deformation of the long valley caldera eastern california from mid 1983 to mid 1988 measurements using a two color geodimeter deformation of long valley caldera in eastern california has been closely monitored since june 1983 with a two color geodimeter which has a precision between 0 12 and 0 20 ppm of the baseline length initial trilateration measurements were concentrated in the south moat of the caldera the source of the january 1983 earthquake swarm since then the network has been expanded from 13 to 42 baselines the additional baselines span the resurgent dome and the western half of the caldera with a combination of measurements made several times per week on 7 11 baselines and monthly or yearly measurements on the remaining baselines we have measured both the temporal and spatial deformation of the caldera although the m 6 4 chalfant valley earthquake was located 35 km from the nearest baseline in the network we observed coseismic strain changes of up to 1 5 ppm on several baselines using fault planes selected to coincide with the hypo centers of the main shock and the m 5 9 foreshock we have inferred a combined moment of 3 0 1025 dyn cm from the two color data this cumulative moment of both earthquakes is equivalent to a magnitude 6 3 shock over the period from mid 1983 to mid 1988 the long term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension during 1983 measurements show a maximum extension rate of 5 0 ppm yr after mid to late 1984 the extension rates decreased by a factor of 2 4 relative to those in the first year with the exception of the coseismic step the extension rates have been fairly constant during the interval between late 1984 and mid 1988 the long term changes in rate can be modeled by using two mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat modeling indicates that both the rates of slip and inflation have decreased since mid 1983 but these two processes are still occurring at a measureable and constant rate

volcanics and structure of the famous narrowgate rift evidence for cyclic evolution amar 1 a near bottom photographic survey on the mid atlantic ridge from the famous region south to transform b reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley extension wedges appear to be propagating south from transform a and north from transform b these merge at mt mars where the rift valley is narrowest and shallowest narrowgate giving the valley a symmetrical hourglass shape three major volcanic en echelon lineaments trend n10 e 10 oblique to the strike of the inner walls faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension this contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths whereas the large scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley using vector analysis the extension wedges are propagating at about 9 cm yr toward each other at this rate a narrow rift valley will be fully transformed into a wide rift in 0 64 m y

the origin of hydrothermal chlorite and anhydrite rich sediments in the middle okinawa trough east china sea abstract during the integrated ocean drilling program iodp expedition 331 five sites were drilled into the iheya north knoll hydrothermal system in the okinawa trough ot a back arc basin characterized by thick terrigenous sediment following up on the previous study by shao et al 2015 we present new mineralogical geochemical and sr nd isotope data to investigate the origin of the hydrothermal sediments and characterize the hydrothermal system the substrate at the iheya north knoll is dominated by pumiceous sediment and other volcanoclastic materials interbedded with hemipelagic terrigenous and biogenous sediments impermeable layers separate the hydrothermal sediments into distinct units with depth that are characterized by various assemblages of alteration materials including polymetallic sulfides sulfates chlorite and kaolinite rich sediments the rare earth elements rees and nd isotope data suggest that the chlorite rich and kaolinite rich layers primarily resulted from the alteration of pumiceous materials in different chemical and physical conditions kaolinite rich sediment likely reflects low ph and low mg concentration fluids while chlorite rich sediment formed from fluids with high ph and increased mg contents likely at higher temperatures the sr isotopic compositions of subsurface anhydrite reflect high seawater hydrothermal fluid ratios in the mid ot hydrothermal area compared with chlorite rich sediments from other sediment covered or felsic hosted hydrothermal systems the chlorite rich sediments in the mid ot are characterized by lower concentrations of al and fe but much higher y zr hf th and rees indicative of the distinct nature of the precursor rocks in this region

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

geochemical implications of rare earth element patterns in hydrothermal fluids from mid ocean ridges abstract rare earth element data are reported for vent fluids collected from eight deep sea hydrothermal areas including samples from sediment hosted and bare rock hydrothermal systems the morb normalized ree patterns from these diverse settings are remarkably similar to each other but distinctly different than seawater or basalt being lree enriched and possessing a strikingly positive eu anomaly the ree concentrations making up these patterns vary widely however both on a spatial and temporal basis when plotted against ionic radii these fluid trends are remarkably linear and show the same fractionation patterns exhibited by plagioclase during magma segregation this analysis suggests that the partitioning of the ree between fluid and rock during high temperature hydrothermal alteration of morb is dominated by crystochemical exchange during dissolution and recrystallization reactions at the exposed surfaces of plagioclase phenocrysts a comparison of ree ionic radii in eight coordination indicate that the partitioning of 3ree and 2eu are dominated by chemical substitution for ca 2 and sr 2 respectively these considerations predict that the total 3ree content of vent fluid varies in a systematic way during the transition from an rich morb plagioclase to ab rich hydrothermal plagioclase and that these parameters can be used as indicators of the average extent of alteration inside active hydrothermal systems

oxygen isotope systematics in a multiphase weathering system in haiti abstract physical and chemical partial dissolution techniques have been applied to a suite of young karst bauxite and laterite samples from the southern peninsula of haiti the 18o values have been obtained for nine mineral species and range from 1 0 for anatase rutile to 33 4 for authigenic quartz on one hand results for quartz calcite kaolinite gibbsite and boehmite compare favourably with 18o values predicted from accepted mineral water fractionation factors assuming modern temperatures 25 2 c and water 18o values 3 1 0 5 on the other hand the 18o values measured for anatase illite smectite chlorite smectite and some iron oxide samples do not compare favourably with predicted values departures from expected values for these minerals may be related to crystallization from water with a different 18o value than modern water an imprecise knowledge of some mineralwater fractionation factors under surficial conditions or nonattainment of isotopic equilibrium between mineral and water during formation partial dissolution techniques hold considerable promise for obtaining quantitative 18o values of individual minerals in single samples of complex fine grained regolith material

an assessment of source parameters of earthquakes in the cordillera of the western united states abstract source parameters for 50 earthquakes occurring between 1915 and 1988 have been used to examine the faulting characteristics of magnitude 5 5 to 7 8 earthquakes within the western u s cordillera an area extending from the eastern sierra nevada to the great plains and from northwestern montana to trans pecos texas source parameters used in this study include seismic information from first motion analyses body and surface waveform modeling geodetic studies and geological studies of surface faulting the principal results of this analysis include 1 all earthquakes occurred on faults dipping 38 or more and no evidence for listric of low angle planar faulting was found 2 all m 7 0 earthquakes occurred at depths 12 km and were composed of multiple subevents 3 most earthquakes 70 per cent had unilateral ruptures and 4 no individual subevent had a rupture length 21 km

recent state of stress change in the walker lane zone western basin and range province united states the nw to north trending walker lane zone wlz is located along the western boundary of the northern basin and range province with the sierra nevada this zone is distinguished from the surrounding basin and range province on the basis of irregular topography and evidence for both normal and strike slip holocene faulting inversion of slip vectors from active faults historic fault offsets and earthquake focal mechanisms indicate two distinct quaternary stress regimes within the wlz both of which are characterized by a consistent wnw 3 axis these are a normal faulting regime with a mean 3 axis of n85 9 w and a mean stress ratio r value r 2 1 3 1 of 0 63 0 74 and a younger strike slip faulting regime with a similar mean 3 axis n65 70 w and r values ranging between 0 1 and 0 2 this younger regime is compatible with historic fault offsets and earthquake focal mechanisms both the extensional and strike slip stress regimes reactivated inherited mesozoic and cenozoic structures and also produced new faults the present day strike slip stress regime has produced strike slip normal oblique slip and normal dip slip historic faulting previous workers have explained the complex interaction of active strike slip oblique and normal faulting in the wlz as a simple consequence of a single stress state with a consistent wnw 3 axis and transitional between strike slip and normal faulting maximum horizontal stress approximately equal to vertical stress or r 0 in both regimes with minor local fluctuations the slip data reported here support previous results from owens valley that suggest deformation within temporally distinct normal and strike slip faulting stress regimes with a roughly constant wnw trending 3 axis zoback 1989 a recent change from a normal faulting to a strike slip faulting stress regime is indicated by the crosscutting striae on faults in basalts 300 000 years old and is consistent with the dominantly strike slip earthquake focal mechanisms and the youngest striae observed on faults in plio quaternary deposits geologic control on the timing of the change is poor it is impossible to determine if there has been a single recent absolute change or if there is rather an alternating or cyclical variation in stress magnitudes our slip data in particular the cross cutting normal and strike slip striae on the same fault plane are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the wlz by wesnousky and jones 1994 the location of the wlz between the deep seated regional extension of the basin and range and the right lateral strike slip regional tectonics of the san andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone in early to mid tertiary time the wlz appears to have had a similarly complex deformational history in this case as a back arc or intra arc region accommodating at least part of the right lateral component of oblique convergence as well as a component of extension

stability of ilmenite and titanomagnetite in the presence of carbon dioxide a thermodynamic evaluation the stability of ilmenite and titanomagnetite in the presence of co2 gas was evaluated thermodynamically ilmenite with a composition ilm90hm10 breaks down to the assemblage siderite hematite anatase below 270 c at 1 000 bars and below 300 c at 2 000 bars titanomagnetite with a composition usp50mt50 breaks down to the same assemblage below 345 c at 1 000 bars and below 385 c at 2 000 bars during the low grade metamorphism of basaltic rocks a co2 partial pressure of only a few hundreds bars is sufficient to alter ilmenite and titanomagnetite to the assemblage siderite hematite anatase

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

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

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

geochemistry of hydrothermal fluids from the ultramafic hosted logatchev hydrothermal field 15 n on the mid atlantic ridge temporal and spatial investigation abstract mantle derived ultramafic rocks commonly occur on the seafloor at slow spreading axes and are tectonically emplaced along shear zones since the early 1990s a growing number of hydrothermal systems have been detected in ultramafic settings but chemical data for fluid compositions in active systems are still limited besides the logatchev field at 15 n on the mid atlantic ridge mar the only other active high temperature 300 c hydrothermal field known to be strongly influenced by ultramafics is the rainbow field at 36 n on the mar the field at logatchev consists of six active vent sites at about 3000 m water depth situated along a nw se trending line with distances of 50 200 m between the individual sites the vent sites were mapped in detail and re sampled during two cruises in 2004 and 2005 using a rov the geochemical composition of the hydrothermal fluids is characterized by very high concentrations of dissolved methane and hydrogen up to 3 5 mm and 19 mm respectively related to serpentinization processes in the reaction zone together with moderate si concentrations of 9 mm a depletion in b compared to seawater and li concentrations lower than in basaltic systems this fluid composition has been identified as characteristic signature of high temperature hydrothermal fluids reacting with ultramafic rocks however additional alteration of gabbroic intrusions is likely the fluid composition is very similar at all vent sites indicating a common source in the reaction zone and little variation during upflow spatial differences in fluid composition were observed between smoking craters and the complex chimney system irina ii but are restricted to elements with strong temperature controlled solubility as cu and co these differences can be related to different exit temperatures up to 350 c and as it is known that more ultramafic hosted hydrothermal systems exist along slow spreading ridges this type of hydrothermal systems might have a significant influence on the elemental budget of the oceans detailed information about the fluid geochemistry and its spatial and temporal variations is an important prerequisite for an estimation of elemental fluxes in ultramafic hosted systems and their relative importance compared to basaltic hosted systems

characteristics and possible origin of native aluminum in cold seep sediments from the northeastern south china sea abstract although native aluminum al has been reported to occur in various geological settings for more than 20 locations but its mechanism of formation still remains to be elucidated we report the occurrence and characterization of al particles recovered from the surface sediment cf4 and a short core sediment rov g 37 cm length obtained at cold seeps in the northeastern continental slope ncs of the south china sea scs x ray diffraction analysis shows that the collected particles are metallic aluminum with the unit cell edge a of 4 059 0 005 a cf4 2 and 4 029 0 004 a t26 28 2 rov g the al particles occur as spherules irregular plates and elongated forms with typical lamellar structures their chemical compositions are 95 07 99 84 al the average values is 98 42 with very small amounts of si fe ti s zn mg ca k na cu co and p and are similar to al particles from the east pacific rise and the central indian basin but differ markedly to those from other locations after ruling out several possibilities of al sources and mechanism of formation an alternative cold seep mechanism is proposed for the origin of al in the scs during the processes of anaerobic oxidation of methane aom and pyrite formation high h2s and h2 partial pressures result in strong reducing micro environments under which al oh 4 is reduced to its metallic state by the microbial bacterial processes

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

source mechanisms of three moderate california earthquakes of july 1986 abstract we studied the source processes of three moderate size earthquakes in california the m s 6 0 event of july 8 1986 near palm springs the m s 5 8 shock of july 13 1986 offshore of oceanside and the m s 6 2 event of july 21 1986 in the chalfant valley on the east side of the sierra nevada the centroidal source parameters are estimated by inversion of long period p and sh waves recorded at teleseismic distances by seismographs of worldwide networks a more detailed description of the source is then obtained by analyzing broadband seismograms the north palm springs earthquake had a simple rupture process most of the seismic moment was released in the first 3 sec in a single step the strike of the source fault plane follows the average trend of the banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks the faulting mechanism of the main shock is right lateral strike slip with a sizable reverse component that is consistent with uplift north of the banning fault the earthquake offshore of oceanside had a slightly more complex rupture process the far field source time function consists of two pulses that represent the release of most of the moment in the first 3 sec the source mechanism indicates reverse faulting with the compressional axis trending northeast almost perpendicular to the trend of mapped faults offshore the event does not appear to be associated with major mapped faults in the area but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike slip fault systems and the idea of a weak san andreas fault system the rupture process of the chalfant valley earthquake was yet more complicated the source time function indicates that in a 4 sec time window three asperities were broken the source mechanism indicates right lateral strike slip faulting with a small normal faulting component on a fault that dips approximately 50 southwest earthquake activity suggests that presently the deformation in the northern part of the owens valley which includes the chalfant valley is dominated by conjugate strike slip faults which strike obliquely to the main range bounding normal faults the waveforms indicate the presence of a low velocity zone at a depth of about 20 km in the crust in the chalfant valley a synthesis of information from waveform modeling first motion fault plane solutions and aftershock distributions indicates that the nucleation and or termination of rupture for the north palm springs and the chalfant valley events were controlled by preexisting fault geometry

distribution speciation and bioavailability of arsenic in a shallow water submarine hydrothermal system tutum bay ambitle island png shallow water hydrothermal vent systems can introduce large amounts of potentially toxic elements such as arsenic as into coastal marine environments the first step in understanding and describing the potential impact of these elements throughout hydrothermally influenced coastal ecosystems is to determine the element s distribution and speciation which in turn influences the availability of the toxin for biological uptake shallow submarine hot springs near ambitle island papua new guinea are discharging as much as 1 5 kg per day of arsenic directly into a coral reef ecosystem we have investigated the bioavailability of the as throughout tutum bay by studying vent fluid seawater pore water precipitates and sediments in addition to measuring as abundance as speciation as iii as v and the methylated species dma and mma was determined in various waters the as concentration for discrete mineral phases in vent precipitates and sediments was determined by sequentially extracting arsenic from the easily extractable carbonate fe oxyhydroxide or hydrous ferric oxide hfo and residual fractions each of which have a different bioavailability diffuse venting seems to play a critical role on the distribution of as throughout tutum bay surface sediments which have a mean as concentration of 527 ppm while excluding the vent precipitates range 1483 to 52 ppm up to 54 ppm as were extracted from the easily extractable fraction of surface sediments mean 19 7 ppm using a k2hpo4 kh2po4 buffer at ph 7 2 arsenic from this fraction is considered to be the most available for biological processes and therefore the most dangerous for biota however sequential extraction shows that 98 6 of the as in vent precipitates and a mean of 93 3 in surface sediments range 88 2 to 96 3 is coprecipitated with the hydrous ferric oxide hfo fraction thus the bulk of the as being discharged into tutum bay is scavenged by the hfo and should remain stable unless the physicochemical conditions surrounding the oxides change in surface seawaters of tutum bay we found as much as four times the average seawater concentration of as 8 4 ag l compared to 2 ag l the abundance of as in seawater just above the sediment water interface is near normal although as iii in both surface and bottom seawater throughout tutum bay is substantially enriched compared to average seawater hydrothermal venting therefore provides bioavailable as by two major pathways throughout tutum bay 1 easily exchangeable as from hydrothermally influenced sediments to as far away as 200 m from focused venting and 2 in surface seawaters which may allow for biological uptake by phytoplankton and transfer up the food web d 2005 elsevier b v all rights reserved

seafloor hydrothermal clay alteration at jade in the back arc okinawa trough mineralogy geochemistry and isotope characteristics seafloor hydrothermal activity at jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs forming mica kaolins kaolinite and halloysite mg rich chlorite talc montmorillonite and a mixed layer mineral of dioctahedral chlorite and montmorillonite chl mont clay mineral assemblages show a vertical variation which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff however mixing alone cannot explain the occurrence of abundant kaolin minerals at jade the formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater low ph values are likely attained by oxidation of h2s either dissolved in the hydrothermal fluid or released from the fluid during decompression the fluid reaching the seafloor is discharged into cold seawater which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas halloysite barite and anhydrite show sr isotope compositions similar to marine sr indicating the derivation of marine sr directly from seawater or by the dissolution of calcareous nannoplanktons the isotopic compositions of kaolinite 18o 7 4 d 23 chl mont 18o 7 0 d 32 and mica 18o 5 4 to 9 9 d 30 to 26 suggest fluids of a heated seawater origin the o isotopic data yielded formation temperatures of 170 c for kaolinite 61 to 110 c for halloysite and 145 to 238 c for mica barite 34s values 21 0 to 22 5 are very similar to the marine sulfate value confirming that the barite formation took place due to mixing of ba bearing hydrothermal fluids and sulfate rich seawater native sulfur shows a large variation in 34s in one hand specimen probably because of rapid disequilibrium precipitation of s during fluid exhalation on the seafloor sulfur in hydrothermal fluids is usually consumed to form metal sulfides therefore abundant native sulfur at jade suggests high h2s metals ratios of the hydrothermal fluids the alteration assemblages and isotopic data of hydrothermal minerals from jade are very similar to those of kuroko type barite deposits of middle miocene age which formed from fluids of high s metals ratios at less than 200 c at jade there is only one black smoker actively discharging high temperature 320 c fluid but there are many fossil sulfide chimneys and mounds in the area the mineralogy and high au and cu in these precipitates suggest highly metalliferous hydrothermal activity in the past these activities likely resulted in discharge of hydrothermal plumes and fall outs of sulfides and sulfates on the seafloor these fall outs were incorporated in sediments far from the vent areas they are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in situ hydrothermal activity some are high as 8 100 ppm for cu 12 500 ppm for zn 1 000 ppm for as 100 ppm for ag and 21 000 ppm for pb detrital grains of montmorillonite in such sediments are coated with fe oxyhydroxides during the suspension in seawater before settling on the seafloor the depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1 800 to 300 ybp

gases and helium isotopes in high temperature solutions sampled before and after odp leg 158 drilling at tag hydrothermal field 26 n mar hydrothermal fluids were sampled for dissolved gases at tag 26 n mar during two alvin dive series in april may 1993 and march 1995 respectively 17 months before and 4 months after ocean drilling program leg 158 total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude even if some increase in h2s c02 and ch4 was noticeable in the 1995 samples no significant difference was observed in helium concentration and helium isotopic ratio 3 he 4 he 8 2 0 1 ra the ch4 3 he ratio found in black smokers sampled in 1993 is around 9 x 10 6 close to ratio found in mar basalts in 1995 this ratio is around 4 times higher due to the ch4 increase the 13 c in c02 measured on two black smokers sampled in 1993 is uniform at 8 4 to 8 8 versus pdb while a 13 c value of 13 is measured at a new site sampled in 1995 located 40 meters east of the main black smokers complex bsc ch4 3 he and 13 c values both point to a mainly magmatic abiogenic origin of the carbon species in the system overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium c02 and ch4 of magmatic origin even if some thermogenic ch4 contribution seems likely in the 1995 samples collected in the new active area we can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the tag mound and temporarily enhanced the hydrothermal circulation by opening new pathways favouring transient input of ch4 rich fluids before coming back to a new steady state

native aluminium spherules and particles in the central indian basin sediments implications on the occurrence of hydrothermal events abstract we have sampled particles of native aluminium al in two sediment cores from the central indian basin cib the cores are geographically separated but are located at the base of two seamounts the native al particles occurring as grains and spherules have an average al content of 95 and are associated with volcanogenic hydrothermal material morphologically and compositionally the specimens are similar to those reported from the east pacific rise after ruling out several processes for the presence of the native al we hypothesize that during progressive melting of magma a basaltic magma is produced which has high contents of reductants such as methane and hydrogen and a low oxygen fugacity during the upward migration of such magma reduction to metallic aluminium and the formation of native al particles takes place

compositions growth mechanisms and temporal relations of hydrothermal sulfide sulfate silica chimneys at the northern cleft segment juan de fuca ridge three active hydrothermal vents forming sulfide mounds and chimneys monolith fountain and pipe organ and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern cleft segment juan de fuca ridge the formation of zoned tubular curich chimneys type i on the monolith sulfide mound is related to focused flow of high temperature to 328 c fluid bulbous chimneys type ii or beehives at the monolith and fountain vents are products of diffuse high temperature to 315 c discharge a broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite rich shells columnar zn sulfide rich chimneys with narrow channelways type iii are constructed where focused and relatively low temperature 261 c fluid vents directly from the basalt substrate the bulk chemistry low cu high pb ag and sio2 contents mineralogy pyrite marcasite wurtzite amorphous silica anglesite colloform and filamentous textures and oxygen isotope characteristics of inactive type iv chimneys indicate a low temperature 250 c origin involving diffuse and sluggish flow patterns and conductive cooling seafloor observations and 210pb data indicate that 1 type iv chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption 2 the high temperature monolith and fountain vents are manifestations of the same heating event shallow emplacement of magma that led to the sheet flow eruption and recent megaplumes and 3 the pipe organ vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption

genesis of kaolin minerals and pyrophyllite in kuroko deposits of japan implications for the origins of the hydrothermal fluids from mineralogical and stable isotope data hydrogen and oxygen isotope and x ray analyses were performed on kaolin minerals kaolinite dickite and nacrite and pyrophyllite from the minamishiraoi and inarizawa deposits of northern japan dickite nacrite and mica rich micasmectite mixed layer minerals occur as envelopes around barite sulfide orebodies in the footwall alteration zones of the minamishiraoi and inarizawa deposits while kaolinite and smectite rich micasmectite mixed layer minerals occur in the outer alteration zones away from the orebodies the hydrogen and oxygen isotope ratios of kaolin minerals from the minamishiraoi and inarizawa deposits fall in a narrow range of 24 to 32 for hydrogen and 7 8 to 9 9 for oxygen these isotope ratios indicate that the hydrothermal fluid responsible for kaolin formation was the same as that which caused mineralization the 18o values of kaolin minerals of the minamishiraoi deposit increase in order of nacrite dickite kaolinite this trend indicates that nacrite formed at the highest temperature among the three kaolin polytypes the oxygen isotope ratio of monoclinic pyrophyllite 18o 5 2 from the inarizawa deposit is lower than that of nacrite 18o 7 5 the difference of 18o values between the pyrophyllite and nacrite may be explained by a higher temperature of formation for the pyrophyllite the mineralogie and isotopic data of the footwall alteration halos are useful as indicators of the size and metal contents of kuroko orebodies the footwall alteration halo of the minamishiraoi deposit small size and barite rich deposit contains kaolin and micasmectite mixed layer minerals while those of the larger polymetallic kuroko deposits contain seriate and chlorite thermodynamic data of these clay minerals indicate that the minamishiraoi deposit formed at relatively lower temperature and farther from the heat source than larger polymetallic deposits the chemical data for hydrothermal solutions from the 13 n east pacific rise hydrothermal site as presented in the literature plot in the kaolinite stability field at 150 c or less therefore active submarine hydrothermal systems such as 13 n east pacific rise should form kaolin minerals in the alteration halo as long as the temperature of the hydrothermal fluid is below sericite and chlorite stability

native aluminium in metalliferous sediments from the east pacific rise axial zone 21 s abstract an endogenous model of the formation of small amounts of 100 m metallic aluminium particles from the metalliferous sediments of the east pacific rise 21 s is proposed on the basis of a detailed study of their morphology chemical composition x ray data spatial and temporal distribution according to this model native aluminium is formed as an accessory mineral during the process of the pre chamber evolution of basic and ultrabasic systems in the spreading zones under high p t and low o 2 conditions as well as during the later serpentinization of basites and ultrabasites from the second and third oceanic layer as a result of the active tectono magmatic processes in the rift zones the native aluminium particles are carried upwards into the seafloor water layer by the upwelling vent fluids of the hydrothermal circulation cells during periods of high hydrothermal activity the number of al particles carried upwards increases the spatial distribution of the particles depends on the energy of the hydrothermal system and the direction and energy of the seafloor currents the thin oxyhydroxide silicate and chloride crusts formed on the surface of the al particles as well as a monoatomic oxide film prevent further alteration of the particles

ree controls in ultramafic hosted mor hydrothermal systems an experimental study at elevated temperature and pressure a hydrothermal experiment involving peridotite and a coexisting aqueous fluid was conducted to assess the role of dissolved cl and redox on ree mobility at 400 c 500 bars data show that the onset of reducing conditions enhances the stability of soluble eu 2 species moreover eu 2 forms strong aqueous complexes with dissolved cl at virtually all redox conditions thus high cl concentrations and reducing conditions can combine to reinforce eu mobility except for la trivalent ree are not greatly affected by fluid speciation under the chemical and physical condition considered suggesting control by secondary mineral fluid partitioning lree enrichment and positive eu anomalies observed in fluids from the experiment are remarkably similar to patterns of ree mobility in vent fluids issuing from basalt and peridotite hosted hydrothermal systems this suggests that the chondrite normalized ree patterns are influenced greatly by fluid speciation effects and secondary mineral formation processes accordingly caution must be exercised when using ree in hydrothermal vent fluids to infer ree sources in subseafloor reaction zones from which the fluids are derived although vent fluid patterns having lree enrichment and positive eu anomalies are typically interpreted to suggest plagioclase recrystallization reactions this need not always be the case

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

helium and methane measurements in hydrothermal fluids from the mid atlantic ridge the snake pit site at 23 n methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the snake pit area mar 23 n as part of the hydrosnake cruise with the submersible nautile the samples contain large quantities of helium and methane with endmember values of 4 6 10 5 cm3 stp g for helium and 1 38 10 3 cm3 stp g for methane the helium isotopic ratio 3he 4he is 8 4 times the atmospheric ratio these values are strikingly similar to those already obtained on the east pacific rise the ch4 3he ratio is also quite similar ch4 3he 2 6 106 suggesting a non biogenic origin of this gas in the fluids assuming that the range of variation of the 3he concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid ocean ridges the global mass flow involved in the hydrothermal circulation at the ridge s axis is estimated between 1000 and 3000 m3 of water sec leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years this estimate implies that in addition to the individual vents smokers there is an extensive diffuse circulation of hydrothermal fluids along the ridges in agreement with recent observations concerning heat budgets at hydrothermal sites in the eastern pacific

geologic setting of hydrothermal activity at the northern gorda ridge abstract the northern end of the northern segment of the rift valley of the gorda ridge was investigated with sea beam bathymetry seamarc ia and ii side scan sonar surface towed magnetics and near bottom camera temperature tows to determine the geologic setting of large chemical anomalies in the water column indicative of intense hydrothermal discharge the magnitude of the water column anomalies is comparable to that associated with the high temperature sea cliff hydrothermal field situated on the east wall of the rift valley 25 km to the south water column andseafloor hydrothermal indicators and tectonic trends converge at a seamount at 42 57 3 n 126 33 0 w situated 4 5 km east of the rift valley axis on a terrace as the most likely source area for the hydrothermal discharge comparison with the geologic setting of the sea cliff hydrothermal field confirms the presence of an anomalous axis oblique 340 357 tectonic trend that permeates the structure of the northern gorda ridge additional sinistral offset along an axis perpendicular structural trend 292 at the sea cliff field indicates a southward increase in deformation of the northern gorda ridge along axis transverse trends

revised dislocation model of the 1986 chalfant valley earthquake eastern california abstract a revised dislocation model is proposed to describe the 1986 chalfant valley rupture leveling data from surveys across the epicentral area are inconsistent with any appreciable dip slip component in the oblique slip model that we had proposed earlier the revised model involves right lateral slip on the northwest trending main rupture surface and left lateral slip on the conjugate foreshock rupture surface

the formation of high temperature clay minerals from basalt alteration during hydrothermal discharge on the east pacific rise axis at 21 n abstract aluminous high temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the east pacific rise at 21 n the clay alteration assemblages are layered crusts up to 1 mm thick completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids the interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of morb the surficial alteration crusts are mixtures of beidellitic smectite aluminous dioctahedral randomly interstratified mixed layer al rich chlorite smectite minor chlorite an x ray amorphous aluminosilicate material and possible minor serpentine amesite a 18 o value of 4 1 0 2 smow is determined for the beidellitic smectite assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater 0 and 350 c hydrothermal fluid from epr 21 n vents 1 6 an equilibration temperature between 290 c and 360 c is calculated for the beidellitic smectite this is substantially higher than any previously reported temperature for an oceanic smectite the mixed layer al rich chlorite smectite has a 18 o value of 3 5 which corresponds to equilibration at 295 360 c the aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration we suggest that this assemblage is largely the product of high temperature interaction between basalt glass plagioclase and mg poor acidic hydrothermal fluids with possibly some contribution of mg from bottom seawater and that the aluminous clays either incorporate al 3 remobilized from basalt by lowph hydrothermal fluids or are residual phases remaining after intense alteration of basaltic glass plagioclase

occurrence of kaolinite and mixed layer kaolinite smectite in hydrothermal sediments of grimsey graben tjornes fracture zone north of iceland the sediments at a site situated among high temperature vents in the grimsey graben tjornes fracture zone north of iceland exhibit features of strong hydrothermal alteration 1 almost total dissolution of the volcaniclastic material composing the background sediment 2 sulfate and sulfide precipitation 3 kaolinitisation smectite precipitated in the shallowest sediment is gradually replaced downward by mixed layer kaolinite smectite and pure well crystallised kaolinite the kaolinite smectite is interstratified with up to 10 swelling smectitic layers according to the oxygen isotope composition kaolinite smectite mixed layer mineral most likely formed at temperatures near 160 degrees c the vertical sequence kaolinite kaolinite smectite smectite as well as the distinct zonation across the kaolinitic veins almost pure kaolinite in the central zone and kaolinite smectite along the rim suggest hydrothermal transformation of initially formed smectite kaolinite smectite kaolinite most probably this conversion occurred in an evolving from alkaline to slightly acidic hydrothermal environment c 2004 elsevier b v all rights reserved

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

oxygen isotope geothermometry and origin of smectites in the atlantis ii deep red sea abstract the sediments underlying the hot brine pool of the atlantis ii deep a localised area of geothermal activity in the red sea comprise a diversity of facies characterised by combinations of one or more of five species assemblages sulphide sulphate silicate oxide and carbonate each including several mineral phases the silicate mineral assemblage is dominated by geothermal authigenic smectites previous studies of these smectites have reported iron rich varieties only nontronite in particular and only one environment of formation in three cores from the southwest basin of the atlantis ii deep of the present study three smectites comprising two species have been distinguished 10 21 evidently from three different environments of formation two of these smectites are nontronites one from sulphide silicate amorphous facies the other from silicate carbonate oxide facies the third is a montmorillonite beidellite from sulphate sulphide silicate oxide facies the oxygen isotopic compositions of samples of the three smectites have been determined from which formation temperatures have been calculated six samples of the anoxic nontronite have formation temperatures in the range 90 140 c a single sample of the oxic nontronite has a formation temperature of about 80 c four samples of the montmorillonite beidellite have formation temperatures in the range 160 200 c the formation temperature range of the two nontronites is intermediate between the temperature of the brine at or prior to discharge up to 250 c 12 and the temperature of the brine pool in the deep about 50 60 c 13 14 the nontronite formation temperature range reflects genesis by combination of isotopically light silica supplied by the incoming brine and isotopically heavier iron oxyhydroxide settling from the upper layers of the brine pool evidently the anoxic nontronite forms at greater depth hotter in the brine pool than the oxic nontronite resulting in a relatively greater contribution from silica but diminished contribution from iron oxyhydroxide in the former compared to the latter the wide range of the formation temperatures for the anoxic nontronite is related to the different actual locations of the samples in the sulphide silicate amorphous facies the formation temperature range of the montmorillonite beidellite is approaching the estimated temperature of the brine at or prior to discharge the montmorillonite beidellite formation temperature range reflects genesis by combination of isotopically light silica and aluminium both supplied by the incoming brine at the site of an active discharge vent the wide range of the formation temperatures for the montmorillonite beidellite may in part reflect a possible thermal event at the brine source likely to have occurred during deposition of the sulphate sulphide silicate oxide facies and which it appears has contributed to the formation of this facies 10 20

mineralogical studies of sulfide samples and volatile concentrations of basalt glasses from the southern juan de fuca ridge sulfide samples obtained from the u s geological survey s dsrv alvin dives on the southern juan de fuca ridge closely resemble those from the same area described by koski et al 1984 major minerals include sphalerite wurtzite pyrite marcasite isocubanite anhydrite and chalcopyrite equilibrium if attained at all during deposition of most sulfides was a transient event over a few tens of micrometers at most and was perturbed by rapid temperature and compositional changes of the circulating fluid two new minerals were found one a hydrated zn fe hydroxy chlorosulfate and the other a mn mg fe hydroxide or hydroxy hydrate both were formed at relatively low temperatures lizardite starkeyite and anatase were found for the first time in such an environment sulfide geothermometry involving the system cu fe s indicates a vent temperature of 328 c for one sample fluid inclusion studies on crystals from the same vicinity of the same sample give pressure corrected homogenization temperatures of 268 and 285 c ice melting temperatures on inclusions from the same sample are about 2 8 c indicating that the equivalent salinity of the trapped fluid is about 50 greater than that of seawater volatile concentrations from vesicle free basalt glass from the vent field are about 0 013 wt co2 and 0 16 wt h2o co2 contents in these samples yield an entrapment depth of 2200 m of seawater which is the depth from which the samples were collected

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

hydrothermal alumina rich clays and boehmite on the gorda ridge abstract hydrothermal alteration of basalt on the northern gorda ridge produced an alteration crust up to 1mm thick that can be divided into two layers based on mineralogy an inner layer next to the basalt surface is an al rich interlayered smectite chlorite similar to high temperature al rich clays recovered from the east pacific rise 21 n with minor quartz zeolite phillipsite and relict plagioclase an exterior layer of boehmite alo oh and minor anatase results from severe leaching of the underlying clays the alteration crust is progressively enriched in al ti v and cr from interior to exterior by residual weathering the presence of boehmite and absence of a silica rich phase in the exterior of the crust indicates the hydrothermal fluids were silica undersaturated or silica was maintained in solution by extremely acidic fluids this constitutes the first reporting of boehmite as a major alteration mineral in a mid ocean ridge hydrothermal environment

structure of hydrothermal plumes at the logatchev vent field 14 45 n mid atlantic ridge evidence from geochemical and geophysical data abstract in the seventh cruise of r v professor logatchev anomalies of natural electric field ef eh and ps were discovered using a towed instrument package rift at 14 45 n on the mar logatchev hydrothermal field the anomalous zone az is situated close 10 35 m to two low temperature venting areas of degrading sulphides and a black smoker irina microsmoke forming a distinct buoyant plume over or close to the main area of high temperature venting situated to the south east from the az no ef or eh anomalies were observed according to the results of mir dives the highly mineralised solutions from smoking craters at the main mound mostly form non buoyant plumes reverse plumes the buoyant plume structure shows the differentiation of the electrical and eh fields within the plume maxima of the ef eh and e h 2 s anomalies were revealed in the lower part 15 m of the plume the negative redox potential plume coupled with a sulphide anomaly is more localized in comparison with the ef this observation indicates a distinct change in the composition of buoyant plume water which may be due to the formation and fallout of early formed fe sulphide particles soon after venting

direct observation of the evolution of a seafloor black smoker from vapor to brine a single hydrothermal vent f vent occurring on very young crust at 9 16 8 n east pacific rise was sampled in 1991 and 1994 in 1991 at the measured temperature of 388 c and seafloor pressure of 258 bar the fluids from this vent were on the two phase curve for seawater these fluids were very low in chlorinity and other dissolved species and high in gases compared to seawater and most sampled seafloor hydrothermal vent fluids in 1994 when this vent was next sampled it had cooled to 351 c and was venting fluids 1 5 times seawater chlorinity this is the first reported example of a single seafloor hydrothermal vent evolving from vapor to brine the 1991 and 1994 fluids sampled from this vent are compositionally conjugate pairs to one another these results support the hypothesis that vapor phase fluids vent in the early period following a volcanic eruption and that the liquid phase brines are stored within the oceanic crust and vent at a later time in this case 3 years these results demonstrate that the venting of brines can occur in the same location in fact from the same sulfide edifice where the vapor phase fluids vented previously

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

trace metals in hydrothermal solutions from cleft segment on the southern juan de fuca ridge concentrations of trace metals in fe and cl rich hydrothermal solutions from the southern juan de fuca ridge sjfr have been determined and corrected for residual precipitates formed in the sampler precipitate corrections for ag cd cu mo pb sb and zn were variable contributing an average of 20 to total solution concentrations whereas corrections for co averaged less than 5 and essentially no corrections were required for fe and mn values for cu co and mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320 c in addition and unlike most other metals studied all vent fluids from the sjfr were almost completely depleted in mo relative to seawater values of about 110 nmol kg 1 in contrast to the cu co mo group concentrations of ag cd sb and pb correlated well with those for zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332 c range encountered in this study and more by the combined chemical processes that control zn levels along the transport pathway from the deep reaction zone to the vent orifice

deformation near the long valley caldera eastern california 1982 1986 annual surveys of trilateration and leveling networks in and around long valley caldera in the 1982 1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right lateral strike slip on a vertical fault in the south moat of the caldera the rate of inflation of the magma chamber seems to have been roughly constant 0 02 km3 yr in the 1982 1985 interval but the slip rate on the south moat fault has decreased substantially in addition there is evidence for a shallow source of dilatation possibly dike intrusion beneath the south moat of the caldera in 1983 and less certain evidence for a deep source possibly magma chamber inflation beneath mammoth mountain in the western caldera in 1983 1985 deformation in the 1985 1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984 1985

mineralogy and geochemistry of hydrothermal sediments from the serpentinite hosted saldanha hydrothermal field 36 34 n 33 26 w at mar abstract the saldanha hydrothermal field is located at the top of a serpentinized massif mount saldanha ms at a non transform offset nto5 along the mid atlantic ridge mar south of the azores it is one of the rare known sites on a worldwide basis where direct evidence of low temperature 7 9 c hydrothermal activity has been provided by direct observation of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover this study focuses on the mineralogy and geochemistry of 14 sediment cores collected at ms for comparison four samples collected at the rainbow site nto6 were also studied mount saldanha hydrothermal sediments are highly diluted within a dominant foraminiferal nanofossiliferous ooze with small fragments of underlying rocks the mineral assemblage of the hydrothermal component is characterized by sulphides nontronite smectites poorly crystallized mn oxyhydroxides and amorphous material cu zn and fe sulphides mn mg oxy hydroxides and putative manganobrucite were also identified in one sample collected at an orifice vent in this sample micro chimneys conduits composed of isocubanite and sphalerite were also identified mount saldanha sediments show a clear enrichment in elements such as mn mg fe cu p and v derived from hydrothermal fluids and ni cr and co derived from ultramafic rocks the geochemical data together with the observed mineral assemblage suggest that the hydrothermal fluids are at a higher temperature than those measured at the escape orifices 7 9 c and a strong enrichment in mg mainly at the top of the mount agrees with extensive mixing of the hydrothermal fluid with unmodified seawater nevertheless the mineral assemblage of ms sediments is consistent with the precipitation from hydrothermal fluids at much lower temperatures than at rainbow the presence of serpentinized and steatitized talcshist ultramafic rocks and the occurrence of a strong methane anomaly within the overlying water column collectively suggest that the hydrothermal circulation at ms is driven by exothermic reactions closely associated with the serpentinization process rainbow sediments have a higher concentration in transition metals and consequently an enrichment in sulphides these differences are likely to be a consequence of the higher temperature of hydrothermal fluids reflected in the composition of hydrothermal solutions and of a stronger hydrothermal flux at the rainbow site

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

gravity magnetic and high precision relocated seismicity profiles suggest a connection between the hayward and calaveras faults northern california 1 gravity magnetic and seismicity data profiled across the hayward fault zone were generated as part of ongoing studies to help determine the geologic and tectonic setting of the san francisco bay region these data combined with previous geophysical studies that indicate that the hayward fault zone dips 75 ne near san leandro and follows a preexisting structure reveal a possible direct connection between the seismogenic portion of the hayward and calaveras faults at depth although the relocated seismicity data are regional in nature they suggest that the dip of the hayward fault zone may vary from near vertical in the northwestern part of the fault to about 75 ne at san leandro in the central part of the fault to about 50 ne in the southeastern part of the fault gravity and magnetic data profiled across the hayward fault zone were processed using standard geophysical techniques cross sections of high precision relocated hypocenters were constructed along each profile from the northwestern to the southeastern end of the hayward fault zone profiles and cross sections are referenced to pinole point where the hayward fault enters san pablo bay and are spaced 2 5 km apart topographic profiles shown on the seismicity cross sections were generated using u s geological survey usgs 7 5 min 30 m digital elevation models relocation of seismicity data was accomplished using a regional double difference method the double difference method incorporates ordinary travel time measurements and cross correlation of p and s wave differential travel time measurements relative locations between earthquakes have hypocentral errors of about 100 m horizontally and 250 m vertically absolute location uncertainties were not determined but are probably dramatically improved compared to the usgs s northern california seismic network catalog data

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed