seismic and magnetic anisotropy of serpentinized ophiolite implications for oceanic spreading rate dependent anisotropy compressional and shear wave anisotropy shear wave birefringence and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the dinardic hellenic ophiolites the densities of these materials ranged from 3330 kg m 3 to 2620 kg m 3 and indicate degrees of serpentinization from 2 3 to 87 9 respectively magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers in all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation but the magnetic anisotropy is not related to the degree of serpentinization the compressional wave anisotropy e monotonically decays from 12 for nearly pure olivine dunite to less than 2 for the most serpentinized sample this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion this loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere the more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy 2007 elsevier b v all rights reserved

crustal large scale serpentinized mantle peridotite body in the sulu ultrahigh pressure metamorphic belt eastern china evidence from gravity and magnetic anomalies abstract from analysis of the geological and geophysical data gravity magnetic seismic and petrophysics we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body smpb situated in the middle to lower crust in the sulu belt the smpb was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh pressure uhp metamorphism our finding suggests an emplacement mechanism for the serpentinized mantle wedge smw early in the subduction process this is different from the classic view which holds that the serpentinized forearc mantle is formed by in situ hydration processes blakely et al 2005 the petrophysical properties of the smpb are similar to those of the serpentinized forearc mantle or smw in modern subduction zones worldwide but the formation mechanisms for smpb and smw are different this observation is important for understanding the geodynamic processes that operated in the large uhp metamorphic belt in the dabie sulu area eastern china

serpentinization of olivine at 300 c and 500 bars an experimental study examining the role of silica on the reaction path and oxidation state of iron abstract the generation of h 2 during serpentinization of ultramafic oceanic crust along earth s mid ocean ridges provides a key energy source for the deep biosphere and has important implications for metal transport and mineral stability in the subseafloor hydrothermal system nonetheless the driving forces and rates of the coupled processes of fe partitioning and h 2 generation during serpentinization are complicated by many factors but especially the activity of sio 2 aq in the serpentinizing fluid to examine the role of this key variable in h 2 generation and fe partitioning between magnetite and fe serpentine during serpentinization san carlos olivine was reacted with a nacl sio 2 aq bearing fluid at 300 c and 500 bars for approximately 90 days using flexible gold cell hydrothermal equipment time series changes in solution chemistry and dissolved h 2 are coupled with magnetic susceptibility mossbauer spectroscopy and fe isotope measurements of reactant olivine and coexisting alteration products to provide a complete description of fe mass transfer and oxidation as a function of reaction progress utilization of talc as reactant not only provided a source of sio 2 aq for serpentinization but effectively prevented brucite formation in keeping with constraints imposed by many natural hydrothermal systems which are inherently open systems moreover this experimental strategy allowed fe derived from olivine dissolution to transfer completely to serpentine and magnetite facilitating reaction path modeling initial changes in solution chemistry indicate buffering of dissolved sio 2 aq owing to the apparently balanced rates of olivine dissolution and serpentine precipitation during this stage of reaction a negligible amount of h 2 is generated suggesting that fe 2 serpentine is stable and the dominant fe bearing product with continued reaction the rate of sio 2 aq production lessens likely due to reduction in surface area of talc such that the continued hydration and hydrolysis of olivine titrates the previously buffered activity of sio 2 aq eventually approaching serpentine brucite coexistence during this secondary stage of reaction the rate of h 2 generation abruptly increases which reflects the formation of magnetite and fe 3 serpentine temporal changes in the isotope composition of dissolved fe throughout these stages likely reflect different mineral alteration and formation processes where initially 1 preferential mass transfer of isotopically light fe into solution occurred during olivine alteration without considerable oxidation as demonstrated by the low h 2 production rate and 2 the preferential removal of isotopically light fe from solution in connection with enhanced magnetite and h 2 formation during the later stages of reaction post experimental analysis of the fe isotopic composition of fe bearing minerals demonstrates statistically negligible enrichment of 56 fe of magnetite relative to olivine and serpentine consistent with observations from natural altered oceanic crust observations from this study demonstrate the utility of using coupled chemical isotopic and analytical approaches to ascertain time series changes in rates of mass transfer and mechanisms of oxidation of fe during serpentinization of olivine ultimately these insights provide important constraints for the chemical and fe isotopic evolution of fluids and minerals in ultramafic hosted hydrothermal systems at mid ocean ridges

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

magnetic properties of serpentinized peridotites from the dongbo ophiolite sw tibet implications for suture zone magnetic anomalies magnetic properties of a suite of variably serpentinized peridotites from the dongbo ophiolite sw tibet china have been investigated to determine the magnetic signatures of suture zones the degree of serpentinization s for these peridotites is mainly in the range of s 60 and s 20 30 serpentinized peridotites indicating that peridotites with such degrees of serpentinization contribute to the aeromagnetic anomalies within the yarlung zangbo suture zone in south tibet

magnetic properties of serpentinized garnet peridotites from the ccsd main hole in the sulu ultrahigh pressure metamorphic belt eastern china 1 magnetic properties of 14 serpentinized garnet peridotites from the interval of 603 20 683 53 m of the chinese continental scientific drilling ccsd hole in the sulu ultrahigh pressure uhp metamorphic belt eastern china were investigated and compared with those of ultramafic rocks from other tectonic settings to relate them to the uhp post metamorphic evolution serpentinized garnet peridotites are characterized by high susceptibility natural remanent magnetization nrm and q koenigsberger ratio nrm h where h is the intensity of the local geomagnetic field values the average nrm and q values are 299 65 10 7 m3 kg 1 19 15 10 3 am2 kg 1 and 16 59 respectively which are generally higher than those of other serpentinized peridotites in the world the serpentine concentration for most samples ranges from 40 to 60 suggesting that the peridotites are of obvious homogeneous serpentinization degree micro raman and x ray diffraction spectra of serpentine for representative samples indicate that lizardite is the only serpentine mineral opaque minerals in samples are mainly nonstoichiometric magnetite and cr spinel with or without minor ilmenite and fe sulfide magnetite occurs as rims around olivine and garnet as well as in clusters a chromian garnet further overgrew on the rims of the garnet the grain size of magnetite in rims is in the pseudo single domain region and favorable for development of high nrm high pressure uhp metamorphic processes possibly lead to enhanced concentrations of fe ti oxides in garnet peridotites and thus increase the magnetism of serpentinized garnet peridotites in the ccsd main hole integrated lizardite formation temperature 450 c and its relationships with the pressure temperature time evolution of the associated uhp rocks permit us to infer that serpentinization of garnet peridotites in the ccsd main hole occurred during late exhumation stages ca 200 215 ma ago under temperatures of 425 c however the exact relationship between magnetic properties of garnet peridotite and the uhp metamorphic process will require further studies

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 magnetic properties of metaperidotitic rocks as a function of metamorphic grade implications for crustal magnetic anomalies seventeen samples from the malenco serpentinite in the swiss alps representing systematic prograde metamorphic conditions from prehnite pumpellyite to upper amphibolite are moderately to strongly magnetic average susceptibility of 6 61 10 2 si units due predominantly to the presence of a magnetite rich spinel some greenschist facies samples contain the metal alloy awaruite ni3fe these contain in addition much greater amounts of magnetite small amounts of pyrrhotite in about half the samples complete the contributions to the total magnetization susceptibilities generally decrease with increasing metamorphic grade because of the production of increasing amounts of chromerich spinel which dilutes the magnetite component these results indicate that significant magnetic anomalies should be generated in suture zones containing metamorphosed serpentinite and in areas where serpentinite is being actively subducted

magnetostratigraphy of the neogene siwalik group in the far eastern himalaya kameng section arunachal pradesh india abstract the siwalik group was deposited from the mid miocene to the pliocene in the foreland of the himalaya and records the unroofing history of the mountain belt in this study we provide the first magnetostratigraphic data for the eastern himalayan foreland basin we analyzed two sections of the lower to upper siwalik group along the kameng river in arunachal pradesh india magnetostratigraphic data were acquired from 395 sites within a 5 8 km thick molassic series thermal demagnetization and magnetic rock property analyses indicate a relatively low temperature 150 340 c characteristic remanent magnetization chrm yielding reliable primary directions carried by iron sulfides such as greigite the results show local counter clockwise rotation of the thrust sheets which is consistent with partitioning of arc normal and left lateral strain along the himalaya nineteen polarity zones have been identified in the two sections detrital fission track data from the top and bottom of the sections are used to constrain correlations with the geomagnetic polarity time scale gpts from chron c5ar 1n to chron c2an 2n from these results we propose that the siwalik formation in arunachal pradesh was deposited between 13 and 2 5 ma the transition between the lower and middle siwaliks is dated at about 10 5 ma and the middle to upper siwaliks transition is dated at 2 6 ma these results coupled with sedimentological observations suggest that the eastern himalayan chrono stratigraphic record is nearly synchronous with that analyzed in other parts of the neogene himalayan foreland basin nevertheless some differences in the evolution of the sedimentation rate and in the thickness of the middle siwaliks suggest that indian plate flexure in the east is different from that observed in other parts of the indian foreland basin and could be affected by the presence of the shillong plateau to the south

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

mineralogy and geochemistry of the berong ni co laterite deposit palawan philippines abstract intense weathering of the serpentinized peridotite resulted in the formation of the berong ni co laterite deposit in palawan philippines the studied laterite profiles are divided from base to top into i serpentinized bedrock ii saprolite horizon and iii limonite horizon olivine in the peridotite contains a significant amount of ni average 0 38 wt nio thus representing the main source of the ore in the laterite horizons six types of ni bearing serpentines up to 6 65 wt nio in the saprock are discriminated based on their textures and raman signatures garnierite a mixture of serpentine kerolite and sepiolite like clays typically occurs as coatings or fracture fillings in the saprock and is characterized by strikingly high nio content up to 34 86 wt goethite and mn oxy hydroxides i e lithiophorite asbolane intermediates are the principal hosts of ni in the limonite up to 2 15 wt nio and 23 89 wt nio respectively with the latter also being the main co carriers up to 12 59 wt coo the whole rock xrf data show typical trends observed in other ni laterite deposits where mg and si are depleted toward the surface and the transition metals i e fe al ni co and mn are retained in the weathered horizons the saprolite horizon is particularly enriched in ni while the lower section of the limonite horizon is enriched in co the mildly acidic environmental condition ph 6 57 in the limonite horizon was found to enhance the mobilization and re distribution of ni from the limonite towards the lower section of the profiles geological observations coupled with mineralogical and geochemical data show that the berong ni co laterite deposit formed in two weathering processes i development of the saprolite and the limonite horizons and ii late precipitation of secondary phyllosilicates i e garnierite mn oxy hydroxides and silica

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

alpine serpentinites ultramafic magmas and ocean basin evolution the ideas of h h hess hess9 ideas related to the tectonic significance of ultramafic rocks ultramafic magmas evolution of oceanic crust and h 2 o outgassing all emanating from his abiding interest in serpentinite hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional alpine type mountain belts they are not magmatic intrusions as he proposed but tectonic remnants of fossil plate boundaries hess was the formulator of the concept of sea floor spreading his ideas on the nature of oceanic crust changed through the years but he repeatedly argued that oceanic crust was partly or predominantly serpentinite despite the lack of subsequent popularity of this view he was at least partly right serpentinite is a part of oceanic crust notably in tectonically thinned regions along rifted ridges near transform faults and along obliquely rifted continental margins such as the mesozoic western alpine continental margin

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

contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres insights from the shape of the anomalies summary detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers including the lower crust and possibly the uppermost mantle is horizontally displaced with respect to that of the upper crust we examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper and lower crustal magnetization patterns thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the nusselt number and the depth of hydrothermal circulation in the crust and the temperature range of serpentinization the models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 dc successfully explain the anomalous skewness and the hook shape of observed sea level magnetic anomalies created at slow and intermediate spreading rates

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

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

serpentinized ultramafics and hydrothermal activity at the mid atlantic ridge crest near 15 n the initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the fifteen twenty fracture zone the ultramafics crop out in block faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m they comprise cumulate harzburgites pyroxenites iherzolites and wehrlites as well as gabbronorites olivine gabbronorites gabbropegmatites and alteration products including serpentinites bastite serpentinites and asbestos the ti zr y relations and relatively constant zr ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust ongoing hydrothermal activity is indicated by chemical anomalies 3he mn in the near bottom water at the ultramafic outcrop the upwelling hydrothermal circulation apparently follows crust penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and or mantle the observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long offset 100 km ridge ridge transform faults where they are serpentinized by hydrothermal processes within the initial 1 106 years of generation of lithosphere at a slow spreading axis the corners formed by the intersections of the walls of a rift valley with both the transform rt corner and the nontransform rn corner portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

effective magnetic susceptibility of the oceanic upper mantle derived from magsat data passive ocean continent boundaries have little magnetic signatures at magsat altitude 400 km whereas subducting oceanic lithosphere has well defined magnetic anomalies over some of the subduction zones based on this observation and a thermal evolution model of the oceanic lithosphere subducted along the kuril kamchatka trench we determined an effective magnetic susceptibility of 0 0085 si for the upper 35 km of the oceanic upper mantle

magnetic properties of synthetic analogues of the altered olivines of igneous rocks summary a laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions three compositions in the forsterite fayalite solid solution series were synthesized by a three firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite fe o as a contaminant 0 005 per cent the prepared olivines were oxidized between 850 and 1280 c at oxygen fugacities of between lo and lop5 atmospheres the composition and concentration of the multiphase oxidation products were studied by magnetic optical electron optical and x ray methods the magnetic constituent of the intergrowth was magnetite below about 1150 c the microstructure is similar to the dendritic intergrowths found for example in the altered olivine of the central layered intrusion of the isle of rhum above about 1150 c the form may be similar to symplectite intergrowths the magnetization process parameters coercive force etc vary systematically with temperature of oxidation to which evidently controls the microstructure of the magnetite the magnetite produced is magnetically hard samples altered at 900 c and 950 c having higher coercive force hc than any magnetite reported in the literature the room temperature properties appear to bridge the published trends found in grown crystal magnetites and crushed grains inferred effective grain sizes of the complex intergrowths range from 0 1 to 3 pm as to increases from 900 to 1200 c the temperature dependence of magnetization process parameters principally h measured between room temperature and the curie point are modelled in terms of i monodomain grains subject to thermal fluctuations ii monodomain grains not subject to thermal fluctuations and iii as multidomain grains model i yields plausible values for effective grain dimensions and length diameter ratios higher to producing larger more equidimensional particles model ii suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure dependent uniaxial particle particle interaction term model iii implies that if domain walls are pinned by both surface and volume defects magnetostriction plays a greater role in volume pinning than in surface pinning plausible values of demagnetizing factors appropriate to grains with few domains are produced by model iii

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

gravity and magnetic modelling of layered mafic ultramafic intrusions in large igneous province plume centre regions case studies from the 1 27 ga mackenzie 1 38 ga kunene kibaran 0 06 ga deccan and 0 13 0 08 ga high arctic events gravity and magnetic data from the global egm2008 and emag2 datasets are used to identify geophysical anomalies in large igneous province lip plume centre regions with the goal of characterizing mafic ultramafic intrusions linked to those lips geophysical anomalies within 18 lips distributed globally are investigated four of these lips are selected for detailed modelling the 1 27 ga mackenzie 1 38 ga kunene kibaran 0 06 ga deccan and 0 13 0 08 ga high arctic lips we recognize three spatial distribution types for intrusions in plume centre regions these are 1 intrusions emplaced along a circular fault system that circumscribes the plume centre 2 intrusions emplaced along linear rifts that in some cases converge towards the plume centre and 3 single or unclassified intrusions modelling supports that the geophysical anomalies associated with these lips tend to be produced by large radius 30 km and deep seated crustal intrusions with densities consistent with mafic ultramafic rock and

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

microstructure of serpentinite mylonites from the josephine ophiolite and serpentinization in retrogressive shear zones california the josephine peridotite has experienced deformational episodes over a wide range of temperatures deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism serpentinization two general types of deformed serpentinites have been identified 1 incohesive serpentinites most common which appear to have experienced low temperature predominantly brittle deformation and 2 serpentinite mylonites which have experienced plastic flow near the upper limit of antigorite stability the distinction between these two is based on textural and mineralogical criteria serpentinite mylonites are cohesive homogeneously foliated and typically strongly lineated rocks composed primarily of antigorite and magnetite they occur in planar bounded zones enclosed within the josephine peridotite that were the loci of both noncoaxial deformation and serpentinization we discuss the development of two important shear sense indicators porphyroclast systems and oblique shear band foliations as observed in these rocks porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy cores are usually composed of primary pyrogenetic minerals whereas mantles including tails are composed of products of the serpentinization reaction these systems are therefore termed porphyroctasts with neocrystallization tails oblique shear bands are common micro structures in serpentinite mylonites shear bands which offset the mylonitic foliation along narrow discrete fractures are termed f type whereas those which offset the mylonitic foliation along narrow ductile micro shear zones are termed d type the latter are considered more reliable for shear sense determinations strains accomplished by movement on shear bands are probably small strain resulting from a volume increase dur ing serpentinization while holding the shear zone width constant is sufficient to produce the shear bands therefore we suggest that oblique shear bands may develop to accom modate volume increases associated with deformation

high temperature hydrothermal alteration of ultrmafic cumulates from the base of the sheeted dikes in the josephine ophiolite nw california ultramafic cumulate pods occur locally just below the base of the sheeted dike complex in the josephine ophiolite in north western california these ultramafic rocks occur with metagabbros and both are extensively intruded by mafic dikes the ultramafic cumulates contain evidence of high temperature hydrothermal alteration early alteration possibly above 900 c which continued to progressively lower temperatures 350 c this hydrothermal alteration has been partially obscured by later low grade regional metamorphism petrographic evidence for the hydrothermal alteration includes 1 hydration of primary clinopyroxene to hornblende 2 supersolvus secondary hornblende 3 subsolvus coexisting hornblende actinolite and actinolite chlorite 4 alteration of hornblende to chlorite 5 alteration of orthopyroxene and olivine to serpentine and 6 serpentine veining the composition of the altering fluid appears to change with temperature

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

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

tertiary tectonics of the patagonian andes retro arc area between 38 15 and 40 s latitude the structure and tectonics of the retro arc area of the andes between 38 degrees 30 and 40 degrees s remains poorly understood because of the scarcity of previous structural studies this paper analyzes the styles distribution and timing of the deformation in the eastern flank of the andes between these latitudes with focus in the tertiary tectonics of the area this andean section can be divided into an internal and an external domain based on its distinctive structural evolution the boundary between them coincides with the bio bio and alumine valleys which separate sectors of the fold and thrust belt with opposite vergence and different structural styles structures developed in the external domain during the andean cycle are associated with a west verging fold and thrust belt which was controlled geometrically by pre existing normal faults of the mesozoic rift phase we also differentiate three segments along the external domain of the inner retro arc zone at these latitudes each segment is associated with a tertiary depocenter related to the western border of a structural high copahue pino hachado block and cordilleras de catan lil and chachil which constitutes an outstanding feature within the study area the recognition and interpretation of the main structures and the spatial and temporal distribution of the tertiary sequences allowed us to establish a tectonic model in which the uplift of this nnw trending block during upper miocene times originated a series of intermontane depocenters well represented between 38 and 40 degrees s as a result of that syntectonic and synorogenic deposits accumulated in a compressive regime these depocenters can be integrated in a narrow deposition zone associated with the western slope of the copahue pino hachado block and its southward prolongation in the cordillera de catan lil and cordillera del chachil on the basis of tectonostratigraphic controls we define the last andean contractional phase of the inner retro arc area at these latitudes between 11 and 5 67 ma for the northern segment between 10 and 6 2 ma for the central segment and between 13 to 8 and 43 ma for the southern segment after this short lived compressional stage deformation migrated to the west associated with the onset of the liquine ofqui strike slip fault zone along the intra arc zone c 2010 elsevier b v all rights reserved

reply to comment by pedreira et al on reconstruction of the exhumed mantle across the north iberian margin by crustal scale 3 d gravity inversion and geological cross section

oxide mineral formation during the serpentinization of a cr spinel seam insights from rock magnetic experiments 1 serpentinization of ultramafic rocks results in the growth of serpentine and magnetite following hydration of magmatic olivine cr spinel seams in ultramafic intrusions that also contain olivine might produce magnetite during the serpentinization process however hysteresis and temperature dependence of susceptibility experiments on serpentinized chromitite 80 cr spinel from the dawros peridotite western ireland reveal the presence of small quantities of a fe ti oxide phase likely titanomaghemite reflected light petrography reveals that the oxides occur as 0 1 m to 20 m grains that are typically intergrown with serpentine it is suggested that the formation of the minute quantities of low susceptibility high curie point titanomaghemite occurred by low temperature alteration of the magnetite produced during serpentinization the presence of such low quantities of the oxide phase may be either a function of the very high mg content of the magmatic olivine or the decoupling of oxide growth from the process of serpentinization where magnetite formed later than the reaction of olivine with the infiltrating fluids our data highlight the usefulness of rock magnetic techniques to the investigation of oxide mineral phases in serpentinized ultramafic rocks

aeromagnetic study of the island of hawaii an aeromagnetic study of the island of hawaii provides new insight on magnetic properties of subsurface rock and geologic structure on a regional scale spectral depth analysis delineates two shallow magnetic zones each roughly 1 5 km thick lying at a depth of 1 km one zone of unknown origin lies in the center of the island and correlates with a regional magnetic high the other zone coincides with pronounced magnetic lows paralleling kilauea s active east rift zone these magnetic lows probably depict rocks chemically altered by hydrothermal fluids in which titanomagnetite has been destroyed analysis of magnetic terrain effects indicates that magnetization also decreases with depth within mauna kea and mauna loa we estimate that magnetization is reduced by about half at a depth of 1 km the magnetic method is particularly useful in delineating the lateral extent of local shield structures such as rifts summit calderas pit craters and vent fissures rifts possess characteristic magnetic patterns primarily long wavelength linear magnetic low zones we propose alteration processes reduce magnetizations along the flanks of rifts on the other hand along young rifts e g kilauea s east rift zone short wavelength magnetic anomalies probably reflect slowly cooled unaltered intrusions altered rock may also produce magnetic lows that help define buried summit calderas

remanent magnetization of the oceanic upper mantle skewnesses of the marine and magsat magnetic anomalies of the atlantic ocean are explained in terms of the remanent magnetization gradually acquired by the oceanic lithosphere during its cooling magnetizations of the lower crust and the upper mantle are about 20 50 and 10 30 percent of the magnetization of the oceanic basaltic layer 2a respectively the oceanic upper mantle controls the skewness of the magsat anomalies whereas the skewness of the marine anomalies arise primarily from the oceanic lower crust

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

abiotic hydrogen h2 sources and sinks near the mid ocean ridge mor with implications for the subseafloor biosphere free hydrogen h2 is a basal energy source underlying chemosynthetic activity within igneous ocean crust in an attempt to systematically account for all h2 within young oceanic lithosphere

joint inversion of gravity and magnetic anomalies of eastern canada the power spectra and degree correlation of the surface topography and free air gravity anomalies of eastern canada show that the gravity anomalies are subdivided into three parts the short wavelength components 30 170 km shorter than 30 km are not well resolved largely arise from density perturbations in the crust and to a lesser extent from the surface topography and moho undulation whereas the contribution of intracrustal sources to the intermediate wavelength components 170 385 km is comparable with that of the topography the long wavelength components 385 1536 km are overcompensated at the moho we present a crustal model for the intermediate and long wavelength components which takes into account the surface topography density perturbations in the crust and moho undulation with a certain degree of isostatic compensation the general characteristics of this model resemble the crustal structure revealed from seismic measurements the reduced to pole magnetic anomalies of eastern canada sh

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading