb overlap zone remains active

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

petrology of baby bare and mama bare lavas the three bares are small volcanoes that host the only known hydrothermal vents active in oceanic crust older than 2 ma major element data for baby and mama bare lavas indicate formation by low extents of partial melting of a depleted mantle source with limited cooling and crystallization prior to eruption these lavas are compositionally unlike all lavas produced at the adjacent spreading center but resemble a subset of lavas from small seamounts nearby we conclude that the three bares represent off axis volcanism rather than normal crustal accretion such off axis volcanism increases basement roughness locally and thus plays an important role in hydrothermal circulation in sedimented ridge flank settings

the geochemical controls on vent fluids from the lucky strike vent field mid atlantic ridge abstract hydrothermal vent fluids were collected from the lucky strike site at 37 17 n on the mid atlantic ridge in both 1993 and 1996 seven vents were sampled with the dsv alvin in 1993 and six vents were sampled in 1996 using the rov jason during the lustre 96 cruise as three of the vents were sampled in both 1993 and in 1996 a time series of vent fluid chemistry is also reported measured temperatures ranged from 202 to 333 c at the 1618 1726 m depth of the vent field which is located on lucky strike seamount these fluids are either equal to or less than the local bottom seawater in chlorinity while the range in fluid compositions at lucky strike is generally within that observed elsewhere the unusual aspects of the fluid chemistries are the relatively high ph and low fe mn li and zn we attribute this as well as an usually low sr ca ratio to reaction with a highly altered substrate the high si and cu contents suggest a deep as well as hot source for these fluids the fluid compositions therefore suggest formation by super critical phase separation at a depth not less than 1300 m below the seafloor and reaction with a relatively oxic and previously altered substrate there is temporal variability in some of the vent fluid compositions as li k ca and fe concentrations have increased in some of the vents as has the fe mn molar ratio although the chlorinities have remained essentially constant from 1993 to 1996 while there is not a simple relationship between vent fluid compositions or temperatures and distance from the lava lake at the summit of the seamount the vent fluids from many of the vents can be shown to be related to others often at distances 200 m the most southeasterly vents eiffel tower and the marker mounds vents are distinct in chlorinity and other chemical parameters from the rest of the vents although closely related to each other within the southeastern area similarly all of the vents not in this one area appear closely related to each other this suggests one or two source fluids for many of the vents as is also inferred to be the case at tag but which is in contrast to observations on faster spreading ridges this may suggest inherently different plumbing for hydrothermal systems at slower versus faster spreading ridges

four dimensional upper crustal construction at fast spreading mid ocean ridges a perspective from an upper crustal cross section at the hess deep rift abstract the faulted walls of the hess deep rift in the equatorial pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast spreading mid ocean ridge the 5000 m deep rift graben exposes a slightly dismembered section of oceanic crust created at the east pacific rise 1 ma the upper portion of the north wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes for tens of kilometers along the rift wall basaltic lavas with sparse dikes grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks the geometry of lava flows dikes and fault zones that developed during magmatic accretion provide constraints on the cross axis temporal evolution of the upper crust in this area outcrop scale digital imagery from submersible vehicles provides a well constrained geological framework for samples collected from these upper crustal rock units and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along axis magma transport during accretion integrated results of geological and geochemical investigations suggest a four dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers few tens of thousands of years of a fast spreading ridge axis in this model a sub axial melt lens where it exists is recharged from below with distinct parental melts and limited along axis mixing preserves geochemical segmentation of the melt lens dikes emanate from distinct portions of the melt lens and transport magmas vertically as well as significant distances laterally along axis erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas among other factors including magma overpressure and orientations of local stresses plays a role in the eruption of magma on the surface during spreading and magmatic construction subaxial subsidence accommodates thickening of the basaltic lava unit sub axial subsidence faulting and block rotation result in an upper crust with inward dipping toward the axis lava flows and outward dipping away from the axis dikes substantial differences in upper crustal geology may result from segment scale variations in which vertical magma delivery dominates in areas of robust magma supply likely over sites of recently recharged melt lenses at segment centers but lateral magma transport dominates in areas of relatively lower magma supply possibly toward segment ends

the relationship of hydrothermal fluid composition and crustal residence time to maturity of vent fields on the juan de fuca ridge the 228ra 226ra ratio of hydrothermal fluids reflect the residence time of the fluids within the crust as well as the age since melting of the basalt with which the fluids interact we have taken into account the variable 228ra 226ra ratio in basalt source rocks to improve the calculation of residence time in four hydrothermal systems on the juan de fuca ridge jdfr apparent large differences in residence time based on the incorrect assumption that the ra activity ratios are equal to the parent th activity ratios in the rock disappear when the correct rock activity ratios are used fluid residence times from the onset of high temperature 200 c reaction are 3 years which is short compared to the mean life of 228ra it thus appears that on the jdfr variability of the fluid 228ra 226ra ratio reflects the basalt age and not significant differences in fluid residence time within the present data set there is an apparent correlation of increasing water rock ratio with increasing system age which may reflect the increased importance of faulting higher permeability and flow rate with age however within the resolution of the method there is no apparent correlation of water rock ratio with residence time finally the residence times are short relative to hypothesized storage periods for brines within the ocean crust a potential implication of this result is that postulated long lived high chlorinity brine reservoirs must be extensively diluted by young ra bearing fluids that have reacted significantly with hot rock

helium isotope variations in juan de fuca ridge basalts we have measured he 4he ratios and he and ne concentrations on a suite of 24 basalt glasses from the neovolcanic zone of the juan de fuca ridge jdfr from 44 6 n near the blanco transform up to 48 0 n on the endeavour segment the helium isotope ratios exhibit a clear geographic variation with relatively constant values of 7 8 ra along the southern jdfr increasing to a maximum of 8 8 ra at 46 9 n on the cobb segment and then dropping to values of 8 0 ra at the cobb offset ratios along the endeavour segment further north are somewhat higher averaging 8 2 ra basalts dredged from axial seamount have he 4he 7 9 8 4 ra indicating that the seamount does not have a geochemical or isotopic signature distinct from other portions of the jdfr this confirms that while axial seamount is the locus of excess magma generation it is devoid of any hotspot or ocean island basalt geochemical signature for the whole sample set he 4he ratios show a negative correlation with 87sr 86sr we attribute the geographical variations in he 4he to broad scale heterogeneity in the mantle source region beneath the jdfr beginning at axial seamount and further north he 4he shows a positive correlation with fe8 0 suggesting that melting dynamics in the underlying mantle may exert some control on the helium isotope ratios along part of our survey area

volatiles in basaltic glasses from a subglacial volcano in northern british columbia canada implications for ice sheet thickness and mantle volatiles

comment on crustal anisotropy from local observations of shear wave splitting in west bohemia czech republic by vaclav vavrycuk

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

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

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

geochemistry of dikes and lavas from the north wall of the hess deep rift insights into the four dimensional character of crustal construction at fast spreading mid ocean ridges 1 an investigation of 1 m y old dikes and lavas from the north wall of the hess deep rift 2 15 n 101 30 w collected during alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust the study area encompasses 25 km of an east west flow line representing 370 000 years of crustal accretion at the east pacific rise samples analyzed exhibit depleted incompatible trace element abundances and ratios la sm n 1 indices of fractionation mgo and incompatible element ratios la sm nb ti show no systematic trends along flow line rather over short 4 m and long 25 km distances significant variations are observed in major and trace element concentrations and ratios modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions these findings combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift dominated systems suggest a more realistic model of the magmatic system underlying the east pacific rise relative to the commonly assumed two dimensional model in this model melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir dikes emanating from these distinct reservoirs transport magma along axis resulting in interleaved dikes and host lavas with different evolutionary histories this model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin

petrologic and geologic variations along the southern explorer ridge northeast pacific ocean mid ocean ridge basalts morb from the well defined southern explorer ridge segment ser in the northeast pacific ocean are moderately to strongly enriched in incompatible elements enriched morb were erupted at the highest and widest part of the segment culmination where the magma supply may be the greatest and also at the northern and southern ends of the ser variations in basalts incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale the variations also preclude the existence of a long lived well mixed magma chamber beneath the robust culmination instead magma chambers are probably temporally and spatially isolated as evidenced by the presence of highly differentiated lavas less enriched transitional morb were erupted along the central part of the ser 11 27 km south of the culmination and were supplied to the ridge separately from a less enriched pan of the mantle a more continuous magma chamber cannot be ruled out for this section on geochemical grounds all of the morb from the ser have undergone significant amounts of fractional crystallization unlike certain segments of the east pacific rise the most evolved morb were erupted near the culmination of the ridge segment aphyric lavas have been recovered only from the vicinity of the culmination while phyric lavas have erupted along the entire length of the ser there is no correlation between composition and total crystal abundances despite the requirement of multiple parental magmas most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent indicating that they formed at similar depths and extents of partial melting the transitional morb from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of na and moderately incompatible elements such as sm and zr enriched high melt fraction lavas at the magma starved southern end of the ser closely resemble lavas from the culmination either the southernmost lavas have flowed laterally within the crust from near the culmination or there is another melting anomaly that supplies the southern end of the ser if the latter case is true correspondence between ridge morphology and mantle thermal state is poor

geological indexes of hydrothermal venting hydrothermal venting occurs on mid ocean ridge axes with a diverse array of morphological structural and petrological characteristics consequently it is not clear if certain geological environments are more conducive than others to hydrothermal activity this paper uses complementary data sets recently collected on multiple tectonic segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue the spatial density of venting is gauged by plume incidence the fraction of linear ridge crest overlain by hydrothermal plumes five geological indexes are compared with plume incidence ridge axis elevation ridge axis cross sectional area the percentage of ridge axis underlain by an axial magma chamber amc reflector mgo wt in basaltic glass and spreading rate a mean value for plume incidence and each index where available is calculated for each of 14 second to fourth order tectonic segments from the juan de fuca ridge 44 30 to 48 45 n and the northern 9 to 11 50 n and southern 13 50 to 18 40 s east pacific rise plumes are most common on segments with a cross sectional area 3 5 km2 a net elevation 0 35 km amc coverage 60 and mgo 7 wt spreading rate is not deterministic on the segment scale the data suggest two classes of ridge segments segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget those with high index values have a plume incidence ranging from low to complete coverage apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity the best individual predictors of hydrothermal activity are cross sectional area and net elevation applying these indexes to the east pacific rise between 18 n and 33 s suggests that the unexplored segments with the highest probability of activity are those at 15 30 16 n 6 9 n 5 6 s 27 28 s and 30 30 33 s

establishing a new era of submarine volcanic observatories cabling axial seamount and the endeavour segment of the juan de fuca ridge abstract at least 70 of the volcanism on earth occurs along the 65 000 km network of mid ocean ridge mor spreading centers within these dynamic environments the highest fluxes of heat chemicals and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions however because underwater volcanoes are difficult and expensive to access researchers are rarely if ever in the right place at the right time to characterize these events therefore our knowledge is limited about the linkages among hydrothermal chemical and biological processes during seafloor formation and crustal evolution to make significant advancements in understanding the evolution of mor environments the united states and canada have invested in the first plate scale submarine cabled observatory linked through the global internet spanning the juan de fuca tectonic plate these two networks include 1700 km of cable and 14 subsea terminals that provide 8 10 kw power and 10 gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column resulting in a 24 7 365 presence in the oceans data and imagery are available in real to near real time the initial experimental sites for monitoring volcanic processes include the mor volcanoes called axial seamount and the endeavour segment that are located on the juan de fuca ridge axial a hot spot influenced seamount is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years in contrast the endeavour segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known a non eruptive spreading event lasting 6 years was documented at endeavour between 1999 and 2005 hydrothermal venting intensity chemistry and temperature as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events this paper presents the similarities and differences between the axial and endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies the u s has made a 25 year commitment for sustained observations using the cabled infrastructure the highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences robotics genomics and sensor miniaturization comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins

small non overlapping offsets of the east pacific rise high resolution bathymetric surveys1 2 of the east pacific rise epr reveal that in addition to overlapping spreading centres oscs the crest of the epr is offset tiny amounts by smaller features recently langmuir et al have shown that despite their subtle bathymetric expression these small offsets bends and kinks of the epr collectively termed deviations from axial linearity devals represent important petrological and tectonic boundaries some devals are small non overlapping offsets snoos that offset the epr axis by up to a few hundred metres bathymetric contours near snoos are usually disturbed from their axis parallel trend in ways that we interpret as evidence for strike slip faulting and volcanism at high angles to the trend of epr axis we offer a model for the formation of snoos and their associated faults that invokes independent magma supply stochastic active spreading and limited non rigid behaviour of the neovolcanic zone

asymmetric deep crustal structure across the juan de fuca ridge a multichannel line was shot across the endeavour segment of the juan de fuca ridge careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric an intermittent reflector from the top 1 km of crust follows topography on both the pacific and the juan de fuca plates it could be either the pillow basalt diabase dike contact or a metamorphic front within the pillow basalts a reflection exists 2 3 km below the rift valley floor diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks the deep crustal structure is highly asymmetric moho exists within 4 km of the rift valley on the pacific plate in contrast to the juan de fuca plate which shows a reflector at intermediate depths dipping away from the rise to reach moho depth 12 km from the rift valley this reflector9s origin is enigmatic we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly

axial and off axial heterogeneity of basaltic rocks from the east pacific rise at 12 35 n 12 51 n and 11 26 n 11 30 n surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the epr at 12 35 n 12 51 n and at 11 26 n 11 30 n and from four adjacent seamounts centered at less than 18 km from the rise axis the basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition there is no correlation between lava flow morphology geological settings and compositions based on mineral and compatible element composition these basalts were classified into three types the least evolved olivine basalts and highly phyric plagioclase basalts type a have high fo87 89 high mg 66 70 and high ni 100 contents the most evolved plagioclase olivine basalts with or without pyroxene type b and c have relatively low fo78 86 low mg 55 65 and low ni 60 100 ppm contents simple crystal fractionation 3 18 accounts for some of the observed compositional range however based on their incompatible element contents the above basalts are divided into three groups the various types a to c are found in each group depleted basalts with low k ti 0 04 0 15 ratios low zr 100 ppm and low nb 0 4 ppm contents undepleted basalts with the highest k ti 0 25 0 46 zr 150 170 ppm and nb 8 16 ppm contents and the transitional basalts having compositions between these two extremes these three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3 4 km in width near 12 43 n 12 51 n on the epr similar volcanic diversity in a more limited area 3 km2 is observed on off axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle the transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting we suggest that a lherzolitic mantle with minerals having a variable behavior under partial melting constitutes this type of composite mantle indeed clinopyroxene enriched in incompatible elements will be consumed at an earlier stage of melting than the olivine plus orthopyroxene eruption of compositionally distinct magma batches in close proximity on axial and off axial structures implies periods of quiescence between successive magmatic stages and cycles

petrology and sr nd and pb isotope geochemistry of mid ocean ridge basalt glasses from the 11 45 n to 15 00 n segment of the east pacific rise 1 basaltic glasses from the geophysically well studied section of the east pacific rise epr between 11 45 n to 15 00 n range from normal mid ocean ridge basalts morb to transitional morb and their major element variations correlate with isotopic and trace element indices of enrichment to first order basalts enriched in na8 0 incompatible elements 87sr 86sr and 206pb 204pb but low in fe8 0 and 143nd 144nd are more prevalent along the shallow portions of the ridge axis in detail the samples can be divided into two chemical and geographical groups the southern bathymetric dome extending from the 11 45 n overlapping spreading center to 14 10 n and the northern group extending from 14 10 n to the orozco transform the boundary between these two groups is apparent in a change in isotopic composition results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11 45 n to 15 00 n segment of the epr a depleted mantle component a seamount type enriched mantle component and an indian morb like mantle component south of 14 10 n the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near ridge seamounts north of 14 10 n the low 206pb 204pb high 207pb 204pb indian morb like component exerts a major influence on the geochemical variability of the axial lavas regional averages of major element composition e g na8 0 and fe8 0 show relatively limited variability consistent with the restricted range in depth for this region and plot within the pacific field of the previously defined global trends major element variations among individual samples however parallel the global array and their correlation with indices of mantle enrichment supports the idea that the pacific type local trend results from small scale heterogeneities in the mantle beneath the epr our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

axial processes along a segment of the east pacific rise 10 12 n axial segments of the east pacific rise are made up of individual volcanoes each volcano has a distinct magma composition and shows a systematic variation in the fractional crystallization with distance along the axis from the central chamber hydrothermal venting lava morphology and tectonics also vary along the axis lavas erupted near transform faults from the tips of propagating rifts in overlapping spreading centres and from near axis seamounts have different and variable compositions

mantle dynamics beneath the discrete and diffuse plate boundaries of the juan de fuca plate results from cascadia initiative body wave tomography we use the delay times of teleseismic s phases recorded by ocean bottom seismometers during the plate scale cascadia initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere our study area covers the entire juan de fuca jdf and gorda plates from their creation at the jdf and gorda ridges to their subduction beneath the north american continent and the entire length of the blanco transform fault the range of the observed vs anomalies requires variations in the melt fraction of the asthenosphere the data require that low vs anomalies extend to depths of at least 200 km which is within the carbonatite melting regime in the upper 200 km of the mantle vs increases rapidly to the east of the jdf ridge while there is no clear relationship with the age of the lithosphere in the gorda region the distribution of melt is asymmetric about both the jdf and gorda ridges dynamic upwelling due to the buoyancy of the mantle and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the jdf ridge the asymmetry about the jdf ridge and the sinuous pattern of upwelling near the blanco transform fault finally mantle flow beneath the diffuse gorda and explorer plate boundaries is distinct from that beneath the discrete plate boundary of the jdf ridge in particular shear between the pacific and jdf plates appears to dominate mantle deformation over seafloor spreading beneath the gorda ridge

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