chemistry of hydrothermal fluids from the 17 s active site on the north fiji basin ridge sw pacific abstract in june 1989 a diving cruise of the french submersible nautile was made along the north fiji basin spreading ridge the objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the kaiyo cruise in 1988 active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge the transparent venting fluid showed significant salinity depletion the extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the cl depleted hydrothermal fluid from the ashes vent site in the juan de fuca ridge these features suggest that the present hydrothermal end member resulted from subcritical phase separation which generated a low salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

phosphogenesis in the immediate aftermath of the great oxidation event evidence from the turee creek group western australia abstract phosphatic peloids and pebble sized microbially bound sediment clasts have been identified within dolomitic sediment that separates centimetric columnar stromatolites in a 20 150 cm thick horizon from the 2 4 2 2 ga turee creek group in western australia petrographic examination of the phosphatic clasts reveals complex internal textures that are defined by different mixtures of fine grained apatite medium grained dolomite irregular domains of partly euhedral micro quartz and carbonaceous matter phosphatic sand sized peloids observed within microbially bound sediment clasts are similar in size texture and composition to the individual peloids in the intercolumn sediment both occurrences have finely dispersed kerogen throughout and zoned cores that are packed with both microscopic apatite granules and larger euhedral quartz crystals the percentage of apatite vs quartz vs kerogen varies between studied peloids similar sized white peloids in the intercolumn sediment and in the microbially bound sediment clasts contain illite and are lined internally by euhedral apatite crystals 20 100 m in size some peloids are kerogenous in one domain and illite bearing in another the phosphatic microbially bound sediment clasts and peloids are interpreted to represent fragments of a peritidal phosphorite redeposited in an offshore setting during high energy events peloids formed within the microbially bound sediment and were subsequently eroded out and redeposited in the intercolumn sediment white peloids are interpreted to represent peloids affected by surficial weathering during temporary exposure when the fine grained kerogenous interior was replaced by clays these weathered peloids were then re sedimented and captured both within microbially bound sediment and in the intercolumn sediment euhedral apatite crystals that line the insides of illite bearing white peloids were dated to 2104 70 ma and 2041 33 ma using u pb and th pb dating methods respectively this age is interpreted to represent the time of metamorphic fluid circulation during a period of known regional metamorphism and provides a minimum age of the reef complex

multi scale morphologic variability of the north fiji basin ridge southwest pacific abstract two cruises of the french japanese starmer joint project carried out with the submersibles nautile 1989 and shinkai 6500 1991 have been devoted to the in situ exploration of the north fiji basin ridge four areas located near 16 20 17 18 50 and 19 s have been explored during 42 dives at 16 20 s the axial zone trending n160 corresponds to a prominent graben with walls 1000 m high tectonic features and recent screes are present and the hydrothermal activity is characterized by sporadic fluid discharge at 17 s present accretion is confined to a high dome less than 1900 m deep and cut in its axial part by a wide graben trending n15 at this site hydrothermal features of two types are observed fossil chimneys of sulfides and active vents made of anhydrite and expelling water at 285 c the site at 18 50 s is located on top of an axial dome very similar to those of the east pacific rise small fissures and very fresh volcanic flows argue for a present day activity with weak evidence of tectonics hydrothermal emanations with animal colonies are found scattered over the whole area the fourth site at 19 s shows old and viscous lavas and a very mature tectonic activity compared to the preceding station which is located only 20 km northward large scale variations have been detected between three segments different thermal regimes can explain the distinct morphologies observed decakilometric scale variations do occur between stations 14 with volcanic predominance and 19 s where tectonic activity is dominating finally small scale variations are observed on sites 4 and 14 where volcanic features gradually give way to tectonic structures the north fiji basin ridge with an intermediate average spreading rate shows tectonic volcanic and hydrothermal variability ranging from 1 km to tens of kilometres and related to a temporary fine scale organisation of the accretionary processes

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

helium and carbon geochemistry of hydrothermal fluids from the north fiji basin spreading ridge southwest pacific the north fiji basin is one of the active marginal basins in the southwestern pacific hydrothermal fluid emanations were located at two sites on the central ridge of the basin high temperature fluids 230 290 c venting from anhydrite chimneys at a 17 s site have end member compositions of 11 0 14 5 mmol kg of co2 30 4 43 5 mol kg of ch4 and 2 3 4 5 10 5 cm3stp g of he taking phase separation processes into account the gas abundances are not enriched compared with the mid oceanic ridge hydrothermal systems isotopic compositions of co2 13c 6 2 to 5 7 pdb ch4 13c 20 to 18 pdb and helium rra 9 0 10 0 are comparable to the mid oceanic ridge signature together with basalt helium data the helium isotopic signature may be attributed to the incorporation of a hotspot like primitive component low temperature shimmering fluids the highest measured temperature was 5 2 c associated with biological communities at a site at 18 50 s show slight chemical anomalies in some species in sio2 mn li ph and ch4 and helium isotope ratios distinct from ambient seawater evaluated helium isotopic compositions prior to dilution fall between rra 8 4 and 8 9 supporting the suggestion of high temperature hydrothermal activity at this site although this was not observed by dive expeditions the gas geochemistry of these two different types of fluids show several similar characteristics to the mid oceanic ridge hydrothermal systems this result is in accordance with previous petrological studies which demonstrated a dominant n morb source signature and a co existing oib source influence of the north fiji basin magmatism

resetting monazite ages during fluid related alteration abstract fluid related coupled dissolution reprecipitation processes can modify the composition of monazite and other orthophosphate minerals at temperatures well below diffusional closure temperatures it is critical to evaluate the effects of alteration processes on th u pb dates especially because incomplete removal of pb during alteration could lead to spurious dates either older or younger than the actual age of the monazite formation experimental alteration of monazite at 450 mpa and 450 c has resulted in the production of altered domains in the vicinity of the grain rim as well as in the vicinity of inclusions and microfractures without any obvious structural control within grains the composition of the altered domains is relatively constant from domain to domain and from grain to grain thorium was reduced from ca 7 wt tho 2 to ca 2 wt uranium and pb were almost entirely removed from the altered domains while unaltered areas of the monazite grains as well as the original monazite grains before the experiment yield very consistent dates of ca 350 ma dates from the altered domains are consistently near zero the implication is that fluid aided alteration of monazite for the conditions of this experiment has resulted in a near complete resetting of the monazite age at temperatures well below the closure temperature because alteration textures in monazite are relatively common in nature careful analysis of such textures offers a means of recognizing and potentially dating fluid rock interactions

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

neoproterozoic peritidal phosphorite from the sete lagoas formation brazil and the precambrian phosphorus cycle the neoproterozoic sete lagoas formation ca 610 ma of the sao francisco basin brazil is a succession of siltstone limestone and phosphorite phosphorite forms part of a previously unrecognized 150 to 200 m thick unconformity bounded depositional sequence lithofacies stacking patterns indicate that deposition was punctuated by higher order fluctuations in base level that produced aggradational parasequences these shallowing upward cycles record the progradation of phosphate rich intertidal flats over shallow subtidal deposits as accommodation filled the presence of mudcracks authigenic chert nodules lack of coarse terrigenous clastics and the abundance of silt with fine abraded quartz grains suggests accumulation along an arid coastline with significant aeolian input delivery of phosphorus adsorbed on aeolian fe oxyhydr oxide and clay is interpreted as having stimulated phosphogenesis in peritidal environments lithofacies associations indicate that windblown phosphorus promoted the establishment of cyanobacterial communities along the coast which produced photosynthetic oxygen and the suboxic conditions necessary for the precipitation of authigenic carbonate fluorapatite as in other precambrian phosphatic systems nearshore oxygen oases were a prerequisite for phosphorite accumulation because redox sensitive phosphogenic processes were pushed into the sediment to concentrate phosphorus in more distal anoxic environments phosphorite could not form because these biotic and abiotic processes were suspended in the water column which cycled phosphorus in sea water rather than at the sediment water interface such shallow water phosphorite is unlike larger younger neoproterozoic phanerozoic phosphatic deposits inferred to have formed in deeper upwelling related environments the increasing size of phosphatic deposits through the latest precambrian is interpreted as reflecting the progressive ventilation of the oceans during the neoproterozoic oxygenation event and resultant expansion of phosphogenic environments into distal settings the widespread cycling of bioavailable phosphorus at the sea floor not only produced the first true phosphorite giants but may have also been an important precondition for the evolution of multicellular animals

800 km long n s spreading system of the north fiji basin abstract the 800 km long n s spreading system in the north fiji basin consists of six contiguous fan shaped rift segments each 100 200 km long with various structural styles such as structural overprinting triple junction rift propagation voluminous magmatism and transform or strike slip fault influence deformation of the plate boundary system within the young hot weak lithosphere occurred under local stress conditions resulting in a unique rift segmentation distinct from that of midoceanic ridges spreading across a series of short and variably oriented segments produces series of fanning spreading centers whose rotation pole is located at the end of each segment the surrounding seafloor basement has adjusted to this setting with non rigid deformation or with fragmentation furthermore the changing tectonic framework caused by the arc rotation probably makes it difficult to keep the same stress condition for a long period small scale fan shaped sea floor spreading is short lived and results in a rapidly evolving plate boundary geometry this suggests a more complicated tectonic style for marginal basins than mid ocean ridge spreading centers

geology and geochemistry of a 800 m section through young upper oceanic crust in the north fiji basin southwest pacific lagabrielle y auzende j m eissen j p janin m c and cotten j 1994 geology and geochemistry of a 800 m section through young upper oceanic crust in the north fiji basin southwest pacific in j m auzende and t urabe editors north fiji basin starmer french japanese program mar geol 116 113 132 we report the results of geological and structural observations made during one dive of the french submersible nautile during the starmer cruise in 1989 in the vicinity of the 16 40 s triple junction in the north fiji basin this dive provided a spectacular 800 m continuous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction a number of volcanic facies have been encountered including massive and pillowed lavas sheeted dikes and spectacular columnar jointed massive lavas pillowed lavas have been observed only at the base and at the top of the section the ratio between massive and pillowed basalts is high suggesting that the crust in this area grew mostly during a stage of high eruptive rate dikes have been observed in the middle part of the section they probably do not represent the top of a classical dike complex but an isolated set of sheeted dikes possibly the feeders of overlying flows the abundance of vertical tectonic breccias observed during the first part of the dive confirms that the basal part of the wall can be regarded as a major tectonic boundary along which occurred significant vertical and strike slip motions micropaleontological data from sedimentary rocks collected during the dive bring new constraints to the evolution of the triple junction the oceanic crust in the surveyed area is at least as old as 1 9 1 3 ma based on the age of sedimentary rocks collected in talus at the base of the wall and observed at the summit of the section the collected basalts include 1 n type morbs 2 e type morbs and 3 babbs this emphasizes the heterogeneous nature of the mantle beneath the central nfb some portions of the depleted mantle underlying the central nfb triple junction area have recorded contamination from melts or fluids from an ancient subduction zone probably the new hebrides during the opening of the nfb whereas adjacent mantle areas show the influence from an alkali enriched source such alkali enriched characters are reported from recent basalts of the northern nfb south pandora ridge rotuma island

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

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

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

recent geodynamic evolution of the north fiji basin southwest pacific a cruise of the r v jean charcot seapso iii december 1985 in the north fiji basin between 16 and 22 s obtained new structural and magnetic data concerning the central part of the basin these have been used for the reinterpretation of the national oceanic and atmospheric administration and u s naval research project aeromagnetic survey 1979 the structural surface ship magnetic and aeromagnetic data allow us to present a sketch of the evolution in four stages of the north fiji basin for the past 10 m y

giant iron ore deposits of the hamersley province related to the breakup of paleoproterozoic australia new insights from in situ shrimp dating of baddeleyite from mafic intrusions banded iron formations of the ca 2770 2405 ma hamersley province of western australia were locally upgraded to high grade hematite ores during the early paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen ore genesis was associated with the stratigraphic break between the lower and upper wyloo groups of the ashburton province and has been variously linked to the ophthalmian orogeny late orogenic extensional collapse and anorogenic continental extension small spot in situ pb pb dating of baddeleyite by sensitive high resolution ion microprobe shrimp has resolved the ages of two key suites of mafic intrusions constraining for the first time the tectonic evolution of the ashburton province and the age and setting of iron ore formation mafic sills dated as ca 2208 ma were folded during the ophthalmian orogeny and then cut by the unconformity at the base of the lower wyloo group a mafic dike swarm that intrudes the lower wyloo group and has a close genetic relationship to iron ore is ca 2008 ma slightly younger than a new syneruptive 2031 6 ma zircon age for the lower wyloo group these new ages constrain the ophthalmian orogeny to the period between ca 2208 and 2031 ma before lower wyloo group extension sedimentation and flood basalt volcanism the ca 2008 ma dikes pre s ent a new maximum age for iron ore genesis and deposition of the upper wyloo group thereby linking ore genesis to a ca 2050 2000 ma period of continental extension similarly recorded by paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca 2320 ma

in situ geological and geochemical study of an active hydrothermal site on the north fiji basin ridge abstract in june 1989 a diving cruise by the submersiblenautile was carried out on the north fiji basin spreading ridge the objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes this operation was the third cruise of the french japanese starmer project following the two cruises of the r v kaiyo in 1987 and 1988 six dives along the spreading axis between 16 58 s and 17 00s show that the axial graben consists of alternating n15 trending horsts and grabens extinct hydrothermal sites have been observed all along the graben these consist of fossil chimneys oxide staining and dead shells at 16 59 s an active chimney has been discovered and called the white lady because of its almost exclusively anhydrite composition this chimney expels a peculiar water characterized by low chlorinity and a 285 c maximum temperature in the northern extremity of the n15 axis a wide fossil hydrothermal site has been explored and sampled it is located in an area cut by n15 n140 and n60 trending faults and fissures

multiple active spreading centres in the hot north fiji basin southwest pacific a possible model for archaean seafloor dynamics abstract the north fiji basin nfb in the southwest pacific is one of the most evolved back arc basins within the basin active spreading is taking place along several ridges some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates segmented accretion centres organized magnetic lineations and propagators for example however the great cumulate length of the spreading centres of the nfb relative to the basin s surface makes it an unique example among the world s oceanic basins we have reviewed a comprehensive body of geological geophysical and geochemical evidence multiple spreading centres of various trends high heat flow high geoid and slow upper mantle seismic velocities which collectively demonstrate that a hot upper mantle underlies the north fiji basin active upper mantle convection is the key process controlling the large crustal and lithospheric production in the nfb which would be thus largely independent from the evolution of even the largest structural features such as the vitiaz and new hebrides subduction zones petrological studies of the archaean greenstone belts have led to a similar picture for the entire earth during the archaean numerous short active ridges connected by unstable triple junctions would have at this time provided an efficient convective mechanism allowing for a large heat dissipation we propose that the presently active processes in the north fiji basin are modern analogues to archaean seafloor dynamics

sedimentology of the paleoproterozoic kungarra formation turee creek group western australia a conformable record of the transition from early to modern earth abstract this paper presents the first detailed sedimentological analysis of the paleoproterozoic kungarra formation the lowermost of three formations comprising the turee creek group in western australia which was deposited across the rise in atmospheric oxygen the great oxidation event or goe and the transition from early to modern earth the data show that the kungarra formation has a gradational conformable lower contact with underlying banded iron formation of the hamersley group and predominantly comprises an upward shallowing succession from deepwater shales and siltstones through rippled fine grained sandstones and stromatolitic carbonates to tidal flat deposits that immediately underlie coastal fluvial deposits of the overlying koolbye formation at the base of the kungarra formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron formation that pass upsection to units of non magnetic shale and ferruginous chert and grey chert reflecting a gradual loss of iron from the world s oceans accompanying the rise of atmospheric oxygen a falling stage systems tract is recognised above this transition in the hardey syncline area capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock two glacio eustatic cycles are recognised within the middle to upper parts of the kungarra formation each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest in contrast to previous models of a north northeastward deepening foreland basin the lack of seismites or internal unconformities within the formation precludes a foredeep setting rather deposition is interpreted as having occurred within an intracratonic basin with detritus sourced from erosion of uplifted bedrock to the southeast

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

crystal chemical controls on rare earth element concentrations in fossil biogenic apatites and implications for paleoenvironmental reconstructions abstract fossil biogenic apatites display rare earth element ree patterns which have been proposed to reflect environmental and biological controls on past seawater composition these patterns can be separated in two groups 1 patterns that are similar to those of open ocean and epicontinental waters and 2 patterns that exhibit a strong enrichment in intermediate ree bell shaped patterns because ree trapping in biogenic apatites occurs mostly post mortem it is essential to understand the crystal chemical inorganic factors that can influence the fractionation of ree between apatite and water a model is proposed by which partition coefficients of ree between apatite and water for a substitution mechanism can be extrapolated from mineral melt partition data to seawater or sedimentary basin conditions these are compared with available experimental partition coefficients for ree adsorption mechanism and used to discuss ree patterns in fossil biogenic apatites calculations of ree patterns for apatites at equilibrium with water show that the bell shaped patterns in fossil apatites can be explained by fractionation with seawater or continental fluids at low temperature under crystal chemical control involving a substitution mechanism and in the context of extensive or late diagenesis in that case the equilibrium composition of the fluid cannot be unequivocally determined from observed ree concentrations in fossil apatites this type of sample is inappropriate to discuss the past environmental or biological controls on ree concentrations in the ocean other sample compositions suggest little to moderate alteration of the original oceanic to freshwater pattern through the adsorption mechanism in the context of weak or early diagenesis and may be used with caution for that purpose

stromatolite reef crusts early cretaceous spain bacterial origin of in situ precipitated peloid microspar peloidal crusts are significant components of early cretaceous aptian reef carbonates in eastern spain the crusts form steep sided laminated deposits on coral and other skeletal surfaces their microfabric consists almost entirely of silt sized peloids in fenestral microspar matrix this microfabric contrasts with more poorly sorted and generally finer grained detrital wackestone packstone fabrics of the adjacent reef matrix scarcity of incorporated grains indicates that the crusts did not trap many particles it is proposed that the crusts are stromatolites and that peloids and inter peloid space were created concurrently by bacterial degradation of organic matter as they developed inter peloid voids were protected from infiltration of extraneous sediment by the organic rich exterior surface of the stromatolite even spacing of the peloids within microspar may reflect self organization of bacterial colonies in the decaying organic matrix compressed and partly amalgamated peloids marginal to burrows in the stromatolites suggest that the peloid fabrics were initially only partially lithified the grainstone like peloid fabric is therefore interpreted as having formed in situ by very early diagenetic processes driven by heterotrophic bacteria

formation of hydrothermal vents in the north fiji basin sulfur and lead isotope constraints abstract hydrothermal sulfides were recovered from the 16 50 s triple junction area in the north fiji basin at a water depth of ca 1900 m the chimney samples can be divided into three groups based on their major metal contents 1 type 1 fe cu rich 2 type 2 fe cu rich with minor zn and 3 type 3 zn rich chimneys type 1 chimneys are mainly composed of chalcopyrite and pyrite and are enriched in elements commonly precipitated under high temperature conditions 300 c such as cu co mo and se type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite and are enriched in elements commonly associated with relatively low temperatures 150 to 250 c such as zn cd pb as and ga type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples type 2 chimneys appear to be the products of low temperature replacement which might be related to formation of type 3 chimneys of type 1 chimneys the trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources n morb and e morb sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys the type 1 sulfides 34 s 2 8 1 2 206 pb 204 pb 18 082 to 18 132 207 pb 204 pb 15 440 to 15 481 208 pb 204 pb 37 764 to 37 916 are depleted in 34 s and have lower pb isotope ratios compared to types 2 and 3 chimneys 34 s 4 1 0 9 206 pb 204 pb 18 122 to 18 193 207 pb 204 pb 15 475 to 15 554 208 pb 204 pb 37 882 to 38 150 the higher 34 s values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater the more radiogenic pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to e morb volcanism that shows higher pb isotope composition than n morb rocks however ferromanganese crust samples have radiogenic pb isotope compositions that are identical to those of e morb therefore input of hydrogenous pb is also a possible source for radiogenic pb isotopes in the types 2 and 3 chimneys

possible ree constraints on the depositional and diagenetic environment of doushantuo formation phosphorites containing the earliest metazoan fauna abstract the neoproterozoic doushantuo formation south china preserves a unique assemblage of what are probably the earliest metazoan fossils sponges cnidarians and bilaterians in the world that could contribute to a better understanding of early faunal evolution on earth the formation spans an interval of dramatic biological chemical and climatic change in weng an mine in southwest china extensive paleontological investigations show embryonic larval and adult animal fossils occur in the upper ore bed of the doushantuo phosphorite but no animal forms have been found in the lower ore bed taphonomy and experiments on fossilization suggest that no fauna lived on or in the lower ore bed as it was deposited rare earth element ree data for the doushantuo phosphorites in weng an document the paleoenvironmental condition under which the phosphorites were deposited the shale normalized ree patterns of the upper phosphorite bed upb are characterized by old vendian early cambrian phosphorite patterns the ree patterns of the lower phosphorite bed lpb are significantly different with less negative ce anomalies and less lree and hree depletion the ree characteristics suggest that the doushantuo phosphorites record seawater condition at the time of sediment deposition and are not affected by subsequent diagenesis the ree compositions of the doushantuo phosphorites show that the marine depositional environment changed from anoxic in the lower ore bed to oxic in the upper ore bed the uniquely diverse and abundant doushantuo eukaryote fossil accumulations at weng an thus may be related to a change from anoxic to oxic bottom water conditions

lateritic weathering of pyroxenites at niquelandia goias brazil the supergene behavior of nickel two typical lateritic weathering profiles jacuba ahd angiquinho from the niquelandia ni deposits brazil were studied in order to establish the petrological relationship between the supergene ni products and the parental pyroxenes frbm the base to the top of the profiles pyroxenes are replaced by goethite and kaolinite through a series of transitional ni bearing phyllosilicates the mineralogy and the chemical composition especially the ni content of these clay minerals depends on the degree of fracturing and serpentinization of the pyroxenite and the location of the pyroxenite with respect to neighboring dunite within the jacuba profile smectite and pimelite pseudomorphs after pyroxene are especially ni rich and in fact are the most ni enriched clay minerals now known in lateritic weathering profiles

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

sedimentary phosphate and associated fossil bacteria in a paleoproterozoic tidal flat in the 1 85 ga michigamme formation michigan usa abstract phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release concentration and mineralization preserved fossil bacteria in phosphate crusts and grains from the ca 1850 million year old bijiki iron formation member of the michigamme formation michigan provide insight into the longevity and nature of this relationship the michigamme formation accumulated near the end of the earth s initial phosphogenic episode ca 2 2 and 1 8 ga to produce one of the first granular phosphorites phosphatic lithofacies consist of fine to medium sand sized francolite peloids concentrated on bedding surfaces in peritidal facies granular beds are up to 2 cm thick and peloids are often partially to completely replaced by dolomite and chert the grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction the peritidal nature of phosphorite in the michigamme formation is in sharp contrast to phanerozoic phosphogenic environments in deeper coastal upwelling settings peritidal settings were well suited for phosphogenesis under the very low oxygen and low dissolved sulfate levels of the paleoproterozoic as cyanobacteria produced oxygen in shallow water and evaporation led to increased sulfate concentrations such concomitant processes helped establish focused redox interfaces in the sediment that chemosynthetic bacterial communities sulfur oxidizers reducers forms that concentrate p and possibly iron oxidizers could exploit phosphate released from organic matter by heterotrophic bacteria and fe redox pumping was further concentrated by these chemotrophs a process that forms late neoproterozoic to phanerozoic phosphorites but on a much larger scale this early example of a granular phosphorite demonstrates that like their phanerozoic counterparts paleoproterozoic phosphorites are the concentrated indirectly biomineralized products of bacterial communities but unlike younger analogs which accumulated across subtidal shelves and shelf margins these ancient deposits formed only in tidal flat settings where phosphogenic redox processes could be established in the sediment from this early beginning the zone of phosphogenesis likely migrated into deeper water settings as oxygen and sulfate levels rose expanding the zone of chemosynthetic bacterial and associated phosphogenesis across the shelf

a pre 2 2 ga age for giant hematite ores of the hamersley province australia one current model for the origin of giant hematite ores of the hamersley province requires a two step process formation of martite and goethite by supergene enrichment followed by burial metamorphism to convert goethite to microplaty hematite our new structural stratigraphic and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower wyloo group a shrimp u pb zircon age from volcaniclastic breccia within the lower wyloo group suggests that hematite detritus in that group was derived from hematite ores older than 2209 or 15 ma the action of pre 2 2 ga oxidizing fluids is implied although their origin supergene versus hypogene remains controversial

petrology and geochemistry of the central north fiji basin spreading centre southwest pacific between 16 s and 22 s abstract the north fiji basin nfb is a 12 m y old back arc basin that has a complex multi stage history the presently active spreading system can be divided into four segments between 16 s and 22 s which from north to south trend n160 n15 and n s the fourth segment is the n s trending segment located near 174e the main n s segment is morphologically similar to other medium rate oceanic ridges whereas the other segments have rougher morphologies which have been severely disturbed by a triple junction at 16 45 s and several instability features such as overlapping spreading centres oscs and propagating rifts the spreading rate seems to diminish from 7 8 cm yr near 20 s to 4 6 cm yr near 18 s mineralogical pertological and geochemical data were obtained on 24 new stations located along all the four segments the petrogenesis of the basalts collected is essentially controlled by low pressure crystal fractionation of plagioclase olivine clinopyroxene plagioclase olivine clinopyroxene with 52 of the nfb basalts reaching the four phase cotectic locally some magma mixing occurs but this is limited to magma batches of closely related composition as might be expected to occur inside a magma reservoir the n s segment which since 3 m y is the only steady state segment is also petrologically and geochemically very comparable to other medium rate oceanic spreading centres producing moderately evolved lile and lree depleted n morb in contrast the three other segments produce basalts of much more variable petrology and geochemistry characterized by lile and slightly lree enriched magmas cf back arc basin basalt babb affinity but not as enriched as for example the mariana babb morb is however also found on the n160 n15 and 174e segments diagrams using ba rb k p and k ti n normalized to the chondrites plotted against latitude clearly show along strike variations beneath the recently formed segments the mantle source is heterogeneous and locally has some babb affinities whereas beneath the more steady state n s segment the magma source is more homogeneous being generally depleted in lile and ree as is the case for classical n morb mantle sources simple evolution from an early stage of babb production to morb described in the lau basin and proposed for the nfb does not seem to occur present day activity still produces large amounts of babb along the less stable and more recently created segments and morb was produced in the earlier stages of the development of the nfb

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

fractionation of silicon isotopes during biogenic silica dissolution abstract silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization primarily by diatoms in surface waters of the ocean interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean as driven by biological fractionation during the production of biogenic silica bsio2 or opal by diatoms however other factors which may influence the silicon isotope composition of diatom opal particularly post formational aging and maturation processes remain largely uninvestigated here we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bsio2 suspended in seawater under closed conditions this fractionation acts counter to that occurring during bsio2 production and at about half its absolute magnitude with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor edsi bsi of 0 55 corresponding to a fractionation factor 30 28 of 0 99945 the enrichment factor did not vary with source material indicating the lack of a significant species effect or with temperature from 3 to 20 c thus the dissolution of bsio2 produces dissolved silicon with a 30si value that is 0 55 more negative than its parent bsio2 an effect that must be accounted for when interpreting oceanic 30si distributions the 30si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed implying a measurable 0 1 change in the relative 30si of opal samples whenever the difference in preservation efficiency between them is 20 this effect could account for 10 30 of the difference in diatom 30si values observed between glacial and interglacial conditions it is unlikely however that the inferred maximum possible change in b30sio2 of 0 55 would be manifested in situ as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

propagating rift and overlapping spreading center in the north fiji basin abstract the present day geometry of the north fiji marginal basin at the pacific and indo australian plate boundary results from a polyphased tectonic evolution since the beginning of the opening spreading occurred along several ridge axes that are characterized by highly variable trends from 15 s to 21 40 s the present day trends of the axis are mainly n20 w n20 e and n n5 e the structural analysis of multi beam echo sounder and seismic data combined with submersible observations 18 dives in this area allow a precise study of relationships between the n n5 e and n20 e axes near 18 30 s two main oceanic features occur in this area the most striking one consists in a broad northward propagating rift of the n n5 e spreading ridge the northern tip of the propagator is located at 18 10 s on 173 30 e and is characterized by a northern v shaped end and a slight camber the diving observations corroborate the present day magmatic and tectonic activities along the n n5 e spreading axis located between 18 10 s and 20 30 s and its northward propagation to the detriment of the n20 e axis which is less active at its southern end moreover the youngest axis trends inside the n20 e axial domain confirms this northward propagation of the n n5 e structural trends the n n5 e spreading axis shows very close similarities with those of an intermediate to fast spreading ridge either in its morphology or in its spreading rate on the other hand the n20 e axis displays a large tectonically active overlapping arm that is counterposed to the northern end of the n n5 e propagating system the transform zone between the two overlapping arms is characterized by typical oblique and curved structural patterns in the sea floor the whole tectonic system constitutes thus a typical large overlapping spreading center the average propagating rate is calculated at 5 7 cm yr that is compatible with backward migration of the n20 e overlapping arm this study confirms the high instability of the north fiji basin spreading system through time and space due to the regional tectonic stress produced by the plate convergence and the double sphenochasmic opening of the basin

stratigraphic architecture and evolution of the early paleoproterozoic mcgrath trough western australia abstract palaeocurrents provenance and stratigraphic analysis of the upper part of the mount bruce supergroup and the wyloo group in the southwestern part of the hamersley province of the pilbara region of western australia have been used to define an early paleoproterozoic foreland basin the mcgrath trough in front of the northwards advancing ophthalmian fold belt the foreland basin succession commences with the boolgeeda iron formation and includes the overlying turee creek and lower wyloo groups the basin geometry is asymmetric thinning northwards away from the ophthalmian fold belt and the trough axis trends east southeast parallel to the ophthalmian folds multiple intraformational unconformities in the upper turee creek group record the advance of the fold belt into the axis of the mcgrath trough in the hardey syncline area east southeast trending folds and steep southward dipping cleavage post date deposition of the beasley river quartzite a shallow marine to fluvial sandstone that records an interval of tectonic quiescence during basin evolution deposition of the beasley river quartzite was followed by renewed uplift of the southern hinterland resulting in continued deposition of fluvial siliciclastics into the foreland basin which was eventually filled with the eruption of the cheela springs basalt the age of the mcgrath trough is constrained broadly between 2 45 ga the age of the underlying woongarra rhyolite and 2 2 ga the age of the cheela springs basalt at the top of the succession the sedimentary fill of the mcgrath trough was derived mainly from the south and becomes texturally and compositionally more mature upward reflecting its recycled orogen provenance two main sources are identified a dominant cratonic source interpreted to be similar to the granitoid greenstone basement of the pilbara craton and a volumetrically lesser source from banded iron formation bif and volcanic and sedimentary rocks of the mount bruce supergroup felsic volcanic detritus likely to have been derived from the underlying woongarra rhyolite is most common in the lower turee creek group and becomes increasingly less common upsection although it is reworked from underlying formations chert and bif clasts appear first in the upper turee creek group and are important components of the most proximal alluvial fan to braided stream deposits in the upper turee creek and lower wyloo groups grains of microplaty haematite identical in appearance to microplaty haematite in the giant hamersley iron ore deposits appear in conglomeratic units from the base of the beasley river quartzite upwards these fluvial sediments were derived from the south and record erosion of ore grade deposits from the uplifted ophthalmia fold belt during lower wyloo group times

palaeoproterozoic terrestrial sedimentation in the beasley river quartzite lower wyloo group western australia abstract pre 2 2 ga aeolianite deposits are relatively rare in the geological record due to reworking of aeolianites either by fluvial systems transgression or non recognition here we present high resolution sedimentary facies analysis of a section through the 2 2 ga beasley river quartzite lower wyloo group western australia the unambiguous presence of terrestrial fluvial aeolian deposition is documented in the form of fluvial architectural elements channel bar lateral accretion and overbank deposits and aeolian features dune pin stripe lamination wind streaks and adhesion features these observations contrast strongly with a previous interpretation of marine deposition which is discounted our data is consistent with the dominantly terrestrial depositional mode of the rest of the lower wyloo group including the basal three corners conglomerate member and the subaerial cheela springs basalt we conclude that the lower wyloo succession formed in a terrestrial regime during continental rifting

marine peloids a product of bacterially induced precipitation of calcite high magnesian calcite peloids are a common constituent of cemented marine carbonate accumulations they are abundant and well developed within both isolated microcavities and surface crusts the nuclei of marine peloids are composed of fossil bacterial clumps encased within anhedral submicron to micron sized high magnesian calcite crystals the marine peloids are similar to silt sized particles found in travertine deposits that also are composed of fossil bacterial clumps and their surrounding calcite coronas these similarities include size 20 60 mu m a cloudy brown nucleus and rims of clear euhedral crystals in addition previous studies have established the presence of fatty acids of bacterial origin within peloidal deposits and enriched stable isotopic signatures have been interpreted as recording organic fractionation concomitant with calcite precipitation these characteristics all indicate that the nuclei of many peloids originate as calcite precipitates within and around clumps of bacteria and that this precipitation was induced by the vital activity of bacterial colonies

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

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

constraining atmospheric oxygen and seawater sulfate concentrations during paleoproterozoic glaciation in situ sulfur three isotope microanalysis of pyrite from the turee creek group western australia abstract previous efforts to constrain the timing of paleoproterozoic atmospheric oxygenation have documented the disappearance of large mass independent sulfur isotope fractionation and an increase in mass dependent sulfur isotope fractionation associated with multiple glaciations at least one of these glacial events is preserved in diamictites of the 2 4 ga meteorite bore member of the kungarra formation turee creek group western australia outcrop exposures of this unit show the transition from the boolgeeda iron formation of the upper hamersley group into clastic glaciomarine sedimentary rocks of the turee creek group here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution as well as the occurrence of detrital pyrite in the meteorite bore member the 15 3 range of 33 s in one sample containing detrital pyrite 3 6 to 11 7 is larger than previously reported worldwide and there is evidence for mass independent sulfur isotope fractionation in authigenic pyrite throughout the section 33 s from 0 8 to 1 0 the 90 range in 34 s observed 45 5 to 46 4 strongly suggests microbial sulfate reduction under non sulfate limiting conditions indicating significant oxidative weathering of sulfides on the continents multiple generations of pyrite are preserved typically represented by primary cores with low 34 s 34 s 4 7 and enrichments in as ni and co the preservation of extremely sharp sulfur isotope gradients 30 p o 2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides yet remained low enough to permit the production and preservation of mass independent sulfur isotope fractionation

propagating rifts in the north fiji basin southwest pacific the accretion system in the north fiji marginal basin currently consists of four major differently oriented axes multibeam bathymetry recently acquired over the central north south axis between 18 and 21 s reveals a peculiar tectonic framework fanned patterns of the sea floor topography at the northern and southern tips of this segment are the general shape of a rugby ball these data and the interpretation of magnetic anomalies indicate that this central sector of the ridge has lengthened northward at the expense of the axis aligned n15 e both spreading and propagation rates are about 70 80 mm yr a southward propagation is also envisaged on the basis of the similar morphotectonic framework observed at the southern tip of the north south ridge