porosity evolution and crystallization driven fragmentation during weathering of andesite 1 a 10 m thick andesitic sill intrusion from the neuquen basin argentina shows spectacular examples of spheroidal weathering and liesegang banding the liesegang patterns demonstrate how andesite blocks initially cut out by a preweathering joint set are subdivided by fractures formed during the spheroidal weathering process the stresses that cause fracturing originate from the growth of ferrihydrite and calcite in the pore space of the andesite partly at the expense of original ilmenite amphibole and plagioclase the porosity evolution and fracture formation during progressive weathering was characterized by scanning electron microscopy studies x ray computed tomography and he and hg porosimetry fresh andesite has a porosity of approximately 8 and a major fraction 80 of the pore volume is composed of pores less than 10 m in diameter the extent of pore filling during weathering increases with pore size pores more than 100 m are almost completely filled by an intimate intergrowth of calcite and ferrihydrite whereas pores less than 10 m are filled less than 50 the fracturing associated with spheroidal weathering is caused by mineral growth in the largest pores which account for 10 20 of the total porosity the periodic precipitation of the weathering product to form liesegang bands indicates a significant supersaturation threshold before nucleation commences the increase in the weathering product growth rate with increasing size is therefore most likely due to higher nucleation probabilities in larger pores

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

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

competitive hydration and dehydration at olivine quartz boundary revealed by hydrothermal experiments implications for silica metasomatism at the crust mantle boundary abstract serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust given that such a fluid has a high silica activity compared with mantle peridotite it is thought that serpentinization and silica metasomatism occur simultaneously at the crust mantle boundary in this study we conducted hydrothermal experiments in the olivine ol quartz qtz h 2 o system at 250 c and vapor saturated pressure under highly alkaline conditions naoh aq ph 13 8 at 25 c to clarify the mechanism of silica metasomatism at the crust mantle boundary composite powders consisting of a qtz layer and an ol layer were set in tube in tube vessels after the experiments the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the ol qtz boundary the mineralogy of the reaction products in the ol hosted region changed with increasing distance from the ol qtz boundary from smectite serpentine smc zone to serpentine brucite magnetite brc zone olivine hydration proceeded in both zones but the total h 2 o content in the products was greater in the brc zone than in the smc zone mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone in contrast the smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions in the smc zone smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration and around the boundary of the smc and brc zones serpentine formation occurred by olivine hydration and brucite dehydration the relative extent of hydration and dehydration reactions controlled the along tube variation in the rate of h 2 o production consumption and the rate of volume increase our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure possibly affecting the mechanical behavior of the crust mantle boundary

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

observations of titan s northern lakes at 5 μm implications for the organic cycle and geology abstract since titan entered northern spring in august 2009 the north pole has been illuminated allowing observations at optical wavelengths on june 5 2010 the visual and infrared mapping spectrometer vims onboard the cassini spacecraft observed the northern pole area with a pixel size from 3 to 7 km since as we demonstrate little of the solar flux at 5 m is scattered by the atmosphere these observations were obtained at relatively large incidence angles and allowed us to build a mosaic covering an area of more than 500 000 km 2 that overlaps and complements observations made by the synthetic aperture radar sar in 2007 we find that there is an excellent correlation between the shape of the radar dark area known as ligeia mare and the vims 5 m dark unit matching most of the radar shoreline the 2010 vims observations suggest that the 125 000 km 2 surface area of ligeia mare measured by radar in 2007 has not significantly changed the vims observations complement the radar observations to the west of ligeia mare and suggest that ligeia mare is connected to kraken mare by either a diffuse network similar to a swamp area or by well defined sub pixel rivers considering the results of recent evaporation models of methane our preferred interpretation of the relative constancy in surface area of ligeia is that it is principally composed of ethane although we cannot rule out the possibility that methane evaporation is balanced with replenishment by either precipitation or underground seepage there is also strong correlation between the location of the small radar lakes and the small vims 5 m dark patches the geographic location of the small lakes are within a vims pixel of the sar location suggesting that the non synchronous component of titan s spin rate if it exists was less than 2 3 10 4 deg day between 2007 and 2010 in agreement with the recent t64 radar observations these observations question the existence of non synchronous rotation two radar bright features appear dark at 5 m the simplest interpretation is that these are very shallow lakes less than one meter deep three new small lakes named freeman cardiel and towada by the iau are found outside of the area mapped with the sar a single scattering model describing reflection of sunlight at 5 m suggests that the lake surface is mirror like and that the albedo of the solid surfaces surrounding the lakes is about 8 these observations together with information of the haze aerosols allow us to show that titan s lakes atmospheric ethane and aerosol haze are smaller carbon reservoirs than titan s sand dunes and atmospheric methane a simple model involving an outburst of methane a few hundreds of myr ago followed by the dissociation of methane in the atmosphere leading to the formation of the haze particles that constitute the dune fields would be consistent with both the present observations and recent measurements of isotopic ratios in atmospheric methane mandt k e et al 2012 astrophys j 749 160 14

low silica activity for hydrogen generation during serpentinization an example of natural serpentinites in the mineoka ophiolite complex central japan the textural evolution in the serpentinite of the mineoka ophiolite complex has been investigated to constrain the natural environment for hydrogen production in the serpentinite hosted hydrothermal vent systems textural relations of the serpentinites from the mineoka ophiolite indicate at least two stages in the process of serpentinization with the replacement of olivine by a mesh texture of serpentine and brucite followed by the development of magnetite bearing or free serpentine veins the generation of hydrogen during serpentinization which accompanies the formation of magnetite involves a silica depletion reaction as evidenced by the low abundance of serpentine in the magnetite bearing veins and the absence of magnetite in pseudomorphs of orthopyroxene direct evidence for the production of hydrogen and strongly reducing conditions is provided by ch4 and h2 bearing inclusions in relic olivine crystals the production of methane and hydrogen may have provided a suitable environment for microbial activity in hydrothermal vent systems along the seafloor our results indicate that low silica activity plays a key role in the generation of hydrogen during serpentinization and that low silica activity environments are possible in olivine rich rocks such as dunite or during local disequilibrium in other silica poor rocks in the mantle lithosphere

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

u th systematics and 230th ages of carbonate chimneys at the lost city hydrothermal field the lost city hydrothermal field lchf is a serpentinite hosted vent field located 15 km west of the spreading axis of the mid atlantic ridge in this study uranium thorium u th geochronological techniques have been used to examine the u th systematics of hydrothermal fluids and the 230th ages of hydrothermally precipitated carbonate chimneys at the lchf fluid sample analyses indicate that endmember fluids likely contain only 0 0073 ng g u or less compared to 3 28 0 03 ng g of u in ambient seawater for fluid samples containing only 2 21 ambient seawater 1 1 11 mmol kg mg th concentration is 0 11 0 13 pg g and surrounding seawater concentrations average 0 133 0 016 pg g the 230th 232th atomic ratios of the vent fluids range from 1 10 10 6 to 11 5 10 6 are less than those of seawater and indicate that the vent fluids may contribute a minor amount of non radiogenic 230th to the lchf carbonate chimney deposits chimney 238u concentrations range from 1 to 10 g g and the average chimney corrected initial 234u is 147 2 0 8 which is not significantly different from the ambient seawater value of 146 5 0 6 carbonate 232th concentrations range broadly from 0 0038 0 0003 to 125 16 ng g and 230th 232th atomic ratios vary from near seawater values of 43 8 10 6 up to 530 25 10 3 chimney ages corrected for initial 230th range from 17 6 yrs to 120 13 kyrs the youngest chimneys are at the intersection of two active steeply dipping normal faults that cut the atlantis massif the oldest chimneys are located in the southwest portion of the field vent deposits on a steep fault bounded wall on the east side of the field are all 4 kyrs old indicating that mass wasting in this region is relatively recent comparison of results to prior age dating investigations of submarine hydrothermal systems shows that the lchf is the most long lived hydrothermal system known to date it is likely that seismic activity and active faulting within the atlantis massif and the atlantis fracture zone coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site the longevity of venting at the lchf may have implications for ecological succession of microorganisms within serpentinite hosted vent environments

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

oxygen bulk diffusion measurements and tem characterization of a natural ultramylonite implications for fluid transport in mica bearing rocks oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample c 5 m grain size 15 20 biotite 20 quartz 60 65 feldspars and minor fe oxides from the gerrish island shear zone se maine usa the diffusion experiments were performed at 250 550 c and 100 mpa water pressure oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample the arrhenius parameters for transport parallel to the foliation are dbulk0 2 0 10 11 m2 s 1 and q 30 6 kj mol 1 the bulk diffusivity perpendicular to the foliation is about a factor of 3 5 less than that parallel to the foliation with the same activation energy the values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions the microstructure of the ultramylonite was characterized using transmission electron microscopy tem the nature and distribution of the interconnected fluid cannot be completely resolved from the tem analysis however the low percentage of three grain channels and open grain interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces the results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates strong lithological control and pervasive nature of fluid transport in mica bearing rocks

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

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

paleobiogeographic relationships of andean angiosperms of cretaceous to pliocene age abstract a review of the geological data suggests that mountains existed in western south america since the late cretaceous these data combined with a review of the earliest occurrences of angiosperm fossils indicate that the floral elements centered in the south andes lived there during the late cretaceous while elements centered in the north andes existed there by the mid tertiary examinations of reports elsewhere in south america and in surrounding continents show that each andean fossil flora is mostly composed of andean or amazonian centered taxa in addition the south andean fossil flora also shares many taxa with the antarctica australia paleoflora while the fossils from the north andes are also found in meso and north america in combination these geological and fossil data suggest that the mountain centered floras have had a long history

reaction enhanced permeability during retrogressive metamorphism abstract retrogressive metamorphism is in most cases associated with influx of external fluids into igneous or metamorphic rocks with low to negligible initial permeabilities thus fluid migration is associated with some process that creates the necessary channels for migration one such mechanism is associated with the volume changes resulting from volatilization reactions petrographic studies of a partly serpentinized troctolite from the duluth complex in minnesota reveal how the increase in solid volume associated with hydration of olivine phenocrystals generates an extensive network of microfractures in the surrounding plagioclase dominated matrix this microfracture network allows hydrous fluids to penetrate further into the rock a 2 d spring network model illustrates how expanding polygons or non spherical grains in an elastically deforming matrix cause dilatant microfracturing and porosity generation even if the overall stresses are compressive the model emphasises how fluid infiltration may be promoted by stress heterogeneities set up by metamorphic reactions

permeability profiles across the crust mantle sections in the oman drilling project inferred from dry and wet resistivity data permeability profiles in the crust mantle sequences of the samail ophiolite were constructed based on onboard measurements of the electrical resistivity of cores recovered during the oman drilling project for each sample we measured dry and brine saturated resistivity during the description campaign on the drilling vessel chikyu owing to the conductive brine in the pore space wet resistivity is systematically lower than dry resistivity the difference between dry and wet resistivity is attributed to the movement of dissolved ions in brine that occupies the pore space we applied effective medium theory to calculate the volume fraction of pores that contribute to electrical transport using an empirical cubic law between transport porosity and permeability we constructed permeability profiles for the crust mantle transition zone and the serpentinized mantle sections in the samail ophiolite the results indicate that 1 the gabbro sequence has a markedly lower permeability than the underlying mantle sequence 2 serpentinized dunites have higher permeability than serpentinized harzburgites and 3 discrete sample permeability is correlated with ultrasonic velocity suggesting that the permeability variations predominately reflect crack density and geometry

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

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

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

inhibition of subduction thrust earthquakes by low temperature plastic flow in serpentine abstract serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults however recent frictional experiments have shown a marked increase in its shear strength with increasing depth implying its deformation via unstable sliding at a plate interface in subduction zones here we report the results of a series of simple shear experiments designed to determine the dominant deformation style of low temperature serpentine species under p t conditions that correspond to the mantle wedge corner in cool subduction zones ca p 1 gpa and t 200 c we found that with increasing shear strain the dominant flow mechanism of the serpentinite evolves from 1 inhomogeneous semi brittle flow by strain localization into sample scale shear bands to 2 homogeneous ductile flow by intracrystalline deformation within individual serpentine grains with the development of a planar shape fabric and a strong crystallographic preferred orientation tem observations revealed that grain size reduction in lizardite crystals occurs due to 001 interlayer glide represented by the occurrence of tiny sheet like sectors oriented parallel to 001 liz lattice fringes which evolved from large elongated sectors with long axes oriented normal to 001 liz given that a low viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains the above results indicate that the presence of low temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes as stress is preferentially accommodated by plastic flow this hypothesis explains the occurrence of an anomalous non seismic region devoid of large thrust earthquakes in locally hydrated forearc mantle within northeast japan

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

an intraslab seismic sequence activated by the 2011 tohoku oki earthquake evidence for fluid related embrittlement 1 using quantitative analysis of waveforms we investigate a seismic cluster that occurred at a depth of 67 km in the philippine sea slab 8 months after the 11 march 2011 megathrust tohoku oki earthquake mw 9 0 the sequence started with an m 4 1 normal fault event on 14 november in the deepest part of the cluster and subsequent earthquakes migrated upward by 6 km along a narrow conduit like zone the earthquakes have stress drops of 0 5 40 mpa and groups of earthquakes with coherent waveforms are observed we explain these observations in terms of fluid related embrittlement and the migration of overpressured fluids the low permeability plate interface of the pacific plate may have been broken by the coseismic or postseismic slips of the tohoku oki earthquake with the fluids subsequently being liberated from the underlying crust of the pacific slab and migrating into the philippine sea slab due to the pore pressure gradient the tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area the heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress the lag of 8 months may represent the time needed for the unsealing of the plate interface the upward migration of fluids and the increase in pore pressure to become sufficient to overcome the lithostatic pressure

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

pore fluid effects on elastic wave velocities of serpentinite and implications for estimates of serpentinization in oceanic lithosphere abstract recent seismic refraction surveys at various outer rise regions have identified low seismic velocities in the uppermost mantle which is indicative of mantle hydration serpentinization related to water infiltration along bending related faults prior to subduction serpentinization could lead to the development of pore spaces during hydration reaction which influences seismic velocity here we measured the elastic wave velocity of low temperature serpentinites with various porosities and reevaluated the degree of serpentinization of the oceanic mantle in outer rise regions accounting for pore fluid effects experimental results show that the elastic wave velocity of serpentinite decreases with increasing porosity this relationship indicates that the estimated degree of serpentinization decreases with increasing serpentinite porosity we then applied these experimental data to the geophysical observations of the outer rise regions to reevaluate the amount and the type of water carried into earth s interior at subduction systems

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

lizardite versus antigorite serpentinite magnetite hydrogen and life the serpentinization of peridotite operates according to one or the other or a combination of two end member mechanisms in low temperature environments 50 300 c where lizardite is the predominant serpentine mineral olivine is consumed by reaction with h2o but its composition mg remains unchanged mg rich lizardite magnetite and dihydrogen gas brucite are products of the reaction at higher temperatures 400 600 c rates of mgfe diffusion in olivine are orders of magnitude faster with the result that the growth of mg rich antigorite can be accommodated by a compositional adjustment of olivine eliminating the need to precipitate magnetite and evolve hydrogen this latter end member mechanism probably best reflects the situation in the forearc mantle wedge

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

contrasting fracture patterns induced by volume increasing and decreasing reactions implications for the progress of metamorphic reactions abstract hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock and some of these reactions proceed with fracturing for this paper we have developed a model for coupled hydraulic chemical mechanical processes using the distinct element method in order to understand the relationships among chemical reactions fluid flow and fracturing during metamorphism the model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure with the help of the model volume decreasing dehydration reactions and volume increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions respectively both types of reaction proceed inwards from drained boundaries or fractures but they show contrasting fracture patterns a volume decreasing dehydration reaction produces tree type fractures with the new fractures generated as branches of a pre existing fracture a volume increasing hydration reaction produces a polygonal network of fractures where new fractures nucleate at sites far from the pre existing fracture and extend to form t junctions these contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient in the case of the tree type fractures the fractures are continuously generated at the reaction front and the reaction proceeds smoothly by positive feedbacks between reaction fracturing and fluid flow in contrast in a volume increasing hydration reaction fracturing initially occurs in response to the irregular boundary shape but as the reaction progresses a compressive stress field is generated which inhibits further fracture generation the compressive stress field also prevents fluid flow by closing the pre existing fractures which slows down the reaction these contrasting feedback systems between volume decreasing dehydration and volume increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved

feedbacks between mantle hydration and hydrothermal convection at ocean spreading centers hydration of the oceanic lithosphere is an important and ubiquitous process which alters both the chemical and physical properties of the affected lithologies one of the most important reactions that affect the mantle is serpentinization the process of serpentinization results in a drastic decrease in the density up to 40 seismic velocity and brittle strength as well as water uptake of up to 13 wt of the ultramafic rock in this paper we use numerical models to study the amount and extent of serpentinization that may occur at mid ocean ridges and its effects on fluid flow within the lithosphere the two dimensional fem model solves three coupled time dependent equations i mass conserving darcy flow equation ii energy conserving heat transport equation and iii serpentinization rate of olivine with feedbacks to temperature exothermic reaction fluid consumption and variations in porosity and permeability volume changes the thermal structure of the ridge is strongly influenced by rock permeability in addition to the spreading velocity of the ridge increased rock permeability enhances hydrothermal convection and results in efficient heat mining from the lithosphere whereas higher spreading velocities result in a higher thermal gradient serpentinization of the oceanic mantle in turn depends on the aforementioned competing processes however serpentinization of mantle rocks is itself likely to result in strong variations of rock porosity and permeability here we explore the coupled feedbacks increasing rates of serpentinization lead to large volume changes and therefore rock fracturing thereby increasing rock porosity permeability while as serpentinization reaches completion the open pore space in the rock is reduced due to the relative dominance of mineral precipitation although variations in the relation between porosity and permeability and serpentinization before the reaction reaches completion do not significantly affect the degree of serpentinization we find that unreasonably large portions of the mantle would be serpentinized if rock closure does not occur at the final reaction stage the amount of water trapped as hydrous phases within the mantle shows a strong dependency on the spreading velocity of the ridge with water content ranging from 0 18 105 kg m2 to 2 52 105 kg m2 additionally two distinct trends are observed where the water content in the mantle at slow spreading ridges drops dramatically with an increase in spreading velocity the amount of water trapped in the mantle at fast spreading ridges on the other hand is lower and does not significantly depend on spreading velocity

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

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