post collisional southeastern beishan granites geochemistry geochronology sr nd hf isotopes and their implications for tectonic evolution abstract bandaoshan granites are exposed in the southeastern beishan area in the central part of the central asia orogenic belt caob la icp ms u pb zircon dating indicates that bandaoshan granites were emplaced into precambrian basement at 285 4 ma and their geochemistry indicates that they are alkali rich potassium high granites initial end values 4 3 to 2 7 and ehf values 2 7 to 0 7 suggest that bandaoshan granites were derived from mantle derived melt and an upper continental crustal or sedimentary component the latter plays a significant role in their genesis in combination with regional geology the early permian bandaoshan pluton is interpreted to form in a post collisional environment in the southeastern beishan area late carboniferous qiaowan granites early permian yin aoxia granites and middle permian xijianquanzi granites are also considered as post collisional granites and together with bandaoshan granites indicate that the region was in a post collisional stage from the end of late carboniferous to middle permian

structural control on sea floor hydrothermal activity at the tag active mound the tag active mound1 13 on the mid atlantic ridge near 26 n is one of the largest known actively forming volcanogenic massive sulphide deposits construction of such a deposit requires that the upflow of hydrothermal fluids be localized at this site over tens of thousands of years but the cause of this localization has been controversial two popular hypotheses propose that it results from young volcanism immediately adjacent to the mound6 or from the intersection of a transfer fault orthogonally crossing the rift valley floor with ridge parallel faults8 here we present high resolution sonar and photographic data which provide no evidence for young eruptions nearby or for transverse through going faults intersecting the tag mound instead our data suggest that the localization of hydrothermal activity at the tag mound is strongly controlled by permeable conduits at the intersection of an actively developing ridge parallel fissure fault complex with a newly recognized but older oblique fault system these observations may aid the interpretation of similar structures in ancient sea floor sulphide deposits where the tectonic disruption caused by emplacement on land has obscured the original geometry

detailed morphology of the tag active hydrothermal mound insights into its formation and growth the detailed structure and morphology of the tag active hydrothermal mound mid atlantic ridge are investigated using near bottom high resolution bathymetry data and digital photographic imagery the mound comprises two distinct circular platforms with one superposed asymmetrically on the other areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales years we propose a model for the growth of the tag active mound in which changes in the near surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed

high resolution bathymetry of the east pacific rise axial summit trough 9 49 51 n a compilation of alvin scanning sonar and altimetry data from 1991 1995 bathymetric data acquired during six cruises to the east pacific rise crest 9 49 51 n are compiled into a single bathymetric database and used to create high resolution bathymetry maps of two geologically and biologically active areas scanning sonar and alvin altimetry were collected during 23 alvin dives to the p vent bio9 and biomarker 141 areas during 1991 95 bathymetry data were filtered edited and processed as individual dives then combined into a single database for each cruise and statistically evaluated cruise data sets were numerically and visually compared and cross correlated to known bottom features and fixed seafloor reference markers then horizontally shifted to establish a consistent database with 3 m vertical and 5 m horizontal resolutions the compiled and corrected bathymetry was used to produce bathymetric maps of two areas within the axial summit collapse trough a 2 104 m area around p and bio9 vents and a 1 1 10 m area around biomarker 141 the resulting detailed bathymetry maps reveal topography dominated by low relief volcanic constructional features and post eruptive collapse of lobate and sheet lava surfaces

formation of large sulfide mineral deposits along fast spreading ridges example from off axial deposits at 12 43 n on the east pacific rise abstract submersible investigations with nautile near the 13 n hydrothermal field on the epr were made in 1992 we selected diving areas based on deep towed side scan sonar imaging close to a pit crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest 70 m high and 200 m in diameter sulfide mounds formed at a fast spreading ridge 12 cm yr despite an overlap in composition between axial and off axial deposits the general trend reflects real differences in compositions off axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment sulfur isotope variation is minimal for the off axis seamounts and suggests that sulfides precipitated from unmodifed end member hydrothermal solutions axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different pb sources and separate convective systems for the seamount and the axial deposits our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount this configuration is by far more efficient than unstable axial processes to create during several hydrothermal episodes major sulfide deposits near a fast spreading ridge off axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis thus we suggest that off axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast to medium spreading ridges

late ordovician to early devonian adakites and nb enriched basalts in the liuyuan area beishan nw china implications for early paleozoic slab melting and crustal growth in the southern altaids abstract we report newly defined nb enriched basalts adakites and dacites from the beishan nw china of the southern altaids based on field geochemical isotopic and geochronology studies two phases of adakites adakite i and adakite ii have been defined which are calc alkaline and characterized by high na 2 o k 2 o ratios 1 49 1 71 and 2 32 3 64 and sr contents 494 1213 ppm and 325 494 ppm negligible to positive eu anomalies strong depletion of hree e g yb 0 48 0 93 ppm and 0 50 0 99 ppm and y 6 87 9 80 ppm and 6 02 10 30 ppm and enriched in rb sr ba k and depleted nb and ti they are characterized by relatively low e nd t values 0 8 to 0 9 and 0 6 to 3 8 and relatively constant high 87 sr 86 sr i ratios 0 70635 0 70636 and 0 70583 0 70651 the zircons of adakite i have relatively low e hf t 0 8 to 2 7 the nb enriched basalts are sodium rich n 2 o k 2 o 1 31 4 44 with higher tio 2 p 2 o 5 zr and nb contents and nb th pm nb la pm and nb u ratios than typical arc basalts they are relatively enriched in rb ba u pb and k depleted in nb and minor negative to positive ba zr sr and ti they have low positive e nd t 0 9 to 2 3 and relatively high 87 sr 86 sr i 0 70556 0 70691 ratios the dacites are typical arc magmas with moderately enriched lile distinctly negative eu nb sr and ti anomalies they have positive e nd t 2 2 and relatively high 87 sr 86 sr i 0 70786 we argue that the liuyuan adakites were most probably related to the melting of young hot subducted crust of the paleo asian ocean which included tectonically subducted radiogenic crustal material and or inheritance from highly radiogenic oceanic crust e g oib the nb enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and or further changed by components other than adakites e g minor slab derived fluids based on own zircon sims u pb dating of these key rock types we further propose that from the late ordovician to early devonian large volumes of magma consisting of late ordovician nb enriched basalts 451 ma and dacites 442 ma late silurian adakite i 424 ma early devonian adakite ii 374 ma and i s a type granites 436 ma 380 ma developed in the southern altaids together with other geochronological data from the literature we conclude that subducted oceanic slab melting was frequent from 470 ma to 370 ma our results suggest that frequent hot and or young oceanic crustal subduction and slab melting were important mechanisms in the accretionary growth of the southern altaids

resultats preliminaires d une etude sur la dispersion de l or en milieu lateritique autour de l indice aurifere de misseni au mali a study of gold dispersion in laterite above a gold zone is based on 58 analyses of soil samples from two shafts and one trench located in the misseni gold anomaly area mali the bedrock in this a

the liuyuan complex in the beishan nw china a carboniferous permian ophiolitic fore arc sliver in the southern altaids the tectonic history and time of closure of the palaeo asian ocean of the altaids are issues of lively current debate to address these issues this paper presents detailed geological petrological and geochemical data of the liuyuan complex lc in the beishan region in nw china located in the southernmost altaids in order to constrain its age origin and tectonic setting the lc mainly comprises massive basalts pillow basalts basaltic breccias gabbros and ultramafic rocks together with cherts and tuffs most prominent are gabbros and large volumes of basaltic lavas these mafic rocks have high tio 2 contents flat rare earth element ree patterns and show high field strength elements hfses similar to those of mid ocean ridge basalts morb the mafic rocks exhibit positive e nd t 6 6 9 0 values representing magmas derived from the mantle but these basic rocks are also enriched in th relative to rees and are systematically depleted in nb ta ti relative to rees there is also a large range in initial 87 sr 86 sr 0 7037 0 7093 all these variables indicate that mantle derived magma was contaminated by fluids and or melts from a subducting lithospheric slab and formed in a supra subduction zone ssz setting a gabbro intruded in the complex was dated by la icp ms on 20 zircons that yielded a 206 pb 238 u weighted average age of 286 2 ma considering the fact that all these basalts are imbricated against permian tuffaceous sediments and limestone we propose that the lc formed as an ophiolite in a fore arc in carboniferous permian time this indicates that the palaeo asian ocean still existed at 286 2 ma in early permian time and thus the time of closure of the palaeo asian ocean was in or after the late permian

variable morphologic expression of volcanic tectonic and hydrothermal processes at six hydrothermal vent fields in the lau back arc basin ultrahigh resolution bathymetric maps 25 cm grid are used to quantify the physical dimensions of and spatial relationships between tectonic volcanic and hydrothermal features at six hydrothermal vent fields in the lau back arc basin supplemented with near bottom photos and nested within regional dsl 120a side scan sonar data these maps provide insight into the nature of hydrothermal systems along the eastern lau spreading center elsc and valu fa ridge vfr along axis transitions evident in localized volcanic morphology and tectonic characteristics include a change from broad low relief volcanic domes hundreds of meters wide 10 m tall that are dominated by pillow and lobate lava morphologies and are cut by faults and fissures to higher aspect ratio volcanic domes tens of meters wide tens of meters tall dominated by aa type lava morphologies with finger like flows and few tectonic structures these along axis differences in localized seafloor morphology suggest differences in hydrothermal circulation pathways within the shallow crust and correlate with regional transitions in a variety of ridge properties including the large scale morphology of the ridge axis shallow axial valley to axial high seafloor lava compositions and seismic properties of the upper crust differences in morphologic characteristics of individual flows and lava types were also quantified providing an important first step toward the remote characterization of complex terrains associated with hydrothermal vent fields

recent volcanic events and the distribution of hydrothermal venting at the lucky strike hydrothermal field mid atlantic ridge 1 we present new high resolution bathymetry and backscatter data acquired in 2006 with the rov victor 6000 over the lucky strike hydrothermal field mid atlantic ridge as long term monitoring of the lucky strike area momar project is being implemented these new high resolution data offer an unprecedented view of the distribution of hydrothermal edifices eruptive facies and small scale tectonic features in the lucky strike vent field we show that vents located in the nw and ne correspond with wide expanses of lumpy seafloor which we interpret as primarily made of broken chimneys and sulfide edifices they are found above scarps with relief 50 m or on associated mass wasting deposits by contrast the se and sw vents correspond with small expanses of lumpy seafloor and are located near smaller scarps which we interpret as more recent faults hydrothermal edifices in the sw venting area appear very recent postdating the emplacement and faulting of the most recent lava we propose that this difference in the age of hydrothermal edifices does not mean that hydrothermal venting itself is more recent in the southern part of the lucky strike field because preexisting sulfide deposits there may have been buried by recent volcanic deposits instead the older edifices in the northern part of the hydrothermal field may have been allowed more time to grow because they are set above the level of recent lava flows the formation of a lava lake is the most recent eruptive event detected at lucky strike lava drainback is evidenced by benches and lava pillars suggesting a close connection with an underlying magma reservoir which probably corresponds to the melt body imaged by singh et al 2006 we have found no evidence that this lake was active for months to decades as lava lakes at terrestrial volcanoes it may instead have formed as a lava pond with successive lava flows covering the eruptive vents as proposed for similar features at the epr the horizontal surface of the lake is deformed only near its southwestern shore along a nne trending set of faults and fissures which appear to control the distribution of hydrothermal chimneys

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

east pacific rise near 13 n geology of new hydrothermal fields abundant massive sulfide deposits are present at the crest of the east pacific rise near 13 north where the opening rate is about 12 centimeters per year large manganese and helium 3 anomalies in seawater samples evidence of intense present day activity of hydrothermal springs indicate that sulfides are still being produced along this segment of the rise massive sulfides also occur on adjacent off axis seamounts

optical image and microchemical analysis of gold grains from a weathered profile of the minvoul greenstone belt northern gabongold grain analysis minvoul greenstone beltm iglesias martinez et al morphological characterization and quantification of gold particles by optical image analysis oia and by compositional analysis of microprobes using scanning electron microscope and electron microprobe analysis techniques were carried out on gold grains from the minvoul area archean greenstone belt in gabon large grains of almost pure gold were found throughout a weathering profile which consisted of saprolite mottled clay zone iron duricrust pisolitic gravels and yellow latosol in the deeper horizons gold was dissolved as shown by corrosion features on the surface of particles with average sizes of 2 6 and 2 35 in the saprolite and mottled clay zones respectively the occurrence of secondary gold in the duricrust was indicated by the larger size of the nuggets average 1 8 the high fineness 995 in average and the close textural relationship between gold and neoformed iron oxyhydroxides the uppermost horizons composed of yellow latosol and pisolitic gravels were interpreted as transported materials based on their size distribution average 0 85 and 1 34 sorting and shape parameters the best morphological parameter to describe the whole weathering profile was found to be the perimeter area ratio the highest ratios were recorded in the saprolite average 0 192 m 1 and decreased towards the surface average 0 057 m 1 the combination of the oia technique and the microchemical analysis of gold grains allowed us to define specific morphological and compositional characteristics of the gold particles for each horizon both methods proved to be of great utility to understand gold concentration dissolution and dispersion processes in supergene environments

the effect of arc proximity on hydrothermal activity along spreading centers new evidence from the mariana back arc 12 7 n 18 3 n back arc spreading centers bascs form a distinct class of ocean spreading ridges distinguished by steep along axis gradients in spreading rate and by additional magma supplied through subduction these characteristics can affect the population and distribution of hydrothermal activity on bascs compared to mid ocean ridges mors to investigate this hypothesis we comprehensively explored 600 km of the southern half of the mariana basc we used water column mapping and seafloor imaging to identify 19 active vent sites an increase of 13 over the current listing in the interridge database irdb on the bathymetric highs of 7 of the 11 segments we identified both high and low i e characterized by a weak or negligible particle plume temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic active sites are concentrated on the two southernmost segments where distance to the adjacent arc is shortest 48 mm yr and tectonic extension is pervasive re examination of hydrothermal data from other bascs supports the generalization that hydrothermal site density increases on segments 90 km from an adjacent arc although exploration quality varies greatly among bascs present data suggest that for a given spreading rate the mean spatial density of hydrothermal activity varies little between mors and bascs the present global database however may be misleading on both bascs and mors the spatial density of hydrothermal sites mapped by high quality water column surveys is 2 7 times greater than predicted by the existing irdb trend of site density versus spreading rate

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

magmatic subsidence of the east pacific rise epr at 18 14 s revealed through fault restoration of ridge crest bathymetry 1 the fault restoration technique of de chabalier and avouac 1994 is applied to an ultra high resolution bathymetry data set from the east pacific rise epr at 18 14 s fault offsets are calculated and subtracted from the original seafloor bathymetry to examine the morphology of the unfaulted seafloor surface within an area encompassing the ridge axis 400 1600 m in dimension the restored topography reveals a gently sloping seafloor 200 m wide which slopes 5 inward toward the spreading axis we attribute this inward sloping seafloor to subsidence within the axial trough due to subsurface magmatic deflation the vertical deformation field extracted from the bathymetry is used to characterize the ridge axis fault population present in the area median fault throws 9 m for inward facing and 8 m for outward facing faults are comparable to values measured for nearby mature abyssal hill fault populations suggesting a genetic link however median fault spacings 70 and 46 m are an order of magnitude smaller and estimated total extensional strain is 3 4 greater than values measured for ridge flank faults these differences indicate the axial fault population at 18 14 s cannot be rafted onto the ridge flanks to form abyssal hill faults without significant modification presumably via volcanic burial we attribute the dense faulting observed in this area to slip on axial fissures during subsidence of the crestal region the surface subsidence of a volcanic edifice can be modeled in terms of volume change in the magma source reservoir and volume of magma withdrawn from the reservoir using the relationship derived for a sill like magma body we estimate that the axial depression at 18 14 s could represent magma withdrawal associated with a small number 4 22 of the frequent dike injection and eruption events required to build the upper crust during the evolution of the trough the subsidence volumes represented by the axial topography at 18 14 s are a small percentage of the underlying upper crustal sections 3 4 suggesting that only a minor mismatch between magma recharge and withdrawal from the axial melt lens during ongoing plate separation could account for this pronounced axial depression

concentration of gold in in situ laterites from mato grosso the gold concentration studied is located in lateritic soils overlying precambrian schists of the cuiaba group in mato grosso brazil the following five horizons may be recognized from bottom to top 1 a gray blue altered schist horizon 2 a red argillaceous alterite 3 a horizon characterized by iron oxihydroxide rich pebbles and quartz fragments in an iron oxihydroxide rich matrix and clays 4 an iron crust and 5 the present soil the most significant gold content is found in the third horizon just below the iron crust according to geological study and morphological observations of the gold particles the gold ore mined today is the result of two combined processes i e the ferrallitic alteration of quartz lodes enclosed in schists and the effect of the red argillaceous alterite which acts as an impervious structure preventing the largest gold grains from migrating downward during their mechanical concentration

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

geomorphological variations at hydrothermal sites in the southern mariana trough relationship between hydrothermal activity and topographic characteristics abstract this study presents the first detailed geomorphological characterization of field scale geological features associated with hydrothermal systems in the southern mariana trough using near bottom swath mapping data collected by the autonomous underwater vehicle auv urashima during cruise yk09 08 and dive observation data acquired by the submersible shinkai6500 during cruise yk10 11 the motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area two of the hydrothermal sites in the study area near 12 57 n 143 37 e are located on the active backarc spreading axis the snail and yamanaka sites one is located at the eastern foot of the axial high the archean site and two are located on an off axis knoll about 5 km from the spreading axis the pika and urashima sites the on axis area is divided into tectonically dominant and volcanically dominant zones volcanically dominant zones are characterized by mounds height 5 30 m diameter 250 320 m cut by fissures the snail and yamanaka sites are located adjacent to these fissures and are possibly represented local activity associated with a 4th order segment scale diking event on the basis of comparisons with previously studied cases on the east pacific rise with similar on axis geological characteristics in contrast to the on axis sites the off axis sites show no evidence of faulting the archean site at the foot of the axial high is characterized by a single mound height 50 100 diameter 250 300 m pronounced off axis lava flows and the presence of high amplitude rugged seafloor features the site is located at the top of the mound numerous ridge lines height mainly 2 6 m extend radially from the top of the mound and several chimney like structures up to approximately 6 m high occur on the top and slopes of the mound the pika site is located on the western peak of an off axis knoll and the newly discovered urashima site is located at the northern foot of the western peak of the same knoll the western peak is characterized by bumpy seabed textures formed by numerous smaller scale mounds and ridge lines however the eastern peak has a very smooth top and slope and shows no signs of hydrothermal activity numerous mounds heights 5 75 m diameters 50 350 m are developed on the comparatively gentle slope of the knoll in contrast to the numerous ridge lines height mainly 1 6 m developed on the relatively steep slopes of the knoll on the basis of the associated geomorphological features the three off axis sites archean pika and urashima were identified as localities created by relatively long term large scale hydrothermal activity as compared with sites in the on axis area the sustained activity at off axis sites appears closely related to an off axis upwelling magma system the three off axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting these areas are characterized by numerous ridge lines conical mounds and bumpy seabed texture whereas the on axis sites are characterized by the absence of ridge lines and the presence of white smoker and shimmering observed on dome shaped pillow mounds hence the distribution of ridge lines mound morphology and bumpy seabed texture is likely to correlate with hydrothermal activity

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

high resolution magnetic signature of active hydrothermal systems in the back arc spreading region of the southern mariana trough high resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back arc spreading region of the southern mariana trough using shinkai 6500 a deep sea manned submersible a new 3 d forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization the results revealed that magnetic anomaly derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region the distribution of magnetic anomaly derived absolute magnetization suggests that all five andesite hosted hydrothermal fields are associated with a lack of magnetization as is generally observed at basalt hosted hydrothermal sites furthermore both the pika and urashima sites were found to have their own distinct low magnetization zones which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension the spatial extent of the resulting low magnetization is approximately 10 times wider at off axis sites than at on axis sites possibly reflecting larger accumulations of nonmagnetic sulfides stockwork zones and or alteration zones at the off axis sites

eruption styles of basalt in oceanic spreading ridges and seamounts effect of magma temperature and viscosity basalts erupted along oceanic zones of spreading and on intraplate seamounts at depths where hydroexplosions are not likely follow various styles of eruption quiet eruptions produce a variety of forms including lava lakes and sheet pillow and pavingstone lavas reactive eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites the stresses induced in a rapidly cooling lava flow are important in determining the different lava forms with lava lakes and sheet flows resulting from low stresses pillows from higher stresses and hyaloclastites from even higher stresses which cause rupture calculations show that if we consider basaltic lavas of the same composition the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption the viscosity is strongly dependent on the crystal liquid ratio of the magma at eruption the temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts within a given ridge segment temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary thus the model predicts that going from fast to slow spreading ridges i e from east pacific rise to mid atlantic ridge the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases moreover within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts pillows and hyaloclastites in later stages the probability of forming hyaloclastites should be low in spreading ridges higher in intraplate seamounts recent high resolution observations on the east pacific rise the galapagos rift the mid atlantic ridge and the red sea are consistent with the model because they show that 1 the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges 2 within a given ridge segment it decreases approaching a transform and 3 hyaloclastites are important on seamounts only

geochronology petrogenesis and tectonic implications of triassic granitoids from beishan nw china abstract five triassic granitic plutons have been identified from the beishan orogen by zircon u pb dating that defines two age groups 240 238 ma and 225 217 ma they belong to the high k calc alkaline and shoshonitic series and have alkalic and alkali calcic characteristics and are weakly peraluminous metaluminous these granitoids are mainly highly fractionated i type granite and a type granite they have high i sr values 0 7049 0 7096 and negative e nd t values of 0 9 to 2 8 with nd model ages of 0 89 1 13 ga suggesting a crustal contribution they also exhibit relatively high zircon e hf t values 1 3 to 6 1 and two stage hf model ages of 1 34 0 87 ga suggesting a mixture of mantle and various amounts of crustal components the mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts possibly derived from pre existing subducted materials the mid triassic 240 238 ma pluton has relatively high sr y 15 9 41 8 ratios and mgo 1 22 3 77 contents with negligibly to weakly negative eu anomalies eu 0 57 0 80 indicating adakite like features this was likely the result of partial melting of crustal components in response to voluminous underplated mantle derived magmas probably linked to lithospheric delamination and asthenospheric upwelling the late triassic 217 225 ma granitic plutons show strong negative anomalies of eu ba nb ta sr p and ti these were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post orogenic setting

volcanism and metallogenesis of axial and off axial structures on the east pacific rise near 13 degrees n a fast spreading segment 12 cm yr super 1 of the east pacific rise near 12 degrees 50 n explored in detail by surface ship and manned submersible displays intense hydrothermal activity the rise crest about 1 500 m in width with a regional depth of 2 600 m is occupied at its center by an axial graben 1 3 titania alkali enriched olivine ti k basalts with high tio 2 1 4 and high alkali content na 2 o 3 0 k 2 o 0 3 occur on the southeastern seamount the fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir asymmetrical with respect to the axis of the graben which supplied both axial and off axial lava flows the ti k enriched basalts are believed to have been produced from an alkali enriched magmatic source

gold and pathfinder elements in ferricrete gold deposits of the yilgarn craton of western australia a review with new concepts abstract secondary mineral deposits have played an important role in the global mineral resource economy for over 50 years with lateritic au al fe and ni deposits having a significant input to global metal production and reserves in the yilgarn craton of western australia a deeply weathered mantle is commonly capped with 2 10 m of lateritic residuum residual lateritic gravels and duricrust and or ferricrete fe oxide cemented sediment which formed under seasonally humid tropical and sub tropical climates during the cenozoic the principal constituents of these units are goethite hematite maghemite kaolinite and quartz they are commonly overlain by younger 2 10 m thick transported cover deposited under later semi arid conditions both ferricrete and lateritic residuum may host exploitable secondary gold deposits typically small

volcanic mound fields on the east pacific rise 16 19 s low effusion rate eruptions at overlapping spreading centers for the past 1 myr 1 volcanic mound fields identified on seamarc ii and hmr1 12 khz side scan data from the southern east pacific rise sepr occur near overlapping spreading centers oscs and migration traces of oscs the volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side scan backscatter reflections from clusters of mound shaped reflectors the lack of growth of the mound fields away from the ridge axis and their occurrence in association with osc traces suggests that mound fields form along the ridge crest near oscs volcanic mound fields are found where 120 khz side scan and visual observations find fields of pillow mounds since pillow mounds are constructed by low effusion rate eruptions the volcanic mound fields found near the oscs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions this tendency for low effusion rate eruptions at oscs is documented for the past 1 myr three independent measurements of ridge segmentation 1 volcanic segment boundaries marked by the low effusion rate eruptions 2 tectonic segments defined by oscs and 3 magmatic segment boundaries based on continuity of parental magma composition all coincide in the study area high backscatter off axis lava fields not associated with seamounts are found on seafloor younger than 0 2 ma the 0 2 ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off axis eruptions mainly occur on crust younger than 0 2 ma

morphology and distribution of lava flows on mid ocean ridges a review abstract both pillow lavas and sheet flows are important features of oceanic volcanism at mid ocean ridges submersible surveys of the neovolcanic zone and off axis areas of these complex seafloor volcanic terrains conducted during the past two decades have uncovered a wide array of basaltic surface forms with pillow basalts displaying bulbous spherical and elongate tubular patterns and sheet flows exhibiting smooth lobate rippled wrinkled ropy whorly hackly and jumbled configurations the genesis of pillow lavas and sheet flows appears to be coupled to variations of spreading rates within the plate boundary zone while pillow lavas predominate at slow spreading centers i e mid atlantic ridge sheet flows prevail along the active volcanic zones at intermediate i e galapagos rift and juan de fuca ridge fast and superfast i e east pacific rise spreading centers yet both types of flows are present at each setting the distribution of pillow lavas and sheet flows along a ridge segment is related to the topographic gradient of the ridge axis hence the formation of sheet flows tends to be favored by the development of topographic highs between ridge segment boundaries where the crustal lid is thinnest and rifting directly taps the underlying magma chamber fissure eruptions here release copious amounts of surface fed fluid lavas which decrease in volume down the topographic gradient away from the bathymetric high of the ridge segment lava flows become more distal and channelized thereby fostering greater concentrations of tube fed pillow flows toward the segment boundaries the type of lava flow morphologies encountered at any specific site on the seafloor is contingent upon the effusion rate cooling rate and slope as demonstrated by laboratory simulations

fine scale segmentation of volcanic hydrothermal systems along fast spreading ridge crests abstract fine scale 3rd and 4th order segments of the mid ocean ridge mor are morphologically defined units of crustal accretion with enigmatic origins at fast spreading centers 3rd order segments are bounded by discontinuities in the structure of the axial high most 3rd order segments are 20 10 km long and terminate at ridge axis discontinuities rads exhibiting 0 2 3 km lateral offsets and or 20 m axial depth increases nested within 3rd order segments are shorter 4th order segments bounded by smaller rads along the fast spreading east pacific rise epr at 10 02 9 08 n and 17 11 42 s spatial distributions of active high temperature hydrothermal vents biological communities and lava flow morphology are known accurately from extensive near bottom surveys and submersible dives along a total of seven 3rd order segments and twenty four 4th order segments the cumulative northern and southern epr data show that 3rd order segments but not 4th order segments consistently exhibit apparently higher rates of lava effusion near segment middles in comparison to segment ends supporting the hypothesis that each 3rd order segment is a discrete centrally fed volcanic system or volcanic segment spatial analyses of the cumulative hydrothermal data reveal a striking increase in abundance of hydrothermal features within the middle portions of 3rd order segments approximately 60 of ongoing high temperature focused flow is concentrated within the middle 40 of 3rd order segments and twice as many biological communities are found at mid segment in comparison to segment ends the two independent data sets volcanic and hydrothermal both indicate that magmatic heat sources are focused beneath the central portions of 3rd order segments and that magmatic hydrothermal systems are spatially and temporally segmented at 3rd order length and time scales we propose that 3rd order segmentation arises from processes controlling melt supply to the middle to lower crust and that 4th order segmentation largely arises from more rapid frequent upper crust processes of crack formation and dike intrusion future work is needed to test the applicability of our results along portions of the fast spreading epr notched by fault bounded axial troughs and in areas farther south where spreading rate reaches a maximum

geologic form and setting of a hydrothermal vent field at lat 10 56 n east pacific rise a detailed study using angus and alvin a hydrothermal vent field here called the feather duster site occurs on the eastern marginal high near the edge of a narrow 95 m and shallow 15 20 m axial graben within an area dominated by sheet flows and collapse features the sheet flows are intermediate in relative age between younger fluid flow lavas on the floor of the axial graben and older pillow constructional lavas on the marginal highs hydrothermal activity occurs in two zones within a 65 by 45 m area the main zone is located where a fissure system and sulfide sulfate chimneys vent warm 9 47 sup 0 c and hot 347 sup 0 c hydrothermal fluids here two mounds of massive sulfide totaling about 200 t are forming one occurs at the base of a 3 m high scarp which is the wall of a drained lava lake the other is perched on top of the scarp 19 references 4 figures

distinct regional differences in crustal thickness along the axis of the mariana trough inferred from gravity anomalies we have compiled extensive gravity and bathymetry data for the whole mariana trough which were collected during several japanese scientific cruises over the last few years this study aims to clarify the lateral distribution of the local differences in geochemical signatures which have been observed locally in the mariana trough shipboard free air gravity anomaly data from eight japan agency for marine earth science and technology jamstec cruises were compiled with those crossover errors of 2 85 mgal mantle bouguer anomalies mba were calculated by subtracting the predictable gravity signal due to the seawater crust and crust mantle density boundaries the crustal thickness variation along the spreading axis was estimated from the mba different features in crustal thickness its variation and segment length for each segment allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the mariana trough segment in region a to the north of 20 35 n shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources a variety of crustal thickness values and of along axis crustal thickness variations in region b between 15 38 n and 20 35 n suggests two types of segments one is similar to a slow spreading ridge segment that has a plume like mantle upwelling under the spreading axis and the other is a magma starved segment region c between 14 22 n and 15 38 n is a less magmatic region individual crustal thickness averages of 3 4 4 1 km region d to the south of 14 22 n has higher individual crustal thickness averages of 5 9 6 9 km suggesting higher magmatic activity with a sheet like mantle upwelling under the spreading axis different features in the mba for off axis areas suggest that these four regions have existed since the mariana trough started spreading moreover comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust which are locally distributed in both regions b and c probably result from mantle source depleted of water and incompatible elements this suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the mariana trough

hydrothermal plumes imaged by high resolution side scan sonar on a cruising auv urashima an autonomous underwater vehicle auv is an ideal platform that enables a closer approach to the seafloor than other platforms the auv provides remarkable advancement not only in imaging small scale bathymetry with high resolution but also in imaging the spatial distribution of scattering signals within a water column acoustic echoes can be recorded continuously by auvs potentially providing information related to hydrothermal plumes within a water column in may 2007 such an auv urashima of the japan agency for marine earth science and technology captured fantastic acoustic images within the water column above an active hydrothermal field the iheya north field off okinawa numerous filamentous and hyperbolic echoes were recorded on the side scan sonar images according to the temperature and geochemical anomaly the scattering signals within the water column were hydrothermal plumes upwelling from the vent area this proof of concept has implications for long range exploration for vent field with auvs

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

correlation between volcanic and tectonic segmentation of fast spreading ridges evidence from volcanic structures and lava flow morphology on the east pacific rise at 9 10 n 1 combined analyses of volcanic features in dsl 120 sonar data and argo i images along the ridge crest of the east pacific rise 9 09 54 n reveal a consistent decrease in inferred lava effusion rate toward the ends of third order segments the correlation of tectonic segmentation and volcanic style suggests that third order segmentation corresponds to the volcanic segmentation of the ridge along axis changes in volcanic structures from collapse troughs to basaltic lava domes and lava morphology from sheet to pillow flows coincide with the boundaries of morphologically defined third order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry pillow lava flows cover 25 of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third order segment ends an additional 25 of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough the remaining terrain consists of lobate lava flows we interpret the spatial correlations of morphologic structural seismic and petrologic data as evidence that individual volcanic plumbing systems are organized at 20 km spacing along the ridge axis third order segment scale in agreement with the hypothesis that volcanic and tectonic segmentations are correlated for fast spreading ridges we estimate that the longevity of volcanic segments is 104 105 years 1 3 orders of magnitude longer than fourth order segments 102 103 years this implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable

distribution of axial lava domes along a superfast overlapping spreading center 27 32 s on the east pacific rise 1 deep towed dsl 120 bathymetric data are used to investigate the pattern of lava dome formation along a superfast spreading portion of the southern east pacific rise epr including the overlapping limbs of a giant 120 120 km propagator near 29 s along the 670 km of the axis surveyed 1172 small domes were identified using a closed contour algorithm their abundance defined by spatial density is well correlated with the along axis relief of the ridge crest where the western and eastern limbs plunge toward the overlap zone densities are high 3 6 km 2 however where the axial depth profile is shallow and flat densities are comparably low 0 4 km 2 volcanic domes within the low abundance areas are characterized by lower ratios of height to basal radius 0 15 versus 0 22 smaller maximum heights 18 versus 40 m and a larger relative percentage of small versus large mounds the zone of high dome abundance encompasses the overlapping limbs of the rift and extends more than 100 km to the north and south beyond the overlap zone domes form dominantly during low effusion rate point source eruptions which suggests that discontinuous melt lenses underlie the ridge axis proximal to the overlapper conversely fissure fed sheet flows dominate along the more distal segments implying the presence of a more continuous axial magma lens throughout the survey area dome abundance increases systematically near second order segment boundaries within the high abundance zone some third order offsets also correlate with increased dome production but local peaks in abundance are not tied exclusively to higher order ridge offsets where dome abundance is low domes are clustered tightly near second order offsets and there is no increase in dome abundance near third order segment boundaries

geotectonic setting of hydrothermal activity on the summit of lucky strike seamount 37 17 n mid atlantic ridge we have investigated the relations between volcanic tectonic and hydrothermal activity on lucky strike seamount 37 17 n mid atlantic ridge using a nested survey strategy involving collection of data from different deep sea mapping systems the highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic pillow lava and hydrothermal sulfide rubble and hydrothermally cemented breccias substrates the large volume of sulfide rubble together with the nature of the sulfide structures indicates that hydrothermal activity has been episodic but ongoing for a long period of time hundreds to thousands of years on the basis of the distribution of hydrothermal deposits we propose a model of alternation between tectonic and volcanic control on hydrothermalism at lucky strike seamount midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high temperature hydrothermal activity over long time periods during periods of amagmatic extension active faulting within the summit depression provides multiple near surface fluid flow pathways for discharge of high temperature fluids and widespread deposition of massive sulfides during eruptive events rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits the new sheet flows form a cap on the hydrothermal system and fluid upflow is reorganized discharge of high temperature fluids is restricted to isolated sites with relatively high permeability for example the edges of the lava lake much of the upwelling hydrothermal fluid pools in the subsurface conductively cools and mixes with entrained seawater before discharging as widespread low temperature diffuse flow hyaloclastites become cemented further augmenting the sealing of the system present day activity at lucky strike seamount represents this locally volcanically controlled phase of activity despite the segment as a whole being dominantly amagmatic

quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment mobility of gold throughout the global weathering system of dondo mobi south gabon has been quantatively addressed by calculating the gold mass transfer during lateritic weathering of auriferous lisvenite archean gneiss and proterozoic schist and by proposing a thermodynamic approach to gold solubility under equatorial rainforest acidic conditions both chemical and short distance translocation processes govern the mobility of gold and gold particles during the long term evolution of collapsed open weathering systems which result from the combined interaction of rock meteoric water and biological agents free gold particles are translocated from the lisvenite derived soil towards formerly barren archean gneiss and porous proterozoic formations derived from schist to form a 200 m wide supergene gold dispersion halo which took shape over about 0 86 1 26 million years simultaneously with physical translocation of gold dissolution of gold makes the total balance of gold transfer through the whole weathering system strongly negative which is the converse of what is generally believed to occur in lateritic systems taking into account hydrolysis reactions three main gold complexes are proposed as a function of corresponding inorganic and organic ligand concentrations these are au oh h o aucioh and au oh fa these gold complexes leave the dondo mobi weathering mantle and move towards the river system in the course of both physical and chemical processes involved in the weathering of the formation

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

malaguana gadao ridge identification and implications of a magma chamber reflector in the southern mariana trough 1 six channel seismic reflection data reveal a magma chamber reflector beneath the malaguana gadao ridge the southernmost segment of the spreading center in the mariana trough for most of its length the spreading center in this active back arc basin is morphologically similar to slow spreading mid ocean ridges having a deep central graben flanked by a zone of abyssal hill fabric this southernmost segment however has a broad smooth cross section lacks a deep central graben and is thus similar in morphology to fast spreading ridges e g the east pacific rise we identify a magma chamber at 1 5 s two way travel time below the crest of the ridge observations from remotely operated vehicles along the ridge reveal not only fresh pillows lobate and sheet lava flows but also an abundance of volcaniclastic debris and intense hydrothermal activity these observations together with the fast spreading morphology of the ridge suggest that this segment has a considerably higher magma supply than is typical in the mariana trough we suggest that the volcanic arc or enhanced melting of a hydrated mantle is supplying volatile rich magma as evidenced by a highly negative coefficient of reflection 0 42 for this mcr and the presence of evolved highly vesicular lava and volcaniclastic materials the southeastern mariana back arc basin spreading ridge does not compare readily with mechanical models for global mid ocean ridge data sets because of marked asymmetry in both volcanism and deformation that may be the consequence of slab related geometry in this part of the convergent margin system

asymmetric accretion along the slow spreading mariana ridge 1 we report the results of several surface geophysical surveys of the central part of the mariana basin western pacific analysis of bathymetric and magnetic data along three distinct spreading segments indicates highly asymmetric spreading processes the half spreading rate being two to three times more important on the western side of the basin than on its eastern side such an asymmetry is as strong at the centers of segments as at their ends and does not systematically correlate with an asymmetric shape of the rift valley a half graben shape of the rift valley is observed only at segment ends at segments centers a symmetric graben shape characterizes the valley the asymmetric shape of the valley at segment ends probably reflects the localization of the tectonic strain due to changes in lithospheric rheology and thickness asymmetric spreading is likely accomplished through a different amount of deformation and magma emplacement on both sides of the spreading axis it may result from the combined effects of 1 asymmetric rheologic melting and stress conditions imposed by the influence of the pre existing volcanic arc and its melt sources on the eastern side of the basin 2 the northwestward movement of the philippine sea plate while the almost vertical pacific slab in the mariana subduction zone resists to this motion 3 a roll back component at the mariana subduction zone

submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the mariana back arc basin from wadatsumi side scan sonar images 1 high resolution deep tow side scan sonar data were collected over two distinct spreading segments in the central part of the mariana back arc basin these data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows using these data we observe spatial variations in seafloor deformation and volcanic activity within each segment and also significant differences in the degree of tectonic deformation between the two segments one segment characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes is in fact currently dominated by intense deformation rather than volcanism the other segment which exhibits morphology wide and deep linear axial valley typical of magma starved segments is subjected to very limited deformation and is covered by mostly unfaulted recent flows each segment also displays along axis variations in the degree of tectonic deformation and in lava flows freshness we observe a decrease of lava effusion rate from segment centers toward their ends we also investigated the apparent asymmetry of the mariana basin on the southern segment of the study area azimuths of tectonic structures are divided into two groups one segment parallel and one 15 oblique to strike of the segment these two trends of linear features developed synchronously with volcanic activity currently oblique structures seem to be the most active ones the tectonic structures that are parallel to the overall valley trend are distributed over the entire valley whereas the oblique structures are only located proximal to the eastern valley wall they are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall asymmetric character such as nonuniform spacing and throw of faults was not observed these observations suggest that the asymmetry of the basin is not due to asymmetric spreading but rather to eastward ridge jumps of several kilometers such small amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin

basaltic lava domes lava lakes and volcanic segmentation on the southern east pacific rise meter scale dsl 120 sonar mapping and coregistered argo ii photographic observations reveal changes in eruptive style that closely follow the third order structural segmentation of the ridge axis on the southern east pacific rise 17 11 18 37 s near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology the ubiquity of pillow lava suggests low effusion rate eruptions the abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices near segment centers we observe fewer but larger lava domes voluminous drained and collapsed lava lakes and smooth lobate and sheet lava flows with very little pillow lava the abundance of sheet flows suggests that high effusion rate eruptions are common fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources this pattern was observed on eight third order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale the third order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across axis width of the region of seismic layer 2a thickening along the one segment where sufficient cross axis seismic lines exist the geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries although rock sampling density is not adequate to constrain the variation across all the third order volcanic segments that we identify these observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading 140 km myr full rate mid ocean ridges

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

morphological and compositional analysis of alluvial gold the fresnedoso gold placer spain abstract the fresnedoso creek gold placer fgp in the moraleja basin w iberian massif has been investigated in terms of morphological textural and compositional evolution of gold particles a mixture of two populations has been identified being coherent with primary sources located at a proximal 2 5 10 km and distal 20 50 km range shape and compositional evidences suggest recycling of paleoplacers tertiary has to be considered to some extend primary laminar morphologies point to lode deposits in small flat veins hosted in the metasediments of the schist greywacke complex all these features suggest the fresnedoso creek gold placer is a deposit constituted by with mono and polycyclic particles transport distance flattening indexes cfi shilo models have been tested and result in useful information for exploration of undiscovered deposits even with a limited dataset compositional analysis of gold particle morphotypes has revealed that the fresnedoso gold is a auag bimetallic alloy textural and compositional groups include type 1 au1 au88 94ag12 6 type 2 au2 au99 ag1 and type 3 au3 au99ag1 variations in composition of type 1 particle s cores could reflect differences in primary composition spatial dispersion of sources or secondary processes but fit within the compositional range of orogenic gold deposits secondary processes have been explored to explain compositional heterogeneity in the particles gold type 2 rim and 3 micro aggregates represents two different de alloying stages from initial ag leaching at the rim and or through grain boundaries and microfractures au2 to complete resetting of primary chemical imprint pervasive porous texture and au re precipitation with fe ooh and clays au3 a model is proposed which combines ag de alloying enhanced in a cl fe rich environment lateritic and the deformation recrystallization processes related to mechanical cold work in the river load bed during transport

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

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

distribution of isolated volcanoes on the flanks of the east pacific rise 15 3 s 20 s volcanic constructions not associated with seamount or volcano chains are abundant on the flanks of the east pacific rise epr but are rare along the axial high the distribution of isolated volcanoes based on multibeam bathymetric maps is approximately symmetric about the epr axis this symmetry contrasts with the asymmetries in the distribution of volcano chains more abundant on the west flank the seafloor subsidence rates slower on the west flank and the distribution of plate motion parallel gravity lineaments more prominent on the west flank most of the isolated volcanoes complete their growth within 14 km of the axis on crust younger than 0 2 ma while seamount chain volcanoes continue to be active on older crust volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities the volume of isolated near axis volcanoes correlates with ridge axis cross sectional area suggesting a link between the magma budget of the ridge and the eruption of near axis volcanoes within the study area off axis volcanic edifices cover at least 6 of the seafloor and contribute more than 0 2 to the volume of the crust the inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the mid atlantic ridge 4 km northern epr 20 km and southern epr 28 km so that isolated volcanoes form primarily on lithosphere younger than 0 2 ma 4 6 km brittle thickness independent of spreading rate this suggests some form of lithospheric control on the eruption of isolated off axis volcanoes due to brittle thickness increased normal stresses across cracks impeding dike injection or thermal stresses within the newly forming lithosphere

the fina nagu volcanic complex unusual submarine arc volcanism in the rapidly deforming southern mariana margin in the mariana convergent margin large arc volcanoes disappear south of guam even though the pacific plate continues to subduct and instead small cones scatter on the seafloor these small cones could form either due to decompression melting accompanying back arc extension or flux melting as expected for arc volcanoes or as a result of both processes here we report the major trace and volatile element compositions as well as the oxidation state of fe in recently dredged fresh pillow lavas from the fina nagu volcanic chain an unusual alignment of small closely spaced submarine calderas and cones southwest of guam we show that fina nagu magmas are the consequence of mantle melting due to infiltrating aqueous fluids and sediment melts sourced from the subducting pacific plate into a depleted mantle wedge similar in extent of melting to accepted models for arc melts fina nagu magmas are not as oxidized as magmas elsewhere along the mariana arc suggesting that the subduction component responsible for producing arc magmas is either different or not present in the zone of melt generation for fina nagu and that amphibole or serpentine mineral destabilization reactions are key in producing oxidized arc magmas individual fina nagu volcanic structures are smaller in volume than mariana arc volcanoes although the estimated cumulative volume of the volcanic chain is similar to nearby submarine arc volcanoes we conclude that melt generation under the fina nagu chain occurs by similar mechanisms as under mariana arc volcanoes but that complex lithospheric deformation in the region distributes the melts among several small edifices that get younger to the northeast

mineralogy and composition of kuroko deposits from northeastern honshu and their possible modern analogues from the izu ogasawara bonin arc south of japan implications for mode of formation the kuroko deposits of ne honshu are a key type deposit for the study of volcanogenic massive sulfide deposits however these deposits have not been studied in detail since the early 1980 s and knowledge of their mode of formation is now dated in this study we present the analysis of 12 samples of the kuroko deposits 12 samples of submarine hydrothermal minerals from the sunrise deposit and 6 samples from suiyo seamount both of which are located on the izu ogasawara bonin arc for 27 elements for the kuroko deposit cd sb ag pb hg as zn cu are highly enriched au te bi ba mo are moderately enriched in tl are somewhat enriched and fe is not significantly enriched relative to the average continental crust within each of these deposits a similar pattern of element associations is apparent zn pb with as sb cd ag hg tl and au fe cu ba with as sb ag tl mo te and au si ba with ag and au caso4 the enrichment of the chalcophilic elements in these deposits is consistent with hydrothermal leaching of these elements from the host rocks which are dominantly rhyolite dacite in the case of the kuroko deposits rhyolite in the case of the sunrise deposit and dacite rhyolite in the case of the suiyo seamount deposit however this pattern of element enrichment is also similar to that observed in fumarolic gas condensates from andesitic volcanoes this suggests that there may be a significant magmatic contribution to the composition of the hydrothermal fluids responsible for the formation of the kuroko deposits although it is not yet possible to quantify the relative contributions of these two sources of elements the compositional data show that sunrise and suiyo seamount deposits are much closer compositionally to the kuroko deposits from ne honshu than are the submarine hydrothermal deposits from the jade site in the okinawa trough which contain on average significantly higher concentrations of pb zn sb as and ag than each of these deposits in spite of the greater similarity in tectonic setting of the hokuroku basin in which the kuroko deposits formed to the okinawa trough intracontinental rifted back arc basin compared to myojin knoll and suiyo seamount active arc volcanoes it appears that submarine hydrothermal deposits from myojin knoll and suiyo seamount are closer analogues of the kuroko deposit than are those from the okinawa trough the present data are consistent with the magmatic hydrothermal model for the formation of kuroko type deposits as formulated by urabe and marumo urabe t marumo k 1991 a new model for kuroko type deposits of japan episodes 14 246 251

relationships between lava types seafloor morphology and the occurrence of hydrothermal venting in the ashes vent field of axial volcano deep towed and submersible photographic surveys within the caldera of axial volcano have been integrated with high resolution bathymetry to produce a geological map of the most active vent field axial seamount hydrothermal expeditions ashes in the caldera the ashes vent field encompasses an approximately 200 m 1200 m area of active venting which is located adjacent to the southwest caldera wall locations for over 2000 photographs in and near the vent field were determined using a seafloor transponder network then each photograph was described utilizing a classification system which provides detailed information concerning lava type hydrothermal activity sediment cover geological structure and biology resulting data were entered into a digital data base and computer generated maps were created that portray spatial relationships between selected geological variables in general the entire ashes field is characterized by pervasive low temperature venting the most vigorous venting is concentrated in an approximately 80 m 80 m area where there are several high temperature vents including some which are producing high temperature vapor phase fluids derived from a boiling hydrothermal system lava types within the ashes vent field are grouped into three distinct morphologies 1 smooth flat surfaced ropy and whorled sheet flows 2 lobate flows and 3 jumbled sheet flows the most intense hydrothermal venting is concentrated in the smooth sheet flows and the lobate flows the location of the ashes field is mainly attributable to faulting which defines the southwest caldera wall but the concentration of intense venting appears to be related also to the spatial distribution of lava types in the vent field and their contrasting permeabilities other structural trends of faults and fissures within the field also influence the location of individual vents

bending as a mechanism for triggering off axis volcanism on the east pacific rise we propose that bending stresses play a key role in triggering volcanism on the flanks of the east pacific rise by simultaneously opening tensile cracks near the surface and increasing the pore pressure of melt bodies trapped in the lower crust in addition crack tip stress intensities remain high during tensile opening such that bending cracks propagate downward efficiently and thus have the potential to tap overpressured melt bodies trapped in the lower crust we find that bending increases the vulnerability of the lithosphere to magmatic penetration out to distances of 20 km from the rise axis corresponding to the region of abyssal hill formation and isolated seamount generation

volatile element isotopes of submarine hydrothermal mineral deposits in the western pacific the abundance and isotopic compositions of volatile elements trapped in fluid inclusions of submarine hydrothermal mineral deposits in western pacific subduction zones okinawa trough izu bonin arc mariana trough and lau basin and in kuroko ores in northeastern japan are presented the helium isotopic compositions corrected for air contribution of the okinawa and mariana troughs ranging 4 49 7 68 ra are lower than those of the izu bonin and lau basin 7 62 8 91 ra this characteristic might reflect the differences in regional tectonic setting the okinawa and mariana troughs are related to back arc spreading with strong graben sedimentary signature whereas the izu bonin arc is associated with island arc magmatism the arc contribution to the lau basin volcanism is significantly strong even though it is assigned to back arc spreading nitrogen isotopes can also be explained by a similar hypothesis whereas argon and carbon isotopes cannot be used to discriminate tectonic setting 13c co2 3he and 15n n2 36ar diagrams elucidate the source of carbon and nitrogen the mor type mantle contributions to carbon are mostly smaller in the okinawa and mariana troughs ranging 0 06 8 9 with the average of 2 4 than in the izu bonin and lau basin 2 1 25 with the average of 7 7 the sedimentary contributions to nitrogen are larger in the okinawa and mariana troughs 11 65 with the average of 35 than in the izu bonin and lau basin 4 24 with the average of 15 and the kuroko samples agree well with the latter carbon and nitrogen fluxes are again higher in okinawa trough than in izu bonin arc

quantitative study of the deformation at southern explorer ridge using high resolution bathymetric data abstract we present preliminary results of a morphological study of the summit of the southern explorer ridge ser the ser is an inflated intermediate rate spreading center located in the northeast pacific off the west coast of canada that hosts a large hydrothermal vent complex known as magic mountain a quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high resolution near bottom bathymetric data these data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures the ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer populations of small faults exhibit an exponential size frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain 3 7 to 18 4 accommodated by the normal faults we propose that the 500 to 600 m wide and 60 m deep asymmetric axial summit graben of the ser formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben we find a well defined geographic distribution in the types of lava flows which indicates a general decrease of the eruption rate through time we also find that the magic mountain hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile transition depth this fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature

location of instruments on the seafloor by joint adjustment of instrument and ship positions the problem of determining accurate locations for arrays of underwater instruments is similar to the problem of locating local earthquakes by utilizing a limited number of receivers but is complicated by the fact that estimated errors in longitude and latitude of a given receiver ship position are nonzero and are typically correlated estimates of ship positions at sea are commonly determined by fixes from the navy transit navigation satellite system this system calculates position error ellipses which vary in size and orientation from fix to fix deviations in the orientations of the principal axes of the error ellipses from north south and east west result in the introduction of off diagonal elements in the ship position variance matrix tying each ship fix to a stationary array of ocean bottom seismometers obs s using acoustic ranging allows averaging of fixes thus solving simultaneously for ship and obs positions puts tight constraints on the system as a whole and especially on the relative positions of the obs s in addition to ship positions the data required to locate the instruments are two way travel times between the ship and transponders on the instruments and initial depth estimates of obs s read from the bathymetry the parameters which we solve for are the locations of the instruments including depth and of the ship at each fix the forward problem calculating acoustic travel times from distances between ship fixes and obs s is solved by a ray shooting algorithm this system of equations is linearized and transformed to diagonalize the covariances it is then solved iteratively by inverting the data jointly using singular value decomposition and step length damping the solution includes the covariances of the parameters we show calculations of positions of four obs s for deployments during three experiments including the rivera oceanic seismic experiment rose typical fix error ellipses 1 are 300 m by 200 m and resulting instrument position uncertainties are approximately 100 m relative instrument positions are accurate to about 50 m relative errors can be decreased to 10 to 20 m by using simultaneous rather than sequential ranging

mobility of gold and silver in lateritic weathering profiles some observations from western australia experimental evidence suggests that very acid chloride solutions generated in lateritic profiles by ferrolysis oxidation and hydrolysis of iron are responsible for the dissolution of gold and silver au is subsequently reprecipitated by reduction of the aucl 4 super ion with fe super 2 reduction of the agcl complex does not similarly occur because of the relative redox potentials of fe super 2 fe super 3 and ag agcl thus chemical refinement of gold occurs during lateritization the development of low ph and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences modified journal abstract

chemical and physical transfers in an ultramafic rock weathering profile part 2 dissolution vs accumulation of platinum group minerals the chemical weathering of ultramafic rocks has resulted in eluvial concentration of pt group minerals pgm in lateritic weathering profiles of southern new caledonia the pt mineralization interpreted as being primary consists of pt group minerals included within chromite crystals the oc currence of pgm as free particles in the weathering profile results from the supergene dissolution of pt bearing chromite traore et al 2008 following their release in the profile supergene dissolution processes variably affect the pgm particles the behavior of pt group elements in the weathering profile is characterized by significant loss of pd and relative enrichment of pt indicating that pd is more mobile than pt in the exogenous cycle unstable pt fe cu pd alloys and pge oxides undergo chemical and mineralogical changes to acquire the chemical configuration of isoferroplatinum pt3fe which is the most stable pt phase in a lateritic environment the isoferroplatinum phase may also be dispersed throughout the weathering mantle and or accumulated in the lower parts of profiles ac cording to a translocation mechanism of residual pt rich fine particles driven by percolation of water through the connected pore spaces