unraveling the exhumation history of high pressure ophiolites using magnetite u th sm he thermochronometry abstract magnetite is a ubiquitous oxide in ultramafic and mafic rocks which is present in a large range of geological and tectonic settings in the case of high pressure ultramafic rocks exhumation timing is commonly constrained by geochronometers in nearby lithologies the development of the magnetite u th sm he method mghe has opened new perspectives to refine the exhumation history of such rocks however no thermal history has been deduced using mghe data so far here we applied mghe dating to magnetite from an hp lt alpine ophiolite body rocher blanc from the schistes lustres units western alps france where part the p t t history is constrained to further refine the thermal history and demonstrate validity of the method textural and geochemical characterizations of the magnetite grains reveal two crystallization environments leading to distinct grain morphologies euhedral and pseudo euhedral with different amounts of mineral inclusions titanite chlorite and augite aegirine both magnetite types crystallized during the same retrograde path at t 250 c under hydrothermal conditions shown by si and v oscillatory zoning and low ti content mghe ages derived from euhedral magnetite crystals range from 14 8 2 1 to 20 9 3 0 ma between zircon and apatite fission track zft and aft ages deduced from the literature pseudo euhedral grains display older ages interpreted as resulting from the implantation of he from u rich neighboring minerals such as titanite for the first time mghe data are inverted along with aft and zft data to refine the thermal history of an ophiolite related to their exhumation the mghe data suggest that the rocher blanc sample has undergone an exhumation in ductile conditions related to subduction dynamics and underplating processes followed by an exhumation reflecting denudation by erosion under brittle conditions this exhumation regime change is found to take place at 20 ma as opposed to a less constrained 26 ma cooling as would be inferred from the zft and aft ages alone considering published thermochronometric data along the western alps we propose that the exhumation is controlled by the upward indentation of the apulian mantle producing regional tilting and associated erosion with the present schistes lustres ophiolite example we demonstrate that the mghe method can be used as a low temperature thermochronometer closure temperature in the 200 250 c range that aids in unraveling the cooling history associated with exhumation of high pressure ultramafic and mafic rocks at the brittle to ductile transition

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

occurrence of graphite in serpentinized olivines in kimberlite abundant 6 to 9 wt graphite has been found in the serpentinized regions of olivine grains from several kimberlites if the graphite originally was dissolved as atomic carbon in high pressure olivine this factor must affect petrologic models of mantle melting in the presence of co 2 on the other hand if the carbon originally existed as co 2 filled fluid inclusions in olivine grains or was introduced during serpentinization the development of graphite provides f o 2 constraints on kimberlite serpentinization there is evidence for all three mechanisms of graphite formation however it appears most likely that graphite developed during reduction of serpentinizing fluids probably methane bearing

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

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

chemistry of springs across the mariana forearc shows progressive devolatilization of the subducting plate abstract cold springs upwelling through large serpentinite mud volcanoes in the outer half of the mariana forearc provide a unique window into processes of devolatilization of the subducting pacific plate we have sampled upwelling pore waters with lower chlorinity than seawater from six sites on five serpentinite mud volcanoes by conventional gravity and piston coring by push coring from the rov jason by drilling on odp legs 125 and 195 and by manned submersible the sites range from 13 47 n to 19 33 n and 52 to 90 km from the mariana trench axis corresponding to approximate depths to the top of the downgoing plate of 16 to 29 km the composition of the springs varies systematically over this distance nearer the trench the upwelling waters have much higher ca and sr than seawater and much lower carbonate alkalinity sulfate na cl k rb and b farther from the trench the waters show the opposite trends relative to seawater chlorinity is consistently lower than in seawater and shows large variations that are not systematic with distance from the trench cs is consistently higher than in seawater and increases with distance from the trench all of the waters have high ph and are heavily depleted in mg si li f and 87sr 86sr relative to seawater they tend to be enriched in o18 o16 except for odp drilling none of the cores was long enough to produce an asymptotic compositional trend with depth we have inferred the end member compositions of the upwelling waters by extrapolation against mg at two sites we were able to compare data from gravity cores with data from drill cores or push cores collected at springs to estimate the effects of reactions that occur at shallow depth below the seafloor on mixing of the upwelling waters with seawater these effects are different for sites high in dissolved ca nearer the trench vs those high in alkalinity farther from the trench common to both are large losses from solution of 1 ca as caco3 and in exchange for na 2 mg in exchange for na or ca and as brucite 3 sulfate probably reduced by microbes or possibly precipitated as gypsum 4 sr ba si and f na is consistently leached from the solids into solution whereas k and o18 o16 are relatively unreactive we infer that the upwelling waters are uniformly saturated with caco3 and that the excess h2o and the trends in ca sr alkalinity and sulfate with distance from the trench result from introduction of h2o and dissolved carbonate and sulfate from an external source the sediment and altered basalt at the top of the subducting plate the concurrent trends in na cl b cs and especially k and rb indicate that these species originate from the top of the subducting plate in response to increasing temperature these systematic variations across the outer forearc imply that the solutions ascend more or less vertically from the source region and do not travel long distances laterally along the decollement before ascending based on leaching of k the 150 c isotherm is crossed approximately beneath big blue seamount at a depth of 22 km below the seafloor 70 km behind the trench by this point it appears that carbonate dissolution has joined dehydration as a significant process at the top of the subducting plate

rodingitization and hydration of the oceanic lithosphere as developed in the leka ophiolite north central norway abstract ophiolite complexes in mountain chains may give supplementary information on the hydration of the oceanic lithosphere to that obtained from dredged and drilled samples from the ocean floor the ultramafic mantle and the layered ultramafic to anorthositic crustal sequences of the cambrian 497 ma leka ophiolite are variably serpentinized and chloritized grossular rodingite rodingite s s has been found over a c 500 m long and tens of meters wide zone in the layered crustal section of the complex and is developed in both pyroxenites and gabbro anorthosite layers shear zones and meter wide fracture zones where the rock has developed a fracture cleavage are oriented at high angel to the layering and these zones were the main conduits for transport of fluid and solute between the various lithologies some 5 15 cm thick layers of anorthosite or leucogabbro have been rodingitized around such a fractures zone with the development of three distinct metasomatic zones along the plagioclase layer a central grossular dominated zone with clinopyroxene clinozoisite prehnite chlorite and minor titanite rodingite zone extends for up to 3 m along strike and gives way to a clinozoisite dominated zone typically 0 5 m wide with additional grossular clinopyroxene and chlorite which is followed outward by a lile enriched zone lile zone with clinozoisite phlogopite k feldspar plagioclase and preiswerkite the lile zone extends more than 3 m out from the clinozoisite dominated zone clz zone assuming constant volume the rodingite formed from the plagioclase layer by addition of 20 g of cao per 100 g of rock all na 2 o c 2 g was removed from both rodingite and clz zones ti and v increase almost 10 in the rodingite compared to its protolith k ba rb and cs are strongly enriched in the lile zone compared to the protolith and suggest interaction with sea water the lithologies alternating with the plagioclase layers clinopyroxenite wehrlite websterite and dunite display textures indicating a number of ca releasing cpx chl cpx srp cpx amph and ca consuming opx cpx2 ol cpx2 cpx1 cpx2 reactions the replacement textures are distributed around fracture and shear zones with the ca releasing reactions in the core and the ca consuming reactions in distal parts forming a metasomatic column out from the fluid pathways serpentinization and chloritization of clinopyroxene was the main ca source for the rodingitization process this first description of rodingite in a layered sequence of an ophiolite complex indicates that the hydration of the oceanic lithosphere occurred at various structural levels and was associated with ca metasomatism also in places where rodingite s s is lacking the different lithologies exchanged elements through transport on shear and fracture zones

thallium as a tracer of fluid rock interaction in the shallow mariana forearc abstract fluids driven off the subducting pacific plate infiltrate the shallow mariana forearc and lead to extensive serpentinization of mantle peridotite however the sources pathways and chemical modifications of ascending slab derived fluids remain poorly constrained and controversial in this study we use thallium tl concentrations and isotopic ratios of serpentinized peridotite and rodingitized diabase from the south chamorro and conical seamounts to discriminate between potential fluid sources with distinct tl isotope compositions serpentinite samples from the mariana forearc all display e 205 tl 0 5 where e 205 tl 10 000 tl 205 tl sample 203 tl 205 tl srm 997 203 tl 205 tl srm 997 203 which is significantly enriched in 205tl compared to the normal mantle e 205 tl 2 given that high temperature hydrothermal processes do not impart significant tl isotope fractionation the isotope compositions of the serpentinites must reflect that of the serpentinizing fluid pelagic sediments are the only known slab component that consistently displays e 205 tl 0 5 and therefore we interpret the heavy tl isotope signatures as signifying that the serpentinizing fluids were derived from subducting pelagic sediments a rodingitized diabase from conical seamount was found to have an e 205 tl of 0 8 suggesting that sediment sourced serpentinization fluids could also affect diabase and other mafic lithologies in the shallow mariana forearc forearc rodingitization of diabase led to a strong depletion in tl content and a virtually complete loss of k na and rb the chemical composition of hybrid fluids resulting from serpentinization of harzburgite with concomitant rodingitization of diabase can be highly alkaline depleted in si yet enriched in ca na k and rb which is consistent with the composition of fluids emanating from mud volcanoes in the mariana forearc our study suggests that fluid rock interactions between sedimentary mafic and ultramafic lithologies are strongly interconnected even in the shallowest parts of subduction zones we conclude that transfer of fluids and dissolved elements at temperatures and pressures below 400 c and 1 gpa respectively must be taken into account when elemental budgets and mass transfer between the subducting plate the forearc the deep mantle and the ocean are evaluated

thermodynamic constraints on mineral carbonation of serpentinized peridotite abstract carbonation of serpentinite appears to be a promising process to mitigate rising concentrations of atmospheric co2 however given that chemical affinities of mineral carbonation reactions in serpentinized peridotite change drastically with temperature protolith and the activities of dissolved constituents this process is incompletely understood in this report numerical models are used to investigate the heterogeneous phase equilibria and the speciation of intergranular fluids during mineral carbonation of serpentinite dunite at 100 200 300 and 400 c the results suggest that temperature dependent activity gradients of both aqueous carbon dioxide aco2 aq and aqueous silica asio2 aq determine the carbonation potential of serpentinite at low to moderate temperatures there is ample thermodynamic driving force for the carbonation of serpentinite as the gradients in asio2 aq and aco2 aq set between talc quartz magnesite and serpentine brucite equilibria are steepest with increasing temperature these activity gradients become smaller because the stability fields of brucite serpentine and talc expand toward higher aco2 aq and asio2 aq while the stability field of magnesite contracts in addition the enhanced solubility of quartz toward higher temperatures shifts the talc quartz magnesite equilibrium to very high aco2 aq field observations of carbonated serpentinite reveal a rather simple sequence of arrested carbonation reactions which is typically fossilized as spatially distinct metasomatic zones a zone of serpentinized peridotite is followed by a zone of serpentine carbonate then talc carbonate and finally quartz carbonate our modeling results not only reproduce these observations they also suggest that complete carbonation is favored at low to moderate temperatures since complete carbonation of serpentinite is achieved at relatively small fluid to rock ratios at temperatures 200 c prolonged fluid influx at low temperatures can result in decarbonation and silicification of the altered rock in contrast at temperatures 200 c decarbonation and silicification reactions are unlikely even at very high fluid to rock ratios since serpentinized peridotite is highly reactive in the presence of co2 aq its widespread exposure at slow and ultraslow spreading mid ocean ridges may represent a significant sink for co2 in the global carbon cycle our modeling results can be used to evaluate fluid rock interactions in active or fossil mineral carbonation systems if the mineralogy is known and the temperature of formation can be constrained independently the asio2 aq aco2aq oxygen fugacity fo2 g and ph can be estimated in turn if the alteration temperature asio2 aq and aco2 aq are known governing phase equilibria can be approximated the latter is a promising tool to monitor the reaction progress during co2 sequestration into serpentinized peridotite

magnetite free yellow lizardite serpentinization of olivine websterite canyon mountain complex n e oregon we document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite but instead relatively fe rich yellow colored lizardite x fe 0 08 to 0 17 and the native fe ni co metals awaruite and wairauite lizardite s identity was confirmed by micro raman spectroscopy although peaks are broad electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of fe 3 exclusively on m sites where it is charge balanced by vacancies although these observations are unusual this secondary mineral assemblage can be explained in terms of the likely intensive variables t f h 2 o f h 2 and a sio 2 attending the alteration the absence of magnetite in serpentinization does not signify a lack of oxidation by forming the hydrated phase component ferri lizardite instead of magnetite from the fayalite and ferrosilite components the yield of hydrogen is reduced by two thirds the usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization

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

the petrology of seafloor rodingites insights from geochemical reaction path modeling abstract thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic ultramafic boundaries the models were set up to investigate the shifts in fluid rock equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 c 300 c and 400 c phase assemblages typical of rodingite grossular diopside chlorite are predicted to form at 200 c and 300 c but only in areas where the fluid is essentially unaffected by reactions with gabbro i e near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid as the fluid becomes more affected by reactions with the gabbro prehnite or epidote ss replace garnet and tremolite replaces clinopyroxene once the fluid chemistry is completely reset by reactions with gabbro the predicted assemblage is typical of greenschist facies albitic plagioclase actinolite chlorite and epidote ss these transitions predicted in the model are very similar to what is observed in natural rodingites from different settings our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids mass transfer of ca is likely by diffusion of the caoh species which is predicted to show a very steep concentration gradient across a mafic ultramafic boundary rodingites act as ca traps because of the great stability of diopside and garnet at low sio 2 activities our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 c and 300 c suggesting that very high temperatures on the order of 800 c are not required in the formation of diopsidite veins

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

microstructural study of deformation zones during cone penetration in silt at variable penetration rates during conventional cone penetration testing in silt the soil will normally be partially drained if the penetration rate varies time for drainage is altered and therefore the measured cone resis

how does hurricane edouard 2014 evolve toward symmetry before rapid intensification a high resolution ensemble study abstracta 14 member high resolution ensemble of edouard 2014 a moderately sheared tropical storm that underwent rapid intensification ri is used to determine causes of vortex alignment and pr

the origin and implications of clay minerals from yellowknife bay gale crater mars the mars science laboratory msl rover curiosity has documented a section of fluvio lacustrine strata at yellowknife bay ykb an embayment on the floor of gale crater approximately 500 m east of the bradbury landing site x ray diffraction xrd data and evolved gas analysis ega data from the chemin and sam instruments show that two powdered mudstone samples named john klein and cumberland drilled from the sheepbed member of this succession contain up to 20 wt clay minerals a trioctahedral smectite likely a ferrian saponite is the only clay mineral phase detected in these samples smectites of the two samples exhibit different 001 spacing under the low partial pressures of h2o inside the chemin instrument relative humidity 1 smectite interlayers in john klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 a but largely remain open in the cumberland sample with a basal spacing of 13 2 a partial intercalation of cumberland smectites by metal hydroxyl groups a common process in certain pedogenic and lacustrine settings on earth is our favored explanation for these differences the relatively low abundances of olivine and enriched levels of magnetite in the sheepbed mudstone when compared with regional basalt compositions derived from orbital data suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine mass balance calculations are permissive of such a reaction moreover the sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim early diagenetic fabrics suggest clay mineral formation prior to lithification thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants allowing circum neutral ph the kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years mineralogical evidence of the persistence of benign aqueous conditions at ykb for extended periods indicates a potentially habitable environment where life could establish itself mediated oxidation of fe 2 in olivine to fe 3 in mag

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

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

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

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

olivine carbonate mineralogy of the jezero crater region a well preserved ancient delta deposit in combination with ample exposures of carbonate outcrops makes jezero crater in nili fossae a compelling astrobiological site we use compact reconnaissance imaging spectrometer for mars crism observations to characterize the surface mineralogy of the crater and surrounding watershed previous studies have documented the occurrence of olivine and carbonates in the nili fossae region we focus on correlations between these two well studied lithologies in the jezero crater watershed we map the position and shape of the olivine 1 m absorption band and find that carbonates are found in association with olivine which displays a 1 m band shifted to long wavelengths we then use thermal emission imaging spectrometer themis coverage of nili fossae and perform tests to investigate whether the long wavelength shifted redshifted olivine signature is correlated with high thermal inertia outcrops we find that there is no consistent correlation between thermal inertia and the unique olivine signature we discuss a range of formation scenarios for the olivine and carbonate associations including the possibility that these lithologies are products of serpentinization reactions on early mars these lithologies provide an opportunity for deepening our understanding of early mars and given their antiquity may provide a framework to study the timing of valley networks and the thermal history of the martian crust and interior from the early noachian to today

chemical and isotopic analyses of hydrocarbon bearing fluid inclusions in olivine rich rocks we examined the mineralogical chemical and isotopic compositions of secondary fluid inclusions in olivine rich rocks from two active serpentinization systems the von damm hydrothermal field mid

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

control of the products of serpentinization by the fe2 mg 1 exchange potential of olivine and orthopyroxene it is argued that the high mg content mg number1 4 95 3 of the serpentine minerals in serpentinized peridotite is a consequence of the environmental femg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite this causes hydrogen to be evolved in an oxygen conserved reaction the low variance mineral assemblage ol srp brc mag sets the chemical potentials of h2o sio2 and o2 internally at an early stage in the process but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown fe3 content of the serpentine minerals particularly lizardite whole rock analyses of highly to completely serpentinized peridotites reveal fe sfe ratios40 4 with an average value 0 69 similar to that of magnetite 0 67 this feature may be attributed to the presence of high fe lizardite as has been found in mo ssbauer spectroscopy studies electron microprobe and scanning electron microcope analyses in the literature exhibit element trends e g decreasing si vs sfe a p f u for olivinepseudomorph lizardite and with some exceptions for bastite lizardite that show a substitution of the cronstedtite component fe charge balanced ontand m sites cronstedtite substitution will be favoured at low temperature and or low hydrogen fugacity and in these circumstances less magnetite will be evolved during serpentinization in some cases none at all some bastite lizardites from sea floor settings show evidence of m site vacancy substitution of fe for fe in the course of progressive serpentinization micrometer to millimeter scale variations in sio2 potential may well be present but their influence on fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene chrysotile is on average more mg rich and less variable in fe mg ratio than lizardite facts that may be attributed to the greater fe content of lizardite chrysotile veins provide the best record available to us of the environmental femg 1 exchange potential in the pore fluid attending serpentinization this potential serves as a robust control on serpentine and brucite compositions although it may fail after olivine and orthopyroxene have been armoured or eliminated and in more open system environments high water rock ratio such as on the sea floor or at serpentinite host rock contacts the default assumption in microprobe analyses that measured iron is all fe can lead to inappropriate petrological conclusions in the case of serpentinites

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

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

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

magnetic signatures of serpentinization at ophiolite complexes we compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes pindos greece oman chenaillet france and the mid atlantic ridge near the kane fracture zone mark the pindos and oman sites show low susceptibility and remanence k 250 3008c occurring at the axis i e chenaillet similar to serpentinites from magmatically poor mid ocean ridges from lower temperature serpentinization 200 2508c likely occurring off axis and possibly during obduction i e pindos and oman at both settings serpentinization can result in significant hydrogen release

reaction induced porosity and onset of low temperature carbonation in abyssal peridotites insights from 3d high resolution microtomography abstract in a drillcore sample of serpentinized harzburgite from the uppermost oceanic crust mid atlantic ridge odp leg 209 site 1270 we demonstrate using high resolution 3d microtomography that micron sized open cavities are present the development of porosity is interpreted to result from dissolution of brucite and or olivine petrographic observations indicate that voids are integrated in a network of carbonate veins the formation of which is linked to changing alkalinity in conjunction with dissolution reactions partial carbonate filling of pore spaces indicates that under static conditions low temperature carbonation leads to clogging of fluid pathways and thus to a reduction in permeability electron microprobe analyses show that the inner walls of open voids are lined with fe rich precipitates we propose that the iron in those phases was released by brucite or olivine dissolution and was subsequently oxidized and precipitated as ferric hydroxide thermodynamic computations show that this process may be a potential source of catabolic energy for microorganisms inhabiting serpentinites the proposed carbonation mechanism implies that carbonate precipitation may start soon after exposure of the abyssal peridotites when dissolution of brucite and weathering of olivine begin and continue until the phases become inaccessible to seawater predicting carbonation rates of abyssal peridotites will hence require understanding of permeability reactions

origin of magnetic highs at ultramafic hosted hydrothermal systems insights from the yokoniwa site of central indian ridge high resolution vector magnetic measurements were performed on an inactive ultramafic hosted hydrothermal vent field called yokoniwa hydrothermal field yhf using a deep sea manned submersible shinkai6500 and an autonomous underwater vehicle r2d4 the yhf has developed at a non transform offset massif of the central indian ridge dead chimneys were widely observed around the yhf along with a very weak venting of low temperature fluids so that hydrothermal activity of the yhf was almost finished the distribution of crustal magnetization from the magnetic anomaly revealed that the yhf is associated with enhanced magnetization as seen at the ultramafic hosted rainbow and ashadze 1 hydrothermal sites of the mid atlantic ridge the results of rock magnetic analysis on seafloor rock samples including basalt dolerite gabbro serpentinized peridotite and hydrothermal sulfide showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of yokoniwa these observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite comparisons with the rainbow and ashadze 1 suggest that in ultramafic hosted hydrothermal systems strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite after the completion of serpentinization and production of hydrogen pyrrhotites convert into pyrite or nonmagnetic iron sulfides which considerably reduces their levels of magnetization our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic hosted hydrothermal systems furthermore the results highlight the use of near seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems

serpentinization of olivine gabbro in central indian ridge insights into h2 production during alteration in lower oceanic crust and sustenance of life at slow spreading ridges abstract we investigated the textural compositional characteristics across olivine interfaces in variably serpentinized olivine gabbro samples dredged from the vityaz megamullion in the northern central indian ridge cir the results are used to understand the role temperature and different chemical properties in infiltrating fluid e g seawater hydrothermal solution on the pattern of compositional evolution during progressive alteration in olivine gabbro the development of two distinct types of replacement rims i e type 1 and type 2 are conspicuous in our samples type 1 pseudomorphic mesh rim that formed within individual olivine grains occurs in two different textural modes a as successive layers of fe rich xmg 0 89 0 02 lizardite magnetite clay and calcite towards the olivine interface and b as homogeneous rim of comparatively fe poor lizardite xmg 0 93 0 01 magnetite replacement of olivine by talc and chlorite is evident towards the clinopyroxene and plagioclase grain boundaries respectively local precipitation of clay minerals is also observed at the inner interface of talc layer and within intra olivine fractures type 2 rim is characterized by the development of inter olivine mg rich lizardite xmg 0 97 0 01 magnetite layer cross cutting type 1 veins thus post dates type 1 replacement the process of cir olivine gabbro alteration records at least two stages of serpentinization any presence of brucite has not been observed suggesting relatively higher silica activity at reacting interfaces during olivine gabbro alteration than that of peridotite serpentinization at temperature log a sio 2 aq log a h 2 o log a al 3 a h 3 and log a ca 2 a h 2 and or temperature during progressive alteration of olivine type 2 replacement attests greater extent of magnetite precipitation as compared to type 1 serpentinization which may indicate increasing fluid rock ratio in the system and or decreasing magnitude of fe intake into serpentine structure with ongoing alteration importantly it implies release of h2 aq during olivine gabbro serpentinization which is consistent with that observed during peridotite serpentinization this corroborates that alteration in the deepest part of the oceanic crust may play an important role on the sustenance of biosphere in slow spreading ridges

serpentinites of the zermatt saas ophiolite complex and their texture evolution the zermatt saas serpentinite complex is an integral member of the penninic ophiolites of the central alps and represents the mantle part of the oceanic lithosphere of the tethys metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean floor hydration subduction related high pressure overprint meso alpine greenschist facies metamorphism and late stage hydrothermal alteration the early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures the primary serpentine minerals were completely replaced by antigorite the stable assemblage in subduction related mylonitic serpentinites is antigorite olivine magnetite diopside the mid tertiary greenschist facies overprint is characterized by minor antigorite recrystallization textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high pressure metamorphism of up to 2 5 gpa and 650 c the zermatt saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth

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

fluid mixing and the deep biosphere of a fossil lost city type hydrothermal system at the iberia margin subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle i e serpentinite however geosphere biosphere interactions in serpentinite hosted subseafloor mixing zones remain poorly constrained here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a cretaceous lost city type hydrothermal system at the magma poor passive iberia margin ocean drilling program leg 149 hole 897d brucite calcite mineral assemblages precipitated from mixed fluids ca 65 m below the cretaceous paleo seafloor at temperatures of 31 7 4 3 c within steep chemical gradients between weathered carbonate rich serpentinite breccia and serpentinite mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity dense microbial colonies are fossilized in brucite calcite veins that are strongly enriched in organic carbon up to 0 5 wt of the total carbon but depleted in 13c 13ctoc 19 4 we detected a combination of bacterial diether lipid biomarkers archaeol and archaeal tetraethers analogous to those found in carbonate chimneys at the active lost city hydrothermal field the exposure of mantle rocks to seawater during the breakup of pangaea fueled chemolithoautotrophic microbial communities at the iberia margin possibly before the onset of seafloor spreading lost city type serpentinization systems have been discovered at midocean ridges in forearc settings of subduction zones and at continental margins it appears that wherever they occur they can support microbial life even in deep subseafloor environments

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

the process of serpentinization in dunite from new caledonia abstract dunite from new caledonia displays three types of serpentine dominated veins the earliest type 1 veins are narrow 50 100 m wide and rarely extend across more than a single olivine grain they are lizardite contain abundant brucite and never contain magnetite type 2 veins are 0 01 to 0 1 mm wide extend across several olivine grains and cut across the type 1 veins they are lizardite contain magnetite often in vein interiors and contain less brucite than type 1 veins type 3 veins are dominantly chrysotile cm scale have a magnetite rich core and extend for meters or more analyses of two representative samples indicate that the type 1 veins have relatively fe rich serpentine xmg 0 92 and brucite xmg 0 82 these minerals are less magnesian than those in the type 2 veins serpentine has xmg 0 93 0 94 and brucite has xmg 0 84 in the magnetite rich core to the type 3 vein both serpentine xmg 0 94 0 97 and one of the two brucite populations xmg 0 94 are mg rich opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0 05 mm wide the serpentine veins after opx lack talc or magnetite and as with veins cutting olivine the older veins are more fe rich xmg 0 84 than the younger veins xmg 0 90 we conclude that the formation of magnetite was accompanied by the extraction of iron from the early formed serpentine and brucite thermodynamic calculations suggest that the type 1 veins formed in a rock dominated system where the activities of feo mgo and sio2 were dictated by the compositions of olivine and orthopyroxene in contrast the type 2 veins were formed in a more fluid dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite serpentine assemblage reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen by producing magnetite this reaction extracted iron from brucite and serpentine making them both more magnesian this would drive the brucite serpentine magnetite assemblage to higher oxygen fugacity progressively decreasing the efficiency of the magnetite forming reactions