exceptionally fast growth rate of 100 yr old tufa big soda lake nevada implications for using tufa as a paleoclimate proxy large tufa mounds 3 m tall with a basal circumference of 5 m have been discovered on the margin of big soda lake nevada usa these tufa mounds are rooted at a maximum of 4 m below the current lake surface and are actively forming from groundwater seepage which can be seen emanating from the top of the tufa mounds big soda lake is a volcanic crater lake whose water level is maintained exclusively by groundwater the age of the tufa mounds is well constrained because prior to the development of the newlands irrigation project in 1907 the water level was 18 m lower than the current lake level the vertical columnar nature of the tufa mounds indicates that they formed under the lake and not subaerially thus the tufa mounds are 100 yr old and have grown at a rate 30 mm yr stable oxygen and carbon isotope analyses of tufa carbonate compared to isotopic analyses of groundwater and lake water and hydrochemical data indicate that the fluids responsible for their precipitation are a simple mixture of modern groundwater and lake water and do not reflect a recent climate signature the exceptionally fast growth of the tufa mounds indicates that large tufa deposits may form almost instantaneously in geologic time given this potential for rapid growth and the fact that variations in isotopic compositions of tufa deposits have been interpreted in terms of changes in paleoclimate and changes in the composition of recharge water over thousands of years care should be taken when trying to determine the significance of variations in isotopic or chemical compositions of tufas that may have been caused by mixing with groundwater

rare earth patterns with positive cerium anomaly in alkaline waters from lake van turkey abstract the distribution of rare earth elements has been studied for aerobic alkaline ph 9 6 carbonate rich waters from lake van turkey shale normalized ree ree sn patterns display an almost linear increase from the light to the heavy rees and strong positive ce sn anomalies there exist neither positive nor negative gd sn anomalies the ree sn patterns are consistent with domination of ree speciation in solution by poly carbonato complexes generating enrichment of the heavy rees that is even more pronounced than that seen in seawater this is the first report of positive ce sn anomalies observed in ree sn patterns of aerobic natural waters these anomalies apparently result from stabilization of penta carbonato ce iv complexes in solution leading to enhanced abundances of ce in comparison with its trivalent ree neighbours thus in remarkable contrast to seawater positive ce sn anomalies may be general features of alkaline carbonate rich and aerobic lake waters

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

rare earth element concentrations and speciation in alkaline lakes from the western u s a the rare earth element ree concentrations of lake water from five alkaline lakes i e mono walker abert summer and goose lakes within the great basin of the western u s a are reported all of the lakes exhibit hree enriched shale normalized patterns the speciation of the rees was determined and in all cases species greater than 99 of each of the rees exist as ln co3 2 complexes the only exception to this is summer lake where minor amounts of phosphate species were also predicted the large enrichments in the hrees observed in these lakes are the result of the formation of stable carbonate complexes in solution where the hrees preferentially form stronger carbonate complexes than the lrees activity product calculations for ree phosphate coprecipitates in these lakes indicate the lakes are close to saturation with respect to these phases and suggest that these precipitates may limit the maximum dissolved ree concentrations in the lakes

carbonate deposition pyramid lake subbasin nevada 1 sequence of formation and elevational distribution of carbonate deposits tufas abstract during the late quarternary the elevation of terrace cutting and carbonate deposition in the pyramid lake subbasin were controlled by constancy of lake level imposed by spill to adjoining subbasins sill elevations are 1177 1183 m mud lake slough sill 1207 m emerson pass sill and 1265 m darwin pass sill carbonate deposition was favored by 1 hydrologic closure 2 proximity to a source of calcium 3 elevated water temperature and 4 a solid substrate the thickness and aspect of tufa are a function oflake level dynamics relatively thin sheets and pendant sheets were deposited during a rising or falling lake the upper parts of thick reef form tufas have a horizontal aspect and were deposited in a lake which was stabilized by spill to the carson desert subbasin the lower parts of the reef form tufas are thinner and their outer surface has a vertical aspect indicating that the lower part formed in a receding lake the thickest and most complete sequences of tufa are mounds that border the pyramid lake shore the tops of the tallest mounds reach the elevation of the darwin pass sill and many mounds have been eroded to the elevations of the mud lake slough sill of the emerson pass sill the sequence of tufa formation from oldest to youngest displayed in these mounds is 1 a beachrock containing carbonate cemented volcanic cobbles 2 broken and eroded old spheroids that contain thinolitic tufa and an outer rind of dense laminated tufa 3 large cylindrical tubular tufas capped by 4 coatings of old dense tufas and 5 several generations of old branching tufa commonly associated with thin platy tufas and coatings of thinolitic tufa 6 young spheroids that contain poorly oriented young thinolitic tufa in the center and several generations of radially oriented young thinolitic tufas near the outer edge 7 a transitional thinolite to branching tufa 8 two or more layers of young branching tufa 9 a 0 5 cm thick layer of fine grained dolomite 10 a 2 cm thick layer of young dense laminated tufa 11 a 0 1 cm thick layer of encrusting tufa that was covered by a beach deposit and 12 a 1 0 cm thick layer of porous encrusting tufa that coated the beach deposit and the sides of tufa mounds the elevational ranges of the principal varieties of tufa are not related to terrace or spill elevations it seems likely that the distribution of tufa varieties is related to variation in the thermal structure or chemistry of lake lahontan it is hypothesized that the thinolitic tufa ikaite formed in the near freezing hypolimnion and the braching tufa calcite formed in the seasonally warmer epiliminion of the lake

hydrothermal carbonate chimneys from a continental rift afar rift mineralogy geochemistry and mode of formation carbonate chimney like deposits up to 60 m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting lake abhe afar rift djibouti the chimneys formed sub aqueously in the lake water body at a higher water level than observed today alternating calcite and low mg calcite silica concentric layers compose the chimney structures mineralogical and geochemical investigations of the chimneys lake water and hot spring hydrothermal fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water in contrast to the hot spring fluid lake water is enriched in hree and possesses a pronounced positive ce anomaly features that are preserved in the carbonate chimney layers mixing calculations based on sr isotope and concentration data indicate a hydrothermal fluid contribution of 45 in the chimney interior which decreases to 4 in the external chimney layer sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks whereas the lake s sr budget is dominated by riverine input considering the fluid mixing ratios calculated by sr data the measured c and o isotope compositions indicate that chimney carbonates precipitated at temperatures between 14 c internal part and 22 c external part with 13c carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric co2 the low mg calcite layers including the outermost layer have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements ree and sr and ca isotope compositions ca isotope data reveal that internal chimney layers formed by non equilibrium calcite precipitation with a predominantly hydrothermal ca source the external low mg calcite layer received ca contributions from both hydrothermal fluid and lake water with the latter being the dominant ca source highly positive 44 40ca of lake water likely reflects non equilibrium ca carbonate precipitation during lake water evaporation with resulting 44ca enrichment of residual lake water the strong degree of 44ca enrichment may point towards multiple lake drying and ca reservoir depletion events coupled c o ca isotope data of the sampled carbonate chimney suggest late stage low temperature hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low mg calcite precipitation u th age dating suggests that the chimneys formed no earlier than 0 82 kyr bp 0 28 0 54

hydrothermal activity in the northern tanganyika rift east africa abstract hydrothermal fluids from sub lacustrine and sub aerial sites were sampled in the north tanganyika rift during the 1991 tanganydro expedition two types of hydrothermal end member fluids a na hco 3 type and a na cl type have been identified within the sample set the na hco 3 hydrothermal fluids are emitted at the level of the sub lacustrine pemba and sub aerial kabezi and mashuzi sites which are located in the northernmost bujumbura sub basin the na cl hydrothermal fluid vents at the level of the sub lacustrine cape banza field on the southeastern edge of the ubwari ridge the two types of hydrothermal fluids display isotopic and chemical evidence of interaction with magmatic volatiles the magmatic effects are strongest in the fluid discharging on the pemba hydrothermal field where abundant co 2 bubbling accompanies fluid emission the 13 c values of the co 2 gas sampled at pemba and at cape banza indicate a magmatic origin the pemba fluid shows the highest alkalinity and boron content and the lowest ph of the hydrothermal end member fluids and it has equilibrated at depth at the highest temperature 214 7 c in addition the sulfide mineralizations associated with the pemba hydrothermal vents are enriched in as and tl which similarly to b can be transported as volatiles in magmatic fluids the northern tanganyika na hco 3 fluids are similar to the na hco 3 fluid of the geothermal system of lake bogoria in the eastern branch of the east african rift and they can be compared with the na hco 3 hydrothermal water that is inferred to flow into lake kivu the cape banza na cl fluids may derive from the mixing between meteoric water and a deep seated basement brine similar to the ones found in old continental shields such as the canadian shield the high equilibrium temperatures and the geochemistry of the north tanganyika rift hydrothermal fluids suggest that degassing magmatic bodies are present at depth in the area these magmatic bodies may be either the southern expression of the south kivu magmatism or a more recent activity not yet expressed as surface volcanism the hydrothermal sites are located along major active rift bounding faults which can be associated with deep and strong seismicity as in the case of the pemba field such deep fractures may have favoured the uprise of the magmatic bodies that are the source of heat and volatiles to the hydrothermal systems the circulation of hydrothermal fluids within the tanganyika lacustrine sedimentary pile could play an important role in the maturation of the organic matter enclosed in the lake sediments and the subsequent oil generation and migration

u th stratigraphy of a cold seep carbonate crust on continental margins oxidation of methane rich fluids from the sediment often leads to formation of authigenic carbonate pavements on the seafloor the biogeochemical processes involved during this carbonate precipitation are increasingly understood but little is known about the duration or mode of carbonate crust formation here we report uranium and thorium concentrations and isotope compositions for a set of 14 samples drilled across an authigenic carbonate pavement which provide the first stratigraphy for a cold seep carbonate crust the 5 5 cm thick crust nl7 cc2 was collected by submersible on the nile deep sea fan in an area of active fluid venting u th analyses must be corrected for initial th and measurement of co existing sediments indicates the presence of both scavenged and detrital initial 230th which must be considered during this correction the calculated 230th u age depth profile for nl7 cc2 provides evidence for continuous downward carbonate precipitation at the studied location over the last 5000 years three distinct phases can be distinguished from top to bottom with average growth rates of 0 4 5 and 0 8 cm kyr respectively corresponding to carbonate precipitation rates ranging from 7 to 92 mol m2 h 1 rates consistent with previous estimates high resolution 13c profiles gontharet s pierre c blanc valleron m m rouchy j m fouquet y bayon g foucher j p woodside j mascle j the nautinil scientific party 2007 nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the nile deep sea fan eastern mediterranean sea deep sea res ii 54 1292 1311 and major elements across nl7 cc2 show that the variations in carbonate precipitation rates were also accompanied by changes in carbonate mineralogy and fluid composition we suggest that these changes primarily reflect modification of the diagenetic environment i e a progressive depletion of dissolved sulphate through anaerobic oxidation of methane caused by the initial carbonate crust formation and the resulting reduction in bioirrigation overall u th dating of cold seep carbonates offers a promising tool to bring new insights into biogeochemical processes at cold seeps and to assess the timing and duration of fluid venting on continental margins

kinematics of the asal rift djibouti determined from the deformation of fieale volcano because of its subaerial exposure the asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes the present topography of the asal rift results from the tectonic dismemberment during the last 100 000 years of a large central volcanic edifice that formed astride the rift zone 300 000 to 100 000 years ago three dimensional deformation of this volcano has been quantified from the combined analysis of the topography and geology the analysis indicates that spreading at 17 to 29 millimeters per year in a n40 5 e direction accounts for most of the separation between arabia and somalia the small topographic subsidence relative to extension suggests that tectonic thinning of the crust has been balanced by injection and underplating of magmatic material of near crustal density the methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas where the main available information is topography or bathymetry

the dynamics of central main ethiopian rift waters evidence from δd δ18o and 87sr 86sr ratios abstract water samples from cold and geothermal boreholes hot springs lakes and rivers were analyzed for d 18o and 87sr 86sr compositions in order to investigate lake water groundwater mixing processes water rock interactions and to evaluate groundwater flow paths in the central main ethiopian rift mer of the ziway shala basin different ranges of isotopic values were recorded for different water types hot springs show 18o 3 36 to 3 69 and d 15 85 to 24 23 deep aluto langano geothermal wells show 18o 4 65 to 1 24 and d 12 39 to 9 31 groundwater wells show 18o 3 99 to 5 14 and d 19 69 to 32 27 whereas the lakes show 18o and d in the range 3 98 to 7 92 and 26 19 to 45 71 respectively the intersection of the local meteoric water line lmwl d 7 18o 11 2 r2 0 94 n 42 and the local evaporation line lel d 5 63 18o 8 n 14 r2 0 82 was used to estimate the average isotopic composition of recharge water into the basin d 5 15 and 18o 2 34 these values are depleted if compared with the modern day average precipitation presumably indicating paleo groundwater components recharged during previous humid climatic phases the measured stable isotope values indicate that the geothermal wells some of the hot springs and groundwater wells mainly consist of meteoric water the sr isotopic signatures in all waters are within the range of the sr isotopic composition of the rift basalts and rhyolites the variability of sr isotopic data also pinpoints complex water rock interaction and mixing processes in groundwater and surface water the 87sr 86sr ratio ranges from 0 70445 to 0 70756 in the hot springs from 0 70426 to 0 70537 in two deep geothermal wells and from 0 70673 to 0 70721 in the rift lakes ziway langano shala and awasa the radiogenic composition recorded by the lakes indicates that the input water was predominantly affected by progressive interaction with rhyolitic volcanics and lacustrine sediments there is a change in mer water composition between the recharge infiltration zone surrounding highlands and the discharge areas from depleted d 18o and less radiogenic sr isotope compositions to enriched d 18o and more radiogenic sr isotope compositions isotope analyses are therefore useful in providing further understanding of the hydrologeological processes occurring within a productive aquifer and the surface water system

strontium isotopes and rare earth element geochemistry of hydrothermal carbonate deposits from lake tanganyika east africa at cape banza north tanganyika lake fluids and aragonite chimneys have been collected many times since the discovery of this sublacustrine field in 1987 this sampling has been investigated here for the sr isotopic compositions and the rare earth element features of the carbonates and a few fluid samples the 87sr 86sr ratios of the chimneys indicate that they have precipitated from a mixture of lake water more than 95 and hydrothermal fluids no zoning in the chimneys was detected with our sr data for the rare earth elements the situation is more complex the external walls of the chimneys are rare earth element poor la 500 ppb yb 200 ppb la yb 2 to 3 4 their shale normalised rare earth element patterns suggest that they are in equilibrium with the inferred carbonate depositing fluids the rare earth element concentrations of the internal walls of the chimneys are significantly light rare earth elements lree enriched with la contents sometimes up to 5 ppm we suggest that they contain more vent fluid rare earth elements than the external wall samples possibly adsorbed on the surface of growing crystals or simply hosted by impurities it was not possible to constrain the nature of these phases but the variations of the compositions of the internal wall materials of the active chimneys with time as well as data obtained on an inactive chimney indicate that this rare earth element excess is mobile partition coefficients were calculated between the external wall aragonite and carbonate depositing fluid the results are strikingly similar to the values obtained by sholkovitz and shen 1995 on coral aragonite and suggest that there is no significant biologic effect on the incorporation of rare earth elements into coral aragonite and that the various carbonate complexes involved me co3 complexes are the main lree carriers in seawater cantrell and byrne 1987 instead of me co3 2 in banza fluids have the same behaviour during aragonite precipitation

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

water rock interaction between basalt and high salinity fluids in the asal rift republic of djibouti two deep geothermal boreholes were drilled in the asal rift in the republic of djibouti the first hole asal 1 produced water and steam temperatures of up to 250 c being recorded at the well head the second hole asal 2 produced minute amounts of geothermal fluid but no vapor although similar temperatures were measured the fluid from asal 1 gives a good representation of the deep geothermal water this is a nacl brine with a tds content of 128 g l 1 the isotopic composition d 5 5 18o 0 6 is only slightly different from the isotopic composition of seawater mineralogic study of cuttings from asal 2 showed extensive and pervasive rock alteration the main neoform minerals are calcite and quartz chlorite illite montmorillonite fe hydroxides albite and adularia are present together with minor amounts of epidote and prehnite calcite and chlorite are systematically present to a depth of 424 m alteration is minor and corresponds to a low water rock wr a ratio of 2 5 silicification and neoform albite are absent neoform quartz appears at a depth of 600 m at a temperature of 175 c adularia and albite occur at a depth of 820 m at a temperature of 210 c epidote appears at higher temperatures of 250 c 1350 m and propylitic alteration is indicated by the presence of albite epidote chlorite and calcite oxygen isotope compositions range from 4 6 to 12 2 for whole rock samples from 7 3 to 14 3 for neoform quartz and from 7 8 to 20 4 for neoform calcite 18omineral values tend to decrease with depth quartzwater isotopic temperatures show good agreement with temperatures measured in situ calculated water rock atomic ratios vary from 0 7 in the less altered zone to 50 in zones of greatest circulation results for sulfur in sulfide deposits and whole rock samples indicate that the sulfur of magmatic origin present in the geothermal fluid has been contaminated by sulfur from sulfates of marine origin present in seawater and in the evaporite layers through which the fluid passes

the three dimensional geometry and growth of forced folds above saucer shaped igneous sills forced folds formed at the seabed immediately overlying shallow 1 km saucer shaped sills along the ne atlantic margin during the early paleogene examples of this sill fold relationship are exceptionally well imaged by high resolution 3d seismic datasets from the ne rockall basin the forced folds are domal in shape 2 4 km in diameter exhibit a structural relief of up to 350 m and comprise sediment volumes of ca 1 km3 a comparison of the thickness distribution across and volume of a saucer shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence this has the important implication that the structural relief on intrusion related forced folds can be used as an estimate of the thickness of the underlying sill the analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer shaped sills their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden these structures represent a new type of four way dip closed hydrocarbon trap

hydrothermal silica chimney fields in the galapagos spreading center at 86 w silica chimneys were discovered in 1985 at 86 w in the rift valley of the galapagos spreading center at 2600 m depth cauliflower garden the inactive chimneys lack any sulfides and consist almost entirely of amorphous silica up to 96 wt sio2 opal a fe and mn oxides are minor constituents oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32 c 29 9 18o and 42 c 27 8 18o th udating reveals a maximum age of 1440 300y amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid seawater mixture assuming equilibrium with quartz at 500 bars initial fluid temperatures of more than 175 c i e a concentration of 182 ppm sio2 were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40 c and 260 bars if the silica chimneys originate from the same or a similar fluid as higher temperature 300 c sulfide silica precipitates found nearby i e 2 5 km away then subsurface deposition of sulfides may have occurred

discovery of sublacustrine hydrothermal activity and associated massive sulfides and hydrocarbons in the north tanganyika trough east african rift massive sulfides and carbonate mineral deposits associated with sublacustrine thermal springs were recently discovered along the zaire side of the north tanganyika trough western branch of the east african rift this hydrothermal activity investigated by scuba diving at a maximum depth of 20 m is located at the intersection of major north south normal faults and northwest southeast faults belonging to the tanganyika rukwa malawi trm strike slip fault zone the preliminary results presented here come from analyses of sulfide deposits hydrothermal fluids and associated hydrocarbons that result from geothermal activity in this part of the east african rift filled by a thick pile of sediment the north tanganyika trough

hydrothermal vents in lake tanganyika east african rift system sublacustrine hydrothermal vents with associated massive sulfides were discovered during april 1987 at pemba and cape banza on the zaire side of the northern basin of lake tanganyika east african rift system new investigations by a team of ten scuba divers during the multinational france zaire germany and burundi tanganydro expedition august october 1991 found hydrothermal vents down to a depth of 46 m along north trending active faults bounding the tanganyika rift on the western side temperatures from 53 to 103 degrees c were measured in hydrothermal fluids and sediments veins of massive sulfides 1 10 cm thick pyrite and marcasite banding were found associated with vents at the pemba site at cape banza active vents are characterized by 1 70 cm high aragonite chimneys and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes hydrothermal fluid end members show distinctive compositions at the two sites the pemba end member is a nahco sub 3 enriched fluid similar to the nahco sub 3 thermal fluids form lakes magadi and bogoria in the eastern branch of the rift the cape banza end member is a solution enriched in nacl such brines may have a deep seated basement origin as do the uvinza nacl brines on the eastern flank ofmore the tanganyika basin geothermometric calculations have yielded temperatures of fluid rock interaction of 219 and 179 degrees c in the pemba and cape banza systems respectively abundant white or reddish brown microbial colonies resembling beggiatoa mats were found surrounding the active vents thermal fluid circulation is permitted by opening of cracks related to 130 degrees n normal dextral faults that intersect the north south major rift trend the sources of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift which is suggested by the isotopic composition of carbon dioxide released at pemba and cape banza 21 refs 2 figs less

controls on magmatism in an island arc environment study of lavas and sub arc xenoliths from the tabar lihir tanga feni island chain papua new guinea the tabar lihir tanga feni tltf islands of papua new guinea mainly comprise high k calc alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction related magmatism numerous sedimentary mafic and ultramafic xenoliths recovered from tubaf seamount located on the flank of lihir island provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region the sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with pacific affinity a majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the tltf arc leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50 or more of elements such as cu zn rb u pb and light ree rare earth elements most likely introduced in the xenoliths via hydrous fluids released from a subducted slab the leaching experiments demonstrated that the light ree enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit ree patterns that range from flat to light ree depleted sr nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87sr 86sr with decreasing 143nd 144nd ratios the labile phases in the ultramafic xenoliths represented by the leachates show decoupling between sr and nd with distinctly more radiogenic 87sr 86sr than the residues both leachates and residues exhibit very wide range in their pb isotopic compositions indicating the involvement of three components in the mantle wedge under the tltf islands two of the components can be identified as pacific oceanic mantle and pacific sediments some of the ultramafic samples and clinopyroxene separates however exhibit relatively low 206pb 204pb at elevated 207pb 204pb suggesting that the third component is either indian ocean type mantle or australian subcontinental lithospheric mantle geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized it did not significantly contribute to the isotopic and incompatible trace element compositions of tltf lavas compared to the mantle samples the tltf lavas have very restricted pb isotopic compositions that lie within the pacific morb range indicating that magma compositions were dominated by melts released from a stalled subducted slab with pacific morb affinity interaction of slab melts with depleted peridotitic component in the mantle wedge followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area

rare earth elements in cold seep carbonates from the niger delta abstract the circulation of methane rich fluids at cold seeps often leads to precipitation of authigenic carbonates mediated by microbial activity which can be used as archives of fluid seepage on continental margins in this study we have investigated the potential of using rare earth element ree concentrations in authigenic carbonates for tracing fluid sources and providing information on associated biogeochemical processes at cold seeps we have examined various chemical procedures for the analysis of ree in authigenic carbonates and present a robust method using dilute acetic acid solution that allows the extraction of ree from carbonate phases with reduced contamination from fe mn oxides and detrital minerals using this method we analysed a series of carbonate samples collected at various deep sea sites of active fluid seepage on the niger delta for their ree and trace element concentrations 13c and 18o values and 87sr 86sr isotope compositions we show that the ree composition of cold seep carbonates is controlled primarily by that of the fluid from which they have precipitated our results also suggest that significant ree fractionation can occur during carbonate precipitation which can be related to various degrees of ree complexation by organic versus carbonate ligands in response to changing pore water chemical composition for parameters such as carbonate alkalinity

formation of carbonate chimneys in the mediterranean sea linked to deep water oxygen depletion marine sediments at ocean margins vent substantial amounts of methane1 2 microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3 4 the factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained here we characterize the petrology and geochemistry of rocks sampled from metre size build ups of methane derived carbonate chimneys located at the amon mud volcano on the nile deep sea fan we find that these carbonates comprise porous structures composed of aggregated spherules of aragonite and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the black sea5 using u series dating we show that the amon carbonate build ups formed between 12 and 7 thousand years ago contemporaneous with the deposition of organic rich sediments in the eastern mediterranean the so called sapropel layer s1 we propose that the onset of deep water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor and thus the formation of carbonate chimneys

east african rift system offset age and tectonic significance of the tanganyika rukwa malawi intracontinental transcurrent fault zone abstract the 1000 200 km tanganyika rukwa malawi fractures area is shown to be a right lateral intracontinental transform zone belonging to the east african rift system using the results carried out from complementary techniques remote sensing field structural analysis and k ar isotopic dating it appears that tectonics in this area have acted at different times the oldest being at 2000 1400 m y the youngest being still active today and are mainly due to the reactivation of nw se structures the area consists mainly of large nw se en echelon dextral transcurrent faults along which the southern tanganyika rukwa and northern malawi troughs are opening ne sw transverse faults showing a curved horizontal shape end these troughs axially eocene to recent volcanic activity is associated with the intersection of the northeastern main nw se transcurrent fracture the livingstone fault

a high precision age estimate of the holocene plinian eruption of mount mazama oregon usa the climactic eruption of mount mazama in oregon north america resulted in the deposition of the most widespread holocene tephra deposit in the conterminous united states and south western canada the tephra forms an isochronous marker horizon for palaeoenvironmental sedimentary and archaeological reconstructions despite the current lack of a precise age estimate for the source eruption previous radiocarbon age estimates for the eruption have varied and greenland ice core ages are in disagreement for the mazama tephra to be fully utilised in tephrochronology and palaeoenvironmental research a refined precise and accurate age for the eruption is required here we apply a meta analysis of all previously published radiocarbon age estimations n 81 and perform bayesian statistical modelling to this data set to provide a refined age of 7682 7584 cal yr bp 95 4 probability range although the depositional histories of the published ages vary this estimate is consistent with that estimated from the gisp2 ice core of 7627 150 yr bp zdanowicz et al 1999

ikaite precipitation by mixing of shoreline springs and lake water mono lake california usa abstract metastable ikaite caco3 6h2o forms abundantly during winter months along the south shoreline of mono lake where shoreline springs mix with lake water ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate ion inhibition of calcite and aragonite during the spring some of the ikaite is transformed to anhydrous caco3 and is incorporated into tufa but most is dispersed by wave action into the lake where it reacts to form gaylussite na2ca co3 2 5h2o spring waters have low ph values are dominantly ca na hco3 have low radiocarbon activities and are mixtures of deep seated geothermal and cold groundwaters chemical modeling reveals that precipitation of caco3 can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96 spring water and 4 lake water under these conditions all the ca and a significant fraction of the co3 of the precipitate is spring supplied a radiocarbon age of 19 580 years obtained on a natural ikaite sample supports this conclusion with the springs supplying a large and probably variable portion of the carbonate and with apparent 14c age of the carbonate varying from spring to spring tufa of similar actual antiquity may yield significantly different 14c dates making tufa at this location unsuitable for absolute age dating by the radiocarbon method

the protracted development of the continent ocean transition in afar continental breakup and the transition to seafloor spreading is characterized by extensional faulting thinning of the lithosphere and at magmatic margins voluminous intrusive and extrusive magmatism1 2 3 4 it is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent ocean transition is buried beneath thick layers of volcanic and sedimentary rocks5 6 and the tectonic activity that characterized breakup has ceased instead the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed ancient rifted margins1 5 7 8 ongoing rifting in ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in ethiopia we show that although intrusion of magma maintains crustal thickness during the early stages of the continent ocean transition subsidence of the margin below sea level and eruption of voluminous basalt flows is initiated by late stage thinning of the heavily intruded weakened plate just before the onset of seafloor spreading we thus conclude that faulting stretching and magma intrusion are each important but at different times during breakup

calcium isotope δ 44 40 ca fractionation along hydrothermal pathways logatchev field mid atlantic ridge 14 45 n we investigate the logatchev hydrothermal field at the mid atlantic ridge 14 degrees 45 n to constrain the calcium isotope hydrothermal flux into the ocean during the transformation of seawater to a hydrothermal solution the ca concentration of pristine seawater ca sw increases from about 10 mm to about 32 mm in the hydrothermal fluid endmember ca hydend and thereby adopts a delta ca 44 40 hydend of 0 95 0 07 parts per thousand relative to seawater sw and a sr 87 sr 86 isotope ratio of 0 7034 4 we demonstrate that delta ca 44 40 hydend is higher than that of the bedrock at the logatchev field from mass balance calculations we deduce a delta ca 44 40 of 1 17 0 04 parts per thousand sw for the host rocks in the reaction zone and 1 45 0 057 parts per thousand sw for the isotopic composition of the entire hydrothermal cell of the logatchev field the values are isotopically lighter than the currently assumed delta ca 44 40 for bulk earth of 0 92 0 187 parts per thousand sw skulan j depaolo d j and owens t l 1997 biological control of calcium isotopic abundances in the global calcium cycle geochim cosmochim acta 61 12 2505 2510 and challenge previous assumptions of no ca isotope fractionation between hydrothermal fluid and the oceanic crust zhu p and macdougall j d 1998 calcium isotopes in the marine environment and the oceanic calcium cycle geochim cosmochim acta 62 10 1691 1698 schmitt a d chabeaux f and stille p 2003 the calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance earth planet sci lett 6731 1 16 here we propose that ca isotope fractionation along the fluid flow pathway of the logatchev field occurs during the precipitation of anhydrite two anhydrite samples from the logatchev hydrothermal field show an average fractionation of about delta 44 4c ca 0 57 parts per thousand relative to their assumed parental solutions ca isotope ratios in aragonites from carbonate veins from odp drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average delta ca 44 40 of 1 54 0 08 parts per thousand sw the relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine ca isotope budget in particular with regard to ocean crust alteration c 2008 elsevier ltd all rights reserved

biological control of calcium isotopic abundances in the global calcium cycle measurements of 44ca 40ca expressed as 44ca were made on igneous rocks and on shell and bone material from modern organisms to investigate the magnitude and origins of calcium isotopic fractionation in nature the results document a span of 4 in 44ca measured with the double spike technique to a precision of 0 15 volcanic rocks including basalt and rhyolite show little variability and cluster near 44ca 0 0 2 systematic analysis of biological samples indicates that biological processing of calcium discriminates against heavy isotopes and that biological fractionation is the primary generator of calcium isotopic fractionation in nature preliminary data suggest that calcium becomes isotopically lighter as it moves through food chains calcium carbonate shells of marine microorganisms and deep sea carbonate ooze have 44ca about 1 0 lower than seawater this fractionation causes seawater to be enriched in heavy calcium 44ca 0 9 relative to igneous rocks marine organisms consequently are isotopically heavier than their terrestrial counterparts at similar trophic level the calcium isotopic composition of living and fossil organisms may record information on diet and environment