abrupt climate change in southeast tropical africa influenced by indian monsoon variability and itcz migration 1 the timing and magnitude of abrupt climate change in tropical africa during the last glacial termination remains poorly understood high resolution paleolimnological data from lake tanganyika southeast africa show that wind driven seasonal mixing in the lake was reduced during the younger dryas inter allerod cool period older dryas and heinrich event 1 suggesting a weakened southwest indian monsoon and a more southerly position of the inter tropical convergence zone over africa during these intervals these events in lake tanganyika coeval with millennial and centennial scale climate shifts in the high latitudes suggest that changes in itcz location and indian monsoon strength are important components of abrupt global climate change and that their effects are felt south of the equator in africa however we observe additional events in lake tanganyika of equal magnitude that are not correlated with high latitude changes indicating the potential for abrupt climate change to originate from within tropical systems

microstructural changes accompanying the opal a to opal ct transition new evidence from the siliceous sinters of geysir haukadalur iceland opaline silica opal a has formed in marine lacustrine and geothermal environments throughout geological time during diagenesis opal a normally changes to opal ct then opal c and finally to quartz such changes commonly destroy the original fabrics and any fossils that opal a contained the physical changes that accompany the opal a to opal ct transition however are known poorly x ray diffraction analyses electron microprobe analyses and high resolution high magnification scanning electron microscope imagery of siliceous sinters from the geysir geothermal area in iceland show that opal a is formed of heterometric arrays of randomly packed microspheres up to 5 m diameter with neighbouring spheres commonly being joined by small connection pads in contrast enlarged spheres lepispheres inverse opal two types and spindle frameworks with hexagonal motifs characterize opal ct the textures in opal ct which vary on a microscale reflect the complex interplay between dissolution e g inverse opal and precipitation e g enlarged spheres spindle frameworks that probably was mediated by groundwater in a near surface environment the processes deciphered from these young rocks should however be applicable to sedimentary opal a and opal ct of all ages irrespective of their origin

geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems mixing of hydrothermal fluids and seawater at the ocean floor combined with slow reaction kinetics for oxidation reduction reactions provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving s c fe and mn compounds during mixing of hydrothermal fluids with seawater for the vent fluid used in the calculations epr 21 n obs the model indicates that mixing environments are favorable for oxidation of h2s ch4 fe2 and mn2 only below 38 c with methanogenesis and reduction of sulfate or s favored at higher temperatures suggesting that environments dominated by mixing provide habitats for mesophilic but not thermophilic aerobes and thermophilic but not mesophilic anaerobes a maximum of 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal kg vent fluid is available from methanotrophy methanogenesis oxidation of fe or mn or sulfate reduction the total potential for chemosynthetic primary production at deep sea hydrothermal vents globally is estimated to be about 1013 g biomass per year which represents 0 02 of the global primary production by photosynthesis in the oceans thermophilic methanogens and sulfate and s reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34s32s s ratios of vent sulfide minerals the metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the earth

evidence for a long lived accommodation transfer zone beneath the snake river plain a possible influence on neogene magmatism geochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the snake river plain yellowstone province srp y magmatism crustal flow metamorphism and extensional exhumation of core complexes north of the srp occurred mostly between 55 and 42 ma as compared to 42 25 ma south of the srp with final exhumation of the southern complexes occurring only during younger miocene 20 0 ma basin and range faulting these significant differences in the timing of events suggest that the now lava covered srp which is at a high angle to cordilleran trends may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south following previous suggestions we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab shallower slab angle to the south used to explain both the locus of late cretaceous paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the srp during the cenozoic the details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused miocene srp y hot spot magmatism

hydrothermal venting in magma deserts the ultraslow spreading gakkel and southwest indian ridges 1 detailed hydrothermal surveys over ridges with spreading rates of 50 150 mm yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting but the validity of this relation at slow and ultraslow ridges is unproved here we compare hydrothermal plume surveys along three sections of the gakkel ridge arctic ocean and the southwest indian ridge swir to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges along the gakkel ridge most apparent vent sites occur on volcanic highs and the extraordinarily weak vertical density gradient of the deep arctic permits plumes to rise above the axial bathymetry individual plumes can thus be extensively dispersed along axis to distances 200 km and 75 of the total axial length surveyed is overlain by plumes detailed mapping of these plumes points to only 9 10 active sites in 850 km however yielding a site frequency fs sites 100 km of ridge length of 1 1 1 2 plumes detected along the swir are considerably less extensive for two reasons an apparent paucity of active vent fields on volcanic highs and a normal deep ocean density gradient that prevents extended plume rise along a western swir section 10 23 e we identify 3 8 sites so fs 0 3 0 8 along a previously surveyed 440 km section of the eastern swir 58 66 e 6 sites yield fs 1 3 plotting spreading rate us versus fs the ultraslow ridges and eight other ridge sections spanning the global range of spreading rate establish a robust linear trend fs 0 98 0 015us implying that the long term heat supply is the first order control on the global distribution of hydrothermal activity normalizing fs to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster spreading ridges in supporting active vent fields this increased efficiency could derive from some combination of three dimensional magma focusing at volcanic centers deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle and nonmagmatic heat supplied by exothermic serpentinization

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

sublacustrine precipitation of hydrothermal silica in rift lakes evidence from lake baringo central kenya rift valley abstract many lakes in volcanic regions are fed by hot springs that in some basins can contribute a large percentage of the annual recharge especially during times of aridity it is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record hot spring deposits travertine sinter provide the most tangible evidence for thermal recharge to lakes although subaerial spring deposits have been widely studied lacustrine thermal spring deposits especially sublacustrine siliceous sinters remain poorly known detailed field petrographic and scanning electron microscope sem studies have been made of fossil sublacustrine sinter exposed at soro hot springs along the northeastern shoreline of ol kokwe a volcanic island in lake baringo kenya modern hot springs at soro which discharge na hco3 cl waters from a deep reservoir 180 c have thin 1 10 mm friable microbial silica crusts around their subaerial vents but thicker 1 cm sinter deposits are not forming the fossil sinter which is present as intergranular cements and crusts in littoral conglomerates and sandstones is composed mainly of opaline silica opal a three types of fossil sinter are recognized 1 massive structureless silica which fills intergranular pores and forms crusts up to 5 cm thick 2 pore lining silica some of which is isopachous and 3 laminated silica crusts which formed mainly on the upper surfaces of detrital particles all three types contain well preserved diatoms including lacustrine planktonic forms microbial remains mainly filamentous and coccoid bacteria including cyanobacteria and extracellular polymeric gels are locally abundant in the opaline silica together with detrital clays and thin laminae composed of authigenic chlorite most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water silica precipitated upon rapid cooling of thermal 90 c waters at and just below their interface with the overlying cooler 25 c lake waters microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids some of the silica originally may have been soft and partly gelatinous planktonic diatoms and detrital clay rained down then became incorporated in the amorphous silica following a fall in lake level the opal a lithified and partly altered to cristobalite opal c and chalcedony the lowest fossil sinters were later encrusted by calcite stromatolites with calcite and quartz forming late pore filling cements the age of the sublacustrine sinters is unknown but some of the deposits could date back to the late pleistocene similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring lake bogoria such rocks may provide evidence for deep hot fluid recharge to lakes when encountered in the geological record

eocene melting of precambrian lithospheric mantle analcime bearing volcanic rocks from the challis kamloops belt of south central british columbia abstract potassic silica undersaturated mafic volcanic rocks form a minor portion of the predominantly calc alkaline eocene challis kamloops volcanic belt which extends from the northwestern united states into central british columbia canada their major occurrence is in the penticton group in south central british columbia where they reach a thickness of up to 500 m and form the northwestern edge of the montana alkaline province these analcime bearing rocks 53 52 ma old are typically rhomb porphyries of ternary feldspar an28ab52or20 additional phenocryst phases include clinopyroxene analcime phlogopite and rare olivine the rocks are characterized by high total alkalis particularly k2o 4 5 wt as well as by a distinct enrichment of large ion lithophile elements versus heavy rare earth elements and high field strength elements they have unusual isotopic compositions compared to most other rocks of the challis kamloops belt particularly high negative nd values and elevated but relatively uniform initial 87sr 86sr ratios 0 7065 the potassic silica undersaturated rocks overlie precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere which was modified in a precambrian subduction setting the alkaline rocks of the challis kamloops belt are related to a slab window environment in particular they were formed above the southern edge of the kula plate adjacent to the kula farallon slab window whereas the montana alkaline province situated well to the southeast was formed directly above the kula farallon slab window upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

seafloor bioalteration of sulfide minerals results from in situ incubation studies we present results of incubation studies conducted at low temperatures 4 c in the vicinity of a seafloor hydrothermal vent system we reacted fe s cu and zn bearing minerals including pyrite marcasite chalcopyrite sphalerite elemental sulfur and a portion of a natural chimney sulfide structure for 2 months at the main endeavour segment of the juan de fuca ridge in the pacific ocean our study utilizes fluorescent in situ hybridizations fish scanning and transmission electron microscopy sem tem and light microscopic analysis the surfaces of these minerals are solely colonized by bacteria and not by archaea colonization densities vary over an order of magnitude with the following sequence elemental sulfur chimney sulfide marcasite pyrite sphalerite chalcopyrite and correspond well with the abiotic oxidation kinetics of these materials excepting elemental sulfur which is both the least reactive to oxidizing species and the most heavily colonized colonization densities also correspond with apparent degree of reaction dissolution pitting accumulation of secondary alteration products heavy accumulations of secondary fe oxides on fe bearing minerals most notably on the chimney sulfide form in situ as the result of mineral dissolution and the activity of neutrophilic fe oxidizing bacteria results suggest that mineral oxidizing bacteria play a prominent role in weathering of seafloor sulfide deposits and that microbial utilization of mineral substrates contributes to biomass production in seafloor hydrothermal environments

a rare earth element signature of bacteria in natural waters abstract this study shows that rare earth element ree distribution patterns between bacteria and water for five different bacterial strains bacillus subtilis escherichia coli alcaligenes faecalis shewanella putrefaciens and pseudomonas fluorescens exhibit a steep increase in the heavy ree hree part of the distribution pattern and a weak peak around the middle ree mree part though the ree patterns for b subtilis and e coli were already reported in our previous paper takahashi y ch tellier x hattori k h kato k fortin d 2005 adsorption of rare earth elements onto bacterial cell walls and its implication for ree sorption onto natural microbial mats chem geol 219 53 67 the present results clearly show that the characteristic ree pattern is commonly observed for various bacterial species in addition similar ree patterns were observed for bioflims containing neutrophilic iron oxidizers and other bacteria formed in a groundwater discharge area in the budo pond in hiroshima japan suggesting that the ree distribution pattern found in the laboratory experiments was reproduced in naturally occurring bacteria in fresh waters in addition the ree pattern characterized by a maximum at mree and an increase at hree was also observed in fe oxyhydroxide precipitates formed below the same biofilm which contained bacterial remnants these results show that the presence of bacteria in natural fe oxyhydroxides and possibly in other natural minerals can be reflected in the shape of ree patterns such results could potentially be extended to geological materials such as banded iron formation bif and used to discuss the effect of microbial activity on the formation of bif although there are still many problems related to the extrapolation of the results obtained for fe oxyhydroxides in modern water systems to bif such as abiotic processes and post depositional alteration of the ree patterns our study suggests that the ree pattern can be used as another tool to assess the role of bacteria in mineral formation in natural waters

coexistence of fe ii and mn ii oxidizing bacteria govern the formation of deep sea umber deposits the genesis of umber deposits has remained controversial for several decades recently microbial fe ii oxidation associated with low temperature diffuse venting has been identified as a key process for the formation of umber deposits but the exact biogeochemical mechanisms involved to the precipitation of mn oxides in umber deposits still remain unknown here we used nano secondary ion mass spectrometer synchrotron based x ray absorption spectroscopy electron microscopy and molecular techniques to demonstrate the coexistence of two types of metal oxidizing bacteria within deep sea hydrothermal umber deposits at the south mid atlantic ridge where we found unique spheroids composed of biogenic fe oxyhydroxides and mn oxides in the deposits our data show that fe oxyhydroxides and mn oxides are metabolic by products of lithotrophic fe ii oxidizing bacteria and heterotrophic mn ii oxidizing bacteria respectively the hydrothermal vents fuel lithotrophic microorganisms which constitute a trophic base that might support the activities of heterogenic mn ii oxidizing bacteria the biological origin of umber deposits shed light on the importance of geomicrobiological interaction in triggering the formation of metalliferous deposits with important implications for the generation of submarine mn deposits and crusts c 2015 elsevier ltd all rights reserved

low angle faulting and the origin of kettle dome a metamorphic core complex in northeastern washington the low angle eastward dipping kettle river fault has been mapped for 45 km along the eastern flank of kettle dome the fault juxtaposes lower plate mylonitic sillimanite grade metamorphic rocks against upper plate greenschist grade metamorphic rocks and unmetamorphosed but folded eocene volcanic and sedimentary rocks the geometry of high angle listric faults which cut upper plate rocks suggests that the upper plate moved eastward relative to the lower plate cataclasis and chloritization spatially related to the kettle river fault cut the mylonitic foliation in the lower plate indicating that the gneisses in the lower plate were not part of a hot mobile diapir during at least the latest stage of their emplacement the similarity of the kettle river fault to the folded newport fault along the northern contact of spokane dome suggests that these faults postdate the mylonitization of the lower plate but predate regional doming

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

mineralized microbes from giggenbach submarine volcano 1 the giggenbach submarine volcano which forms part of the kermadec active arc front is located 780 km nne of the north island of new zealand samples collected from chimneys associated with seafloor hydrothermal vents on this volcano at a depth of 160 180 m contain silicified microbes and microbes entombed in reticular fe rich precipitates the mineralized biota includes filamentous rod shaped and rare coccoid microbes in the absence of organic carbon for rdna analysis or preserved cells the taxonomic affinity of these microbes in terms of extant taxa remains questionable because of their architectural simplicity and the paucity of taxonomically significant features the three dimensional preservation of the microbes indicates rapid mineralization with a steady supply of supersaturated fluids to the nucleation sites present on the surfaces of the microbes the mineralization styles evident in the microbes from the giggenbach submarine volcano are similar to those associated with mineralized microbes found in terrestrial hot spring deposits in new zealand iceland yellowstone and kenya these similarities exist even though the microbes are probably different and the fluids become supersaturated with respect to opal a by different mechanisms for ancient rocks it means that interpretations of the depositional settings cannot be based solely on the silicified microbes or their style of silicification

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

siliceous sublacustrine spring deposits around hydrothermal vents in lake taupo new zealand chimneys around hydrothermal vents at a depth of c 140 m in the horomatangi reefs area in lake taupo new zealand are composed of amorphous silica opal a with lesser amounts of mn fe and hg rich precipitates detrital quartz and feldspar grains are found in the surrounding sediments the associated biota includes fish bacteria sponges crayfish amphipods copepods ostracodes annelids and other unidentified organisms much of this biota however is not preserved in the opal a deposits instead the silicified biota includes filamentous microbes and diatoms scattered coccoid microbes fungal hyphae and spores rare sponge spicules and rare worm tubes the diatoms coccoid microbes and fungi were brought into the area with the detrital sediment in ancient successions distinction between sublacustrine terrestrial and some marine hot spring deposits may be difficult because the precipitates share many similarities distinction based on the biota is viable only if palaeoecological information can be inferred from accurately identified organisms unfortunately silicification commonly disguises microbes and precludes their accurate identification diagenetic transformation of opal a to its more stable polymorphs also destroys many original depositional fabrics and silicified microbes distinction between the different types of spring deposit may therefore depend on the interpretation of their overall sedimentological and stratigraphic setting

geology geochemistry and isotopic character of the colville igneous complex northeastern washington the colville igneous complex in northeastern washington comprises paleocene orthogneisses and leucocratic granitic rocks and eocene volcanic plutonic suites which were emplaced during early tertiary extension the colville igneous complex is divided into three components 1 65 4 ma orthogneisses of the okanogan dome or metamorphic complex and similar gneisses in the kettle and lincoln complexes 2 61 51 ma leucocratic granitic rocks and 3 53 47 ma intermediate to felsic volcanic plutonic rocks the leucocratic granites and younger volcanic plutonic rocks are divided on the basis of age geochemical and isotopic characteristics and emplacement mechanism the older leucocratic granites are silica rich corundum normative and sr and light rare earth element ree enriched and they form several large discrete compositionally zoned plutons and intrusive suites the younger eocene volcanic plutonic rocks can be divided into two suites an older compositionally restricted 68 72wt sio2 sr enriched medium k suite and a younger intermediate to felsic high k calcalkalic suite sr isotopic measurements of volcanic and plutonic rock in the colville igneous complex yield variable initial 87sr 86sr ratios sri ranging from about 0 704 to values greater than 0 710 in general the volcanic rocks are especially variable with little or no correlation between sri and geographic location the leucocratic granites however show a systematic west to east increase across the area which probably reflects an increased involvement with precambrian crust the variable nature of sri in the colville igneous complex may also be controlled by crustal thickness and or depth of melting within individual volcanic and plutonic suites there is a crude correlation of mean sri with age there is a systematic decrease in sri with increasing age which may reflect either a migration of the site s of melting or a decreased upper crustal residence time possibly related to the onset of extension the leucocratic granites may have formed by small degrees of partial melting in the lower crust perhaps facilitated by thermal blanketing of an initially overthickened crust the ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper more radiogenic crust to be melted

diversity of biogenic minerals in low temperature si rich deposits from a newly discovered hydrothermal field on the ultraslow spreading southwest indian ridge 1 a newly discovered hydrothermal field on the ultraslow spreading southwest indian ridge expands the known ranges of biogenic minerals microbes and hydrothermal deposits the deposits from this vent site show typical characteristics of low temperature hydrothermal deposits rare earth element fractionation might be partly influenced by bacterial activity a physicochemical gradient is present across the layer of deposits providing suitable microhabitats for various microbes molecular phylogenetic analysis demonstrates a diverse range of bacteria some of which are involved in sulfur iron and nitrogen metabolism a high diversity of biogenic fe and si minerals are present in the deposits biogenic silica fe sulfides and fe oxides with distinctive morphologies are closely related to microbes several novel fe oxide structures are found and attributed to new types of biogenic minerals the precipitation of porous silica spherules and fe sulfides can be strongly promoted by extracellular polymer saccharides biomineralization features indicate that these deposits evolved through four stages these results help us to better understand the formation of biogenic minerals and low temperature deposits associated with microbes in the earth s modern and ancient seafloor hydrothermal vent fields

sulfide impregnated and pure silica precipitates of hydrothermal origin from the central indian ocean abstract this is the first report about silica rich hydrothermal precipitates which were sampled together with hydrothermal sulfides chimney fragments in an extinct vent field in the central indian ocean there are two kinds of silica rich rocks a jasper which is impregnated and replaced to various degrees mainly by sphalerite and to a lesser extent by barite pyrite and traces of chalcopyrite and an opalite which is an almost pure silica phase without any sulfide or sulfate impregnations but which is sometimes covered by manganese crusts no internal concentric zoning indicating typical chimney structures can be recognized in the jasper and or opalite samples the textures rather suggest a sedimentary silica and or iron deposition from diffuse low temperature 60 c vent fluids partly with still visible indications of former bacterial mats and synsedimentary deformation structures the sphalerite and barite impregnations within the jasper however are considered to have precipitated from white smoker type fluids since they were deposited under intermediate temperatures between 155 and 265 c according to fluid inclusion studies the sulfur isotopic composition 34 s of our sulfide samples has mean values of 6 1 for sphalerite and 5 7 for pyrite indicating a mixture of predominantly basaltic sulfur with subordinate amounts of reduced seawater sulfur the oxygen isotope signals of some pure jasper concentrate samples indicate that the mean formation temperature calculated from these values lies at 63 2 c the relationship between the massive pyrite and chalcopyrite ores from the extinct chimney structures and the silica rich precipitates can be explained by different cycles of hydrothermal activity one high temperature above 300 c cycle dominated by pyrite and chalcopyrite formation and one later epithermal below 300 c cycle which resulted in sphalerite and silica dominated precipitates furthermore zonation and zone refining processes are part of the evolution of the mineralized field

shuswap terrane of british columbia a mesozoic core complex the shuswap terrane of the northwest cordillera owes its distinctive characteristics to deformation and high grade regional metamorphism that occurred primarily during the mesozoic within the terrane the monashee complex the overlying selkirk allochthon and the intervening monashee decollement record strain and metamorphism that occurred in middle to late jurassic time during and subsequent to the accretion of a western allochthonous terrane the selkirk allochthon moved eastward across the monashee complex after emplacement of the western allochthonous terrane but before late mesozoic to early cenozoic telescoping of the rocky mountain belt in this latter event the monashee complex and overlying allochthonous slices were transported eastward relative to the north american craton on a sole fault that developed during listric thrusting of the rocky mountain foreland synmetamorphic to late metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the monashee decollement zone were generated by shear strain during the middle jurassic emplacement of the selkirk allochthon and are not related to any upper crustal extension that occurred in the tertiary uplift normal faulting brittle reactivation of the mylonitic decollement zone widespread resetting of k ar and rb sr mineral dates and arching of the terrane are events that culminated in the eocene

subduction style magmatism in a non subduction setting the colville igneous complex ne washington state usa of the klondike mountain formation which show no decoupling the colville igneous complex is located within the eocene magof the lile and hfse i e no subduction signature precluding matic belt of the north american cordilleran interior it straddles the presence of a subduction slab beneath this part of the north the us canadian border in northeast washington and southern american cordillera during the eocene we propose the alternative british columbia the complex consists of three intrusive and two model that the colville igneous complex formed as a result of extrusive phases the first extrusive phase being contemporaneous decompression melting of crust and mantle during post laramide with the latter two intrusive phases as a consequence of suborogenic collapse of an overthickened crust solidus re equilibration in the plutonic rocks this study concentrates on the two extrusive phases the sanpoil volcanic formation and the klondike mountain formation the sanpoil volcanic formation consists of andesites dacites and rare trachyandesites sio2

a reduced crustal magnetization zone near the first observed active hydrothermal vent field on the southwest indian ridge 1 inversion of near bottom magnetic data reveals a well defined low crustal magnetization zone lmz near a local topographic high 37 47 s 49 39 e on the ultraslow spreading southwest indian ridge swir the magnetic data were collected by the autonomous underwater vehicle abe on board r v dayangyihao in february march 2007 the first active hydrothermal vent field observed on the swir is located in area a within and adjacent to the lmz at the local topographic high implying that this lmz may be the result of hydrothermal alteration of magnetic minerals the maximum reduction in crustal magnetization is 3 a m the spatial extent of the lmz is estimated to be at least 6 7 104 m2 which is larger than that of the lmzs at the tag vent field on the mid atlantic ridge mar as well as the relict field bastille dante grotto and new field vent sites on the juan de fuca ridge jdf the calculated magnetic moment i e the product of the spatial extent and amplitude of crustal magnetization reduction is at least 3 107 am2 for the lmz on the swir while that for the tag field on the mar is 8 107 am2 and that for the four individual vent fields on the jdf range from 5 107 to 3 107 am2 together these results indicate that crustal demagnetization is a common feature of basalt hosted hydrothermal vent fields at mid ocean ridges of all spreading rates furthermore the crustal demagnetization of the area a on the ultraslow spreading swir is comparable in strength to that of the tag area on the slow spreading mar

hydrothermal nontronite formation at eolo seamount aeolian volcanic arc tyrrhenian sea abstract a sediment core containing a yellowish green clay bed was recovered from an area of extensive hydrothermal deposition at the se slope of the eolo seamount tyrrhenian sea the clay bed is composed of pure nontronite described for the first time in the tyrrhenian sea which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits the high al content suggests precipitation from al containing hydrothermal solutions the ree distribution of the eolo nontronite has a v shape pattern the heavy ree enrichment is in part due to their preferential partitioning in the nontronite structure this enrichment was possibly further enhanced by the hree preferential sorption on bacterial cell walls the light ree enrichment is the result of scavenging uptake by one of the nontronite precursors i e poorly ordered fe oxyhydroxides from the hydrothermal fluids oxygen isotopic composition of the nontronite yields a formation temperature of 30 c consistent with a low temperature hydrothermal origin the relatively radiogenic nd isotopic signature of the nontronite compared to the present day mediterranean seawater indicates that approximately half of nd and presumably the rest of the lree are derived from local volcanic sources on the other hand 87 sr 86 sr is dominated by present day seawater sr scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube like filaments which are indicative of bacteria assisted precipitation bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly ordered hydrothermal fe oxyhydroxides and silica precipitated upon aging the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite finally the well balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

microbial silicification in iodine pool waimangu geothermal area north island new zealand implications for recognition and identification of ancient silicified microbes silicified microbes provide evidence for some of the earliest life forms on earth they are extremely important to understanding the early development of life and the conditions that allowed its development such discussions commonly rely on comparisons with extant taxa and therefore depend upon the preservation style of the microbes and in particular the preservation of the taxonomically important features silicified microbes are deceptive they commonly appear to be well preserved even though their taxonomically critical features have been destroyed by silicification an understanding of the early taphonomic processes that influence microbial silicification can be obtained by studying extant microbes that are being silicified in modern hot spring pools iodine pool located in the waimangu geothermal area on the north island of new zealand is ideal for this purpose the spring water has a temperature of 69 100 c a ph of 8 3 9 0 and 440 457 ppm sio 2 glass slides left in the shallow marginal waters of this pool for 90 h became covered with thin layers of opaline silica discrete opal a spheres unsilicified microbes pseudofilaments partly silicified microbes and silicified microbes the rapidly silicified microbes appear well preserved with their general morphology diameter length and presence or absence of septa being readily apparent most of the silicified microbes however lack the key features that would allow accurate comparisons with extant taxa only two of the silicified microbes can be tentatively allied with the unsilicified forms despite being found side by side on the same glass slide these problems in identifying modern rapidly silicified microbes suggest that identifications of ancient silicified microbes can be problematic and must be treated with caution

the colville batholith tertiary plutonism in northeast washington associated with graben and core complex gneiss dome formation the colville batholith in the central okanogan highlands of washington is composed of numerous individual granitoid intrusions plutons most of which may be grouped on the basis of field evidence modal analysis and major element chemical composition into three successive intrusive suites the keller butte suite kbs the devils elbow suite des and the herron creek suite hcs the kbs is distinguished from the younger intrusive suites by 1 modes restricted to the fields of granite and granodiorite 2 lack of horn blende 3 a relatively fe rich afm variation trend 4 an alkali lime index classification of calcic as opposed to calc alkalic and alkali calcic for the younger suites 5 compositions 64 sio2 and 6 low mgo relative to sio2 the des is distinguished from the hcs by 1 its lower modal alkali feldspar and correspondingly low k2o relative to sio2 and 2 a slightly more fe rich afm variation trend biotite and hornblende k ar ages the most believable and geologically significant of which are concordant ages from plutons of the des indicate the batholith is early tertiary in age norms of highly evolved residual aplitic granite suggest that the magmas completed crystallization at 2 to 3 km depth the structural relations of the plutonic rocks suggest that associated core complex and graben structures were active both before and after emplacement of kbs and des plutons and that the graben structures continued to develop until after emplacement of the hcs the structural relations and radiometric ages taken together lead to the conclusion that the batholith core complexes and graben structures are all broadly contemporaneous and chiefly of early tertiary age

the tuscarora au ag district eocene volcanic hosted epithermal deposits in the carlin gold region nevada the tuscarora mining district contains the oldest and the only productive eocene epithermal deposits in nevada the district is a particularly clear example of association of low sulfidation deposits with igneous activity and structure and it is unusual in that it consists of two adjoining but physically and chemically distinct types of low sulfidation deposits moreover tuscarora deposits are of interest because they formed contemporaneously with nearby giant carlin type gold deposits the tuscarora deposits formed within the 39 9 to 39 3 ma tuscarora volcanic field along and just outside the southeastern margin of the caldera like mount blitzen volcanic center both deposit types formed at 39 3 ma contemporaneous with the only major intrusive activity in the volcanic field no deposits are known to have formed during any of the intense volcanic phases of the field intrusions were the apparent heat source and structures related to the mount blitzen center were conduits for hydrothermal circulation the ore forming fluids interacted dominantly with eocene igneous rocks the two deposit types occur in a northern silver rich zone that is characterized by relatively high ag au ratios 110 150 narrow alteration zones and quartz and carbonate veins developed mostly in intrusive dacite and in a southern gold rich zone that is typified by relatively low ag au ratios 4 14 more widespread alteration and quartz fissure and stockwork veins commonly developed in tuffaceous sedimentary rocks the deposit types have similar fluid inclusion and pb and s isotope characteristics but different geochemical signatures quartz veins from both zones have similar thermal and paragenetic histories and contain fluid inclusions that indicate that fluids cooled from between 260 and 230 c to less than 200 c fluid boiling may have contributed to precious metal deposition veins in both zones have relatively high as and sb and low bi te and w the silver zone has high ca pb mn zn cd tl and se the gold zone has high hg and mo a few samples from an area of overlap between the two zones share chemical characteristics of both deposit types the deposit types could represent a single zoned or evolving system in which hydrothermal fluids rose along structures within the silver zone preferentially deposited ag and base metals and then spread into the gold zone alternatively the deposit types could represent two distinct but temporally indistinguishable hydrothermal cells that only narrowly overlapped spatially as noted in previous studies the hydrothermal fluids that generated the tuscarora and other epithermal deposits could have evolved from carlin type fluids by boiling and mixing with meteoric water if so the tuscarora deposit may represent epithermal conditions above carlin type deposits and carlin type deposits may lie beneath the district

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

formation of fine grained metal and silicate precipitates on a bacterial surface bacillus subtilis abstract the ability of the gram positive bacterium b subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of fe and a1 at concentrations close to those levels in soils and at slightly acid 5 5 and basic 8 0 ph in all cases formation of silicate crystallites quasi crystalline precipitates on the bacterial surfaces was observed bacterially mediated minerals were more diverse in composition and morphology less crystalline smaller and sometimes more abundant than those that were abiotically formed fe pretreatment of the bacterial cells enhanced the binding of silicate at ph 8 0 walls which were not pretreated with fe bound silicate more favourably at acid values when heavy metals pb cd zn cr ni cu were added to the mixture at ph 4 5 silicate retention was greatly favoured giving greater retention of either si or metals than was seen in abiotic controls experiments with only heavy metals showed a high affinity of the bacterial walls for the metals even at low temperatures 4 c it is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls

the role of the shuswap metamorphic complex in cordilleran tectonism a review the shuswap metamorphic complex consists of three parts each with unique stratigraphy and orogenic evolution separated by major faults of diverse nature and having in common only a post late mesozoic tectonic history the first part the monashee complex is a possible extension of the precambrian shield that contains limited evidence of mesozoic orogenesis and that was rapidly uplifted during the cretaceous to paleogene the monashee decollement a warped mylonite zone interpreted as a regional thrust fault active through the middle jurassic separates this complex from the second part which contains rocks correlative with hadrynian to late paleozoic strata of the pericratonic prism the third part the okanagan complex straddling the 49th parallel from the okanagan valley to kootenay lake contains the probable exhumed roots of a mesozoic magmatic arc built upon possible north american continental and transitional crust and includes late paleozoic and early mesozoic suspect terranes the columbian or