high magnesium staurolite in a sapphirine garnet rock from the limpopo belt southern africa unusually mg rich staurolite with f fe fe mg 0 51 occurs in a peraluminous silica deficient sapphirine garnet gedrite spinel corundum phlogopite rock as relic inclusions within pyrope almandine garnet f 0 43 0 48 from which it is generally separated by sapphirine f 0 19 gedrite has f 0 24 0 27 phlogopite 0 19 and spinel 0 43 the gradual disappearance of staurolite is due to a shift of the 3 phase afm assemblage staurolite garnetsapphirine to more fe rich compositions according to probably prograde reactions between staurolite and garnet to form sapphirine together with either quartz or gedrite thus still more mg rich staurolite seems to have existed in the rock at earlier stages of metamorphism characterized by lower temperatures and perhaps higher pressures the local equilibria now frozen in indicate minimum pressures of 7 8 kbar and temperatures in excess of about 800 c

hydrogen zoning in zinc bearing staurolite from a high p low t diasporite samos greece a combined emp sims fib ftir study li rich zincostaurolite occurs as millimeter long crystals at the marble footwall of a meta karst bauxite on eastern samos the samos rocks have been metamorphosed during an early alpine high p low t metamorphism m1 followed by a late alpine greenschist grade overprint m2 textures and mineral chemistry indicate that staurolite formed from gahnite cookeite and pyrophyllite during the early m1 stage staurolite crystals show growth zoning with cores enriched in zn concentrations of fe mg co and to a minor extent li increase toward the rims hydrogen concentrations were analyzed by sims they are significantly higher in cores up to 5 97 atoms h per 48 o compared to rims 3 9 to 4 5 atoms h and clearly negatively correlated with al synchroton light polarized ftir spectra on oriented fib prepared foils show the same zonation effect the absolute hydrogen concentrations being systematically lower by about 25 the discrepancy is caused by sub micrometer scale hydrogen loss at the crystal surface during fib thinning this staurolite is unique as from the three available hydrogen sites the h3 site has the highest occupation ever observed whereas the h2 site is not occupied this is probably due to the high li content the zonation in hydrogen is interpreted as reflecting the two stage growth m1 staurolite that formed a low t of about 400 450 c and high p of 1 5 gpa incorporated nearly the maximum amount of hydrogen allowed by the staurolite structure 6 h pfu and was subsequently overgrown and marginally replaced during the m2 stage by less hydrous fe co richer staurolite hydrogen zoning in staurolite is facilitated by the sensitivity of its structure to changing p t conditions water in staurolite is maximized at high p and low t cores of staurolite from samos represent the most hydrous staurolite compositions reported to date

distributions of hydrothermal activity along the mid atlantic ridge interplay of magmatic and tectonic controls abstract hydrothermal activity has been investigated along three different sections of the slow spreading mid atlantic ridge mar 11 20 30 n 36 38 n and the reykjanes ridge 57 45 63 06 n when considered in total the incidence of venting along these three sections of the mar compares well with the predictions of a model in which frequency of venting is linearly related to ridge crest spreading rate at the scale of individual study areas however departure from the model is observed by up to an order of magnitude venting is anomalously rare along the reykjanes ridge but anomalously abundant along the mar 36 38 n whilst such variability may be within the error of the linear spreading rate model we note that the interplay between magmatic and tectonic processes also differs between the three study areas in the case of the reykjanes ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling investigative strategy because the style of venting which predominates may not give rise to conventional black smoker hydrothermal plumes along the oblique and broadly segmented mar 36 38 n we propose that vigorous hydrothermal venting in broad segment end non transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

second order segmentation the relationship between volcanism and tectonism at the mar 38 n 35 40 n deep tow sidescan sonar data acquired along 240 km of the mid atlantic ridge mar between 35 40 n and 38 n have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second order segments and their bounding non transform offsets ntos the segments show a range of volcanic and tectonic types but in general the northernmost segments i e those with greater influence from the azorean hotspot are shallower and more volcanically robust than those to the south with hydrothermal activity in segment centres for example menez gwen and lucky strike nonetheless this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment the ntos are broad right stepping discontinuities locally up to 25 km wide and accommodate offsets between 10 and 50 km the discontinuities are mostly sediment floored and link the spreading segment tips with a range of structures these include locally dense arrays of en echelon extensional normal faults short lengths of linear strike slip fault strands occasionally cutting basement blocks apparently stranded within the offsets basement blocks within the offsets are cross cut by complexes of intersecting faults suggesting that deformation is distributed across the zone the pervasive faulting taking place at the ntos favours fluid circulation and associated hydrothermal activity as at the rainbow site at 36 17 n

different tdm ch4 hydrothermal plume signatures tag site at 26 n and serpentinized ultrabasic diapir at 15 05 n on the mid atlantic ridge as a part of the 1988 noaa vents program ch4 and mn tracers were used to identify and compare hydrothermal plumes found above the tag field 26 n and in the rift valley at 15 n close to the eastern intersection of the ridge axis with the 15 20 n fracture zone at the mid atlantic ridge mar active hydrothermal venting was confirmed at tag based on elevated concentrations of total dissolved mn tdm up to 30 nmol kg high ch4 concentrations up to 200 nl l and elevated nephelometry signals plumes of a different composition were identified at 15 n with high ch4 concentrations up to 400 nl l low total dissolved mn concentrations tdm 1 nmol kg and no significant nephelometry signal the different properties of these tracers and the different tracer ratios can be used to deduce vent fluid characteristics and compare one hydrothermal area to another tdm ch4 and nephel ch4 ratios at tag are of the same order of magnitude as those observed at other spreading axis hydrothermal fields at 15 n the low tdm ch4 ratio provides evidence of fluid circulation into ultrabasic rocks and offers a potentially useful and single method of exploring for hydrothermal activity associated with serpentinization mantle degassing through hydrothermal activity associated with serpentinization is an important process with respect to chemical and thermal exchanges between the upper mantle and the ocean different ratios of hydrothermal tracers i e tdm ch4 provide a useful framework for identifying subseafloor processes along mid oceanic ridges

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

non transform offsets along the mid atlantic ridge south of the azores 38 n 34 n ultramafic exposures and hosting of hydrothermal vents ten contiguous non transform offsets ntos along the mid atlantic ridge mar south of the azores between 38 n and 35 40 n have been studied in detail using swath bathymetric acoustic backscatter and deep tow high resolution sidescan sonar tobi data in contrast with discontinuities studied elsewhere at slow spreading ridges these left lateral ntos are consistently broader and larger with complex structural fabrics accommodating the offset they are characterized by a range of elevated and faulted massifs detached from their segment flanks with an irregular acoustic backscatter pattern some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks and sometimes associated with high temperature hydrothermal venting water column surveys adjacent to these massifs show high ch4 and low tdm total dissolvable manganese concentrations possibly resulting from the process of serpentinization of ultramafic rocks the correlation between the shallow dome like shaped massifs and the high concentrations of ch4 associated with low levels of mn is of particular interest to predict the outcrop of ultramafic rocks within the ntos where no geological data are available the exposure of the ultramafic massifs within the ntos is favored by low magmatic supply and low angle detachment faulting occurring at segment ends the pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high temperature vent sites

depositional setting of the 2 1 ga francevillian macrobiota gabon rapid mud settling in a shallow basin swept by high density sand flows the depositional setting of the 2 1 ga fill to the franceville basin of gabon is important for understanding the habitat energy and availability of light and oxygen and taphonomy of recently discovered early macro organisms buried in black shales in unit fb the available data bearing on the stratigraphy and sedimentology of unit fb provide new insight into processes acting on the palaeo seafloor the shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands the latter poubara sandstones were emplaced during a forced regression during the terminal infill of fault bounded sub basins following a stage characterised by a ferruginous to anoxic water column the structureless sandstones were deposited from high density gravity currents along with a locally strong bottom oscillation of the water column tuft structures preserved in cyanobacterial mats together with the position of the macro organisms at the top of the sandstone beds within associated black shales point to a water depth of less than 80 m the relative sea level fall that drove deposition of the poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows this article is protected by copyright all rights reserved

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

high resolution magnetics reveal the deep structure of a volcanic arc related basalt hosted hydrothermal site palinuro tyrrhenian sea high resolution magnetic surveys have been acquired over the partially sedimented palinuro massive sulfide deposits in the aeolian volcanic arc tyrrhenian sea surveys flown close to the seafloor using an autonomous underwater vehicle auv show that the volcanic arc related basalt hosted hydrothermal site is associated with zones of lower magnetization this observation reflects the alteration of basalt affected by hydrothermal circulation and or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids to discriminate among these inferences estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area the stockwork we use high resolution vector magnetic data acquired by the auv abyss geomar above a crater shaped depression hosting a weakly active hydrothermal site our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit forming lobe shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

origin of red beds in the paleoproterozoic franceville basin gabon and implications for sandstone hosted uranium mineralization red beds are extensively used as evidence for an oxygenated atmosphere in the paleoproterozoic red beds in the unmetamorphosed ca 2 15 ga fa formation of the franceville basin gabon were geochemically petrographically and mineralogically characterized to constrain the process of their formation and their relationship to atmospheric oxygenation in the early paleoproterozoic petrographic observations indicate that ferric oxides are dispersed in clay filling intergranular pores and along platy cleavage in altered phyllosilicates grain coating hematite is generally rare to absent in most samples suggesting hematite precipitation after sediment deposition and during burial diagenesis textures and geochemical data suggest that iron was likely sourced internally by alteration of iron bearing minerals such as biotite and chlorite in sediments and redistributed in rocks during late diagenesis positive correlation between fe mg ratio and 56fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron bearing silicates biotite and chlorite the lack of relationship between fe3 fet ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis and was incorporated into green reduced facies that likely replaced red facies during diagenesis and burial large range of 56fe values extending towards positive values up to 1 permil is similar to that observed in archean and paleoproterozoic iron formations or modern groundwater derived fe oxyhydroxides suggesting partial oxidation of fe under mildly oxidizing conditions during early diagenesis

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

uranium deposits of franceville basin gabon role of organic matter and oil cracking on uranium mineralization abstract the lower proterozoic uranium deposits in the franceville basin gabon are mainly known to have hosted natural nuclear reactors however this basin also witnessed a former petroleum system which shows major occurrences of uranium ores associated with organic matter the uranium mineralization is the result of several episodes of oil generation and migration fluid mixing and hydrofracturing during burial at around 2 0 ga black shales were heated and expelled hydrocarbons which migrated in underlying sandstones the first episode of oil migration was synchronous with a silicification event produced by a hot low salinity fluid as shown by fluid inclusions trapped in quartz overgrowths the second episode was contemporaneous with quartz dissolution and is characterized by the mixing of hydrocarbons and oxidizing u bearing brine which resulted in the precipitation of uo2 inclusions in oil then oil to gas conversion generated overpressured gas trapped in pores with uraniferous bitumen nodules changes in the stress regime probably favored radial hydrofracturing around bitumen nodules and subsequently the development of a pervasive micro fracture network there uo2 precipitated together with cu fe sulfides through thermochemical sulfate reduction favored by the presence of hydrocarbons and methane

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

textural and paleo fluid flow control on diagenesis in the paleoproterozoic franceville basin south eastern gabon abstract the paleoproterozoic 2 15 ga franceville basin south eastern gabon is a continental sedimentary basin that host unmetamorphosed sediments this study involve detailed mineralogy sedimentology and petrography of the basal sedimentary units of fa and lower fb fb1 formations from the basin margin to centre in relation to mineral paragenesis and fluid flow the fa formation conglomerate sandstone and mudstone consists lithofacies of mixed fluvial and fluvio deltaic transitional origin while the overlying fb1 formation includes alternating organic rich black shale and sandstone of marine deposit the medium to coarse grained fluvio deltaic quartz arenite in upper part of fa formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history this provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis in contrast the clay and unstable detrital grains rich fluvial arkosic to sub arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts these arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities from the proximal to distal basin and within sample suites there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore fluid the predominance of 1mt illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid rock ratio dolomite anhydrite barite and fe oxides are the main crystallized authigenic minerals aside illite and fe rich chlorite clay minerals the mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the fa formation in the franceville basin are mainly controlled by depositional facies

early quartz cements and evolution of paleohydraulic properties of basal sandstones in three paleoproterozoic continental basins evidence from in situ δ18o analysis of quartz cements quartz cement microstratigraphy and high precision in situ 18 o values obtained by secondary ion mass spectrometry sims from m size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant paleoproterozoic basins the athabasca and thelon basins canada and the mcarthur basin australia in these examples the earliest quartz cements have the highest 18 o values up to 33 mean 26 0 v smow indicative of low temperature precipitation at depths of 0 to 1 1 km some paragenetically early cement phases however have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures subsequent generations of quartz cements have progressively lower 18 o values mean 16 1 3 consistent with precipitation at higher temperatures these data combined with petrographic observations indicate quartz cementation began in near surface vadose and phreatic zones the next stage of cementation is characterized by syntaxial burial cement overgrowths regionally and stratigraphically well sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation which resulted in cement bridged pore throats at relatively shallow depths of 0 2 km and very early in their burial history transforming these units into diagenetic aquitards as a result these units did not experience mineralization compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement but are instead marked by minor syntaxial overgrowth cement that make upb5 of the intergranular volume formed at depths of 3 5 km and were variably replaced by illite and chlorite during burial these lithologies were still open to fluid movement at critical times in the basin when u and

modelling the thermal evolution of slow spreading ridge segments and their off axis geophysical signature systematic studies conducted between 15 n and 40 n over ridge segments along the slow spreading mid atlantic ridge mar have shown that segment characteristics are related to the thermal state of the segments and gradually vary with their length this paper presents further developments of a 3 d model based on the presence of a hot zone located under the segment centres gac et al 2003 to 1 quantify the thermal structures and the geophysical signatures of segments of various lengths considered as representative of the various mar segments 2 test if a simple and single model of thermal evolution can account for the characteristics of all segments and 3 explain the past evolution of the segmentation as is observed off axis along the mar the modelled thermal structure and three simulated geophysical outputs crustal structure along axis variations of the earthquake maximum depth and the mantle bouguer gravity anomalies mba are found to be directly controlled by the shape geometry and dimensions of the hot zone a consistent fit between model outputs and along axis variations of the geophysical observables over the various segments is obtained by varying solely the length of the hot zone this result shows that segments of different length may in fact constitute the different stages of a single evolution process the axial geophysical characteristics of the segment would progressively evolve from those of shorter segments to those of longer ones as the hot zone lengthens along axis a subsequent shortening of the segment would result from a simultaneous shortening of the hot zone segment characteristics reverting back from those of longer segments to those of shorter ones three geophysical fields topography gravity and magnetic anomalies are simulated as the results of the thermal evolution of aligned and offset segments the length of which evolves through time these simulations succeed in fitting observations for the entire range of observed axis offsets between adjacent mar segments the segment evolution produces peculiar off axis isostatic topography and gravity anomaly mba the rhomb shaped patterns our simulations which model adjacent offset segments having evolved through several cycles of lengthening and shortening yield a good fit to the isostatic topography and mba patterns observed in the off axis region finally the distribution of magnetization depends on the magnetic properties of each type of rocks and on the petrological structure of the lithosphere which in turn results from its thermal structure and evolution modelled magnetic anomalies are shown to be in good agreement with off axis observations along the n21 40 segment tammar of the mar

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

fossil clams from a serpentinite hosted sedimented vent field near the active smoker complex rainbow mar 36 13 n insight into the biogeography of vent fauna hydrothermal circulation at ultramafic hosted sites supports a large variety of high and low temperature hydrothermal vents and associated ecosystems the discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest approximately 2 5 km east of the active rainbow vent field on the mid atlantic ridge mar 36 degrees 13 n increased our knowledge regarding the diversity of vent communities at slow spreading ridges bivalve molluscs of the family vesicomyidae were represented by the genus phreagena here we present the first record of this genus in the atlantic ocean this second vesicomyid species known from the mar phreagena sp was found to be associated with a thyasira species that is affiliated with t southwardae at the logatchev vent field on the mar and with t vulcolutre in the gulf of cadiz these two clams have close relationships with seep taxa along the continental margin and were likely associated with sedimented vent fields delta o 18 and delta c 13 analyses of the shells suggested that the burrowing bivalve thyasira could incorporate isotopically light carbon derived from the oxidation of methane in the sediment while the signature of phreagena sp shells denoted a different carbonate source c 14 dating of the shells denoted that the hydrothermal activity in the rainbow area began at least similar to 25 5 kyr bp which is similar to the model of the hydrothermal vent field distribution that was proposed for the logatchev hydrothermal site the results provide new insight regarding the diversity of chemosynthetic fauna on the mar over geologic time ultramafic hosted on axis sedimented vent fields extend the range of habitats for chemosynthetic communities underlying the need to further explore the geology of these types of environments on slow spreading ridges and to determine their role in the ecology of deep sea vent communities

deep sea magnetic vector anomalies over the hakurei hydrothermal field and the bayonnaise knoll caldera izu ogasawara arc japan 1 we conducted deep sea magnetic measurements using autonomous underwater vehicles in the bayonnaise knoll caldera the izu ogasawara island arc which hosts the large hakurei hydrothermal field we improved the conventional correction method applied for removing the effect of vehicle magnetization thus greatly enhancing the precision of the resulting vector anomalies the magnetization distribution obtained from the vector anomaly data shows a 2 km wide belt of high magnetization trending nnw sse going through the caldera and a low magnetization zone 300 m by 500 m in area extending over the hakurei site comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately especially in areas of sparse data distribution when the vector anomaly rather than the total intensity anomaly is used we suggest a geologically motivated model that basaltic volcanism associated with the back arc rifting occurred after the formation of the caldera resulting in the formation of the high magnetization belt underneath the silicic caldera the hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault the deposit apparently extends beyond the low magnetization zone climbing up the caldera wall this may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall

crustal magnetization of brothers volcano new zealand measured by autonomous underwater vehicles geophysical expression of a submarine hydrothermal system near bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at brothers volcano southern kermadec arc new zealand crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields one of which was unknown prior to our surveys the magnetization pattern is consistent with a model of discrete individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

chloritoid and staurolite stability implications for metamorphism in the archaean yilgarn block western australia the stability of quartz chloritoid staurolite almandine cordierite and aluminium silicates is used to constrain both metamorphic conditions and pressure temperature trajectories for two localities within the 2700 ma archaean yilgarn block in western australia available experimental data are used to calculate thermodynamic data for a self consistent set of equilibria between these minerals a lower amphibolite facies locality from the margin of a lower strain area contains assemblages including quartz chloritoid staurolite garnet biotite with altered cordierite replacing chloritoid quartz staurolite andalusite and quartz cordierite andalusite biotite this locality was heated to 530 560 c in the andalusite field at 4 2 kbar a sample from a mid to upper amphibolite facies highly strained locality contains relict staurolite enclosed by andalusite in turn replaced by cordierite and muscovite with biotite and sillimanite in the matrix the assemblage was heated isobarically from conditions near the maximum experienced by the lower grade locality of 560 c at 4 2 kbar to temperatures in excess of the andalusite sillimanite transition but within the quartz plus muscovite stability field 600 650 c the higher grade locality is close to a granitoid dome and sections based on gravity profiles reveal that this locality is underlain by granitoid at shallow depths the higher grade metamorphism apparently reflects superposition of the thermal aureole on regional metamorphic conditions similar to those in the lower grade areas

from detrital heritage to diagenetic transformations the message of clay minerals contained within shales of the palaeoproterozoic francevillian basin gabon unmetamorphosed and undeformed marine siliciclastics rocks of the fb fc and fd of the francevillian series gabon were deposited in an epicontinental basin clay minerals found in black shale siltstone and sandstone are dominantly illite and chlorite except in two levels of the fb formation which contain smectite rich randomly ordered mixed layers their survival in a 2 1 ga old sedimentary series is not related to the abundance of organic matter total organic carbon or toc nor redox conditions at the time of deposition as indicated by the fe speciation fehr fet and fepy fehr ratios rather it results from an incomplete illitization reaction that reflects potassium deficiency the k2o al2o3 ratio of shale siltstone and sandstone vary along the series and appear to conserve the signature of the original chemical composition of the rocks k feldspars which are present in the fc and fd formations are missing in the fb formation consequently the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the illitization reaction reached during diagenesis

trace element perspective into the ca 2 1 billion year old shallow marine microbial mats from the francevillian group gabon abstract the sedimentary fabrics of precambrian mat related structures mrs represent some of the oldest convincing evidence for early life on earth the ca 2 1 billion year ga old mrs in the fb2 member of the francevillian basin in gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated shallow marine settings but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source despite these insights what remains lacking is a full characterization of the geochemical composition of the mrs to test whether the bulk composition of fossilized mrs is distinct from the host sediments sandstones and shales here we show that the trace element te content of microbial textures belonging to pyritized mrs poorly pyritized mrs and elephant skin textures est is highly variable and differs from that of the host sediments the poorly pyritized mrs contain a unique matrix with embedded ti and zr rich minerals and syngenetically enriched in te the est some of which are developed along the same stratigraphic horizon as the poorly pyritized mrs display a distinct distribution of te bearing heavy minerals suggesting a local difference in physical conditions during sedimentation similarly high chalcophile element ce content in pyritized mrs relative to the host sediments of the fb2 member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis the geochemical relationship between the mrs and the francevillian sediments e g fb fc and fd formations indicates that specific biological pathways for ce enrichments i e microbially controlled accumulation are not apparent our findings highlight bulk rock te distinction between the 2 1 billion year old mrs and their host sediments but also indicate that environmental conditions such as hydrodynamic regime and water column redox chemistry may simply overwhelm any potential biological signal our data suggest that the microbial impact may have only passively influenced te enrichment in the studied sediments implying that te concentrations in mrs are a poor biosignature importantly this work indicates that bulk te geochemistry does not unveil specific microbiological processes in the rock record which is consistent with the observed patterns in modern analogues

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

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

magnetic structure of a slow spreading ridge segment insights from near bottom magnetic measurements on board a submersible 1 near bottom magnetic measurements on board submersible nautile were carried out on the mid atlantic ridge 21 40 n segment and deep sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three component magnetometer data a forward modeling technique using short wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor in the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies whereas less reliable estimations are obtained off axis probably because the sediment buries the basement topography the natural remanent magnetization nrm measured on basalt samples collected during these dives is compared with the magnetization from anomalies though both results give a similar range of magnetization intensity no correlation is confirmed between them possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization whereas the nrm has variations at the scale of individual pillow or lava pile equivalent magnetization inverted from the sea surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends however the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis we ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite the results of three across axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the brunhes epoch although magnetization lows are generally correspondent to periods of low paleointensity they show deeper drop than predicted from the paleointensity variation

seismic imaging of magma sills beneath an ultramafic hosted hydrothermal system hydrothermal circulation at mid ocean ridge volcanic segments extracts heat from crustal magma bodies however the heat source driving hydrothermal circulation in ultramafic outcrops where mantle rocks are exhumed in low magma supply environments has remained enigmatic here we use a three dimensional p wave velocity model derived from active source wide angle refraction reflection ocean bottom seismometer data and pre stack depth migrated images derived from multichannel seismic reflection data to investigate the internal structure of the rainbow ultramafic massif which is located in a non transform discontinuity of the mid atlantic ridge seismic imaging reveals that the ultramafic rocks composing the rainbow massif have been intruded by a large number of magmatic sills distributed throughout the massif at depths of 2 10 km these sills which appear to be at varying stages of crystallization can supply the heat needed to drive high temperature hydrothermal circulation and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting our results demonstrate that high temperature hydrothermal systems can be driven by heat from deep sourced magma even in exhumed ultramafic lithosphere with very low magma supply

experimentally determined femg exchange between synthetic staurolite and garnet in the system mgofeoal2o3sio2h2o the fe mg exchange between synthetic staurolite and garnet was determined experimentally as a function of pressure temperature and bulk composition in the range of 5 25 kbar 600 750 c on the basis of the reaction12 pyrope 14 fe staurolite 13 almandine 14 mg staurolite eq 1 the results of the exchange runs clearly indicate that the fe mg fractionation between staurolite and garnet does not depend much on pressure but on temperature and bulk composition the compositional data on coexisting grtss and stst as determined by microprobe were processed by a nonlinear least squares procedure using different mixing models for staurolite solid solutions and the asymmetric solution model for garnet hackler and wood 1989 in all the models staurolite solid solutions show large negative deviation from ideality the experimental results are best described by a temperature dependent symmetric mixing model with wsst 79 j k one site model such a large negative ws value however is rather unrealistic especially considering that only a relatively small temperature interval has been investigated therefore a symmetric mixing model with wgst 11539 830 j one site model is recommended for use in thermodynamic calculations the expression for the derived geothermometer is based on the temperature dependent mixing parameters because these describe the fe mg fractionation between staurolite and garnet very well the geothermometer is valid for staurolites with xfe 0 6 and is t 18 02 0 044 p rt in fegrt rt in mggrt 87 78 0 057 p xmgst2 xmgst 2 12 55 xcagrt xmngrt r 10 3 in kd 0 021 0 079 xfest 2 xmgst 2 where pressure p is in kbar and temperature in c

high resolution near bottom vector magnetic anomalies over raven hydrothermal field endeavour segment juan de fuca ridge high resolution near bottom vector magnetic data were collected by remotely operated vehicle jason over the raven hydrothermal vent field 47 57 3 n 129 5 75 w located north of main endeavour vent field on the endeavour segment of the juan de fuca ridge the survey was part of a comprehensive heat flow study of the raven site using innovative thermal blanket technology to map the heat flux and crustal fluid pathways around a solitary hydrothermal vent field raven hydrothermal activity is presently located along the western axial valley wall while additional inactive hydrothermal deposits are found to the nw on the upper rift valley wall magnetic inversion results show discrete areas of reduced magnetization associated with both active and inactive hydrothermal vent deposits that also show high conductive heat flow higher spatial variability in the heat flow patterns compared to the magnetization is consistent with the heat flow reflecting the currently active but ephemeral thermal environment of fluid flow while crustal magnetization is representative of the static time averaged effect of hydrothermal alteration a general nw to se trend in reduced magnetization across the raven area correlates closely with the distribution of hydrothermal deposits and heat flux patterns and suggests that the fluid circulation system at depth is likely controlled by local crustal structure and magma chamber geometry magnetic gradient tensor components computed from vector magnetic data improve the resolution of the magnetic anomaly source and indicate that the hydrothermally altered zone directly beneath the raven site is approximately 15 106 m3 in volume

axial magnetic anomalies over slow spreading ridge segments insights from numerical 3 d thermal and physical modelling the axial magnetic anomaly amplitude along mid atlantic ridge segments is systematically twice as high at segment ends compared with segment centres various processes have been proposed to account for such observations either directly or indirectly related to the thermal structure of the segments 1 shallower curie isotherm at segment centres 2 higher fe ti content at segment ends 3 serpentinized peridotites at segment ends or 4 a combination of these processes in this paper the contribution of each of these processes to the axial magnetic anomaly amplitude is quantitatively evaluated by achieving a 3 d numerical modelling of the magnetization distribution and a magnetic anomaly over a medium sized 50 km long segment the magnetization distribution depends on the thermal structure and thermal evolution of the lithosphere the thermal structure is calculated considering the presence of a permanent hot zone beneath the segment centre the best fitting thermal structure is determined by adjusting the parameters shape size depth etc of this hot zone to fit the modelled geophysical outputs mantle bouguer anomaly maximum earthquake depths and crustal thickness to the observations both the thermoremanent magnetization acquired during the thermal evolution and the induced magnetization which depends on the present thermal structure are modelled the resulting magnetic anomalies are then computed and compared with the observed ones this modelling exercise suggests that in the case of aligned and slightly offset segments a combination of higher fe ti content and the presence of serpentinized peridotites at segment ends will produce the observed higher axial magnetic anomaly amplitudes over the segment ends in the case of greater offsets the presence of serpentinized peridotites at segment ends is sufficient to account for the observations

comparison of diagenetic fluids in the proterozoic thelon and athabasca basins canada implications for protracted fluid histories in stable intracratonic basins the paleoproterozoic thelon basin located on the border between nunavut and the northwest territories of canada is a contemporaneous analog of the uranium rich paleoproterozoic athabasca basin in canada early diagenesis resulted in precipitation of extensive hematite on the surfaces of detrital quartz grains throughout the thelon formation and minor hydroxy phosphate in veins locally continued diagenesis then resulted in syntaxial quartz cementation of detrital quartz at 130 c from fluids having ca 17 wt equivalent nacl similar to the athabasca basin cementation of this type is most pronounced in fine grained sequences in the thelon basin a period of extensive desilicification during continued burial was followed by formation at ca 200 c of peak diagenetic illite having ar ar ages of ca 1400 1690 ma in the thelon formation this illite was associated with fluids with 18 oa nd d values of ca 6 and 50 respectively similar to those during peak diagenesis of the athabasca basin although the timing salinity and isotopic composition of the peak diagenetic fluids in the thelon and athabasca basins are similar the peak diagenetic mineral assemblage in the athabasca formation is dickite and illite with minor dravite and goyasite rather than simply illite consequently the fluids at peak diagenesis which in the athabasca basin are synchronous with formation of world class unconformity type uranium deposits had different compositions in each basin post peak diagenesis in the thelon basin was quite distinct from that in the athabasca basin in that illite was replaced in the central portion of the basin by k feld spar and then sudoite which crystallized from saline brines at ca 1000 ma and 100 c evidence for later infiltration of these brines is absent in the athabasca basin although uranium mobilization at ca 900 ma from fluids having the same characteristics as those at peak diagenesis was pronounced in the athabasca basin recent incursion of meteoric waters along reactivated structures into the athabasca basin has variably affected hydrous and uraniferous minerals but evidence for this is lacking in the thelon basin the thelon basin reflects less intensive fluid rock interaction in its early history than that recorded in the basal units of the athabasca basin 132

near seafloor magnetics reveal tectonic rotation and deep structure at the tag trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n we take advantage of geological constraints from ocean drilling program drill holes and high resolution bathymetry to revisit the near seafloor magnetic anomaly at the trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n the dipolar anomaly associated with the site is better reduced to the pole if the magnetization vector is tilted by 34 an observation suggesting that the strongly faulted basalt surrounding the site has been rotated by 53 along an axis parallel to the mid atlantic ridge as a probable consequence of the detachment tectonics inferred in this area the faults and the deeper detachment focus and guide the hot ascending hydrothermal fluid magnetic modeling shows that the nonmagnetic stockwork zone is a significant contributor to the observed negative anomaly the rest being accounted for by a deeper source probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site

what causes low magnetization at basalt hosted hydrothermal sites insights from inactive site krasnov mar 16 38 n high resolution magnetic surveys acquired near the seafloor show that active basalt hosted hydrothermal sites are associated with zones of lower magnetization this observation may reflect the thermal demagnetization of a hot hydrothermal zone the alteration of basalt affected by hydrothermal circulation and or the presence of thick nonmagnetic hydrothermal deposits in order to discriminate among these inferences we acquired vector magnetic data 50 m above inactive hydrothermal site krasnov using the remotely operated vehicle rov victor this deep hydrothermal site located 7 km east of the mid atlantic ridge mar axis at 16 38 n is dissected by major normal faults and shows no evidence of recent hydrothermal activity it is therefore a perfect target for investigating the magnetic signature of an inactive basalt hosted hydrothermal site krasnov exhibits a strong negative magnetic anomaly which implies that the lower magnetization observed at basalt hosted hydrothermal sites is not a transient effect associated with hydrothermal activity but remains after activity ceases thermal demagnetization plays only a secondary role if any in the observed magnetic low forward models suggest that both the nonmagnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low the permanence of this magnetic signature makes it a useful tool to explore midocean ridges and detect inactive hydrothermal sites

hydrogeology sequence stratigraphy and diagenesis in the paleoproterozoic western thelon basin influences on unconformity related uranium mineralization abstract the paleoproterozoic statherian thelon formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the thelon basin a continental basin in northern canada this study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under studied western thelon basin and its relation to uranium mineralizing processes in the vicinity of the boomerang lake unconformity related u prospect four lithofacies comprise the thelon formation in the western thelon basin consisting of breccia conglomerate sandstone and minor mudstone these are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment these unconformity bounded sequences are composite in nature and can be subdivided into six high frequency sequences hfs low accommodation and high accommodation systems tracts are delineated instead of the traditional systems tracts that are linked to changes in relative sea level as no marine deposits are recognized in the thelon formation two significant strataform paleohydrogeological units were distinguished in the thelon formation based on the relative timing of key diagenetic events such as compaction grain dissolution and cementation diagenetic aquicludes became isolated from basinal fluid flow due to stylolite catalyzed early diagenetic quartz cementation and occlusion of secondary intergranular macroporosity by kaolinite and muscovite and minor dickite during later peak diagenesis diagenetic aquifers conducted u bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite sequence stratigraphic correlation indicates that diagenetic aquicludes which were not effective at conducting u bearing basinal fluids overlie the unconformity at the boomerang lake prospect and extend along the entire strike length of a related exploration trend in contrast thick diagenetic aquifers lie approximately 10 km to the north of the boomerang lake prospect along a different exploration trend the western and eastern portions of the thelon basin experienced depositional environments and diagenetic conditions distinct from one another suggesting that they were separate sub basins the western thelon basin contains fewer of the well cemented facies that compartmentalized basinal fluid flow than the eastern thelon basin and contains a kaolin dominated diagenetic mineral assemblage rather than a muscovite dominated assemblage that is common in the eastern thelon basin in these regards the western thelon basin evolved more similarly to the eastern portion of the athabasca basin where thick regional kaolin rich diagenetic aquifers were fluid conduits for the movement of the u rich fluids that formed high grade unconformity related u deposits

ridge propagation oceanic core complexes and ultramafic hosted hydrothermalism at rainbow mar 36 n insights from a multi scale magnetic exploration abstract a high resolution magnetic survey conducted over the apparently inactive ultramafic hosted hydrothermal site clamstone reveals a positive magnetic anomaly in accordance with the magnetic response of other ultramafic hosted sites known to date here this magnetic signature is mainly the result of in place bulk magnetized serpentinite underlying the site rather than any concentration of magnetite within the stockwork zone the latter being probably negligible due to the limited fluid venting conversely the nearby high temperature active ultramafic hosted hydrothermal site rainbow illustrates the key contribution of hydrothermal activity in producing large amounts of magnetite in stockwork zones within ultramafics at a larger scale we propose from the analysis of sea surface magnetic anomalies that the underlying rainbow hill has been formed as an oceanic core complex associated with the amar minor north segment later dismantled by the northward propagation of the amar minor south segment and the coeval recession of the amar minor north segment this model solves the apparent paradox of a high temperature hydrothermal site sitting on an oceanic core complex usually associated with cold poorly magmatic spreading environments the core complex belongs to the cold receding amar minor north segment whereas the heat fueling site rainbow comes from the hot propagating amar minor south segment it also explains why the rainbow hill although interpreted as an oceanic core complex lacks the typical corrugations of such core complexes the latter have been dismantled by the deformation associated with the northward propagation of the non transform offset

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

the magnetic signature of ultramafic hosted hydrothermal sites unlike basalt hosted hydrothermal sites characterized by a lack of magnetization the magnetic signature of ultramafic hosted hydrothermal sites remains poorly known despite their wide occurrence at slow spreading ridges and their strong mineral potential the first high resolution magnetic surveys of such ultramafic hosted sites achieved by deep sea submersible on four sites of the mid atlantic ridge reveal positive magnetic anomalies and therefore a strong magnetization at the largest sites this observation reflects the presence of a wide mineralized zone beneath these sites the stockwork where several chemical processes concur to create and preserve strongly magnetized magnetite beyond pointing out the importance of subsurface chemical processes in hydrothermal activity the aging of oceanic lithosphere and the ocean chemical budget our results have immediate application for detecting and characterizing economically valuable deep sea mineral deposits

a staurolite talc assemblage in tourmaline phlogopite chlorite schist from northern victoria land antarctica and its petrogenetic significance staurolite and corundum are found as inclusions in tourmaline in a talc phlogopite chlorite albite chist near mount bernstein 71 37 s 163 07 e northern victoria land antarctica these inclusions are interpreted as relics of a staurolite talc corundum chlorite assemblage that was stable during an early stage in the metamorphic cycle and subsequently armored by tourmaline probably during the middle stage pressures and temperatures during the middle stage are estimated to be 650 700 c and 5 5 6 4 kbar the transition from the early to the middle stage represents a roughly isothermal decrease in pressure of 2 3 kbar during a late retrograde stage t 300 370 c p 3 5 kbar staurolite was partly replaced by a muscovitic aggregate containing clinozoisite pumpellyite and margarite

magnetic alteration of zero age oceanic basalt the youngest sampled submarine lava flow which erupted june 1993 on the juan de fuca ridge provides the basis for a tight constraint on the initial or zero age magnetization state of morb detailed profiles of magnetic hysteresis parameters curie temperatures and unblocking temperatures of nrm with respect to the chilled margin of a pillow fragment show evidence of significant oxidation which preferentially affected the finest grain size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy the oxidation must have occurred during or immediately after initial cooling implying that morb is already appreciably magnetically altered before aging nevertheless successful results of thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by morb preserves a record of geomagnetic intensity variations that may be reflected in small scale magnetic anomalies

hydrothermal clay mineral formation in the uraniferous paleoproterozoic fa formation francevillian basin gabon abstract the spatial distribution of neoformed clay minerals was investigated in the paleoproterozoic fa formation of the francevillian basin south east gabon which hosts high grade u ore deposits associated with the only known occurrence of natural nuclear reactors illite appears as the main clay phase in the lower fluvial unit in the mineralized upper fluvio deltaic tidal unit the clay assemblage is more diversified and commonly characterized by illite chamosite berthierine and chamosite berthierine mixed layers usually in association with alteration products of u bearing minerals the clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of u ore deposits polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid rock ratio according to crystalline structure mineral chemistry thermodynamic modeling and geochronology clay phases seem to be mainly hygrometer rather than exclusively a thermometer and their formation as well as associated dissolution precipitation of u bearing minerals took place between ca 2040 and 2010 ma ago at temperatures of about 240 30 c using previous burial estimates this suggests the operation of hydrothermal processes controlled by an external heat source likely associated with volcanism during deposition of the fd and fe formations hydrothermal activity involving oxidized fluids would have driven u remobilization and ultimately formation of high grade u ore deposits the thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich u endowment

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

submarine hydrothermal activity and gold rich mineralization at brothers volcano kermadec arc new zealand brothers volcano of the kermadec intraoceanic arc is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world it has two distinct vent fields known as the nw caldera and cone sites whose geology permeability vent fluid compositions mineralogy and ore forming conditions are in stark contrast to each other the nw caldera site strikes for 600 m in a sw ne direction with chimneys occurring over a 145 m depth interval between 1 690 and 1 545 m at least 100 dead and active sulfide chimney spires occur in this field and are typically 2 3 m in height with some reaching 6 7 m their ages at time of sampling fall broadly into three groups 4 23 and 35 years old the chimneys typically occur near the base of individual fault controlled benches on the caldera wall striking in lines orthogonal to the slopes rarer are massive sulfide crusts 2 3 m thick two main types of chimney predominate cu rich up to 28 5 wt cu and more commonly zn rich up to 43 8 wt zn geochemical results show that mo bi co se sn and au up to 91 ppm are correlated with the cu mineralization whereas cd hg sb ag and as are associated with the dominant zn rich mineralization the cone site comprises the upper cone site atop the summit of the recent main dacite cone and the lower cone site that straddles the summit of an older smaller more degraded dacite cone on the ne flank of the main cone huge volumes of diffuse venting are seen at the lower cone site in contrast to venting at both the upper cone and nw caldera sites individual vents are marked by low relief 0 5 m mounds comprising predominately native sulfur with bacterial mats vent fluids of the nw caldera field are focused hot 300 c acidic ph 2 8 metal rich and gas poor calculated end member fluids from nw caldera vents indicate that phase separation has occurred with cl values ranging from 93 to 137 of seawater values by contrast vent fluids at the cone site are diffuse noticeably cooler 122 c more acidic ph 1 9 metal poor and gas rich higher than seawater values of so4 and mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water rock interactions iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from fe rich brines evidence for magmatic contributions to the hydrothermal system at brothers includes high concentrations of dissolved co2 e g 206 mm kg at the cone site high co2 3he negative d and 18oh2o for vent fluids negative 34s for sulfides to 4 6 sulfur to 10 2 and 15n2 to 3 5 vent fluid ph values to 1 9 and mineral assemblages common to high sulfidation systems changing physicochemical conditions at the brothers hydrothermal system and especially the cone site occur over periods of months to hundreds of years as shown by interlayered cu au and zn rich zones in chimneys variable fluid and isotopic compositions similar shifts in 3he 4he values for both cone and nw caldera sites and overprinting of magmatic mineral assemblages by water rock dominated assemblages metals especially cu and possibly au may be entering the hydrothermal system via the dissolution of metal rich glasses they are then transported rapidly up into the system via magmatic volatiles utilizing vertical 2 5 km long narrow 300 m diameter pipes consistent with evidence of vent fluids forming at relatively shallow depths the nw caldera and cone sites are considered to represent stages along a continuum between water rock and magmatic hydrothermal dominated end members

marine magnetic anomalies as recorders of geomagnetic intensity variations in addition to providing a robust record of past geomagnetic polarity reversals marine magnetic anomalies often show shorter wavelength variations which may provide information on geomagnetic intensity variations within intervals of constant polarity to evaluate this possible geomagnetic signal we compare sea surface profiles of the central anomaly with synthetic profiles based on brunhes age 0 0 78 ma paleointensity records derived from deep sea sediments the similarity of the synthetic profiles and observed profiles from the ultra fast spreading southern east pacific rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust this interpretation is further supported by systematic variations in the pattern of the central anomaly at slower spreading ridges which are entirely consistent with a progressively smoother record of the sediment derived paleointensity if the sedimentary records as calibrated to available absolute paleointensity data accurately record variations in dipole intensity over the brunhes it follows that much of the brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0 05 to 5 0 ma furthermore the sediment paleointensity records reflect the significant increase in geomagnetic intensity from a low of 2 1022 am2 near 40 ka to a peak value 11 1022 am2 at 3 ka that has been well documented from absolute paleointensity determinations we suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant central anomaly magnetic high

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

exceptional preservation of expandable clay minerals in the ca 2 1 ga black shales of the francevillian basin gabon and its implication for atmospheric oxygen accumulation abstract clay minerals are exceptionally well preserved in marine black shale of the ca 2 1 ga francevillian group in southeastern gabon the fb formation of the francevillian group is characterized by smectite rich clay minerals including randomly ordered r0 type and ordered r1 type mixed layer illite smectite i s the preservation of r0 type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis which is unique considering the paleoproterozoic age of the sedimentary rocks r0 and r1 type smectite rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures suggesting formation of clay organic matter complexes our data suggests that clay minerals may have enhanced organic matter preservation providing the oldest example where a link between clay minerals and organic matter sequestration can be established our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time

magnetic exploration of a low temperature ultramafic hosted hydrothermal site lost city 30 n mar abstract a 2003 high resolution magnetic survey conducted by the autonomous underwater vehicle abe over the low temperature ultramafic hosted hydrothermal field lost city reveals a weak positive magnetic anomaly this observation is in direct contrast to recent observations of strong positive magnetic anomalies documented over the high temperature ultramafic hosted hydrothermal vents fields rainbow and ashadze which indicates that temperature may control the production of magnetization at these sites the lost city survey provides a unique opportunity to study a field that is to date one of a kind and is an end member of ultramafic hosted hydrothermal systems our results highlight the key contribution of temperature on magnetite production resulting from serpentinization reactions whereas high temperature promotes significant production and partitioning of iron into magnetite low temperature favors iron partitioning into various alteration phases resulting in a magnetite poor rock moreover the distribution of magnetic anomalies confirms results of a previous geological survey indicating the progressive migration of hydrothermal activity upslope these discoveries contribute to the results of 25 yrs of magnetic exploration of a wide range of hydrothermal sites from low to high temperature and from basalt to ultramafic hosted and thereby validate using high resolution magnetics as a crucial parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic based ecosystems and potentially mineral rich deposits

the hydroxyl content of staurolite the composition of forty six staurolite samples spanning a range in metamorphic grade has been determined by electron microprobe analysis and an improved micro analytical technique for water determination the hydroxyl content per unit cell calculated on the basis for fortyeight oxygen and hydroxyl atoms ranges from 3 15 to 3 57 at black mountain western new hampshire 2 45 to 2 94 at jamaica vermont 1 79 to 3 01 in western maine and 1 96 to 3 32 in northwestern connecticut the oh content of staurolite is controlled by homogeneous reactions cation hydroxyl substitutions and heterogeneous reactions redox and dehydration equilibria closely spaced samples in all four areas show differences in oh content due to variations in mu o 2 between adjacent beds a decrease in the oh content of staurolite was observed only in the highest grade rocks