the role of the calcium carbonate calcium phosphate switch in the mineralization of soft bodied fossils authigenic minerals play an important role in the preservation of most soft bodied fossils the greatest detail is preserved in apatite calcium phosphate but its precipitation is usually inhibited by the high concentrations of hco 3 in aqueous settings nonetheless investigations of soft bodied biotas have revealed very early authigenic calcite crystal bundles in close association with phosphatized soft tissues this demonstrates that the geochemical controls on soft tissue mineralization are dynamic and act on a very local scale direct comparisons with experimental results permit the conditions of fossilization to be inferred

the low temperature magnetic signature of fe rich serpentine in cm2 chondrites comparison with terrestrial cronstedtite and evolution with the degree of alteration 1 we have studied the low temperature magnetic properties of one ci and five cm2 chondrites well characterized in the literature and of cronstedtite the fe end member of the fe mg serpentine solid solution cronstedtite is highly anisotropic and its relaxation properties below 10 k suggest a glassy magnetic behavior frustration of the magnetic interactions in octahedral layers would account for the observed magnetic freezing with a possible role of the random distribution of ferric iron in tetrahedral layers based on the comparison between powders and crystals characterized by scxrd and empa we show that grain size and substitution of octahedral fe by mg induce changes in the position of the low tmagnetic susceptibility peak a low tpeak is also found in each of the cm2 chondrites but not in the ci chondrite it is interpreted as the signature of fe serpentines which are considered by most authors to have mostly formed during asteroidal aqueous alteration events the low tmagnetic properties of the less altered meteorites compare well with those of cronstedtite and the position of the susceptibility peak varies with the published degree of alteration by comparing magnetic data on reference cronstedtite magnetic and published mineralogical data on cm2 chondrites we suggest that the evolution of the low tmagnetic signature of the latter reflects bulk changes in fe serpentines chemical composition rather than only mixing between fe rich and mg rich serpentines this is analogous to previous observations on the changes in fe incorporation to mg serpentines during progressive serpentinization of abyssal peridotites

widespread oxidized and hydrated amorphous silicates in cr chondrites matrices implications for alteration conditions and h2 degassing of asteroids abstract the cr chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals they have experienced very variable degrees of aqueous alteration ranging from incipient alteration in their matrices to the complete hydration of all of their components in order to constrain their chemical alteration pathways and the conditions of alteration we have investigated the mineralogy and fe oxidation state of silicates in the matrices of 8 cr chondrites from type 3 to type 1 fe l edge x ray absorption near edge structure xanes was performed on matrix fib sections using synchrotron based scanning transmission x ray microscopy stxm the fe 3 fe ratio of submicron silicate particles was obtained and coordinated with tem observations in all the least altered cr chondrites que 99177 eet 87770 eet 92042 lap 02342 gra 95229 and renazzo we find that the matrices consist of abundant submicron fe rich hydrated amorphous silicate grains mixed with nanometer sized phyllosilicates the fe 3 fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78 in the most altered samples al rais and gro 95577 fine grained phyllosilicates also have a high fe 3 fe ratio around 70 whereas the coarse micrometer sized phyllosilicates are less oxidized down to 55 and have a lower iron content these observations suggest the following sequence submicron fe2 amorphous silicate particles were the building blocks of cr matrices after accretion they were quickly hydrated and oxidized leading to a metastable amorphous gel like phase nucleation and growth of crystalline phyllosilicates was kinetically limited in most type 3 and 2 crs but increased as alteration became more extensive in al rais and gro 95577 the decreasing fe 3 fe ratio is interpreted as a result of the transfer of fe3 from silicates to oxides during growth while aqueous alteration progressed higher temperature longer duration change of fluid composition in a fully closed system equilibrium thermodynamics suggest that the water to rock ratios typically assumed to be low fe 3 fe ratio such a high fe 3 fe value could be accounted for however if the system was partially open at least with respect to h2 and other gases as well rapid degassing of the fluid would have favored more oxidizing fluid conditions recently proposed scenarios involving some degree of water d h increase through rayleigh isotopic fractionation are supported by these results

redox and speciation micromapping using dispersive x ray absorption spectroscopy application to iron in chlorite mineral of a metamorphic rock thin section 1 the first dispersive xanes mapping is presented here the experiments have been conducted at the iron k edge on the dispersive exafs beamline of the european synchrotron radiation facility france the mapping has been performed on a polished thin section of 30 m of a natural metamorphic rock including different types of minerals because of the high x ray absorption due to the thickness of the glass sample holder 1 mm the data have been collected in the fluorescence mode using the so called turbo xafs design the effective spot size was approximately 10 10 microns and an area 390 180 microns in size was mapped improvements of the acquisition process allowed collection of each xanes spectrum in 1 5 s and with a step size of 5 microns so that 2808 spectra with full xanes information were collected then automatic procedures for data reduction and mapping reconstruction were developed using matlab the results are maps of iron content oxidation state and speciation with a 5 m spatial resolution after two dimensional deconvolution subsequent analyses of the reconstructed images provide some quantitative calibrations

serpentinization of oceanic peridotites 2 kinetics and processes of san carlos olivine hydrothermal alteration 1 the kinetics of the reaction mg fe olivine h 2 o serpentine magnetite brucite h 2 were investigated at 500 bars in the 250 350 c range using natural olivine san carlos fo 91 with grain sizes between 1 and 150 mm and for run durations up to 514 d the amount of magnetite produced which directly relates to reaction progress was accurately monitored using up to 24 time resolved magnetic measurements per experiment eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5 15 mm and after 500 d for grain sizes of 50 63 mm serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains they were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization related processes the nature of the serpentine mineral product depended on the initial olivine grain size igs for igs in the 5 150 mm range lizardite formed and olivine dissolution was the rate limiting process at igs below 5 mm chrysotile crystallized instead of lizardite and the relationship between olivine surface area and reaction kinetics no longer held we infer that for such small olivine grain sizes dissolution is no longer the rate limiting process serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes etch pits formation associated with dissolution and grain fracturing for igs above 20 mm interestingly fractures and etch pits with similar geometry and sizes were also observed for residual olivine with a typical grain size of 50 mm in serpentinized peridotite samples from the southwest indian ridge this suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems we therefore suggest that the new kinetic data set that we present here which encompasses a range of olivine grain sizes and reaction temperatures is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available

experimental study of the effects of solute transport on reaction paths during incipient serpentinization abstract this paper presents the results of 4 reactive percolation experiments set up for investigating the impact of flow rate on serpentinization reaction paths for conditions relevant of the oceanic peridotite sub seafloor during the initial stages of its hydrothermal alteration the experiments consisted in injecting artificial seawater into porous compressed olivine powder cores at constant flow rates q 0 24 0 48 1 14 and 5 21 ml h 1 the experiments were conducted at constant temperature 170 c and pressure 25 mpa and lasted 11 to 28 days at the end of the experiments the outlet fluids composition displayed similar compositions buffered by the formation of serpentine amg2 a h 2 9 7 10 asio2 3 9 to 5 2 ph in situ 6 1 these values were achieved in a few to up to 300 h for the high flow rate experiment suggesting that they corresponded to a steady state regime of mass transfer which depended on flow rate differences in the composition of fluid versus time and in the structure of reacted samples during and after the four reactive percolation experiments suggested also various incipient serpentinization reaction paths the low q experiments produced sio2 aq enriched outlet fluids and nodular aggregates were identified covering the reacted olivine surfaces during high q experiments fibrous filaments of proto serpentine were formed on the olivine surfaces and the fluids progressively achieved steady state compositions similar to the other experiments these results together with those of previously published reactive percolation experiments lead us to propose two end member reaction paths for incipient serpentinization of olivine dominated permeable rocks infiltrated by seawater derived hydrothermal fluids 1 a transport controlled reaction path occurring in diffusion dominated zones is characterized by transient brucite precipitation which produces mg trapping and si release in solution followed by serpentine precipitation and 2 a kinetics controlled reaction path occurring in advection dominated zones where transport conditions are favorable to mg leaching and where serpentine precipitates first the occurrence of these two end member reaction paths is determined locally by the composition of the fluid which varies along flow paths thus both reaction paths can coexist in the sample depending on the local pore geometry our study shows that the interplay between fluid transport and reaction kinetics controls the chemical fluxes between the mineral surface and the bulk solution and the incipient serpentinization reaction paths in natural systems the scale and distribution of these reaction domains will depend on the complex structure of the ultramafic basement our results suggest that the precipitation of serpentine and silica rich phases will be favored in fluid focusing zones such as faults and fractures whilst formation of brucite will preferentially occur as part of pervasive background serpentinization

μxanes study of iron redox state in serpentine during oceanic serpentinization serpentinization of ultramafic rocks at mid ocean ridges generates significant amounts of h2 ch4 and supports specific biological communities the abiotic h2 production is attributed to the reduction of h2o during serpentinization which balances oxidation of ferrous iron contained in primary minerals mainly olivines and pyroxenes to ferric iron contained in secondary minerals mainly serpentines and magnetite magnetite has thus far been considered as the sole fe3 carrier for estimating bulk h2 production notably because the valence of iron in serpentine minerals and its relationship with both magnetite abundance and serpentinization degree are usually not determined we show that the serpentine contribution to the fe and fe3 budget has a significant effect on h2 production we performed xanes analysis at the fe k edge on thin sections of peridotites with various degrees of serpentinization from odp leg 153 mark region 23 n fe3 fetot in oceanic serpentines is highly variable from 0 2 to 1 at the thin section scale and it is related non linearly to the local degree of serpentinization a typical value of 0 7 is observed above 60 serpentinization the highest values of fe3 fetot observed within or close to late veins suggest that the fe3 fetot in serpentine record the local water rock w r ratio as previously proposed from thermodynamic modeling we estimate that the w r ratio increased from 0 6 to 25 during serpentinization at mark and locally reached 100 in veins mass balance calculations combining all mineral and bulk rock analyses provide the distribution of fe and fe3 as serpentinization progresses serpentine dominates the fe3 budget of the rock over magnetite during the first 75 of serpentinization contributing up to 80 of the total fe3 at later stages serpentine contribution to the fe3 budget decreases down to 20 while magnetite formation exponentially increases iron transfer from serpentine to magnetite balances the bulk fe3 content of the rock that increases almost linearly with the advance of the reaction formation of serpentine accounts for the majority of fe3 and h2 production at early stages of serpentinization at a depth 2 km at mark where the concentration of h2 can reach more than 100 mm according to the low w r h2 production values and depths can vary from one site to another depending on the evolution of the temperature w r ratio inlet fluid composition and favored formation of serpentine vs magnetite at mark fe3 in serpentine represents 15 27 of the total fe contained in a rock serpentinized to more than 80 and accounts for 25 of the total h2 production that is estimated at 325 335 mmol kg of rock the absence of magnetite does not necessarily mean a negligible h2 production even at low t conditions 150 200 c under which the fe and fe3 richest serpentines have been observed serpentine minerals are important fe3 carrier in the altered ocean lithosphere and may affect mantle redox state while dehydrating at depth in subduction zones

incipient hydration of mantle lithosphere at ridges a reactive percolation experiment a reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 c and 19 mpa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid ocean ridges the experiment was stopped after 23 days when the sample became impermeable the initial flow rate was 0 2 ml h and then was decreased down to 0 06 ml h after 8 days permeability decreased continuously throughout the experiment the analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto serpentine brucite and fe oxides these reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages these compositions are interpreted as resulting from a suite of coupled dissolution precipitation reactions controlled at the pore scale by surface kinetic processes that rapidly reaches steady state constant fluid composition independent of fluid flux hence the effective rate of serpentinization was controlled to the first order by the transport of reactants during the experiment mass balance calculations show that the rate of olivine conversion was fast 0 2 0 5 wt day yet only 8 wt of the olivine sample reacted because the permeability drop limited fluid circulation porosity varied little during the experiment compared to permeability changes the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths the rate at which permeability decreased was the fastest at low flux conditions this suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions but mainly reflected the development of low peclet pore scale zones forming micro environments that control the serpentinization reaction paths these results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere

the solubility of corundum in h2o at high pressure and temperature and its implications for al mobility in the deep crust and upper mantle abstract the solubility of corundum in h 2 o was measured at 700 1100 c and 0 5 2 0 gpa using a piston cylinder apparatus and weight loss methods at 700 c 0 5 gpa al concentration is 0 41 0 15 millimolal 1 analytical error corundum solubility increases strongly with pressure p and temperature t reaching 2 57 0 15 millimolal at 700 c 2 0 gpa and 27 3 1 5 millimolal at 1100 c 2 0 gpa the results were fitted with the equation log m al 1 131 3827 t 2 277 7336 t log h 2 o where m al is molality of al h 2 o is the density of h 2 o in g cm 3 and t is in kelvin the data indicate that al can have significant solubility in h 2 o during metamorphic and metasomatic processes in the deep crustal and upper mantle

using equilibrium thermodynamics in the study of metasomatic alteration illustrated by an application to serpentinites abstract metasomatic rocks in which mineralogical change is driven by infiltration of an externally derived fluid are challenging for the application of equilibrium thermodynamics because fluid driven changes in rock bulk composition can be faster than diffusive within grain re equilibration nevertheless careful definition of systems in terms of appropriate length scales an informed choice regarding the controlling variables and detailed petrological analysis can provide useful results thermodynamic calculations using methods that solve for equilibrium between a set of independent end member reactions produce valid results regardless of the identity of the controlling variables but determination of the relationship between cause and effect in metasomatic rocks is best performed with some knowledge of the parameters that drive the evolution of the system the correspondence between observations of serpentinised harzburgites from new caledonia and the results of the application of thermodynamic techniques suggest that equilibrium thermodynamics can be applied to these rocks on appropriate small length scales varying from micron to cm the primary drivers for mineralogical change are proposed to be 1 infiltration of a h 2 o fluid which is buffered to lower a o 2 by the rock with resultant production of h 2 and the resulting change in the redox budget of the rocks and 2 gradients in a sio 2 caused by mm to cm scale variation in si mg fe inherited from the harzburgite protolith

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

lateral variation in geometry of thrust planes and its significance as studied in the shumar allochthon lesser himalayas eastern bhutan abstract the features mapped indicate a sse striking displacement vector for the shumar thrust of eastern bhutan sections parallel to this direction show that the frontal flat segment of the thrust plane along with the subthrust and the hanging wall rocks has been folded into a synform a longitudinal section however shows a cuspate lobate shape broad concave upward curves meeting along sharp crests of the thrust plane which laterally cuts up through the subthrust rocks the broad salients and sharp re entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate lobate lateral curvature which is a primary feature of the thrust plane each cusp combined with the forward or backward inclination of the thrust plane imparts a scoop like shape several of which are joined by transverse ridges parallel to the displacement vector the s surfaces transposed parallel to the limbs and axial surfaces of the isoclinal early folds in the allochthon have been progressively reoriented parallel to the curviplanar thrust surface later post thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction as a result the bent hinges are congruent to the laterally curved thrust surface below the evolution of the shumar thrust can be explained by lateral propagation along concave upward trajectories thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface the ridges along which the thrusts coalesce have the form of lateral ramps

simultaneous precipitation of magnesite and lizardite from hydrothermal alteration of olivine under high carbonate alkalinity the present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in earth systems e g functioning of hydrothermal fields or in engineered systems e g ex situ and in situ mineral sequestration of co2 for this case specific experimental conditions were examined 200 c saturated vapor pressure 16bar solution solid weight ratio 15 olivine grain size 30 m and high carbonate alkalinity 1m nahco3 under these conditions competitive precipitation of magnesite and serpentine preferentially lizardite type were clearly determined by using conventional analytic tools xrd fesem ftir and tga excluding the fate of the iron initially contained in olivine the alteration reaction for olivine under high carbonate alkalinity can be expressed as follows 2 mg 2 sio 4 2h 2 o h co 3 mg co 3 mg 3 si 2 o 5 oh 4 oh this reaction mechanism implied a dissolution process releasing mg and si ions into solution until supersaturation of solution with respect to magnesite and or serpentine the released iron contained in the olivine has not implied any precipitation of iron oxides or oxy hydroxides in fact the released iron was partially oxidized about 50 via a simple reduction of water 2 fe 2 2h 2 o 2 fe 3 h 2 2 oh in this way the released iron was incorporated in serpentine fe ii and fe iii and in magnesite fe ii this latter was clearly determined by fesem eds chemical analysis on the single magnesite crystals the nucleation and epitaxial growth processes at the olivine fluid interfaces cannot be excluded in our investigated system the experimental kinetic data fitted by using a kinetic pseudo second order model have revealed a retarding process of serpentine formation with respect to magnesite about three times slower in fact the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution we assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given co2 pressure is imposed in the system on the other hand the reactivity of serpentinized olivine chrysotile brucite small amount of residual olivine and high purity chrysotile at the same above investigated conditions and the olivine serpentinization in initial acid ph 0 66 are also reported as complementary information in this study these novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate this could contribute to a better understanding of fluid rock interactions in natural active hydrothermal fields on earth

experimental evidence for perovskite and post perovskite coexistence throughout the whole d region article i nfo since the phase diagram for fe al bearing mgsio3 compositions at the p t conditions of the core mantle boundary remains ambiguous we investigated the fe distribution among the silicate perovskite pv and post perovskite ppv polymorphs using tandem synchrotron analyses of x ray diffraction and x ray absorption spectroscopy we performed measurements at the fe k edge of the partitioning of iron between pv and ppv up to more than 150 gpa after annealing at about 3300 k we obtain a unique solution for kfe pv ppv of 4 2 0 5 our results evidence that the two silicates should coexist over the whole d region with the main post perovskite phase being largely depleted in fe compared to the perovskite as fe and al have a dominant effect on the phase diagram these new results challenge recent determinations of the temperature profile in the lowermost mantle based on the clapeyron slope of the pv to ppv transition for pure mgsio3 composition also it appears clear that variations in the molar fractions of perovskite and post perovskite phases should be expected radially or laterally in the d region in relation with thermal or compositional heterogeneities this can help explaining the seismological anomalies observed for this mantle region finally we predict a significant increase of the feo activity in the d region which should greatly affect the chemical exchange between mantle and core

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

river profiles along the himalayan arc as indicators of active tectonics abstract longitudinal profiles along sixteen major transverse himalayan rivers add important constraints to models of active continental subduction and its evolution these profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases the base of the zone of increased gradients correlates with 1 the topographic front between the lesser and high himalayas 2 the narrow belt of intermediate magnitude thrust earthquakes 3 the main central thrust zone mct these features define a small circle in the central portion of the himalayan arc these correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising high himalayas and the lesser himalayas no sharp increases in gradient are observed near the main boundary thrust mbt except on a few rivers such as the jhelum or kundar where the mbt lies close to both the mct and the seismic belt thus it is unlikely that the mbt is a major tectonic boundary the diversion of river courses along the mbt and around anticlines in the sub himalayas has probably been caused by aggradation near the rosion deposition boundary upstream of uplifts in the mahabharat range and sub himalayas a parallel is drawn between the himalayas and new guinea based on the hypothesis that continent arc collision of the type occurring in northern australia preceded continent continent collision in the himalayas the present sedimentary tectonic phase in new guinea resembles the subathu paleocene eocene phase in the himalayas incipient counterparts of the major himalayan structures including the mct and the mbt are recognized in new guinea the drainage patterns in the himalayas and in new guinea bear a similar relation to major structures this suggests that 1 the tectonic evolution of the himalayas has been rather uniform since early stages of collision and 2 the himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the high himalayas

hydrothermal alteration of olivine in a flow through autoclave nucleation and growth of serpentine phases a single pass flow through experiment was conducted to induce serpentinization and to relate serpentine mineralogy to reaction progress during open system alteration of san carlos olivine by air saturated water the fluid flowed at 300 c and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell chemical analysis of the fluids indicated a steady state composition with respect to mg and ph after 550 hours which was interpreted to reflect saturation with brucite and one or more serpentine group minerals the alteration products were characterized by x ray diffraction xrd scanning electron microscopy sem and high resolution transmission electron microscopy hrtem nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by tem using platinum carbon replica techniques after reaction olivine fragments displayed clear signs of dissolution and growth of neoformed phases brucite and magnetite were distributed uniformly through the reaction cell although magnetite was more abundant there is however a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell lizardite is the dominant serpentine phase at the inlet whereas fibrous serpentines are predominant near the outlet lizardite crystals are more numerous but smaller in the middle of the cell and are topotactically aligned along a single preferential crystallographic orientation of the olivine cylindrical chrysotile formed predominantly on the surfaces of lizardite conical or polygonal serpentine the morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation

significance of forelimb folds in the shumar allochthon lesser himalaya eastern bhutan abstract the shumar allochthon is the most widespread of all the thrust sheets constituting the lesser himalaya of eastern bhutan the pre to syn thrusting folds of the allochthon have been affected by an unusual type of superposed folding called here forelimb folding in this type of superposed folding the alternate limbs of pre existing folds are buckled and thickened while the other set of limbs is stretched and thinned the hinges of these pre to syn thrusting folds are also bent producing plane non cylindrical folds analysis of these fold structures within the allochthon indicate simple shear strain generated in the course of forward movement of the thrust sheet within the shear zones some of the larger folds were oriented such that one set of limbs the forelimbs were inclined towards the shear direction and the other set the backlimbs were inclined in the opposite direction in the course of progressive shearing the forelimbs were shortened and buckled producing the forelimb folds and the backlimbs were stretched and thinned the forelimb folds also indicate the shear sense in the course of progressive shearing the forelimb folds have been boudinaged and rotated producing rootless folds with axes parallel to the thrust direction

the effects of serpentinization on density and magnetic susceptibility a petrophysical model abstract new measurements are presented of initial magnetic susceptibility and density for serpentinized harzburgites from the josephine peridotite oregon an inverse correlation between density and susceptibility which is typical of these rocks and similar rocks from other localities is not explained by simple serpentinization reactions calculations of density and susceptibility as a function of reaction progress for reactions that produce magnetite show susceptibilities that are larger than those observed at low degrees of serpentinization susceptibilities of fe bearing serpentine and brucite are calculated from a molecular field model initially developed for the olivine and orthopyroxene solid solutions at high degrees of serpentinization susceptibilities that are lower than those observed are predicted by reactions where iron is sequestered by serpentine and brucite observed density susceptibility trends of serpentinized harzburgites fall between the values predicted by these limiting reactions the observed properties may be modelled by multiple reactions involved in multi stage serpentinization processes such that the production of magnetite increases with the degree of serpentinization the empirical and calculated properties provide a complementary characterization of density and susceptibility values of serpentinites

rare earth element systematics of fossil bone revealed by la icpms analysis intra bone rare earth element ree fractionation trends were studied by la icpms analysis to put constraints on 1 the mechanisms controlling ree fractionation within fossil bones 2 the relative timing of ree uptake in various parts of fossil bone and 3 the origin of ree in fossil bones we have evaluated ree bone profiles across 54 fossil bones from a broad range of well characterised taphonomic settings ranging in age from triassic to early medieval ree patterns and concentration gradients are highly variable and intra bone fractionation trends in la yb n vs la sm n space of few specimens cover almost the entire range previously observed for bulk samples intra bone variability of ce anomalies as well as variable y ho and la la n is also observed sometimes diagenetic fluids with fractionated hree enriched compositions have entered the bone from the marrow cavity producing secondary ree uptake profiles theoretical intra bone fractionation trends modelled using lattice strain theory concur with most ree data but some trends cannot be modelled using realistic boundary conditions this fact as well as the occurence of positive and negative ce anomalies within the same fossil sample hint towards changing ree compositions of diagenetic fluids over the timescales of ree uptake because apparent ce anomalies frequently evolve over bone profiles and ce anomalies can be inherited from previous fractionation events ce anomalies are a highly ambiguous tracer for constraining ambient redox conditions in general bioapatite ree signatures may not always reliably reflect ambient taphonomic or redox conditions and diagenetic fluid compositions therefore ree patterns and ce anomalies of fossil bones must be interpreted cautiously as they vary spatially within skeletal remains and are affected by intra bone fractionation processes as well as changing compositions of the diagenetic fluid

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

30 000 years of hydrothermal activity at the lost city vent field strontium carbon and oxygen isotope data and radiocarbon ages document at least 30 000 years of hydrothermal activity driven by serpentinization reactions at lost city serpentinization beneath this off axis field is estimated to occur at a minimum rate of 1 2 10 4 cubic kilometers per year the access of seawater to relatively cool fresh peridotite coupled with faulting volumetric expansion and mass wasting processes are crucial to sustain such systems the amount of heat produced by serpentinization of peridotite massifs typical of slow and ultraslow spreading environments has the potential to drive lost city type systems for hundreds of thousands possibly millions of years

role of bacterial mats in oxygen deficient marine basins and coastal upwelling regimes preliminary report bacterial mats play an important role in the sedimentation history of many marine biogenic regimes observations of the living mats suggest that in shallow waters they are dominated by photosynthetic cyanobacteria while in deeper waters the mats are dominated by nonphotosynthetic sulfur oxidizing bacteria fossilized remains of analogous mat structures from the miocene monterey formation are commonly associated with oil producing and phosphate rich zones preliminary observations suggest that bacterial mats may be a source of the organic matter that in time is thermally altered to hydrocarbons

thermochemistry and phase equilibria of hydrous phases in the system mgo sio2 h2o implications for volatile transport to the mantle calorimetric phase equilibrium and in situ equation of state data for antigorite talc and dense hydrous magnesium silicate dhms phases have been obtained to construct a self consistent thermodynamic data set that ensures internal consistency well behaved extrapolation and physically reasonable values of h s v k k and for talc antigorite and hydrous phase a used in conjunction with temperature profiles of subducting oceanic lithosphere the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400 600 km in colder slabs since the slab can remain in the stability field of hydrous phases throughout its descent once water is brought to these depths it can probably diffuse into hotter mantle regions via hydrous mg2sio4 or by other mechanisms indicating that the transition zone is unlikely to be dry

iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15 n on the mid atlantic ridge aqueous dihydrogen h2 aq is produced in copious amounts when seawater interacts with peridotite and h2o oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine poorly understood in this process is the partitioning of iron and its oxidation state in serpentine although both impose an important control on dihydrogen production we present results of detailed petrographic mineral chemical magnetic and mosbauer analyses of partially to fully serpentinized peridotites from the ocean drilling program odp leg 209 mid atlantic ridge mar 15 n area these results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite seawater reaction path models in samples from hole 1274a mesh rims reveal a distinct in to out zoning from brucite at the interface with primary olivine followed by a zone of serpentine brucite magnetite and finally serpentine magnetite in the outermost mesh rim the compositions of coexisting serpentine mg 95 and brucite mg 80 vary little throughout the core about 30 50 of the iron in serpentine brucite mesh rims is trivalent irrespective of subbasement depth and protolith harzburgite versus dunite model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water to rock ratio during serpentinization at temperatures above 330 c the dissolution of olivine and coeval formation of serpentine magnetite and dihydrogen depends on the availability of an external silica source at these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen hence conditions are not reducing enough to form awaruite at t 330 c hydrogen generation is facilitated by the formation of brucite as dissolution of olivine to form serpentine magnetite and brucite requires no addition of silica the model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from 150 to 250 c and bulk water to rock ratios between 0 1 and 5 these conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization the development of the common brucite rims around olivine is either due to an arrested reaction olivine brucite serpentine brucite or reflects metastable olivine brucite equilibria developing in the strong gradient in silica activity between orthopyroxene talc serpentine and olivine serpentine brucite

stability of phase a in antigorite serpentine composition determined by in situ x ray pressure observations abstract here we precisely determined the low pressure stability limit of phase a in the mg end member antigorite bulk composition defined as the reaction forsterite water phase a enstatite water line or water storage line in a multi anvil apparatus pressures were determined by in situ synchrotron x ray diffraction measurements using nacl as an internal pressure standard results demonstrate that the water line is located at 800 c and 8 5 gpa and at 550 c and 5 1 gpa with a clapeyron slope of 13 6 mpa c we also examined the conditions for the formation of phase a by the decomposition of antigorite on the basis of the phase relations in the systems mgo sio 2 h 2 o msh and mgo al 2 o 3 sio 2 h 2 o mash a descending slab peridotite retains water in phase a beyond the stability of antigorite only when temperature in the slab is lower than 550 or 660 c at 5 1 gpa corresponding to 160 km depth in the system msh or mash respectively the double seismic planes observed within the slabs may be caused by the dehydration reaction of antigorite or chlorite their merging depths which could represent the high pressure stability limit of antigorite or chlorite are 160 km or deeper beneath northeast japan aleutian kamchatka and kuril water included in hydrous slab peridotite is transported into the transition zone and deeper mantle in these areas but recycled to the surface in other subduction zones

preservation of early cambrian animals of the chengjiang biota the chengjiang biota of yunnan china documents the earliest extensive radiation of the metazoa recorded in the fossil record gauging preservational bias is crucial in providing an assessment of the completeness of this biota and thereby elucidating whether it represents a comprehensive depiction of early cambrian life we here present a model to explain the nature of the exceptional preservation of the chengjiang biota and details of the decay process this study indicates that chengjiang fossils were preserved through two taphonomic pathways that may have captured tissues of distinct compositions and this finding should provide a foundation for the interpretation of chengjiang fossils many chengjiang fossils are preserved by pyrite later pseudomorphed by iron oxides the clay rich host sediment was deficient in organic carbon but replete in available fe and this composition ensured that a decaying carcass acted as a local substrate for fe and s reducing bacteria pyrite morphology probably reflects contrasts in the decay rate and hence the h 2 s production rate of different tissues in a carcass reactive rapidly decaying tissues would have quickly supplied h 2 s producing many pyrite nuclei resulting in framboidal habits more recalcitrant tissues would have produced h 2 s more slowly so that crystal growth operated on fewer nuclei resulting in larger euhedral pyrite crystals reflective films especially common on chengjiang arthropods represent the remains of degraded carbon

modern water rock reactions in oman hyperalkaline peridotite aquifers and implications for microbial habitability abstract the samail ophiolite in oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere previous investigations of hyperalkaline fluids in oman have focused on fluids released at surface seeps which quickly lose their reducing character and precipitate carbonates upon contact with the o2 co2 rich atmosphere in this work geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers serpentinite rock and hyperalkaline fluids ph 10 which exhibit millimolar concentrations of ca2 h2 and ch4 as well as variable sulfate and nitrate were accessed from wells situated in mantle peridotite near ibra and studied to investigate their aqueous geochemistry gas concentrations isotopic signatures mineralogy fe speciation and microbial community composition the bulk mineralogy of drill cuttings is dominated by olivine pyroxene brucite serpentine and magnetite at depth fe bearing brucite is commonly intermixed with serpentine whereas near the surface olivine and brucite are lost and increased magnetite and serpentine is detected micro raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells fe k edge x ray absorption near edge spectroscopy xanes analysis of the lizardite shows a strong tetrahedral fe coordination suggesting a mixture of both fe ii and fe iii in the serpentine magnetite veins are also closely associated with this second generation serpentine and 2 10 m magnetite grains overprint all minerals in the drill cuttings thus we propose that the dissolved h2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine as well as destabilization of pre existing brucite present in the partially serpentinized dunites and harzburgites in particular we hypothesize that fe bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late stage magnetite additional serpentine and dissolved h2 dissolved ch4 in the fluids exhibits the most isotopically heavy carbon in ch4 reported in the literature thus far the ch4 may have formed through abiotic reduction of dissolved co2 or through biogenic pathways under extreme carbon limitation the methane isotopic composition may have also been modified by significant methane oxidation 16s rrna sequencing of dna recovered from filtered hyperalkaline well fluids reveals an abundance of meiothermus thermodesulfovibrionaceae sulfate reducers and clostridia fermenters the fluids also contain candidate phyla op1 and od1 as well as methanobacterium methanogen and methylococcus sp methanotroph the composition of these microbial communities suggests that low temperature hydrogen and methane generation coupled with the presence of electron acceptors such as nitrate and sulfate sustains subsurface microbial life within the oman ophiolite

water diffusion transport in a synthetic dunite consequences for oceanic peridotite serpentinization abstract a series of san carlos olivine aggregates sintered at high pressure and high temperature with two different porosities around 1 and 10 and grain sizes 1 5 m and 0 38 m were reacted at 300 c and 500 bars in the presence of pure water the reaction progress was monitored magnetically and the composition and distribution of the reaction products were analyzed at the end of each experiment brucite formation mainly occurred at the aggregate surface as a result of both congruent olivine dissolution and aqueous mg and si buffering by the reaction products i e brucite and lizardite the measured reaction progress did not exceed 2 6 after 290 d which strongly contrasts with previous studies performed on san carlos olivine powders i e isolated grains in aqueous solution hence limited water transport through the intergranular region of the aggregate drastically decreased the olivine surface area accessible to water and thus slowed down the whole serpentinization process when extrapolated to peridotite relevant olivine grain sizes our experimental results indicate that the water diffusion rate will become so slow that the first layer of primary minerals exposed to water within a mesh structure must fully react before the next mineral layer starts reacting layer by layer mechanism this type of reaction transport mechanism is obviously not consistent with the micro scale serpentine distribution in the mesh of oceanic peridotite samples therefore additional water transport pathways are required cracks formed under extensional thermal stresses are good candidates since in comparison to grain boundary or reaction induced fractures they are wide enough to drastically enhance water transport in oceanic peridotites and therefore account for the observed textures the layer by layer mechanism inferred here can only set a lower time bound for serpentinization completion assuming a mesh size of 1 mm and an initial grain size of 100 m and considering a temperature ranging from 100 to 300 c with permanent water saturation completion should take place within 100 1000 yr surprisingly this duration represents only 1 to 10 of the estimated timescale of the natural serpentinization process emphasizing the central role played by water availability in the natural reaction process

influence of pyroxene and spinel on the kinetics of peridotite serpentinization hydrothermal experiments were performed at 311 degrees c and 3 0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization the results show that the rates of reaction strongly depend on grain sizes of solid reactants with smaller grain sizes resulting in faster kinetics after 27 days of reaction the reaction extent was 99 for peridotite with grain sizes of 30 mu m and the reaction extent was 28 for grain sizes of 100 177 mu m compared to peridotite olivine is serpentinized at much slower rates e g 5 3 of reaction extent was achieved for olivine with grain sizes of 100 177 mu m after 27 days approximately five times lower than that reached during peridotite serpentinization such contrasting results are due to the presence of pyroxene and spinel an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel the reaction extent achieved in experiments with 3 wt spinel greatly increased to 98 after 27 days much higher than that achieved during olivine serpentinization these results appear to be related to pyroxene and spinel releasing al and cr during serpentinization as indicated by compositions of serpentine orthopyroxene lost similar to 60 of al at a reaction extent of 59 influence of al and cr is suggested by a dramatic increase in reaction extents with the addition of al2o3 and cr2o3 powders olivine in natural geological settings is commonly associated with pyroxene and spinel consequently serpentinization kinetics may be much faster than previously thought

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

compositional controls on hydrogen generation during serpentinization of ultramafic rocks abstract where ultramafic rocks are exposed to water at temperatures 2 formation are not well established we used thermodynamic computations to examine the equilibrium mineral assemblages mineral compositions and the chemistry of fluids during serpentinization of 21 different ultramafic rock compositions and 10 distinct compositions of olivine between 25 c and 400 c at 50 mpa our models predict some systematic differences between serpentinization of olivine dominated lithologies i e peridotite and of orthopyroxene dominated lithologies i e pyroxenite most notably it is predicted that serpentinization of peridotite causes the formation of serpentine having elevated fe 3 fe 3 fe 2 values fe bearing brucite at temperatures ca 320 c and magnetite at temperatures ca 200 c while serpentinization of pyroxenite does not produce magnetite but instead forms fe rich serpentine with relative low fe 3 fe 3 fe 2 values and fe poor talc the predicted activities of dissolved hydrogen a h 2 aq dissolved silica a sio 2 aq as well as the ph vary accordingly at temperatures ca 350 c fluids interacting with peridotite are more reducing have lower a sio 2 aq and higher ph than fluids interacting with pyroxenite a direct correlation between the iron content of olivine its stability relative to water temperature and a h 2 aq is apparent from our calculations in contrast to forsterite rich olivine fayalite rich olivine can be stable to temperatures as low as 180 c in the presence of water as a consequence the predicted ah 2 aq for serpentinization of fayalite is maximal at temperatures 180 c

mineralogical evidence for h2 degassing during serpentinization at 300 c 300 bar abstract hydrogen is produced in large amounts during hydrothermal alteration of peridotite in low spreading rate mid ocean ridges this production is directly linked to reducing conditions in hydrothermal fluids induced by the oxidation of fe2 in primary minerals olivine and pyroxene to fe3 in secondary minerals magnetite and serpentine a better knowledge of iron speciation in serpentine is therefore crucial to the quantification of hydrogen production during the serpentinization process for the first time we have determined the amount of ferric iron in altered peridotite as a function of alteration time we investigated experimentally the alteration of powdered lherzolite in pure water at 300 c 300 bar for each experimental run 0 7 18 34 and 70 days h2 degassing was measured using in situ gas chromatography and the experimental products were analyzed using xrd raman and x ray absorption spectroscopy at the iron k edge in parallel aqueous solutions were analyzed by icp aes our results show quasi complete serpentinization at 70 days with replacement of primary olivine and pyroxene by secondary lizardite and magnetite the fe3 fetotal ratio is linearly dependent on the hydrogen production and ranges from 0 to 0 66 at the end of the experiment our results reveal strong variations in fe3 in serpentine for different alteration times from 0 to 100 of ferric iron including up to 12 of tetrahedral iron hydrogen was produced in three main stages 1 a first stage during which the h2 production rate reaches a maximum at 18 days and is controlled by the crystallization of magnetite 2 an intermediate stage during which serpentine incorporates ferric iron and thus plays a major role up to 50 in the hydrogen formation and 3 a final stage during which magnetite amount increases from 2 to 5 of the mineral assemblage the last alteration stage is accompanied by a slight increase of the fe3 fetotal ratio while the rate of hydrogen production decreases at the end of the experiment consequently variations of the ferric iron contents in natural oceanic peridotites may constitute a good indicator of the hydrogen potential of various ultramafic hydrothermal fields

scaling properties of thrust fault traces in the himalayas and inferences on thrust fault growth abstract map traces of thrust faults in the himalaya show a conspicuous cuspate lobate pattern i e broad lobes joined by sharp angular cusps for each lobe we can draw a chord by joining the apices of two adjacent cusps and define amplitude a by perpendicular distance of apex of the lobe from its chord the lobes show a linear scaling defined by the relationship a cl where l is the chord length and c is a constant the l a ratio remains nearly constant over about 1 71 orders of magnitude spread of the l values of the sampled lobes suggesting that a is proportional to l the trace pattern indicates that the thrust surfaces are laterally curved due to the growth of fault segments from multiple nuclei along concave upward laterally curved trajectories the propagating surfaces coalesce along sharp angular ridges this lateral curvature gives a furrow like shape to each segment and a fluted appearance to the thrust surfaces the invariant nature of the l a ratio supports self similarity and is manifest as a linear scaling of a against l of the lobes the self similarity further suggests the possibility of thrust fault growth along a characteristic curved surface

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

serpentinization of oceanic peridotites 1 a high sensitivity method to monitor magnetite production in hydrothermal experiments 1 a new method using the magnetic properties of magnetite fe 3 o 4 was developed to monitor experimental serpentinization the saturation remanent magnetization signal jrs was measured during the course of experiments designed to react san carlos olivine mg 0 91 fe 0 09 2 sio 4 with water at 250 to 350 c and 500 bars at the end of the experiments the ratio with saturation magnetization jrs js ratio allowed to convert each successive jrs measurement into an in situ amount of magnetite produced by the serpentinization reaction water weight loss was also measured on the end product to determine the final degree of serpentinization the application of this procedure to a series of experiments performed at 300 c 500 bars for various run duration 9 to 514 days and starting olivine grain size 1 to 150 mm shows a linear relationship between magnetite production and reaction progress this relationship can be safely transposed to other experimental conditions using thermochemical modeling and or the fe content of the product phases we show that this high sensitivity magnetic method is a powerful tool to precisely monitor serpentinization kinetics in fe bearing systems it represents in addition a new indirect mean for monitoring the production of hydrogen which is bound to magnetite production rate citation malvoisin b j carlut and f brunet 2012 serpentinization of oceanic peridotites 1 a high sensitivity method to monitor magnetite production in hydrothermal experiments

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

role of microbial mats in the fossilization of soft tissues it has been speculated that microbial mats are an important agent in the fossilization of soft tissues particularly when apatite ca 5 co 3 po 4 3 oh f is involved this has been tested by chemical analyses of the jurassic limestones of cerin france where phosphatized soft tissues are abundant and are associated with unequivocal microbial mats the sedimentary distribution of p k and fe following deposition was controlled by the presence of the mats p concentrations in the mats may approach 2 5 times those elsewhere in the sediment the highest p concentrations correlate with the occurrence of phosphatized soft tissues this is the first analytical evidence to demonstrate a fundamental role for microbial mats in the preservation of soft bodied fossils

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

three dimensional geometry of thrust surfaces and the origin of sinuous thrust traces in orogenic belts insights from scaled sandbox experiments abstract sinuous traces of emerging thrust tips comprising multiple salients and recesses are commonly observed in orogenic belts e g lesser himalayas of india nepal and bhutan and in accretionary prisms e g nankai trough off the coast of japan lateral along the strike of the deformation zone variation in the depths of foreland basins i e variable sediment thickness or in the strength of the basal detachment or presence of a curved indenter has been traditionally cited to explain the formation of salients in fold and thrust belts although they are not applicable in all cases in the present work we have carried out four series of scaled analog model experiments using dry quartz sand changing the dip of the basal decollement 0 or 5 and the basal friction b 0 5 or 0 3 to investigate the 3d shape of thrust surfaces under varying overall boundary conditions but without any lateral variation of these parameters within the models the experimental results show that under all boundary conditions thrust surfaces are curved both in their dip and strike directions i e spoon shaped in 3d multiple concave upward and convex upward segments constitute a thrust surface which produces a sinuous trace when the tip line intersects the earth s surface it is also shown that thrust surface curvatures occur at different scales and the overall thrust surface roughness corrugations has a self affine fractal geometry

iron transformations during low temperature alteration of variably serpentinized rocks from the samail ophiolite oman abstract partially serpentinized peridotites in the samail ophiolite in the sultanate of oman currently undergo low temperature alteration and hydration both at shallow levels with water recently in contact with the atmosphere and at depth with anoxic reducing fluids however it is unclear how changes in the distribution and oxidation state of fe are driving the production of energy rich gases such as hydrogen and methane detected in peridotite catchments we track the fe transformations in a suite of outcrop samples representing a subset of the spectrum of least to most altered end members of the oman peridotites we use microscale mineralogical and geochemical analyses including qemscan raman spectroscopy synchrotron radiation x ray fluorescence xrf mapping and electron microprobe wavelength dispersive spectroscopy the less altered peridotites possess a diversity of fe bearing phases including relict primary minerals e g olivine pyroxene chromite and secondary phases e g serpentine and brucite raman spectroscopy and electron microprobe data si mg fe indicate that much of the serpentine is significantly intergrown with brucite on the sub micron scale these data also indicate that the fe content of the brucite ranges from 10 to 20 wt feo the mineral assemblage of the highly reacted rocks is less diverse dominated by serpentine and carbonate while olivine and brucite are absent magnetite is relatively rare and mainly associated with chromite goethite and hematite both fe iii hydroxides were also identified in the highly altered rocks whole rock chemical analyses reflect these mineralogical differences and show that fe in the partially serpentinized samples is on average more reduced 0 40 0 55 fe3 fetotal than fe in the highly reacted rocks 0 85 0 90 fe3 fetotal we propose that olivine brucite chromite and perhaps serpentine in the less altered peridotites act as reactive phases during low temperature alteration of the oman peridotite the pervasive oxidation of fe ii in the less altered peridotites to fe iii in the most altered peridotites during water rock reaction in the subsurface of the samail ophiolite may produce h2 which will influence the development of microbial energy sources and habitats and carbon cycling and sequestration within the ultra mafic ocean crust

low temperature alteration of serpentinized ultramafic rock and implications for microbial life abstract alteration serpentinization of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere in this study we focus on ongoing low temperature water rock processes in partially serpentinized dunite from the leka ophiolite complex mid norway we aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities the study is based on the chemical and physical alteration features of a 51 m deep rock core as well as the chemical composition of groundwater from the core s borehole the borehole intersects three main fracture zones observations during drilling indicated that water infiltration largely takes place along the deepest zone the rock in the fracture zones is nearly completely altered to serpentine brucite magnetite and minor calcite an increase in ph and decrease in mg si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water rock reactions where dissolution of brucite serpentine and calcite and precipitation of chrysotile hydromagnesite and iron hydroxide are most likely to occur detection of both h 2 and o 2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water rock ratios and to dissolution of fe ii containing brucite and serpentine resulting in the formation and successive diffusion of h 2 into the oxic borehole oxidation of h 2 reduced fe and mn nh 4 and ch 4 are all potential energy yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources methane could result from microbial oxidation of h 2 and reduction of co 2 or from the degradation of organic matter the results demonstrate that low temperature water rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

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

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

formation of h2 ch4 and n species during low temperature experimental alteration of ultramafic rocks abstract hydrogen methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities it is well known that production of h 2 through serpentinization of the primary fe ii containing minerals olivine and pyroxene in ultramafic rocks and reduction of co 2 to ch 4 are processes that occur at higher temperatures knowledge is however limited about these reactions under low temperature conditions and how they are affected by fe ii containing secondary minerals such as brucite and serpentine which are commonly found in these environments in this experimental study we explored the formation of h 2 ch 4 and nh 4 during low temperature 25 c reactions between deionised water and 1 unaltered 2 medium altered and 3 highly altered dunites over a period of 99 days hydrogen was detected in all three experiments and the concentration increased over time the o 2 concentration decreased over time but o 2 was still present at significant levels when h 2 started to form this suggests that the h 2 was formed locally at the surface of mineral grains rather than in the hypoxic solution the highly altered dunite gave the highest and the unaltered dunite the lowest h 2 concentrations and while the o 2 concentration decreased fast with the medium altered dunite significant amounts of o 2 still remained with the unaltered dunite at the end of the experiments both the medium and highly altered dunite gave o 2 concentrations close to or below the detection limit during the last part of the experiment factors that likely affected the h 2 production are fe content mineralogy dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite methane was also detected and increased with time in all experiments the results indicate that ch 4 formation also occurred locally on the surface of minerals detection of both nh 4 and no 3 in the aqueous solutions indicates that previously absorbed n species were leached out during the experiment increasing nh 4 and decreasing no 3 concentrations over time suggest no 3 reduction likely catalysed by mineral surfaces the results imply that low temperature water rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy yielding metabolic processes by subsurface microorganisms such reactions could play an important role in sustaining subsurface microbial communities particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow spreading mid ocean ridges and possibly also in other rocks and sediments the results suggest that brucite and possibly also serpentine may be more important sources of fe ii and thus h 2 formation than olivine in low temperature ultramafic systems formation of h 2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of h 2 formation and the subsurface habitat for aerobic h 2 oxidizing microorganisms

phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2 four sections documenting the impact of the late cenomanian oceanic anoxic event oae 2 were studied in basins with different paleoenvironmental regimes accumulation rates of phosphorus p bound to iron organic matter and authigenic phosphate are shown to rise and arrive at a distinct maximum at the onset of oae 2 with an associated increase in 13 c values accumulation rates of p return to preexcursion values in the interval where the 13 c record reaches its fi rst maximum an offset in time between the maximum in p accumulation and peaks in organic carbon burial hydrogen indices and corg preact molar ratios is explained by the evolution of oae 2 in the following steps 1 an increase in productivity increased the fl ux of organic matter and p into the sediments the preservation of organic matter was low and its oxidation released p which was predominantly mineralized 2 enhanced productivity and oxidation of organic matter created dysoxic bottom waters the preservation potential for organic matter increased whereas the sediment retention potential for p decreased 3 the latter effect sustained high primary productivity which led to an increase in the abundance of free oxygen in the ocean and atmosphere system after the sequestration of co 2 in the form of black shales this oxygen helped push the ocean back into equilibrium terminating black shale deposition and removing bioavailable p from the water column

tectonics of a part of the shumar allochthon in eastern bhutan abstract the lesser himalaya of eastern bhutan is made up of a pile of thin thrust sheets stacked on one another and the shumar allochthon lies at the top of this pile the shumar allochthon is a composite tectonic unit divisible into two structurally discordant sub units the upper sub unit system b is made up of a layered sequence of quartzite phyllite and marble that repeats several times through the sequence several subsidiary thrusts in the lower sub unit system a form a imbricate system arranged as a hinterland dipping duplex this imbricate system is spatially related to a transverse footwall ramp where the shumar thrust has cut up the underlying buxa stratigraphic sequence at the crest of this ramp the shumar thrust and the roof thrust of the duplex join forming a splay a detailed map of a part of the shumar allochthon reveals that alternate blocks parcels or horses of metasediments and sheared granitic gneisses constitute the stack of imbricates in this duplex analysis of the imbricate system indicates accretion of footwall elements in the course of forward propagation of the shumar allochthon and thus provides an idea about the nature of the basement on which the allochthon moved these accreted footwall elements suggest that the thrust sheets of the lesser himalaya of eastern bhutan moved and now rest on a basement of precambrian gneisses the top part of this gneissic basement was mobilized and involved in the thrusting process as is shown by the horses of the basement repeated at several levels within the shumar allochthon isolated patches of younger sediments on the precambrian gneisses were also involved in the thrusting

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

scales of structural heterogeneity within neotectonic normal fault zones in the aegean region abstract structural heterogeneities within neotectonic normal fault zones in the aegean region are important controls on the geomorphic expression of their uplifted footwall blocks contrasting fault rocks and slip plane phenomena corrugations comb fractures slip parallel fractures and pluck holes are responsible for mesoscopic inhomogeneities within fault scarps along strike disruptions in the continuity of fault scarps step over zones step up zones step down zones footwall cross faults and hangingwall cross faults are products of perturbations in slip plane geometry or intersections with transverse faults large scale variations in range front morphology reflect contrasting patterns of deformation within fault zones some major fault zones and many minor fault zones are characterized by a migration of the active slip plane towards the hangingwall resulting in a distributed network of high angle faults and a range front which is stepped in profile the presence of a localized mass of uneroded bedrock hangingwall salients protruding from the hangingwall of some major fault zones results in intense deformation along a principal slip plane and a range front which is ramp like in profile as a consequence of structural heterogeneities within neotectonic normal fault zones fault scarps and range fronts in the aegean region are subject to a temporally and spatially variable pattern of degradation

serpentinization element transfer and the progressive development of zoning in veins evidence from a partially serpentinized harzburgite serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen here we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water h2o and silica sio2 activities on mineral assemblages produced during progressive serpentinization of a harzburgite we describe a harzburgite from the santa elena ophiolite in costa rica that is 30 serpentinized serpentine brucite magnetite veins occur in olivine al rich serpentine talc veins occur in orthopyroxene and al rich serpentine talc brucite veins occur at the boundary of orthopyroxene and olivine bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene and olivine dominated areas specifically the sample records 1 varying brucite composition depending on whether or not it is associated with magnetite 2 formation of magnetite from fe rich brucite fe rich serpentine during olivine hydration where magnetite coexists with brucite mg 96 and serpentine mg 99 3 chemical gradients in si al cr and ca within and between orthopyroxene and olivine hosted veins and 4 local different equilibrium assemblages within different zones of veins the studied sample preserves rarely observed textures documenting continuous replacement of olivine rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites furthermore the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization

garnets within geode like serpentinite veins implications for element transport hydrogen production and life supporting environment formation geochemical micro environments within serpentinizing systems can abiotically synthesize hydrocarbons and provide the ingredients required to support life observations of organic matter in microgeode like hydrogarnets found in mid atlantic ridge serpentinites suggest these garnets possibly represent unique nests for the colonization of microbial ecosystems within the oceanic lithosphere however little is known about the mineralogical and geochemical processes that allow such unique environments to form here we present work on outcrop scale vein networks from an ultramafic massif in norway that contain massive amounts of spherulitic garnets andradite which help to constrain such processes vein andradite spherulites are associated with polyhedral serpentine brucite ni fe alloy awaruite and magnetite indicative of low temperature 200 c alteration under low fo2 and low asio2 aq geochemical conditions together with the outcrop and micro scale analysis geochemical reaction path modeling shows that there was limited mass transport and fluid flow over a large scale once opened the veins remained isolated closed system forming non equilibrium microenvironments that allowed upon a threshold supersaturation the rapid crystallization seconds to weeks of spherulitic andradite the presence of polyhedral serpentine spheres indicates that veins were initially filled with a gel like protoserpentine phase in addition massive fe oxidation associated with andradite formation could have generated as much as 600 mmol h2 aq per 100 cm3 vein although no carboneous matter was detected the vein networks fulfill the reported geochemical criteria required to generate abiogenic hydrocarbons and support microbial communities thus systems similar to those investigated here are of prime interest when searching for life supporting environments within the deep subsurface

relationships between organics water and early stages of aqueous alteration in the pristine cr3 0 chondrite met 00426 abstract in order to investigate the nature of organics at the time of their accretion into chondrite parent bodies as well as their subsequent evolution with aqueous alteration we have conducted a study of the morphologies spatial distribution and relationships between organic particles and the surrounding matrix phases in one of the most pristine carbonaceous chondrites currently known met 00426 cr which is therefore likely to contain the best preserved record of pre accretion features focused ion beam sections were extracted for transmission electron microscope observations from its matrix organic matter om shows a heterogeneous population of grains most of them smaller than 1 m diverse morphologies are observed such as compact rounded aggregates individual and aggregated nanoglobules and micron to nanometer sized veins a common feature is the systematic presence of cracks connected to the grains and filled with om the surrounding matrix groundmass consists of amorphous iron rich silicate particles intimately mixed with phyllosilicates sulfides and occasional tochilinite with sizes ranging from several hundreds of nanometers to below 10 nm a close spatial relationship is commonly observed between some of the organic matter particles and alteration phases such as tochilinite and phyllosilicates phyllosilicates sometimes occur intimately intercalated with organic matter at a scale below 10 nm we used tem eds techniques to quantify the water concentration in the matrix amorphous silicate material and the phyllosilicates the water contents of both materials are identical at 10 6 wt h 2 o and demonstrate that the amorphous silicate material in this meteorite is hydrated therefore even though these cr3 0 chondrites are the least altered objects from a mineralogical point of view their matrices contain significant amounts of water in the amorphous silicate this coupled in situ study of organics and aqueous alteration suggests that a significant population of the om accreted as a mixture of soluble and insoluble molecules together with water ice grains and that the om was mobile at the micrometer scale the spatial distribution of the om grains can therefore in part be attributed to parent body processes we suggest that as accreted ice melted hydration of the amorphous silicates and formation of tochilinite and phyllosilicates occurred in the immediate vicinity of the composite water ice organic matter grains the water soluble component of the organics was likely transported and redistributed in the surrounding porosity cracks grain boundaries as water circulated the textural settings suggest that some of the om material could have been polymerized during aqueous alteration and transformed into insoluble molecules perhaps during the last stages of alteration as water was consumed by silicate hydration reactions

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

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

determination of trace element partition coefficients between water and minerals by high pressure and high temperature experiments leaching technique a refined approach for the experimental determination of fluid composition at elevated pressure and temperature conditions is developed it effectively determines partition coefficients of trace elements between aqueous fluid and minerals this approach is developed on top of many previously known technical achievements however it differs from the previous ones by the use of calibrated base and acid leaching of quenched samples to strip off the precipitates formed from the fluid during the quench this permits the reconstruction of a fluid composition from an experimental charge that consists of a complete mixture of solid and fluid phases this is a significant difference from the previous analogous studies in which the fluid was separated from the solids all through the experiment mass balance calculations and error propagation assess the merits of the present method applied to a fluid antigorite system

oxidative conditions can lead to exceptional preservation through phosphatization exceptional preservation through phosphatization is primarily controlled by a reduction in ph favoring the precipitation of apatite over that of calcite laboratory experiments have suggested that phosphatization results from anoxic decay here we report results of the fine scale mineralogical characterization of cretaceous phosphatized fossils of teleost fishes and crustaceans from the jebel oum tkout lagerstatte morocco data collected using complementary laboratory and synchrotron based x ray techniques reveal that oxidative conditions were established at a certain step of decay supporting these conclusions are the presence covering and embedded in the phosphatized tissues of fe iii rich mineral phases the precipitation of which was likely biologically induced during decay the present study highlights that the establishment of oxidative conditions during decay can be compatible with exceptional preservation of fossils through phosphatization

contrasted effect of aluminum on the serpentinization rate of olivine and orthopyroxene under hydrothermal conditions abstract olivine and pyroxene are the major minerals of ultramafic rocks the hydrothermal alteration of these rocks leads to the serpentinization reaction that mainly forms serpentine and variable amounts of talc brucite and magnetite as well as hydrogen the serpentinization kinetics of pyroxene under hydrothermal conditions has been very little studied in comparison with olivine and both have been evaluated experimentally only in simple aqueous fluids here we evaluate the effect of aluminum on the serpentinization rate of olivine and orthopyroxene at 200 c 340 c and 200 mpa to simulate natural hydrothermal conditions we used low pressure diamond anvil cells lp dac and time resolved x ray diffraction to monitor in situ the progress of the serpentinization reaction in four experiments we also performed two long lasting additional experiments with orthopyroxene for six days at 340 c and 200 mpa for which in situ monitoring was not possible at 340 c in presence of al olivine conversion into lizardite is extremely fast half time reaction t1 2 7 h while orthopyroxene did not react much even after 6 days 11 in contrast to olivine orthopyroxene conversion to serpentine was faster without al 48 in 6 days magnetite was also observed in the run with olivine only at 340 c in experiments run with orthopyroxene only we observed the exclusive formation of proto serpentine instead of lizardite we propose that the contrasted effect of al on the serpentinization rate of olivine and orthopyroxene results from the complexation of al in the solution that reacts differently with the different mineral surfaces during their dissolution the positively charged olivine surface allows the adsorption of the dominant negatively charged al oh 4 complex while the neutral surface of orthopyroxene does not this adsorption process could facilitate both the dissolution of olivine and the nucleation growth of an al enriched lizardite

serpentinization iron oxidation and aqueous conditions in an ophiolite implications for hydrogen production and habitability on mars a b s t r a c t molecular hydrogen produced through iron oxidation during formation of serpentine and magnetite can sustain terrestrial subsurface ecosystems the fe 3 in serpentine partitions into octahedral and tetrahedral sites differently as serpentinization proceeds and tetrahedral fe 3 is present toward the end of serpentinization we map fe oxidation states in a serpentinite to determine the degree to which serpentinization progressed and where hydrogen production has been maximized to assess habitability at an abandoned chrysotile mine in norbestos quebec in association with the canadian space agency s mars methane analogue mission we also analyzed stable isotopes of carbon and oxygen in carbonates to constrain the conditions of water rock interaction during serpentinization iron oxidation and coordination was determined through field imaging of rock walls with a visible hyperspectral imager 420 720 nm and samples collected from imaged rocks and elsewhere in the mine were imaged in the laboratory 420 1100 nm sample chemistry mineralogy and oxidation state were determined with laboratory measurements of visible through mid infrared reflectance spectra major element chemistry mineralogy and mossbauer spectroscopy mapping with hyperspectral imaging of outcrops and hand samples shows that tetrahedral fe 3 is common in serpentinites at this site and results are confirmed through other measurements major element chemistry and mineralogy are consistent with serpentine plus minor carbonate carbonate samples show an exceptional range in 13

the role of decay and mineralization in the preservation of soft bodied fossils abstract fossil deposits that preserve soft bodied organisms provide critical evidence of the history of life usually only more decay resistant materials e g cuticles survive as organic remains as a result of selective preservation and subsequent diagenesis to more resistant biopolymers permineralization the permeation of tissues by mineralizing fluids may preserve remarkable detail particularly of plants however evidence of more labile tissues e g muscle normally requires the replication of their morphology by rapid in situ growth of minerals i e authigenic mineralization this process relies on the steep geochemical gradients generated by decay microbes the minerals involved and the level of detail preserved which may be subcellular depend on a number of factors including the nature of microbial activity and amount of decay availability of ions and the type of organism that is fossilized understanding these controls is essential to determining the conditions that favor exce

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

kinetics and mechanism of antigorite dehydration implications for subduction zone seismicity properties of serpentine minerals are thought to influence the occurrence and location of intermediate depth seismicity in subduction zones which is often characterized by two dipping planes separated by 30 km defining a double seismic zone the seismicity of the lower plane is believed to be provoked by the dehydration of serpentine since the experimentally determined stability limit for antigorite matches hypocenter locations this requires that the fluid produced by dehydration is released much faster than the typical time scale of ductile deformation mechanisms here we measured the kinetics of antigorite dehydration in situ at high pressure and high temperature by time resolved synchrotron x ray diffraction in a closed system antigorite dehydrates in two steps during step 1 it partially breaks down into olivine and a hydrous phyllosilicate closely related to the 10 a phase the modal abundance of the intermediate assemblage is described by 66 wt antigorite 19 wt olivine 12 wt 10 a phase during step 2 at higher temperature the remaining antigorite and the 10 a phase fully dehydrate from the analysis of reaction progress data we determined that the major release of aqueous fluid occurs during step 2 at a fast rate of 10 4 mfluid 3 mrock 3 s 1 this exceeds by orders of magnitude the typical time scale of deformation by ductile mechanisms of any mineral or rock in the subducting slab or in the overlying mantle wedge these results suggest that the fast dehydration of antigorite may well trigger the seismicity of the lower plane of the double seismic zone

life in the hydrated suboceanic mantle in the roots of the ocean crust mantle derived rocks are progressively hydrated by hydrothermal circulation raman spectroscopic analyses of hydrated rocks sampled from the ocean floor reveal accumulations of organic matter which point to the hydration process as a possible energy source

stable isotope composition and rare earth element content of vertebrate remains from the late cretaceous of northern spain lano did the environmental record survive abstract oxygen and carbon isotope measurements have been performed on phosphatic remains from faunal associations dinosaurs crocodiles turtles and fish of the late cretaceous continental and marine sediments from northern spain basque country the environmental meaning of the oxygen isotope record in fossil reptiles is discussed on the basis of known paleoecology modern fauna analogs and apatite chemistry rare earth elements ree co32 and po43 contents fossil remains in sandstones and argillites from two localities urria and cuezva have low down to 16 and scattered 18o po4 co3 and 13c values with ree patterns characterized by strong middle ree enrichments these geochemical compositions of the biogenic apatites have most likely been acquired during a multi stage late diagenesis involving complete recrystallization of the apatite in the presence of aqueous fluids ree contents of fossils preserved in the clayey sands from the site of lano suggest that samples have only been affected by early diagenesis despite the diversity of analyzed taxa and the close association of marine sharks and rays and freshwater species crocodiles and turtles evolving in a littoral environment the narrow range of phosphate 18o values is interpreted as reflecting the temperature and isotopic composition of groundwaters that prevailed in the burial environment the potential information relative to the physiology and thermal regime of the vertebrates has been erased during the process of early diagenesis if all oxygen in the phosphate has exchanged with groundwaters the oxygen isotope composition of vertebrate remains suggests that a warm climate t 20 25 c prevailed in northern spain during the late cretaceous

experimental study of carbonate formation in oceanic peridotite abstract interactions of co2 rich aqueous fluids with mantle peridotite have major implications for geochemical budgets and microbial life in the shallow oceanic lithosphere through the formation of carbonate minerals and reduced carbon species however the underlying mechanisms controlling the transformation of co2 to carbonates in ultramafic hosted hydrothermal systems remain incompletely understood a long term laboratory experiment was conducted at 300 c and 35 mpa to investigate serpentinization and carbonate formation pathways during hydrothermal alteration of peridotite powdered harzburgite was initially reacted with a ca rich aqueous fluid for 14 592 h 608 days and changes in fluid composition were monitored with time once the system reached a steady state a co2 aq rich fluid was injected and allowed to react with the system for 5907 h 246 days fluid speciation and mineral analyses suggest that serpentinization of harzburgite in the co2 poor system led to the precipitation of serpentine brucite magnetite and minor calcite in addition to other minor phases including chlorite and sulfur poor ni sulfides the addition of the co2 aq rich fluid caused dolomite ca rich dolomite and high mg calcite to form at the expense of olivine calcite and brucite while serpentine remained unreactive replacement textures and mineral assemblages mimic those documented in carbonate altered seafloor serpentinites particularly those from the mid atlantic ridge and the iberia margin in contrast to thermodynamic predictions magnesite did not form in the experiment because the dissolution of clinopyroxene in combination with the lack of serpentine reactivity maintained low mg ca ratios in solution clinopyroxene dissolution and unreactive serpentine may similarly maintain low mg ca ratios in submarine serpentinization systems and limit magnesite formation in subseafloor environments results of this study suggest that the formation of ca mg carbonates by mineral carbonation is favorable in subseafloor serpentinization systems and likely represents a significant but poorly quantified carbon sink in hydrothermally altered oceanic lithosphere at slow spreading mid ocean ridges

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems

aluminum speeds up the hydrothermal alteration of olivine the reactivity of ultramafic rocks under hydrothermal conditions controls chemical fluxes at the interface between the internal and external reservoirs of silicate planets on earth hydration of ultramafic rocks is ubiquitous and operates from deep subduction zones to shallow lithospheric environments where it considerably affects the physical and chemical properties of rocks and can interact with the biosphere this process also has key emerging societal implications such as the production of hydrogen as a source of carbon free energy to date the chemical model systems used to reproduce olivine hydrothermal alteration lead to the formation of serpentine with sluggish reaction rates here we use in situ diamond anvil cell experiments and show that the presence of aluminum in hydrothermal fluids increases the rate of olivine serpentinization by one to two orders of magnitude aluminum increases the solubility of olivine and enhances the precipitation of aluminous serpentine after two days olivine crystals were fully transformed to aluminous serpentine under conditions typical for natural hydrothermal environments i e 200 and 300 c 200 mpa this result motivates a re evaluation of the natural rates of olivine serpentinization and of olivine hydrolysis in general in a wide range of settings this discovery also opens the potential of the serpentinization reaction for industrial scale production of hydrogen at economically feasible timescales and temperature