magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes serpentinization of ultramafic rocks during hydrothermal alteration at mid ocean ridges profoundly changes the physical chemical rheological and magnetic properties of the oceanic lithosphere there is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults which eventually results in structures known as oceanic core complexes occs however we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of occs since magnetite is a direct product of serpentinization the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere here we present new results from an integrated rock magnetic paleomagnetic and petrological study of variably serpentinized peridotites from the first fossil occ recognized in an ophiolite integration with existing data from mid ocean ridge related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range 0 100 of degrees of serpentinization variations in a range of magnetic parameters with serpentinization and associated paleomagnetic data provide i key constraints on the mechanism s of serpentinization at mid ocean ridges ii insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies and iii evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at occs

the roles of fluid transport and surface reaction in reaction induced fracturing with implications for the development of mesh textures in serpentinites the distinct element method was used to simulate chemical mechanical hydraulic processes that occur during serpentinization volume increasing hydration within the oceanic lithosphere the proposed model considers water transported in two ways advective flow along fractures and through matrices variations in fracture pattern and system evolution were examined using two nondimensional parameters the ratios of the rates of flow in fracture f and matrix m to the surface reaction rate in cases of fixed f and m with sufficiently low reaction rates the fracture pattern is not dependent on the surface reaction rate otherwise the fracture pattern varies systematically as a function of f and m at low f 1 and low m 1 the reaction proceeds from the boundaries inward and forms fine fractures layer by layer at high f 10 000 and low m 10 the reaction proceeds from the boundaries inward and forms polygonal fracture networks as m increases 100 the reaction tends to proceed homogeneously from the boundaries without fracturing a comparison of natural and simulated textures reveals that the following conditions are necessary to develop mesh textures during serpentinization in the oceanic lithosphere 1 the surface reaction rate must be similar to or higher than the fluid flow rate in the matrix or than the diffusive transport of water and much lower than the fluid flow rate along fractures 2 original olivine grain boundaries act as pathways for fluid transport these pathways may result from thermal or tectonic stress induced cracking prior to serpentinization

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

microstructure and porosity evolution during experimental carbonation of a natural peridotite abstract carbonation experiments t 100 200 c pco2 14 6 19 4 mpa were performed under closed system conditions on a natural peridotite from almklovdalen w norway to investigate the microstructure and porosity evolution as a function of reaction progress the unreacted samples cubes of 2 5 3 cm3 were composed mainly of forsteritic olivine 80 90 vol clinochlore orthopyroxene amphibole and chromite initial porosities were on the order of 6 8 due to the presence of abundant inter and intragranular microfractures microstructural observations after 7 42 days of experimental treatment showed that olivine was by far the most reactive phase after 7 days at 100 c no carbonate products were formed but longer reaction times produced small amounts of magnesite on the surface and within the fracture network of the peridotite cubes increasing temperatures led to faster reactions and pore space filling by magnesite in addition replacement rims up to 50 m thick composed of magnesite and amorphous silica formed around individual olivine grains the replacement began with the formation of an amorphous silica layer that retained the shapes of the initial olivine grains this layer remained permeable during the reaction as it allowed further dissolution of the underlying olivine increasing extents of carbonation were accompanied by decreasing porosities and enhanced cementation of grain boundaries about 50 of the initial porosity was closed when carbonation reached about 10 the reacted samples showed no evidence of reaction induced cracking indicating that the carbonation reaction was self limiting in our experiments these results may have important implications for the efficiency of in situ co2 sequestration in peridotites

initiation of strike slip faults serpentinization and methane the nootka fault zone the juan de fuca explorer plate boundary the nootka fault zone is a ridge trench trench transform fault that was initiated 4 ma when the explorer ridge became independent of the juan de fuca ridge multibeam data around the fault zone and a compilation of several seismic reflection surveys provide insight into initiation of strike slip faults previous interpretations assumed that the two faults seen cutting the seafloor are subparallel to shear between the explorer and juan de fuca plates and formed instantaneously at 4 ma increased data density shows that these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults formed at the ridge they are surrounded by numerous buried steeply dipping small offset growth faults at least some of which are likely still active our observations corroborate analogue models of strike slip fault initiation that predict formation of riedel like shears within a zone of faulting and that displacement localizes over time the existence of several long subparallel faults and a very wide zone of faulting has been predicted by models of distributed shear at depth along the nootka fault zone basement has risen by several hundred meters and bright reversed polarity reflectors some of which are interpreted to be methane hydrate reflectors are common hydration likely as serpentinization of the upper mantle could explain both sets of observations serpentinization can result in a 30 50 volume expansion and methane is observed in vents driven by this process biogenic sources of methane are likely to be present and concentrated by currently active fluid flow in the faulted sediments

windy mars a dynamic planet as seen by the hirise camera 1 with a dynamic atmosphere and a large supply of particulate material the surface of mars is heavily influenced by wind driven or aeolian processes the high resolution imaging science experiment hirise camera on the mars reconnaissance orbiter mro provides a new view of martian geology with the ability to see decimeter size features current sand movement and evidence for recent bedform development is observed dunes and ripples generally exhibit complex surfaces down to the limits of resolution yardangs have diverse textures with some being massive at hirise scale others having horizontal and cross cutting layers of variable character and some exhibiting blocky and polygonal morphologies reticulate fine polygonal texture bedforms are ubiquitous in the thick mantle at the highest elevations

orbital change experiments with a mars general circulation model abstract we use a mars general circulation model to examine the effect of orbital changes on the planet s general circulation and climate system experiments are performed for obliquities ranging from 0 to 60 for two different longitudes of perihelion each experiment simulates a full mars year assuming a fixed atmospheric dust distribution and fixed amount of co 2 in the atmosphere cap system we find that global mean surface temperatures and pressures decline with increasing obliquity due to the increasing extent of the winter polar caps the seasonal co 2 cycle and intensity of the solstice circulation amplify considerably with increasing obliquity such that global dust storms are likely at both solstices the most significant feature of the high obliquity solstice circulations is the development of an intense low level jet associated with the return branch of the hadley circulation model surface stresses are used to map regions of preferred dust lifting which are defined in terms of an annual deflation potential for the present obliquity the model predicted regions of high deflation potential are in good agreement with cantor et al s 2001 j geophys res 106 23653 23688 observations which gives us some confidence in the model s ability to predict where lifting might occur when mars orbit parameters are different than they are today in general we find that the dust lifting potential increases sharply with obliquity and is greatest at times of high obliquity when perihelion coincides with northern summer solstice over an obliquity cycle the model global annual deflation potential ranges from several tenths of a millimeter at 0 obliquity to almost 15 mm at 60 obliquity much higher values are possible when the atmosphere is very dusty we find a strong correlation between the deflation potential and surface thermal inertia regions of high deflation potential correspond to regions of high thermal inertia high rock abundance and regions of low deflation potential correspond to regions of low thermal inertia high dust sand abundance furthermore while the regions of preferred lifting high deflation potential expand somewhat with increasing obliquity and dust loading the central parts of tharsis arabia and elysium show no tendency for significant lifting at any obliquity or longitude of perihelion these regions may therefore be very old and represent net long term sinks for atmospheric dust it is the topography of the planet through its influence on surface pressure and wind systems which ultimately determines where dust accumulates finally as was found by fenton and richardson 2001 j geophys res 106 32885 32909 we find no tendency for the development of east southeasterly winds at the pathfinder site for any of our orbital change experiments this suggests that the ancient wind regime discussed by greeley et al 2000 j geophys res 105 1829 1840 was produced by other factors such as polar wander

native copper formation in mine prop wood from cyprus illustrates displacive growth by force of crystallization abstract a long standing issue in the study of metamorphic rocks concerns the question in how far replacement of the protolith assemblage during the growth of metamorphic porphyroblasts is accompanied by the additional operation of a displacive mechanism due to an alleged crystallization force exerted by the growing porphyroblast whilst in few cases a component of displacive growth can be corroborated unequivocal criteria to identify such displacements are lacking it follows that the concept of a so called crystallization force is surrounded by uncertainties on the viability and operation of displacive growth in natural systems we investigate the growth of cu porphyroblasts in a sample of mine prop wood from a bronze age copper mine in cyprus the pit prop became infiltrated with copper saturated polluted mine water during collapse of an ancient mine gallery the microstructures associated with the copper porphyroblasts strongly indicate displacive growth whilst the matrix wood has not been affected by any externally imposed deformation we suggest that the displacement microstructures associated with the copper porphyroblasts substantiate the existence of crystallization forces

global duricrust on mars analysis of remote sensing data global remote sensing data for mars are analyzed to obtain a simple self consistent model for the surface layer the data sets discussed include radar cross section measurements at several wavelengths radio whole disk thermal emission observations at two wavelengths the global distribution of thermal inertia deviations of diurnal temperatures from those of a homogeneous model and thermal spectral estimates of surface rock abundance and of the thermal inertia of the nonrocky component of the surface the data sets which most constrain the interpretation are the rock abundance map and the correlation of thermal inertia with radar cross section these require the rock abundance to not vary significantly from place to place and simultaneously require the density and thermal inertia of the fines to vary in a consistent manner the simplest model which can explain all of the data involves a global case hardened crust duricrust which varies spatially in its degree of formation in general low thermal inertia regions have a poorly developed crust and highinertia regions have a well developed crust there are however regions that consist of coarse particles and which do not fit this model e g chryse this model is consistent with the ages of low inertia regions and with aeolian mechanisms for their development the duricrust is thought to form via the mobilization of salt ions within a layer of water adsorbed within the regolith and its formation may be associated with the exchange of water between the regolith and atmosphere which occurs on the 105 and 106 year time scale

pore fluid extraction by reactive solitary waves in 3 d in the lower crust viscous compaction is known to produce solitary porosity and fluid pressure waves metamorphic de volatilization reactions can also induce porosity changes in response to the propagating fluid pressure anomalies here we present results from high resolution simulations using graphic processing unit parallel processing with a model that includes both viscous de compaction and reaction induced porosity changes reactive porosity waves propagate in a manner similar to viscous porosity waves but through a different mechanism involving fluid release and trap in the solid by reaction these waves self generate from red noise or an ellipsoidal porosity anomaly with the same characteristic size and abandon their source region to propagate at constant velocity two waves traveling at different velocities pass through each other in a soliton like fashion reactive porosity waves thus provide an additional mechanism for fluid extraction at shallow depths with implications for ore formation diagenesis metamorphic veins formation and fluid extraction from subduction zones

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

formation of iddingsite veins in the martian crust by centripetal replacement of olivine evidence from the nakhlite meteorite lafayette abstract the lafayette meteorite is an olivine clinopyroxenite that crystallized on mars 1300 million years ago within a lava flow or shallow sill liquid water entered this igneous rock 700 million years later to produce a suite of secondary minerals collectively called iddingsite that occur as veins within grains of augite and olivine the deuterium hydrogen ratio of water within these secondary minerals shows that the aqueous solutions were sourced from one or more near surface reservoirs several petrographically distinct types of veins can be recognised by differences in their width shape and crystallographic orientation augite and olivine both contain veins of a very fine grained hydrous fe and mg rich silicate that are 1 2 m in width and lack any preferred crystallographic orientation these narrow veins formed by cementation of pore spaces that had been opened by fracturing and probably in response to shock the subset of olivine hosted veins whose axes lie parallel to 0 0 1 have serrated walls and formed by widening of the narrow veins by interface coupled dissolution precipitation widening started by replacement of the walls of the narrow precursor veins by fe mg silicate and a crystallographic control on the trajectory of the dissolution precipitation front created micrometre scale 1 1 1 serrations the walls of many of the finely serrated veins were subsequently replaced by siderite and the solutions responsible for carbonation of olivine also partially recrystallized the fe mg silicate smectite was the last mineral to form and grew by replacement of siderite this mineralization sequence shows that lafayette was exposed to two discrete pulses of aqueous solutions the first of which formed the fe mg silicate and the second mediated replacement of vein walls by siderite and smectite the similarity in size shape and crystallographic orientation of iddingsite veins in the lafayette meteorite and in terrestrial basalts demonstrates a common microstructural control on water mineral interaction between mars and earth and indicates that prior shock deformation was not a prerequisite for aqueous alteration of the martian crust

dissolution assisted pattern formation during olivine carbonation olivine and pyroxene bearing rocks in the oceanic crust react with hydrothermal fluids producing changes in the physical characteristics and behaviors of the altered rocks notably these reactions tend to increase solid volume reducing pore volume permeability and available reactive surface area yet entirely hydrated and or carbonated rocks are commonly observed in the field we investigate the evolution of porosity and permeability of fractured dunites reacted with co2 rich solutions in laboratory experiments the alteration of crack surfaces changes the mechanical and transport properties of the bulk samples analysis of three dimensional microstructural data shows that although precipitation of secondary minerals causes the total porosity of the sample to decrease an interconnected network of porosity is maintained through channelized dissolution and coupled carbonate precipitation the observed microstructure appears to be the result of chemo mechanical coupling which may provide a mechanism of porosity maintenance without the need to invoke reaction driven cracking

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

quasi 3 d seismic reflection imaging and wide angle velocity structure of nearly amagmatic oceanic lithosphere at the ultraslow spreading southwest indian ridge we present results from 3 d processing of 2 d seismic data shot along 100 m spaced profiles in a 1 8 km wide by 24 km long box during the sismosmooth 2014 cruise the study is aimed at understanding the oceanic crust formed at an end member mid ocean ridge environment of nearly zero melt supply three distinct packages of reflectors are imaged 1 south facing reflectors which we propose correspond to the damage zone induced by the active axial detachment fault reflectors in the damage zone have dips up to 60 and are visible down to 5 km below the seafloor 2 series of north dipping reflectors in the hanging wall of the detachment fault these reflectors may correspond to damage zone inherited from a previous north dipping detachment fault or small offset recent faults conjugate from the active detachment fault that served as conduits for isolated magmatic dykes and 3 discontinuous but coherent flat lying reflectors at shallow depths 1 5 km below the seafloor and at depths between 4 and 5 km below the seafloor comparing these deeper flat lying reflectors with the wide angle velocity model obtained from ocean bottom seismometers data next to the 3 d box shows that they correspond to parts of the model with p wave velocity of 6 5 8 km s suggesting that they occur in the transition between lower crust and upper mantle the 4 5 km layer with crustal p wave velocities is interpreted as primarily due to serpentinization and fracturation of the exhumed mantle derived peridotites in the footwall of active and past detachment faults

control of serpentinisation rate by reaction induced cracking abstract serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water the solid volume increase during serpentinisation can lead to stress build up and trigger cracking which ease fluid penetration into the rock the quantitative effect of this reaction induced cracking mechanism on reactive surface generation is poorly constrained thus hampering our ability to predict serpentinisation rate in geological environments here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid ocean ridges we develop a micromechanical model to quantify the propagation of serpentinisation induced cracks in olivine the maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses the micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting and thus bulk reaction rate our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation we also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the marum ophiolite and the papuan ultramafic belt papua new guinea the natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40 these results are in agreement with our model predictions suggesting that reaction induced cracking controls the serpentinisation rate we use our model to estimate that at mid ocean ridges serpentinisation occurs up to 12 km depth and reaction induced cracking reduces the characteristic time of serpentinisation by one order of magnitude down to values comprised between 10 and 1000 yr the increase of effective pressure with depth also prevents cracking which positions the peak in serpentinisation rate at shallower depths 4 km above previous predictions

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

fracture initiation during volume increasing reactions in rocks and applications for co2 sequestration abstract volume increasing processes in rocks can induce various types of fractures such as hierarchical domain division of the unreacted material and spalling of the reacted material the mechanism responsible for the selection of a particular fracture type is generally not known we study such deformations and fracture mechanisms in a simplified system with a range of numerical techniques and find that sharpness of reaction fronts and magnitude of volume increase are the main parameters controlling which fracture type forms spalling dominates when there is sharp reaction fronts and large magnitude of volume change otherwise we get mainly domain dividing fractures increasing reaction front sharpness and magnitude of volume change also increases the fracture density and we argue that the change in fracture density can be important for the viability of in situ co 2 sequestration in ultramafic rock

reaction induced fracturing during replacement processes replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales the underlying mechanisms are in general poorly understood but both mechanical and chemical processes are thought to be important replacement of leucite kalsi2o6 by analcime naalsi2o6 h2o is common in silica poor igneous rocks a 10 increase in volume is associated with the replacement process and this generates stresses that eventually cause fracturing of the reacting leucite experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric onion skin like layers near the reacting interface and the formation of cross cutting often hierarchically arranged sets of fractures that divide the remaining leucite into progressively smaller domains these structures may explain the patchy alteration patterns observed in natural leucite samples and similar so called mesh textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks they are also strikingly similar to larger scale patterns formed during spheroidal weathering processes a simple discrete element model illustrates the mechanics that control the formation of such systems and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes

diffusion in solid earth systems recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid earth crystals melts and fluids although far from complete the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length and time scales of many natural transport phenomena in which diffusion plays a role conversely observations of diffusion progress in specific natural samples can be used to extract time temperature information for a variety of geologic and geochemical processes ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention despite this undeniable progress several major challenges remain that largely define the frontiers of research in solid earth diffusion perhaps foremost among these is the need to address and understand the multi scale multi path aspects of diffusion in many systems a complication that is not limited to polyphase materials individual mineral grains can exhibit clear indications of multi path behavior even when visible evidence of such paths is lacking many other diffusion frontiers are linked in one way or another to this multi scale issue they include diffusion of molecular h2o and the effect of h species on diffusion in minerals and rocks diffusive fractionation of multiple isotopes of a single element diffusion at the extreme conditions of the deep earth reconciliation of observations from natural samples and laboratory studies and development of theoretical approaches to predict

controls on rock weathering rates by reaction induced hierarchical fracturing abstract weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface the hydrosphere and the biosphere it is still poorly understood how this coupling affects weathering rates and patterns produced by weathering here we show that the observed spheroidal weathering patterns in basaltic intrusions dolerites in the karoo basin in south africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division the fracture processes generates new reactive surface area in a self accelerating manner thus providing a first order control on the total weathering rate

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

surface specific measurements of olivine dissolution by phase shift interferometry natural olivine dissolution and replacement often occurs preferentially along specific crystallographic planes thus olivine reactivity at specific surfaces was examined in situ using phase shift interferometry which has a detection limit 10 5 nm s by dissolving two smoothed olivine crystal faces and a third sample corresponding to a surface that was generated by preferential dissolution along structural defects the experiments were conducted at 22 c and ambient pressure in 0 1 m nacl solutions that were acidified to phs between 1 and 4 using 0 1 m hcl these experiments show that olivine dissolution can vary from one surface to another as well as in different areas of the same surface that have similar characteristics the fastest vertical retreat occurred at the surfaces related to defects however only vertical advancement was observed at ph 1 on this surface consistent with the observation of isolated islands on the surface during atomic force microscopy investigations after the experiment raman analysis of the precipitated phase showed that it was not one of the thermodynamically stable phases expected from phreeqc modeling however the correlation between the siloxane ring peak of amorphous silica with a similar peak in the precipitate spectrum in conjunction with previous experimental and natural observations indicates that the precipitate was a si enriched amorphous phase therefore precipitation can facilitate the further dissolution of olivine on this surface as long as it does not completely armor the surface precipitate formation on surfaces associated with outcropping defects supports the natural observations of preferential dissolution and serpentinization along these defects implying that the fast dissolution of these surfaces will play a critical role during olivine replacement in addition comparison with flow through experiments indicates that outflow fluid chemistry may provide an incomplete picture of processes occurring during olivine dissolution

gone with the wind eolian erasure of the mars rover tracks 1 the wheel tracks left by the mars exploration rovers spirit and opportunity are unique artificial markings on the surface of mars the tracks stretch several kilometers across diverse terrain in two widely separated regions of the planet the initial appearance and characteristics of the tracks were well documented by the science and navigation cameras aboard the vehicles at the time the tracks were formed orbital observations by mars global surveyor and mars reconnaissance orbiter document the erasure of the tracks over a period of more than 2 mars years close up examinations of track crossings where the rovers encountered tracks made hundreds of martian solar days earlier provide insights into the mechanisms and time scales of eolian alteration on mars these observations suggest that fallout of atmospheric dust plays only a minor role in obscuring rover tracks over time instead track erasure is dominated by sediment that is transported by surface winds both deposition and erosion act to erase the rover tracks the length scales for eolian sediment transport are hundreds of meters at least much larger than the size of the tracks gradual processes such as dust devils and sand saltation have minor effects that can nonetheless erase rover tracks over long time periods however short lived strong wind events associated with seasonal dust storms have much more pronounced effects significantly altering the tracks on time scales of days these episodic strong winds tend to occur annually during the perihelion season the time scale for track erasure is typically only 1 martian year

reaction assisted hierarchical fracturing during serpentinization abstract hierarchical fracture patterns are the result of a fracturing process that successively divides rocks into smaller domains in a 2d outcrop section such fracture patterns are characterized by four sided domains and t junctions where new fractures stop growing at right angles to pre existing fractures here we describe fracturing of mm to dm thick serpentinized orthopyroxenite dykes in a dunite matrix from the leka ophiolite complex in nord trondelag norway the fracturing process is driven by volume changes associated with the serpentinization of the dunites during this process the orthopyroxenite dykes are extended by the expanding dunite and undergo considerable fracturing the resulting fracture patterns are hierarchical the cumulative distributions of fracture lengths show a scaling behavior that lies between a log normal and power law fractal distribution this is consistent with a simple fragmentation model in which domains are divided according to a top hat distribution of new fracture positions within unfractured domains reaction assisted hierarchical fracturing is also likely to be responsible for other structures commonly observed in serpentinized ultramafic rocks including the mesh textures observed in individual olivine grains and the high abundance of rectangular domains at a wide range of scales this mechanism may provide first order controls on fluid migration rates in a wide variety of geological settings

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

serpentinization of mantle derived peridotites at mid ocean ridges mesh texture development in the context of tectonic exhumation at slow spreading ridges axial detachment faults exhume mantle derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault at the microscopic scale serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture we present a petrographic study sem ebsd and raman of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the mid atlantic and southwest indian ridges we show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale and in at least one studied location at the 100 m scale we propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids additional reaction induced cracks developed for serpentinization extents 20 the resulting microfracture network has a typical spacing of 60 m but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100 400 m in size apparent mesh rim thickness is on average 33 19 m corresponding to serpentinization extents of 70 80 published laboratory experiments suggest that mesh rims formation could be completed in a few years i e quasi instantaneous at the plate tectonic timescale the depth and extent of the serpentinization domain in the detachment fault s footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution

the interface scale mechanism of reaction induced fracturing during serpentinization peridotite serpentinization has first order effects on geochemical and petrophysical processes in the lithosphere this process induces intensive fracturing generating fluid pathways to facilitate the hydration of vast amounts of originally impermeable rocks but the mechanism linking interfacial reaction processes with fracture propagation has not been understood by combining microstructural characteristics of olivine lizardite serpentinization with fundamental aspects of interface coupled dissolution precipitation and crack growth theory we propose a microstructurally consistent self propagating fracturing mechanism fracturing is driven by stress generated from the growth and transformation of a metastable amorphous proto serpentine phase where stress is localized within surface perturbations etch pits and coalesced etch pits that originate from the anisotropic dissolution of olivine water migration into fractures reiterates the process resulting in hierarchical olivine grain segmentation our results indicate that the advancement of serpentinization at the grain scale is independent of solid state diffusion and does not rely on external forces

sand dune movement in the victoria valley antarctica we use vertical aerial photographs and lidar topographic survey data to estimate dune migration rates in the victoria valley dunefield antarctica between 1961 and 2001 results confirm that the dunes migrated an average of 1 5 m year these values are consistent with other estimates of dune migration from cold climate deserts and are significantly lower than estimates from warm deserts dune migration rates are retarded by the presence of entrained ice soil moisture and a reversing wind regime dune absorption merging and limb extension are apparent from the time series images and account for significant changes in dune form and the field scale dune pattern dune field pattern analysis shows an overall increase in dune field organization with an increase in mean dune spacing and a reduction in total crest length and defect density these data suggest that dunes in other cold desert environments on earth mars or titan that may also have inter bedded frozen laminae still have the potential to migrate and organize albeit at lower rates than dunes in warm deserts

olivine dissolution rates a critical review abstract the dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and the ease in obtaining pure samples for laboratory experiments due to the relative simplicity of its dissolution mechanism most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole olivine dissolution is controlled by the breaking of octahedral m2 oxygen bonds at or near the surface liberating adjoining sio44 tetrahedra to the aqueous fluid aqueous species that adsorb to these bonds apparently accelerate their destruction for example the absorption of h h2o and at some conditions selected aqueous organic species will increase olivine dissolution rates nevertheless other factors can slow olivine dissolution rates notably olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid by for example the presence and or growth on these surfaces of either microbes or secondary phases the degree to which secondary phases decelerate rates depends on their ability to limit access of the reactive aqueous fluids to the olivine surface it seems likely that these surface area limiting processes are very significant in natural systems lowering olivine surface reactivity in many environments compared to rates measured on cleaned surfaces in the laboratory a survey of the literature suggests that the major factors influencing forsteritic olivine dissolution rates are 1 ph 2 water activity 3 temperature and 4 mineral fluid interfacial surface area evidence suggests that the effects of aqueous inorganic and organic species are relatively limited and may be attributed at least in part to their influence on aqueous solution ph moreover the observed decrease in rates due to the presence of secondary mineral coatings and or the presence of microbes can be attributed to their ability to decrease olivine surface area directly exposed to the reactive aqueous fluid as the reactivity of forsterite surfaces are spatially heterogeneous its surface area normalized rates will tend to decrease as it dissolves even in the absence of a mineral or bacterial coating each of these factors limits and or influences the application of forsterite dissolution to 1 enhanced weathering efforts 2 mineral carbonation and 3 the low temperature generation of hydrogen or hydrocarbons via the oxidation of its divalent iron

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

a unifying basis for the interplay of stress and chemical processes in the earth support from diverse experiments the interplay between stress and chemical processes is a fundamental aspect of how rocks evolve relevant for understanding fracturing due to metamorphic volume change deformation by pressure solution and diffusion creep and the effects of stress on mineral reactions in crust and mantle there is no agreed microscale theory for how stress and chemistry interact so here i review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces chemical potential helmholtz free energy normal stress at interface molar volume the experiments encompass temperatures from 100 to 1300 degrees c and pressures from 1 bar to 1 8 gpa the equation applies to boundaries with fluid and to incoherent solid solid boundaries it is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions that document flow laws for pressure solution and diffusion creep that address polymorphic transformations under stress and that investigate volume changes in solid state reactions the accord is not in all cases quantitative but the equation is still used to assist the explanation an implication is that the chemical potential varies depending on the interface so there is no unique driving force for reaction in stressed systems instead the overall evolution will be determined by combinations of reaction pathways and kinetic factors the equation described here should be a foundation for grain scale models which are a prerequisite for predicting larger scale earth behaviour when stress and chemical processes interact it is relevant for all depths in the earth from the uppermost crust pressure solution in basin compaction creep on faults reactive fluid flow systems serpentinisation the deeper crust orogenic metamorphism the upper mantle diffusion creep the transition zone phase changes in stressed subducting slabs to the lower mantle and core mantle boundary diffusion creep

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

influence of amorphous silica layer formation on the dissolution rate of olivine at 90 c and elevated pco2 for mitigating against rising levels of atmospheric co2 carbonation of m2 bearing silicates has been proposed as a possible option for sequestering co2 over long time spans due to its rapid far from equilibrium dissolution rate and its widespread occurrence in mafic and ultramafic rocks olivine has been suggested as a potentially good candidate for achieving this goal although the efficacy of the carbonation reaction still needs to be assessed with this as a goal the present study aims at measuring the carbonation rate of san carlos olivine in batch experiments at 90 c and pco2 of 20 and 25 mpa when the reaction was initiated in pure water the kinetics of olivine dissolution was controlled by the degree of saturation of the bulk solution with respect to amorphous silica this yet unrecognized effect for olivine was responsible for a decrease of the dissolution rate by over two orders of magnitude in long term 45 days carbonation experiments with a high surface area to solution volume ratio sa v 24 600 m 1 the final composition of the solution was close to equilibrium with respect to sio2 am independent of the initial concentration of dissolved salts nacl and naclo4 ranging between 0 and 1 m and with an aqueous mg si ratio close to that of olivine no secondary phase other than a ubiquitous thin 40 nm si rich amorphous layer was observed these results are at odds with classic kinetic modeling of the process due to experimental uncertainties it was not possible to determine precisely the dissolution rate of olivine after 45 days but the long term alteration of olivine was indirectly estimated to be at least 4 orders of magnitude slower than predicted taken together these results suggest that the formation of amorphous silica layers plays an important role in controlling the rate of olivine dissolution by passivating the surface of olivine an effect which has yet to be quantified and incorporated into standard reactive transport codes

planet wide sand motion on mars prior to mars reconnaissance orbiter data images of mars showed no direct evidence for dune and ripple motion this was consistent with climate models and lander measurements indicating that winds of sufficient intensity to mobilize sand were rare in the low density atmosphere we show that many sand ripples and dunes across mars exhibit movement of as much as a few meters per year demonstrating that martian sand migrates under current conditions in diverse areas of the planet most motion is probably driven by wind gusts that are not resolved in global circulation models a past climate with a thicker atmosphere is only required to move large ripples that contain coarse grains

mass transport and fractionation during monazite alteration by anisotropic replacement abstract the present experimental study investigates the causes of anisotropic replacement and its impact on the element mass transfer during monazite alteration by combining in situ characterization of solid and fluid products starting material of standard manangotry monazite was placed at 200 mpa with 18 o doped fluids at temperature t between 300 and 600 c for durations between 14 and 114 days in some cases with hematite magnetite buffer and in presence of pre fractured quartz crystals for all experiments with naoh at t 400 c monazite products show a replacement rim altered domain but with complex microstructures that are best explained by nanoscale observations at 400 c and 500 c replacement propagates anisotropically through the unaltered domain due to preferentially oriented dissolution and or fracturing at the reaction interface the altered domain corresponds to nanochannels of secondary monazite mnz2 intruding primary monazite mnz1 with nanopores filled by amorphous si rich inclusions at the reaction front interface at 600 c the altered domain increases in volume and becomes homogeneous and non porous due to textural and chemical reequilibration however nanoscale observations reveal an anisotropic propagation associated with dislocations and preservation of mnz1 relics within the apparently homogeneous altered domain textural equilibration leads to transient pores inclusions which preferentially move toward the reaction front in all experimental products the abundant porosity attests to significant mass transfer during monazite replacement nanomixture within altered domains hampers accurate determination of mnz2 composition and is assumed to be responsible for the pb measured in the rim due to mnz1 contribution however composition measurements indicate that mnz2 contains higher light rare earth elements lree higher th u ratio but lower y th u pb and ca contents compared to mnz1 elements released in the fluid by monazite replacement are mobilized from the reaction front toward the grain boundary and then redistributed between secondary th silicates and phosphates si rich gel like material and aqueous fluids at t 500 c analyses of inclusions trapped in replaced quartz confirm an efficient ree solubility in the alkaline solutions the si rich gel like inclusions contain high amounts of ree e g 950 4600 ppm ce th 4480 6170 ppm and u 30 110 ppm while aqueous fluid inclusions have high concentrations of ree and th around 100 s of ppm compared to common natural brines there is a fractionation of th and u during mnz1 replacement th u 70 with low th u ratios in the aqueous fluid 7 23 compared to the secondary solid products up to 150 independently of the initial presence of hematite magnetite buffer this accounts for a fractionation within the isotopic th pb and u pb ages within the altered domain in natural samples observed microscopically the complex anisotropic geometry of the replacement can lead to a nanomixture within apparently homogeneous altered domains with partial resetting of the u th pb geochronometer and to misleading interpretations of secondary inclusions within apparently unaltered domain

formation of garnet clusters during polyphase metamorphism pre alpine garnets of variscan age from metapelitic basement units in northern italy were strongly retrogressed at near surface conditions prior to alpine contact metamorphism the replacement by sheet silicates caused a significant volume increase during retrogression producing pervasive fracturing up to several hundreds of angular fragments formed from each crystal electron backscatter diffraction analysis documents a maximum misorientation of 22 of some fragments as a result of local rotation during fracturing new garnet growth is observed on the garnet fragments during contact metamorphic overprinting resulting in garnet clusters fragments can be identified due to calcium rich domains fragment orientations were inherited and only minor new nucleation occurred these garnets develop features typically associated with multiple nucleation models but here they reflect multiple metamorphic events we propose that clusters can be indicative of multiple metamorphic events which were separated by a period of intense retrograde alteration

the fate of b cl and li in the subducted oceanic mantle and in the antigorite breakdown fluids we present an inventory of b cl and li concentrations in a key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle and in b fluid inclusions produced during high pressure breakdown of antigorite serpentinite samples correspond to i nonsubducted serpentinites northern apennine and alpine ophiolites ii high pressure olivine bearing antigorite serpentinites western alps and betic cordillera iii high pressure olivine orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites betic cordillera two main dehydration episodes are recorded by the sample suite partial serpentinite dewatering during formation of metamorphic olivine followed by full breakdown of antigorite serpentine to olivine orthopyroxene fluid ion probe and laser ablation icp ms la icp ms analyses of cl b and li in the rock forming minerals indicate that the hydrous mantle is an important carrier of light elements the estimated bulk rock b and cl concentrations progressively decrease from oceanic serpentinites 46 7 ppm b and 729 ppm cl to antigorite serpentinites 20 ppm b and 221 ppm cl to olivine orthopyroxene rocks 9 4 ppm b and 45 ppm cl this suggests release of oceanic cl and b in subduction fluids apparently without inputs from external sources lithium is less abundant in oceanic serpentinites 1 3 ppm and the initial concentrations are still preserved in high pressure antigorite serpentinites higher li contents in olivine ti clinohumite of the olivine orthopyroxene rocks 4 9 ppm bulk rock li as well as in the coexisting fluid inclusions suggest that their budget may not be uniquely related to recycling of oceanic li but may require input from external sources laser ablation icp ms analyses of fluid inclusions in the olivine orthopyroxene rocks enabled an estimate of the li and b concentrations in the antigorite breakdown fluid the inclusion compositions were quantified using a range of salinity values 0 4 2 wt naclequiv as internal standards yielding maximum average fluid rockdb 5 and fluid rockdli 3 5 we also performed model calculations to estimate the b and cl loss during the two dehydration episodes of serpentinite subduction the first event is characterized by high fluid rock partition coefficients for cl 100 and b 60 and by formation of a fluid with salinity of 4 8 wt naclequiv the antigorite breakdown produces less saline fluids 0 4 2 wt naclequiv and is characterized by lower partition coefficients for cl 25 60 and b 12 30 our calculations indicate that the salinity of the subduction fluids decreases with increasing depths fluid rockdb fluid rockdcl 1 0 5 indicates that cl preferentially partitions into the evolved fluids relative to b and that the b cl of fluids progressively increases with increasing depths and temperatures despite light element release in fluids appreciable b cl and li are still retained in chlorite olivine and ti clinohumite beyond the antigorite stability field this permits a bulk storage of about 10 ppm b 45 ppm cl and 5 ppm li i e concentrations much higher than in mantle reservoirs chlorite is the cl repository and its stability controls the cl and h2o budget beyond the antigorite stability b and li are bound in olivine and clinohumite the subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources potentially introducing anomalies in the upper mantle

in situ oxygen isotope analyses in serpentine minerals constraints on serpentinization during tectonic exhumation at slow and ultraslow spreading ridges abstract peridotites exhumed along detachment faults at slow and ultraslow spreading ridges undergo variable and complex serpentinization here we present in situ oxygen isotope analyses in serpentine minerals from a textural sequence identified in samples drilled at the atlantis massif mid atlantic ridge mar 30 n and dredged along the easternmost southwest indian ridge swir 62 65 e the textural sequence is similar at both locations and involves mesh texture and bastite formation after olivine and orthopyroxene mesh texture recrystallization into chrysotile and antigorite dominated textures and banded and fibrous veins the 18o in serpentine decreases with the textural sequence which we interpret to record an increase in the time integrated water rock ratio under a nearly constant temperature while mesh texture development starts at stoichiometric water rock ratios and creates an isotopic variability at a scale of 100 m recrystallized textures indicate serpentinization temperatures on the order of 260 290 c seawater dominated fluid hypothesis or 320 360 c hydrothermally altered seawater hypothesis the subsequent banded veins record the evolution towards colder and seawater dominated fluids potentially present in shallower levels of the detachment footwall serpentinization leading to the mesh textures appears faster and more heterogeneous in atlantis massif samples than in swir samples we propose that such features are influenced by the amount of gabbros intruded in the peridotite in addition to influencing fluid pathways gabbros also provide a source for observed si enrichments in some atlantis massif samples and control the ph which in turn influences the mobility of al

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

a comparative review of upscaling methods for solute transport in heterogeneous porous media the classical fickian model for solute transport in porous media cannot correctly predict the spreading the dispersion of contaminant plumes in a heterogeneous subsurface unless its structure is completely characterized although the required precision is outside the reach of current field characterization methods the advection dispersion model remains the most widely used model among practitioners two approaches can be adopted to solve the effect of physical heterogeneity on transport first based on a given characterization of the spatial structure of the subsurface upscaling methods allow the computation of apparent scale dependent parameters especially longitudinal dispersivity to be used in the classical fickian model in the second approach upscaled non fickian transport equations with scale independent parameters are used in this paper efforts are made to classify and review upscaling methods for fickian transport parameters and non fickian upscaled transport equations for solute transport with an emphasis on their mathematical properties and their one dimensional analytical formulations in particular their capacity to mode scale effects in apparent longitudinal dispersion is investigated upscaling methods and upscaled models are illustrated in the case of two three dimensional synthetic aquifers with lognormal hydraulic conductivity distributions characterized by variance values of 2 and 8 c 2008 etsevier b v all rights reserved

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

inter mineral mg isotope fractionation during hydrothermal ultramafic rock alteration implications for the global mg cycle abstract both riverine and ocean waters are enriched in 24 mg compared to the homogeneous chondritic mg isotopic composition of the earth s mantle requiring a fractionation step that is generally attributed to low temperature continental crust weathering here we present new observations that indicate that this 24 mg enrichment of surface waters may originate from mg isotope fractionation during the hydrothermal alteration of primary silicate minerals mineral separates of hydrothermally altered ultramafic rocks were collected from three different localities in norway coexisting olivine and serpentine exhibit invariant mg isotope ratios suggesting that serpentinization does not fractionate mg isotopes in contrast carbonation results in significant inter mineral mg isotope fractionation between the antigorite magnesite and talc the carbonation of the natural samples is constrained by o isotope thermometry at 275 c and hence closes the temperature gap between previous investigations of the natural distribution of mg isotopes during surface weathering and magmatic processes the precursor antigorite has an isotopic composition of 26 mg dsm 3 0 11 0 05 whereas the talc is enriched in 26 mg with mean mg 26 0 17 0 08 and the magnesite is depleted in 26 mg with mean mg 26 0 95 0 15 as carbonate minerals dissolve faster than silicate minerals the chemical weathering of carbonated ultramafic and by analogy mafic rocks on the continents will yield isotopically lighter mg to natural surface waters consistent with field observations moreover the mg fractionation observed in this study suggests that sub seafloor hydrothermal carbonation may be a significant contribution to the mg isotopic composition of ocean water

seasonality of present day martian dune gully activity martian slope gullies are argued to be evidence for recent liquid water flow on the surface of mars to explain the source of water a wide range of environmental conditions and processes has been invoked however a lack of information about the environmental context or timing of gully activity makes it difficult to evaluate the theories here we present new observations of extensive gully modification over the past 6 mars years within dune gullies with slope gully morphology observed activity within 18 gullies in 7 dune fields constrains timing to winter which is consistent with observed slope gully activity these observations show that fluvial processes are unlikely to cause present day martian dune gully activity and imply that co 2 frost accumulation may play the dominant role

ripple migration and dune activity on mars evidence for dynamic wind processes 1 in this report we show evidence of widespread ripple migration over the stoss side of dark barchan dunes in nili patera on mars the measured average migration of 1 7 meters in less than 4 terrestrial months clearly indicates that active sand saltation is occurring in the study area in addition we document widespread changes in the dune base ground surface contact and in the slip face structures showing that not only the ripples but the whole dunes are actually migrating in the present day atmospheric setting these results provide unequivocal evidence of recent aeolian activity and suggest that other dunes and ripples on mars may also be active

dissolution precipitation processes governing the carbonation and silicification of the serpentinite sole of the new caledonia ophiolite the weathering of mantle peridotite tectoni cally exposed to the atmosphere leads commonly to natural carbonation processes extensive cryptocrystalline mag nesite veins and stock work are widespread in the ser pentinite sole of the new caledonia ophiolite silica is systematically associated with magnesite it is commonly admitted that mg and si are released during the laterization of overlying peridotites thus the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric co 2 in this study we investigate serpentinite carbonation processes and related silicifica tion based on a detailed petrographic and crystal chemical study of serpentinites the relationships between serpen tine and alteration products are described using an original method for the analysis of micro x ray fluorescence images performed at the centimeter scale our investigations highlight a carbonation mechanism together with precipitation of amorphous silica and sepiolite based on a dis solution precipitation process in contrast with the per descensum mg si enrichment model that is mainly concentrated in rock fractures dissolution precipitation process is much more pervasive thus although the texture of rocks remains relatively preserved this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite

transverse aeolian ridges on mars first results from hirise images abstract three images obtained by the high resolution imaging science experiment hirise were analyzed for the information they could provide regarding transverse aeolian ridges tars on mars tars from five locations in a hirise image of the floor of ius chasma show remarkably symmetric cross sectional profiles with average slopes for the entire feature of 15 these results apply to tars that span an order of magnitude in wavelength and a factor of 6 in height a hirise image of gamboa impact crater in the northern lowlands shows low albedo sand patches

experimental constraints on fluid rock reactions during incipient serpentinization of harzburgite the exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution precipitation and reduction oxidation reactions a process referred to as serpentinization these reactions have major implications for microbial life through the provision of hydrogen h 2 to simulate incipient serpentinization under well constrained conditions we reacted centimeter sized pieces of uncrushed harzburgite with chemically modified seawater at 300 oc and 35 mpa for ca 1 5 yr 13 441 h monitored changes in fluid chemistry over time and examined the secondary mineralogy at the termination of the experiment approximately 4 mol of the protolith underwent alteration forming serpentine accessory magnetite chlorite and traces of calcite and heazlewoodite alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites neither brucite nor talc precipitated during the experiment given that the starting material contained 4 times more olivine than orthopyroxene on a molar basis mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine coupled mass transfer of dissolved si mg and h between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine hydrogen was released in significant amounts throughout the entire experiment however the h 2 release rate decreased with time serpentinization consumed water but did not release significant amounts of dissolved species other than h 2 suggesting that incipient hydration reactions involved a volume increase of 40 the reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and h 2 release while this concept might seem at odds with completely serpentinized seafloor peridotites reaction driven fracturing offers an intriguing solution to the seemingly self limiting nature of serpentinization indeed the reacted sample revealed several textural features diagnostic of incipient reaction driven fracturing we conclude that fracturing must have far reaching impacts on the rates of serpentinization and h 2 release in peridotite hosted hydrothermal systems

evolution of fracture permeability of ultramafic rocks undergoing serpentinization at hydrothermal conditions an experimental study we performed flow through laboratory experiments on five cylindrically cored samples of ultramafic rocks in which we generated a well mated through going tensile fracture to investigate evolution of fracture permeability during serpentinization the samples were tested in a triaxial loading machine at a confining pressure of 50 mpa pore pressure of 20 mpa and temperature of 260 c simulating a depth of 2 km under hydrostatic conditions a pore pressure difference of up to 2 mpa was imposed across the ends of the sample fracture permeability decreased by 1 2 orders of magnitude during the 200 330 h experiments electron microprobe and sem data indicated the formation of needle shaped crystals of serpentine composition along the walls of the fracture and chemical analyses of sampled pore fluids were consistent with dissolution of ferro magnesian minerals by comparing the difference between fracture permeability and matrix permeability measured on intact samples of the same rock types we concluded that the contribution of the low matrix permeability to flow is negligible and essentially all of the flow is focused in the tensile fracture the experimental results suggest that the fracture network in long lived hydrothermal circulation systems can be sealed rapidly as a result of mineral precipitation and generation of new permeability resulting from a combination of tectonic and crystallization induced stresses is required to maintain fluid circulation