magmatism in the iberian pyrite belt petrological constraints on a metallogenic model igneous formations associated with massive sulphide deposits in the iberian pyrite belt ipb are essentially composed of basic lavas and dolerites and dacitic to rhyolitic volcanites intermediate lavas are subordinate the basic rocks show variable geochemical characteristics lavas and dolerites comparable to recent within plate alkaline basalts seem restricted to the western and southern parts of the ipb whereas basic rocks comparable to continental tholeiites or arc related basalts occur across the whole belt the felsic rocks are classified as calc alkaline and belong to the low al2o3 and high yb type at given sio2 al2o3 and tio2 contents they show variable zr nb and hree contents heavy rare earth element fractionation decreases from the dacites to the rhyolites gd yb n 1 whereas the negative eu anomaly becomes more pronounced the characteristics of the rhyolites are typical of sulphide fertile volcanic packages trace element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase partial melting of an amphibolite protolith which appears as the most probable model for the origin of this dacitic magma requires a high t p gradient in the crust the occurrence of alkaline basalts and continental tholeiites is consistent with formation of the ipb in a tensional tectonic setting however the associated island arc tholeiites suggest a location in a domain of plate convergence emplacement in a fore arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints a high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the ipb

evaluation of the role of cambrian granites in the genesis of world class vhms deposits in tasmania abstract an analysis of the distribution composition and alteration zonation of cambrian granites which intrude the mt read volcanics of western tasmania provides evidence that there may have been a direct input of magmatic fluids containing fe cu au and p to form the copper gold volcanic hosted massive sulphide vhms mineralisation in the mt lyell district interpretation of regional gravity and magnetic data indicates that a narrow discontinuous body of cambrian granite 2 4 km wide extends along the eastern margin of the mt read volcanic belt for over 60 km the cambrian granites are altered magnetite series types which show enrichment in barium and potassium and contrast markedly with the fractionated ilmenite series devonian granites related to tin mineralisation elsewhere in the dundas trough copper mineralisation occurs in a linear zone above the apex of the buried cambrian granite body at the southern end of the belt from mt darwin to the mt lyell district over a strike length of 25 km gold and zinc mineralisation are concentrated higher in the volcanic stratigraphy more distant from the granite overlapping zones of alteration extend from the granite into the surrounding volcanic rocks an inner zone of k feldspar alteration is overprinted by chlorite alteration which passes outwards into sericite alteration magnetite pyrite chalcopyrite apatite mineralisation is concentrated in the chlorite alteration zone as veins and low grade disseminations the mt lyell copper gold stringer and disseminated mineralisation is hosted in felsic volcanic rocks 1 to 2 km west of the interpreted buried granite position magnetite apatite pyrite veins in the prince lyell deposit at mt lyell are very similar to the veins in the halo of the granite further south and provide evidence for magmatic fluid input during the formation of the copper gold vhms deposits a model involving deeply penetrating convective seawater mixing with a magmatic fluid released from the cambrian granites best explains the features of vhms mineralisation in the mt lyell district

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

hydrothermal discharge zones beneath massive sulfide deposits mapped in the oman ophiolite the area in the oman ophiolite containing the volcanic hosted bayda and aarja massive sulfide deposits exposes a cross section of ocean crust and reveals to an unprecedented extent the fossil zones of hydrothermal upwelling that fed these sea floor deposits the fossil discharge zones are elongate areas of alteration and mineralization characterized by numerous small metres to tens of metres in length linear discontinuous gossans the gossans result from oxidation of hydrothermal pyrite replacing primary igneous phases and filling voids and fractures in the altered host rocks the two deposits have separate discharge zones that appear to be sub sea floor extensions of their stockworks the bayda zone extends through the volcanic section into the upper sheeted dike complex and is interpreted as having formed on the ridge crest above an axial magma chamber the aarja zone terminates against a plagiogranite pluton that intrudes the lower volcanic section and is thought to have formed after bayda in an off axis environment structural stratigraphic and compositional characteristics of the bayda and aarja massive sulfide bodies are consistent with this interpretation the geometry of the discharge zones suggests that in both cases upfiow occurred in broad zones at least 400 600 m wide that were elongated along strike i e parallel to the spreading axis

hydration and uplift of the oceanic crust on the mid atlantic ridge associated with hydrothermal activity evidence from boron isotopes boron isotope systematics of vent fluids from the mid atlantic ridge mar and the faster spreading east pacific rise epr reveal that at the mar 50 of seawater boron is removed from circulating fluids on the downwelling limb of the hydrothermal convection cell whereas little boron is removed at epr sites geophysical and petrographic studies suggest that downwelling fluids on the mar penetrate deep in the oceanic crust and upper mantle where boron is removed during serpentinization of peridotite the associated hydration of the oceanic crust reduces its density and modelling of the data suggests that this results in uplift leading to significant elevation of the median valley walls in the vicinity of active hydrothermal systems

serpentinization of the martian crust during noachian abstract this paper proposes a model of serpentinization of the southern martian crust that may explain the topographic dichotomy the absence of an associated free air gravity anomaly and the presence of strong magnetic anomalies in the southern hemisphere the thermodynamical conditions for serpentinization were likely met in the lithosphere during the noachian period this process may have decreased the density in the southern crust and created the topographic dichotomy different reactions of serpentinization that can form magnetite have been considered assuming an intense magnetic field core dynamo we obtain chemical remanent magnetizations that are in the order of the estimates deduced from martian magnetic anomaly studies the pertinence and the implications of our model concerning the early thermal evolution of mars are discussed with emphasis on the intensity of the paleo magnetic field

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

seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern east pacific rise a full waveform inversion of two ship wide aperture seismic reflection data from a ridge crest seismic line at the southern east pacific rise indicates that the axial magma chamber here is about 50 m thick is embedded within a solid roof and has a solid floor the 50 60 m thick roof is overlain by a 150 200 m thick low velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation and the roof itself may be the transition zone separating the magma chamber from circulating fluids furthermore enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle the presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

regional alteration systems associated with volcanogenic massive sulfide mineralization at panorama pilbara western australia outstanding exposure across the panorama district of the archean pilbara block of western australia reveals a cross section through a ca 3 24 ga massive sulfide bearing volcanic pile and an underlying coeval subvolcanic intrusion in an area of low metamorphic grade and very low strain this has provided a rare opportunity to map a complete regional scale hydrothermal alteration system associated with volcanogenic massive sulfide mineralization and to assess the role of a subvolcanic intrusion in driving such a system four major alteration facies can be defined in the panorama district these are feldspar bearing background alteration and feldspar sericite quartz alteration and completely feldspar destructive sericite quartz and chlorite quartz alteration these alteration facies affect all of the volcanic pile and the upper parts of the underlying subvolcanic intrusion regional semiconformable alteration zones which are predominantly feldspar bearing are crosscut by broad corridors of transgressive feldspar destructive alteration a convective hydrothermal model is invoked to explain this distribution of alteration facies where semiconformable alteration zones represent seawater recharge and transgressive feldspar destructive alteration zones mark the path of evolved seawater discharging back to the sea floor the interpreted discharge sites are coincident with base metal sulfide deposits at the top of the volcanic pile geologic relationships and the distribution of alteration zones clearly implicate the strelley granite as the subvolcanic intrusion which drove the hydrothermal alteration systems granophyric textures and miarolitic cavities in the upper parts of the intrusion suggest that a magmatic fluid phase was exsolved early in its crystallization history granite hosted copper zinc tin veins near the top of the intrusion are interpreted to be a product of this magmatic fluid this raises the possibility of a substantial contribution of magmatically derived metal to the overlying volcanogenic massive sulfide hydrothermal alteration systems

u pb dating of serpentinization hydrothermal zircon from a metasomatic rodingite shell sudetic ophiolite sw poland abundant zircon grains were found within a metasomatic chlorite corrensite trioctahedral regularly interstratified chlorite smectite shell that surrounds rodingite from the jordanow gogolow jg serpentinite massif sudetic ophiolite bohemian massif the zircon grains bear two types of primary fluid inclusions containing either liquid co2 or aqueous solutions the inclusions were used to assign the p t conditions of the zircon formation 270 300 c ca 1 kbar using the crossed isochore method abraded single zircon grains give a u pb age of 400 4 3 ma which is interpreted as the age of zircon crystallization this zircon formed during rodingitization i e a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process the zircon age from the jordanow gogolow massif is consistent with the crystallization age of metagabbro protoliths from the sudetic ophiolite as determined by oliver et al j geol soc lond 150 1993 355 a ce anomaly combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from sudetic ophiolite suggests that the sm nd whole rock isotopic data of pin et al lithos 21 1988 195 353 21 ma possibly characterize the age of later metamorphic event

analogue models of laccolith formation abstract this paper describes dynamically scaled analogue models of laccolithic intrusions experiments consisted of the injection of a newtonian fluid low viscosity silicone putty into a sandpack with or without an interbedded ductile layer of silicone putty acting as a potential decollement level boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the decollement layer on the pattern of intrusion further experiments were made to examine the effects of an extensional regime during intrusion experiments showed that 1 laccolith formation requires the occurrence of a decollement layer between two competent units 2 the critical thickness of the decollement layer necessary for laccolith formation decreases with increasing depth 3 laccoliths change from lenses to bell shaped with decreasing overburden 4 for a constant thickness of the decollement layer the largest diameter of laccoliths is proportional to the thickness of the overburden and 5 a syn injection gravitational sliding regime results in an asymmetric laccolith with amplification of the bell shape

source and tectonic implications of tonalite trondhjemite magmatism in the klamath mountains in the klamath mountains voluminous tonalite trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 ma early cretaceous it occurred about 5 to 10 m y after the 165 to 159 ma josephine ophiolite was thrust beneath older parts of the province during the nevadan orogeny thrusting from 155 to 148 ma the magmatism also corresponds to a period of slow or no subduction most of the plutons crop out in the south central klamath mountains in california but one occurs in oregon at the northern end of the province compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later more voluminous tonalitic and trondhjemitic intrusions most plutons consist almost entirely of tonalite and trondhjemite poorly defined concentric zoning is common tonalitic rocks are typically of the low al type but trondhjemites are generally of the high al type even those that occur in the same pluton as low al tonalite the suite is characterized by low abundances of k2o rb zr and heavy rare earth elements sr contents are generally moderate 450 ppm by comparison with sr rich arc lavas interpreted to be slab melts up to 2000 ppm initial 87sr 86sr 18o and nd are typical of mantle derived magmas or of crustally derived magmas with a metabasic source compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source transitional mid ocean ridge to island arc basalt but not by fractional crystallyzation of a basaltic parent melting models require a residual assemblage of clinopyroxene garnet plagioclase amphibole residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab such models are consistent with the metabasic part of the josephine ophiolite as the source because the josephine ophiolite was at low t during nevadan thrusting an external heat source was probably necessary to achieve significant degrees of melting heat was probably extracted from mantle derived basaltic melts which were parental to the mafic precursors of the tonalite trondhjemite suite thus under appropriate tectonic and thermal conditions heterogeneous mafic crustal rocks can melt to form both low and high al tonalitic and trondhjemitic magmas slab melting is not necessary

magma mixing in the subvolcanic environment petrology of the gerena interaction zone near seville spain the mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high level subvolcanic setting in the spanish hercynian at gerena near seville the compositions involved are calc alkaline and the situation is one of tonalite quartz diorite synplutonic dykes injected into a granitic magma chamber the resulting hybrids include dykes pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite the present investigation is based on field and petrographic observations of hybridization textures the identification of different stages in the crystallisation history of the tonalite through mineral textures and the characterization of mineral compositions at these various stages proportions of the end member magmas involved were obtained by major oxide mixing models and tested satisfactorily with trace elements a mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber after high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions it is argued that such high energy flow conditions are a necessary requirement for effective hybridization in this environment in contrast to most large scale magma chamber settings where mixing is driven by thermal and buoyancy contrasts

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

sulfur isotopic compositions of sulfides along the southwest indian ridge implications for mineralization in ultramafic rocks the recently explored tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow spreading southwest indian ridge displays high temperature sulfide mineralization isocubanite sphalerite and minor pyrrhotite and low temperature pyrite and covellite phases pyrite can be subdivided into pyrite i and ii with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite sphalerite and pyrite i or occurring as individual grains in quartz veinlets the sulfide minerals have the greatest range of 34s values 23 8 to 14 1 found so far among modern sediment starved ridges with distinct 34s values for low and high temperature mineral phases the high 34s values of isocubanite 9 6 to 12 2 and sphalerite 9 1 to 14 1 suggest that sulfate which precipitated from seawater during an early low temperature phase of hydrothermal circulation was the main sulfur source for these sulfides pyrite ii has the lowest and most variable 34s values 23 8 to 3 6 suggesting microbial sulfate reduction pyrite i has variable and generally positive 34s values 0 1 to 12 0 with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate mixed with volcanogenic sulfur intermittent magmatism represented by gabbroic intrusions and high permeability caused by well developed fractures associated with detachment faults contributed to the formation of sulfides in the tianzuo hydrothermal field these factors possibly control sulfide mineralization in amagmatic segments of ultraslow spreading ridges

geochronological constraints on late archean magmatism deformation and gold quartz vein mineralization in the northwestern anialik river greenstone belt and igneous complex slave province n w t three phases of the anialik river igneous complex aric give u pb zircon ages in the range 2705 2683 ma and three units from the adjacent northwestern anialik river greenstone belt argb give ca 2678 ma ages titanite from unsheared aric rocks crystallized during localized metamorphism and deuteric alteration between 2693 and 2683 ma hydrothermal titanite in wall rocks to gold mineralized shear zones crystallized during early shear zone development 2670 1 ma and was subsequently locally altered to rutile with gold occurring within the rutile bearing assemblage unaltered second generation hydrothermal titanite overgrowing the rutile assemblage crystallized during later brittle ductile movement 2656 2 ma and provides a minimum age for gold mineralization relatively high 207pb 204pb ratios of pb in gold associated galena suggest that it was partly derived from significantly older crustal material possibly underlying the igneous complex and greenstone belt this interpretation is consisten

methane in serpentinized ultramafic rocks in mainland portugal we report a new case of methane ch4 of apparent abiotic origin in continental serpentinized ultramafic rocks multiple analytical techniques on site and in the laboratory revealed methane and ethane degassing from hyperalkaline ph 11 ca2 oh mineral waters in boreholes drilled in the alter do chao igneous intrusion at cabeco de vide in mainland portugal the c and h isotopic composition of ch4 13c 20 2h 283 suggests a dominant abiotic origin although minor thermogenic contributions cannot be excluded similarly low methane to ethane ratios suggest a predominantly non microbial source consistent with previous microbiological data showing the lack of methanogenic archaea in these waters heavier hydrocarbons co2 and h2 are below detection limits this case study confirms that ch4 from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked to deep circulation of meteoric waters maximum depth of cabeco de vide serpentinized rocks is less than 1 km and present temperatures are likely lower than 50 c serpentinization and related gas formation may have occurred at any time during thermal evolution of the igneous intrusion so gas formation temperature cannot be easily determined this case is an opportunity to test thermometry provided by ch4 isotopologue analyses the existence of methane in continental serpentinized igneous rocks is more widespread than previously thought and petroleum systems with similar serpentinized ultramafics in reservoir rocks may have traces of the observed 13c enriched ch4

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

geophysical evidence for hydration of the crust and mantle of the nazca plate during bending at the north chile trench water transported in subducting oceanic plates plays a key role in a number of phenomena including intraslab seismicity and arc magmatism however the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated we present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the nazca plate at the north chile trench crustal seismic velocities at the trench are lower than velocities of mature fast spreading crust and even lower than velocities of highly extended slow spreading crust in addition the nazca plate at the north chile trench may contain an 20 km thick upper mantle layer with 17 serpentine which implies 2 5 wt water these results document pervasive rock alteration by water percolation linked to bending related extensional faulting

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

deformation and surface uplift associated with serpentinization at mid ocean ridges and subduction zones 1 we employ the classical problem of an inclusion in an elastic half space to model effects of sub surface serpentinization on crustal deformation change of stress state and surface uplift at the tag hydrothermal field on the mid atlantic ridge the model suggests that an anomalous salient 3 km in diameter and 100 m high that projects 3 5 km westward from the east valley wall may have resulted from a relatively deep seated well serpentinized body exhibiting transformational strain the associated large strains would likely result in sub surface fracturing or faulting but surface uplift may be relatively insensitive to the exact depth and shape of the serpentinized region serpentinization of a region beneath the footwall of the tag detachment fault will tend to promote slip along some overlying normal faults which may then enhance fluid pathways to the deeper crust to continue the serpentinization process our solution for the miyazaki plain above the kyushu palau subduction zone in sw japan explains the observed uplift of 120 m the small transformational strains associated with serpentinization in this region may promote thrust type events in the aseismic slip zone near the upper boundary of the subducting philippine sea plate as well as intraplate earthquakes associated with normal faulting the rate of serpentinization needed to produce the observed uplift at the miyazaki plain is significantly greater than that needed at tag though significantly smaller on per unit volume basis thermal effects of serpentinization in both regions appear to be small but uplift data provide an additional constraint on inferring serpentinization from geological and seismological observations

seismicity in cascadia abstract we examine spatio geometric patterns in the density of seismicity within the cascadia forearc to gain insight into controls on seismogenesis tremor epicenters exhibit the most regular distribution defining a 40 80 km wide band extending along almost the entire convergent margin from the southern terminus of gorda plate subduction in northern california to the central explorer plate below northern vancouver island based on prior characterization of constituent low frequency earthquakes the up and down dip limits of tremor are assumed to represent slab isodepth contours at nominal values of 28 and 45 km between which fluids at near lithostatic overpressures are trapped within the subducting slab epicenters of earthquakes within the north american crust are anticorrelated with those of tremor and concentrated in washington and northern california where they are sandwiched between tremor and the cascade volcanic arc seismicity within the subducting plate possesses the most limited epicentral distribution seismicity is confined to shallow depths off vancouver island and in northern california and projects to greater depths beneath washington and southern british columbia comparison of seismicity patterns with long wavelength slab geometry and thermo petrologic constraints suggests that seismicity occurrence in cascadia is governed by an interplay between slab strain metamorphic dehydration within the subducting oceanic plate and a plate boundary seal that controls where fluids enter the overriding plate the inferred fluid evolution model harbors interesting implications for mantle wedge hydrology forearc crustal composition and volcanism in cascadia and other warm subduction zones

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

experimental evidence of reaction induced fracturing during olivine carbonation author s zhu w fusseis f lisabeth h xing t xiao x de andrade v karato si abstract 2016 american geophysical union all rights reserved mineral carbonation a process that binds co2 in the form of carbonates by silicate weathering is widespread on the earth s surface because of the abundance of silicate rocks and the permanence of the carbonated solids sequestering co2 via mineral carbonation has generated lots of interests however it is unclear how the fluid rock reaction proceeds to completion in spite of an increasing solid volume we conducted a mineral carbonation experiment in which a sintered olivine aggregate reacted with a sodium bicarbonate solution at reservoir conditions time resolved synchrotron x ray microtomographic images show cracks in polygonal patterns arising in the surface layers and propagating into the interior of the olivine aggregate the nanotomography data reveal that the incipient cracks intersect at right angles we infer that stretching due to nonuniform volume expansion generates polygonal cracking of the surfaces our data shed new lights on the processes that control hydration and carbonation of peridotite

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

oxygen isotope alteration in the noranda mining district abitibi greenstone belt quebec a regional whole rock oxygen isotope survey carried out in the noranda district shows how oxygen isotope alteration can be used to guide exploration for massive sulfide deposits the survey is based on 588 outcrop whole rock oxygen isotope determinations from 478 sites in a approximately 35 by 50 km area between the destor porcupine and cadillac larder lake breaks from east of the horne mine to west of the magusi river mine the mean isotopic composition of all samples is 7 4 or 2 per mil smow nested analysis of variance of 478 isotopic determinations from 371 of these sites shows that 74 percent of the total variance in whole rock delta 18 o r r whole rock is associated with outcrop location 21 percent with outcrop sampling and 5 percent with sample analysis an analysis of 259 determinations indicates that rock type is statistically no more important than outcrop sampling the statistical analysis indicates that isotopic variations greater than or 1 per mil can be used to map the hydrothermal circulation responsible for massive sulfide mineralization specific traverses visually confirm the lack of 18 o dependence on rock type and the utility of a 2 per mil interpretive band the large scale pattern of isotopic alteration in the noranda district is dominated by a nearly continuous 2 to 3 km wide annulus at the margins of the flavrian pluton of delta 18 o r 18 o r 18 o r depletion is surrounded up section by a approximately 10 km wide band of patchy but intense 18 o enrichment 8 5 14ppm the combined pattern of concentric light and heavy alteration has a diameter of approximately 30 km the clericy intrusion shows a similar pattern of isotope alteration on a smaller 10 km diam scale the lake dufault pluton seems to have been isotopically altered by continuing convection driven by the flavrian hydrothermal system modeling suggests that the 18 o depletion is produced by water circulating toward and along the edges of an intrusion and that the surrounding heavy alteration is produced either by seawater recharge or limited hydrothermal discharge the large scale and coherency of the alteration pattern centered on the flavrian pluton is remarkable and indicates that this pluton drove a major long lived approximately 10 m y hydrothermal system in which convection penetrated to an 8 kin depth the intensity and coherency of the 18 o depletion correlates with the tonnage of associated massive sulfide mineralization oxygen isotope depletion may in general mark the margins of plutons that have driven sufficient hydrothermal circulation to produce economic massive sulfide deposits up section it may be possible to screen plutons for up section massive sulfide exploration potential by collecting and isotopically analyzing approximately 50 samples from five traverses across a pluton9s margins additional samples may provide details light fingers that guide exploration to particular deposits

tectonics and seismicity of the southern washington cascade range abstract geophysical geological and seismicity data are combined to develop a transpressional strain model for the southern washington cascades region we use this model to explain oblique fold and fault systems transverse faults and a linear seismic zone just west of mt rainier known as the western rainier zone we also attempt to explain a concentration of earthquakes that connects the northwest trending mount st helens seismic zone to the north trending western rainier zone our tectonic model illustrates the pervasive effects of accretionary processes combined with subsequent transpressive forces generated by oblique subduction on eocene to present crustal processes such as seismicity and volcanism

the influence of spreading rate basement composition fluid chemistry and chimney morphology on the formation of gold rich sms deposits at slow and ultraslow mid ocean ridges seafloor massive sulphide sms deposits are variably enriched in precious metals including gold however the processes invoked to explain the formation of auriferous deposits do not typically apply to mid ocean ridge settings here we show a statistically significant negative correlation between the average gold concentration of sms deposits with spreading rate at non sedimented mid ocean ridges deposits located at slow spreading ridges 20 40 mm a have average gold concentrations of between 850 and 1600 ppb however with increasing spreading rate up to 140 mm a gold concentrations gradually decrease to between 50 and 150 ppb this correlation of gold content with spreading rate may be controlled by the degree and duration of fluid rock interaction which is a function of the heat flux crustal structure faulting and the permeability of the source rocks deposits at ultraslow ridges including ultramafic hosted deposits are particularly enriched in gold this is attributed to the higher permeability of the ultramafic source rocks achieved by serpentinisation and the inherent porosity of serpentine minerals combined with relatively high gold concentrations in peridotite compared with mid ocean ridge basalt variations in fluid chemistry such as reducing conditions and the potential for increased sulphur availability at ultramafic hosted sites may also contribute to the high concentrations observed beehive chimneys which offer more favourable conditions for gold precipitation may be more prevalent at ultramafic hosted sites due to diffuse low velocity venting compared with more focussed venting at basalt hosted sites

seismic imaging of magma sills beneath an ultramafic hosted hydrothermal system hydrothermal circulation at mid ocean ridge volcanic segments extracts heat from crustal magma bodies however the heat source driving hydrothermal circulation in ultramafic outcrops where mantle rocks are exhumed in low magma supply environments has remained enigmatic here we use a three dimensional p wave velocity model derived from active source wide angle refraction reflection ocean bottom seismometer data and pre stack depth migrated images derived from multichannel seismic reflection data to investigate the internal structure of the rainbow ultramafic massif which is located in a non transform discontinuity of the mid atlantic ridge seismic imaging reveals that the ultramafic rocks composing the rainbow massif have been intruded by a large number of magmatic sills distributed throughout the massif at depths of 2 10 km these sills which appear to be at varying stages of crystallization can supply the heat needed to drive high temperature hydrothermal circulation and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting our results demonstrate that high temperature hydrothermal systems can be driven by heat from deep sourced magma even in exhumed ultramafic lithosphere with very low magma supply

co2 sequestration and extreme mg depletion in serpentinized peridotite clasts from the devonian solund basin sw norway the conglomerates of the solund devonian basin of sw norway contain numerous locally up to 20 vol peridotitic clasts with concentric mm to 10 cm thick zones of varying red to black color the peridotite clasts show a clear alteration related textural evolution the least altered rocks are partly serpentinized peridotites showing a typical mesh texture with veins of serpentine magnesite and ni rich magnetite surrounding olivine fo91 relicts and its mg depleted clay like alteration product deweylite assemblage in the more advanced ophicarbonate stage the mesh cells contain calcite silica and are surrounded by talc in the final stage quartz calcite and hematite dominate the mineralogy and occur together with minor amounts of chromite talc cr chlorite and cr hydroandradite in tandem with this textural evolution is a decrease in mgo from 40 to 2 5 wt and a cao increase from 1 to 35 wt all peridotite clasts are characterized by high cr and ni concentrations the chemistry and the textural evolution show that the clasts formed by an extreme mg mobilization from the peridotite with development of secondary porosity and subsequent precipitation of calcite mgo removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where mg free minerals e g almandine are replaced by mg bearing minerals e g talc calculated apparent 87 sr 86 sr ratios of the clasts at 385 ma 0 7124 0 7139 corresponding to the inferred age of sediment deposition and incipient clast alteration indicate interaction with diagenetic basinal fluids we explain the reaction history as a three stage process involving a partial serpentinization of olivine in an oceanic environment b breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of mgo under near surface conditions in a tropical devonian climate and c further mg mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis sedimentary basins with abundant weathered peridotite represent potential sites for a permanent co2 storage by formation of calcite in a low temperature environment 2010 elsevier ltd all rights reserved

experimental evidence for chemo mechanical coupling during carbon mineralization in ultramafic rocks abstract storing carbon dioxide in the subsurface as carbonate minerals has the benefit of long term stability and immobility ultramafic rock formations have been suggested as a potential reservoir for this type of storage due to the availability of cations to react with dissolved carbon dioxide and the fast reaction rates associated with minerals common in ultramafic formations however the rapid reactions have the potential to couple with the mechanical and hydraulic behavior of the rocks and little is known about the extent and mechanisms of this coupling in this study we argue that the dissolution of primary minerals and the precipitation of secondary minerals along pre existing fractures in samples lead to reductions in both the apparent young s modulus and shear strength of aggregates accompanied by reduction in permeability hydrostatic and triaxial deformation experiments were run on dunite samples saturated with de ionized water and carbon dioxide rich solutions while stress strain permeability and pore fluid chemistry were monitored sample microstructures were examined after reaction and deformation using scanning electron microscopy sem the results show that channelized dissolution and carbonate mineral precipitation in the samples saturated with carbon dioxide rich solutions modify the structure of grain boundaries leading to the observed reductions in stiffness strength and permeability a geochemical model was run to help interpret fluid chemical data and we find that the apparent reaction rates in our experiments are faster than rates calculated from powder reactors suggesting mechanically enhanced reaction rates in conclusion we find that chemo mechanical coupling during carbon mineralization in dunites leads to substantial modification of mechanical and hydraulic behavior that needs to be accounted for in future modeling efforts of in situ carbon mineralization projects

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

seafloor hydrothermal systems driven by the serpentinization of peridotite 1 heat balance models predict that heat released upon serpentinization of peridotites can result in a wide range of hydrothermal venting temperatures temperatures ranging from a few to a few tens of c are likely temperatures 300 c may be possible in conjunction with significant heat input from below low fluid flow rate 10 kg s a high rate of serpentinization 100 kg s or some combination of these factors the volume of fresh lithosphere accessible to serpentinization may limit the duration of an episode of hydrothermal venting when the serpentinization rate exceeds 10 kg s hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation given the uncertainty of several parameters the current episode of venting at the lost city vent field on the mid atlantic ridge may range from 102 to 104 years

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

diffuse impact of the mid atlantic ridge with the romanche transform an ultracold ridge transform intersection the romanche is a long offset 950 km slow slip 1 7 cm yr transform thus a hot ridge axis should meet a 50 m y old thick and cold lithosphere at the ridge transform intersection rti a strong thermal topographic transform cold edge effect is therefore predicted a morphobathymetric seismic reflection and petrologic study of the eastern romanche rti shows that as the mid atlantic ridge approaches the transform a well formed axial rift valley disappears about 80 km from the rti and is substituted by short en echelon poorly developed axial ridge segments they too disappear about 30 km from the edge of the transform valley the predicted gradual deepening of the ridge axis toward the transform was not observed an active nodal deep and an inside corner high are also absent these observations and the distribution of earthquake epicenters suggest a poorly developed diffuse rti an inactive rift valley 80 km west of the present rti suggests ridge jumping within the last 4 m y the present poorly developed rti may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform we infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle derived serpentinized peridotites outcrop ubiquitously on the seafloor starting 30 km from the edge of the transform valley the unusually deep 4 km below sea level axial ridge segments the lack of crust and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform these results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold rtis and that ultracold rtis are magma starved short lived and unstable in time and space

a sensitivity study of the dense shelf water formation in the okhotsk sea 1 sensitivity of dense shelf water dsw formation to tidal mixing around the kuril straits wind stress and river runoff in the okhotsk sea is examined by an ice ocean coupled model horizontal resolution of the model is set to 3 8 km in the northern okhotsk sea for well resolving the coastal polynyas which is believed to be the principal region of dsw formation the model shows a good performance in terms of sea ice production and the consequent dsw formation dsw is also found to be formed in the offshore region apart from the coastal polynya dsw is defined independently for each experiment such that it identifies the water influenced by brine by introducing such definition the sensitivity of the dsw formation is assessed separately for change of density and that of formation rate the density of dsw exhibits high sensitivity to all the elements considered herein while its formation rate is sensitive only to winds winds affect the dsw formation rate mainly by influencing that occurs in the offshore region

seismic evidence for a magma chamber beneath the slow spreading mid atlantic ridge seismic reflections from magma chambers have been observed along the fast spreading east pacific rise1 2 and the intermediate spreading valu fa ridge3 4 sub axial reflections also exist beneath the intermediate spreading juan de fuca ridge5 but no magma chambers have been identified beneath the slow spreading mid atlantic ridge suggesting that here magma chambers lie unusually deep or are transient features6 11 seismic reflection profiles acquired in 1989 over the snake pit hydrothermal area in the rift valley of the mid atlantic ridge 25 km south of the kane fracture zone showed no evidence of magmatic activity12 although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1a 2 km13 14 by suppressing in these data high amplitude coherent noise generated at the sea floor i have obtained images in an otherwise non reflective crust of seismic reflections beneath and just south of the snake pit hydrothermal area these reflections define a small 4 km wide dome whose apex is 1 200 m beneath the sea floor as bright reflections from the upper flanks of this dome occur in the depth range suggested by the vent fluid geochemistry i interpret the dome to be the seismic expression of a small magma chamber

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

evolution of the snow lake portion of the palaeoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada abstract understanding growth of early precambrian crust requires knowledge of the evolution of granite greenstone and metasedimentary belts two of the fundamental building blocks of early precambrian cratons u pb geochronologic data from the snow lake area which straddles the flin flon granite greenstone and kisseynew metasedimentary belts of the trans hudson orogen documents a 60 70 ma period of palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding archaean cratons earliest volcanic and plutonic rocks of the flin flon granite greenstone terrane at snow lake formed 1 89 ga mainly in a juvenile oceanic arc tectonic setting sedimentary rocks of the kisseynew belt previously thought to be contemporaneous with snow lake area arc volcanism are up to 30 50 ma younger 1 86 1 84 ga and postdate deformation associated with 1 88 1 87 ga arc arc collisions and 1 87 1 84 ga calc alkaline plutonism that affected much of the western flin flon belt deformation at snow lake occurred during two widely spaced episodes both of which effect the post 1 86 1 84 ga turbidite sequence and thus postdate 1 88 1 87 ga intraoceanic accretion in the western flin flon belt the older deformational episode involved isoclinal folding formed during 1 84 ga convergence of the kisseynew and flin flon belts and thrusting of kisseynew sedimentary lithologies southwards onto the flin flon volcanic rocks the younger deformational episode occurred 15 25 ma later and involved further folding and southwest thrusting accompanied by a 1 81 ga high grade thermal metamorphic episode centred in the kisseynew belt only minor increase of crustal material in the form of late intrusions were added following this episode

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

interactions between magma chambers and hydrothermal systems oceanic and ophiolitic constraints a compilation of published exit temperatures and salinities for seafloor hydrothermal fluids along with fluid inclusion data on oceanic and ophiolitic rocks shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300 360 c thus clearly being in the one phase field they overlie a lower system marked by the generation of highly saline brines at high temperatures the brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers in ophiolites the brines typically occur within the transition zone between the sheeted dike complex and the plutonic sequence i e on top of magma chambers this transition zone relatively well known from ophiolite studies oman troodos bay of islands josephine is characterized by mutually intrusive relationships between gabbro plagiogranite and dikes and xenoliths of altered diabase within plagiogranite and gabbro in fast to medium spreading ophiolites oman it is the locus of a major downward decrease in the density of hydrothermal veins and alteration the vein system is strongly anisotropic and shows a well marked preferred vertical along strike orientation the underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins while the overlying sheeted dike complex is densely veined and pervasively altered this zone is marked by major gradients in the physical parameters over short distances temperature several degrees celsius per meter pressure several hundred kilopascals transition from brittle to ductile in steady state systems direct interactions between the magma chamber and the hydrothermal system are restricted to this zone characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks complications to this steady state model resulting from a non steady state magma chamber e g josephine ophiolite are mainly the development of late stage faulting and more pronounced retrogressive alteration

evolution of the paleoproterozoic snow lake arc assemblage and geodynamic setting for associated volcanic hosted massive sulphide deposits flin flon belt manitoba canada the majority of volcanogenic massive sulphide vms deposits at the east end of the paleoproterozoic flin flon greenstone belt occur in the 1 89 ga snow lake arc assemblage vms deposits in this

the hydrothermal plumbing of a serpentinite hosted detachment evidence from the west iberia non volcanic rifted continental margin this study documents the critical role of structurally induced fluid flow during the evolution of the footwall succession to a major low angle normal detachment fault drilled by the ocean drilling program leg 173 site 1068 beneath the southern iberia abyssal plain the fault zone comprises carbonate altered rodingitized and albitized metabasite rich sedimentary breccias and serpentinized mantle peridotites the brittle infrastructure of the detachment consists of mineralized high dilation breccias and meshes of mineralized extensional and shear veins that root into chlorite and serpentine cataclasite and gouge the fault rocks are underlain by cohesive serpentinite that shows kernel textures indicative of volume expansion accompanying serpentinization of peridotite the texture is disrupted and offset by small scale fractures and faults the distribution of serpentine polytypes carbonates fe ni alloys sulfides oxides and other silicate phases varies across the fault zone in patterns consistent with mineralization and replacement from solutions derived from two end member components seawater and ch4 bearing calcium hydroxide enriched hydrothermal solutions the latter form when heated seawater reacts with peridotite to form serpentinite at low water to rock ratios serpentine mineral chemistries indicate that fracture controlled serpentinite recrystallization and replacement occurred at various fo2 asio2 and ca2 conditions in places this also involved mild prograde thermal events the serpentinite also hosts tochilinite valleriite group minerals and aragonite both are interpreted as indicators of sea water incursions into the upper reaches of the detachment to account for the evidence of coeval hydrothermal mineralization and displacements across the detachment we relate hydrothermal discharge to the buffering of high pore fluid pressures by fault slip localized sources of high fluid pressures at depth are attributed to serpentinization of peridotite around the fault that promotes changes to solution mass density exothermic reactions and swelling pressures sealing of the fault between the serpentinization front and the top of the detachment results from hydrothermal mineralization alteration and serpentine gouges hydrothermal discharges from the detachment accompanying shear failure allow for variable mixing between the hydrothermal solutions and seawater and post slip convective draw down of seawater into the detachment it is suggested that the latter may have been limited in duration by ongoing mineralization leading to the restoration of the fault seal concomitant serpentinization around the detachment at depth provides scope for cyclic hydrothermal discharges and fault slip

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

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

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

the economic significance of the bell river complex abitibi subprovince quebec the exposed part of the bell river complex on the northern edge of the abitibi subprovince represents the upper portion of a layered complex with a massive gabbroic main zone being overlain by a magnetite rich gabbroic anorthositic layered zone and a granophyre the cumulate rocks display cryptic variation in mineral and whole rock compositions towards a more evolved signature with stratigraphic height there is a decrease in an content of plagioclase from an73 et an51 and in mg of augite from mg 75 to mg 56 an increase in la yb ratio of whole rocks from 0 5 to 2 9 and a decrease in cr from 770 to 5 ppm and ni from 315 to 45 ppm contents of whole rocks concentrations of platinum group elements pge are mostly below detection limit compositional comparison with the south african bushveld complex shows that the lowest exposed levels of the complex are equivalent to the critical main zone boundary that is in the immediate hanging wall of the pge reefs based on recent geophysical studies showing that the bell river complex is approximately 5 km thick we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme a comparison of compositional data from the bell river complex and associated basalts and rhyolites of the watson lake and wabassee groups suggests that the three suites are cogenetic based on the fact that the numerous important volcanogenic massive sulphide vms deposits in the region are located around the intrusion and the similar age of the intrusion and the volcanic rocks we support earlier workers in suggesting that the intrusion and the vms deposits are genetically linked that is that the bell river complex provided the heat source that drove the hydrothermal fluids responsible for forming the vms deposits la partie expose du complexe de be11 river sur la bordure septentrionale de la sous province de l abitibi represente la portion superieure d un complexe stratifie incluant une zone principale formee d une masse gabbroique recouverte par une zone stratifiee gabbroique anorthositique riche en magnetite et une unite de granophyre les compositions obtenues sur roches totales et sur mineraux des lithologies b cumulats presentent des variations relativement subtiles neanmoins elles portent la signature d une differenciation magmatique plus prononcee en gravissant les niveaux stratigraphiques on observe une diminution de la composante an du plagioclase de an73 et an51 et du mg de l augite de mg 75 a mg 56 un accroissement pour roches totales du rapport la yb de 0 5 a 2 9 et une decroissance pour roches totales des teneurs en cr de 770 a 5 ppm et en ni de 315 a 45 ppm les concentrations des elements du groupe du platine egp sont quasiment sous leur limite de detection la comparaison de ces compositions avec celles du complexe du bushveld d afrique du sud montre que les niveaux exposes a la base du complexe sont equivalents a la frontiere des zones critique principale c est a dire du toit immediat des filons porteurs des egp les etudes geophysiques recentes ont revele que le complexe de bell river a une epaisseur d environ 5 km et a partir de cette donnee nous inferons l existence d une zone ultramafique cachee et nous proposons un site pour entreprendre un programme de forage d exploration une comparaison des compositions obtenues pour le complexe de bell river avec les basaltes et rhyolites associes des groupes de watson lake et de wabassee suggere que les trois suites sont cogenetiques sachant que de nombreux depots majeurs de sulfures massifs volcanogeniques smv dans la region sont localises tout autour d une intrusion et que l intrusion et les roches volcaniques sont sensiblement du meme ge par consequent nous adherons aux interpretations anterieures suggerant que i intrusion et les depots des smv sont lies genetiquement c est a dire que le complexe de bell river represente la source de chaleur qui activait la circulation des fluides hydrothermaux responsables de la formation des depots de smv

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

petrography mineralogy and geochemistry of the nkamouna serpentinite implications for the formation of the cobalt manganese laterite deposit southeast cameroon the nkamouna co mn ni laterite is an advanced exploration project located in southeast cameroon the deposit comprises a deep and complex laterite profile with high co 0 25 wt and mn 1 33 wt contents subeconomic ni grade and a ni co ratio of less than 3 this makes nkamouna an unusual type of laterite deposit as it will be primarily mined for co the co mn mineralization occurs at the interface between an overlying allochthonous felsic rock sourced laterite and a lower in situ serpentinite derived laterite the serpentinite is massive with a nonpseudomorphic texture and consists of two genetically related phases serpentinite i and serpentinite ii serpentinite i is characterized by antigorite magnetite quartz ferritchromite and magnesite and serpentinite ii by antigorite and magnesite both serpentinite types are extremely low in sulfides serpentinite i is the dominant and oldest phase and was derived from a subcontinental mantle pyroxene rich peridotite emplaced in the crust during the pan african orogeny the ni concentration in the serpentinite is variable 0 07 0 30 and heterogeneously distributed partly reflecting the present dominance of pyroxenes in the protolith however ni is also hosted by magnetite and ferritchromite the differential weathering of these ni hosting minerals is also responsible for the lower ni grade of the deposit importantly the serpentinite is depleted in co and mn and the enrichment of these elements in the laterite profile suggests that these elements may be derived from an exotic source

geophysical controls of chemical disequilibria in europa the ocean in jupiter s moon europa may have redox balance similar to earth s on earth low temperature hydration of crustal olivine produces substantial hydrogen comparable to any potential flux from volcanic activity here we compare hydrogen and oxygen production rates of the earth system with fluxes to europa s ocean even without volcanic hydrothermal activity water rock alteration in europa causes hydrogen fluxes 10 times smaller than earth s europa s ocean may have become reducing for a brief epoch for example after a thermal orbital resonance 2 gyr after accretion estimated oxidant flux to europa s ocean is comparable to estimated hydrogen fluxes europa s ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active as evidence of geologic activity and subduction implies

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

downward continued multichannel seismic refraction analysis of atlantis massif oceanic core complex 30 n mid atlantic ridge 1 detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the atlantis massif oceanic core complex occ contrasting notably with its conjugate ridge flank multichannel seismic mcs data are downward continued using the synthetic on bottom experiment sobe method providing unprecedented detail in tomographic models of the p wave velocity structure to subseafloor depths of up to 1 5 km velocities can vary up to 3 km s over several hundred meters and unusually high velocities 5 km s are found immediately beneath the seafloor in key regions correlation within situand dredged rock samples video and records from submersible dives and a 1 415 km drill core allow us to infer dominant lithologies a high velocity body ies found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the occ indicating a propagating magmatic source as the origin for this pluton s the western two thirds of the southern ridge is capped in serpentinite that may extend nearly to the base of our ray coverage the distribution of inferred serpentinite indicates that the gabbroic pluton s was emplaced into a dominantly peridotitic host rock presumably the mantle host rock was later altered via seawater penetration along the detachment zone which controlled development of the occ the asymmetric distribution of seismic velocities and morphology of atlantis massif are consistent with a detachment fault with a component of dip to the southeast the lowest velocities observed atop the eastern central dome and conjugate crust are most likely volcanics here an updated model of the magmatic and extensional faulting processes at atlantis massif is deduced from the seismic results contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow rate spreading centers

kinematic analysis of serpentinite structures and the manifestation of transpression in southwestern puerto rico abstract faults and shear zones recorded in the monte del estado and rio guanajibo serpentinite masses in southwestern puerto rico show previously unrecognized southwestward tectonic transport the orientations of planar and linear structures and the sense of slip along faults and shear zones determined by offset rock layers drag folds in foliations and steps in slickensided surfaces and or s c fabrics from 1846 shear planes studied at more than 300 stations reveal two predominant groups of faults 1 northwesterly striking thrust faults and easterly striking left lateral faults and 2 northwesterly striking right lateral faults and easterly striking thrust faults shortening and extension p and t axes calculated for geographic domains within the serpentinite reveal early north trending shortening followed by southwestward directed movement during which older structures were re activated the sw directed shortening is attributed to transpression that accompanied late eocene left lateral shearing of the serpentinite a third younger group comprising fewer faults consists of northwesterly striking left lateral faults and north directed thrusts that also may be related to the latest transpressional deformation within puerto rico deformational events in puerto rico correlate to tectonic events along the caribbean north american plate boundary

major and minor element mobilization with implications for nicufe sulphide remobilization during retrograde metasomatism at the vammala mine southwest finland abstract granitic melt derived from migmatitic host rocks intruded a d 3 related fracture system in a 1 89 ga ni cu fe sulphide bearing peridotite sheet near vammala southwest finland zones of wall rock alteration mainly comprising amphibole phlogopite chlorite developed in the peridotite adjacent to the leucocratic dykes the minerals in the alteration zones and in the gneissic host rocks together with the production and intrusion of granitic melt are consistent with metamorphic conditions of at least 600 c and 4 5 kbar later hydration under condtions of lower temperature and pressure resulted in partial serpentinization of the peridotite and dykes this later transition precluded direct investigation of the earlier transition that developed the medium to high grade alteration zones in the peridotite at the interface with the leucocratic dykes because serpentinization was largely accommodated by volume increase rather than mass loss of mgo and sio 2 the relative concentrations of most components in the serpentinized peridotite should mirror those in the peridotite based on this contention two methods are used to document the composition volume behaviour of the transition from now serpentinized peridotite to alteration zones apparent transition and hence infer the mass balance trends of the igneous peridotite to alteration zone transition real transition the methods collectively suggest that in the alteration assemblages k 2 o rb 2 o p 2 o 5 tio 2 and al 2 o 3 form a group of relatively enriched components and that al 2 o 3 was particularly mobile an estimated volume increase of 10 10 accompanied serpentinization and a calculated volume decrease of 10 15 accompanied the apparent transition thus a volume increase of real transition for best fit volume factors gresens equation demonstrates that the apparent transition involved mass gains in sio 2 al 2 o 3 k 2 o p 2 o 5 and tio 2 and mass losses in feo mgo cao ni s and volatiles loss on ignition loi these amounting to net losses of 32 5 kg of volatiles and 10 8 kg of other components per 1000 kg of serpentinized peridotite the real transition involved similar relative gains and losses except for sio 2 gain slightly less mgo loss slightly more and loi loss negligible it respectively records inward transfer of components from a feldspar rich source to the peridotite and outward transfer to that source this is consistent with metasomatic exchange between the peridotite and the probably supra solidus granitic intrusions during the real exchange ni cu fe sulphides were retained and redistributed internally remobilized in the outer alteration zones but transferred into the dyke from the zone abutting it externally remobilized the same genetic interpretations probably apply to similar dyke and alteration systems in other ni cu fe sulphide deposits hosted in ultramafic rocks in southwest finland

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

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

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

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

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

regional stable isotope systematics of hydrothermal alteration and massive sulfide deposition in the west shasta district california the west shasta cu zn district is located in california at the southern end of the klamath mountains and contains numerous pyrite dominant volcanogenic massive sulfide deposits the deposits are hosted in a bimodal suite of intrusive and extrusive basalt to andesite copley greenstone and rhyolite balaklala rhyolite of devonian age regional variations in oxygen hydrogen and sulfur isotope ratios in the copley greenstone balaklala rhyolite and mule mountain trondhjemite record hydrothermal alteration which took place over a large range in temperature from 18 o values vary for the copley greenstone from 3 5 to 11 6 per mil and for the balaklala rhyolite from 7 6 to 13 3 per mil delta d values of the copley range from 71 to 42 per mil and for most samples of the balaklala from 73 to 46 per mil the isotopic variation reflects changing mineralogy in addition to a range of alteration temperatures with progressive chloritization the water content of the copley greenstone increases from 1 51 to 8 34 wt percent h 2 o whereas the delta 18 o values decrease water rock mass ratios calculated from the oxygen isotope data are 1 8 to 2 8 for high temperature alteration regional variations in delta d and delta 18 o values of the copley greenstone indicate higher temperatures of alteration and greater water rock interaction adjacent to the mule mountain stock beneath the area of the iron mountain mine largest of the massive sulfide deposits this suggests that the stock is the heat source responsible for the iron mountain deposit the alteration pattern associated with the stock and the low temperature isotopic alteration of the capping porphyry upper unit of the balaklala indicate that the mule mountain stock was coeval with the middle unit of the balaklala estimates of the oxygen isotope compositions of the balaklala and mule mountain magmas delta 18 o approximately 6 5ppm from analyses of quartz phenocrysts suggest that the two magmas or magma types are consanguineous and were derived from the same source as the copley greenstone sulfur isotope data support this interpretation there is no clear evidence for temporal variations in the isotopic composition of the magmas mass balance considerations and data on the isotopic composition and content of sulfur in the copley greenstone most 95 percent of the sulfur in the ore deposits could have been leached from the copley greenstone during hydrothermal alteration the variation in delta 34 s values of whole rock samples is larger for the copley 4 0 to 22 4ppm most 0 0 to 6 0ppm than for either the balaklala 3 3 to 4 9ppm or the massive sulfide deposits 0 5 to 6 0ppm but most 3 0 to 5 0ppm no correlation is found between the abundance and isotopic composition of sulfur in the copley greenstone isotopic fractionation effects during transport and local reprecipitation of leached sulfur can explain the sulfur isotope variability of the copley greenstone sulfur isotope data also support degassing of the mule mountain or related magmas as a sulfur source

natural deformation related to serpentinisation of an ultramafic inclusion within a continental shear zone the key role of fluids abstract the role of fluids in the deformation of continental serpentinites is investigated from structural microstructural and petrographic analyses applied to a km scale porphyroclast mantled in a viscous matrix of amphibolites the clast is sited within a shear zone of the palaeozoic maures massif france syntectonic fluid rock interactions occurred from km to mm scales at first on the clast borders along the main rheological boundaries then within the clast they are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack seal veins typifying an intermediate brittle ductile behaviour of serpentinites three main stages of deformation serpentinisation processes occurred in relation with the left lateral movement of the hosted shear zone they developed under metamorphic conditions evolving from amphibolites to green schists facies conditions 400 mpa 550 c to 200 mpa

zircon thermometry of high temperature rhyolites near volcanic associated massive sulfide deposits abitibi subprovince canada high temperature high silica rhyolites are commonly found near cu zn volcanic associated massive sulfide vms deposits in the archean abitibi subprovince they are characterized by anhydrous phenocrysts feldspar quartz in concentrations of generally 73 sio 2 high incompatible element contents rb sr 1 zr y

the role of reacting solution and temperature on compositional evolution during harzburgite alteration constraints from the mesoarchean nuasahi massif eastern india abstract we investigate the microtextural chemical features of partially serpentinized harzburgites from the lower ultramafic unit of the mesoarchean nuasahi massif eastern india in order to understand the role of reacting fluid composition and temperature on the phase evolution across replacement interfaces during progressive alteration two distinct types of pseudomorphic replacement textures are identified type 1 replacement texture was developed after primary orthopyroxene and is composed of talc olivine lizardite tremolite magnetite primary olivine was replaced by mesh textured mg rich lizardite magnetite at the center of the olivine grains and successive layers of relatively fe rich lizardite magnesite and calcite toward olivine rims defining type 2 replacement texture the nuasahi harzburgite was initially out of equilibrium with respect to h 2 o co 2 bearing reacting solution and the secondary compositions have mainly evolved in the cao mgo feo sio 2 al 2 o 3 h 2 o co 2 system as a result of fluid rock interaction the alteration process across orthopyroxene interfaces has started at relatively higher temperature conditions 400 h 2 o c o 2 aq ratio in the solution across replacement interfaces the fe 2 mg 1 chemical exchange potential of the equilibrating system plays an important role in dictating the fe mg ratio of the secondary compositions and the molar proportions of magnetite the precipitation of tremolite and calcite in some isolated areas reflects a variation in ca activity in the reacting fluid the precipitation of carbonates may also be associated with an increase in ph in the interfacial solution the pattern in phase evolution across type 1 interfaces suggests a greater extent of diffusion of solutes toward inner interfaces relative to the rim center during progressive alteration of the nuasahi harzburgite possibly due to an increase in permeability in replaced phases with increasing magnitude of diffusion toward type 1 inner reacting interfaces the widespread presence of arrested replacement features suggests an alteration event under low fluid rock ratio and or sluggish reaction kinetics with decreasing temperature

magma traps and driving pressure consequences for pluton shape and emplacement in an extensional regime abstract the level of emplacement and final form of felsic and mafic igneous rocks of the wichita mountains igneous province southwestern oklahoma u s a are discussed in light of magma driving pressure lithostatic load and crustal magma traps deposition of voluminous a type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma intruded along this crustal magma trap are the a type sheet granites length thickness 100 1 of the wichita granite group of which the mount scott granite sheet is typical and smaller plutons of biotite bearing roosevelt gabbro in marked contrast to the subhorizontal granite sheets the gabbro plutons form more equant stocks with flat roofs and steep side walls late diabase dikes cross cut all other units but accompanying basaltic flows are extremely rare in the volcanic pile based on magmastatic calculations we draw the following conclusions concerning the level of emplacement and the shape of these intrusions 1 magma can rise to a depth at which the magma driving pressure becomes negligible magma that maintains a positive driving pressure at the surface has the potential to erupt 2 magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy i e crustal magma trap if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy 3 subhorizontal sheet intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load under such conditions the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion 4 thicker steep sided stocks or batholiths with flat roofs form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load under these conditions the necessary space for the intrusion must be created by other mechanisms e g stoping 5 subvertical sheets i e dikes form when the magma driving pressure is less than the lithostatic load at the level of emplacement

kinematic history of a reactivated oceanic suture the mamonia complex suture zone sw cyprus the mamonia complex suture zone in sw cyprus marks an arcuate tectonic suture between an upper cretaceous oceanic microplate of supra subduction zone origin troodos complex and a deformed triassic passive margin sequence mamonia complex mapping of the serpentinite filled fault zones that define the suture zone combined with microstructural studies of its constituent fault rocks has revealed evidence of three main tectonic events of upper cretaceous age 1 high temperature c 600 c transform related dextral strike slip and localized metamorphism of mamonia complex units c 90 83 ma 2 retrograde hydration c 83 73 ma 3 regional contractional reactivation at low temperatures which ceased by the latest maastrichtian c 65 ma the present day structural geometries are interpreted as the result of maastrichtian contractional reactivation of a pre existing steep and irregular structural architecture that formed during transtension the resultant orientation of finite strain axes and thrust displacements display a radial distribution documenting near fault normal contraction along the whole length of the arcuate suture zone this pattern of displacements suggests a model whereby the troodos microplate impinged on the mamonia complex during westerly to southwesterly directed plate motion contractionally reactivating the mechanically weak suture zone