diffusion of ti and some divalent cations in olivine as a function of temperature oxygen fugacity chemical potentials and crystal orientation the diffusion of titanium along with ni co and mn in pure synthetic forsterite has been studied as a function of temperature oxygen fugacity crystallographic orientation and chemical potentials in the four component system mgo sio2 tio2 tio1 5 in over 100 different experimental conditions no ti diffusion profiles with the classical shape of the error function were observed instead the profiles vary between stepped at high fo 2 probably owing to diffusion on octahedral sites and trapping in tetrahedral sites to hockey stick shapes at low fo 2 owing to concentration dependent diffusion the profile shapes vary systematically between these end members according to oxygen fugacity these profile shapes are also reproduced in experiments using natural san carlos olivine the change in profile shape is interpreted to be a function of valence state change from ti4 to ti3 shown by a sigmoidal log fo 2 diffusivity relationship to distinguish between ti4 to ti3 in forsterite a diffusion profile was also investigated using hydroxylation spectroscopy whereby a crystal from a diffusion experiment was hydroxylated by annealing in an h2o rich fluid at moderate pressure and temperature the point defects associated with the added structural oh were then determined by infrared spectroscopy providing evidence for the presence of ti4 to ti3 at intermediate oxygen fugacity the transition from ti4 to ti3 occurs at considerably higher fo 2 in these diffusion experiments than in equilibrium experiments in similar systems titanium diffusion qualified using a mobility parameter m is fastest along the c axis at high activity of silica low oxygen fugacity and high temperature the rate of ti diffusion is broadly similar to those of mn ni and co and closer to published rates ofmg self diffusion in olivine than si self diffusion the observed ti4 to ti3 diffusion occurs on the m sites from statistical examination of the large mn ni and co diffusivity dataset we determine that there is a 10 2 3 dependence of diffusion on asio 2 the cations diffuse more rapidly at high asio 2 in addition diffusivity has a 10 1 8 dependence on fo 2 suggested to be the result of increased concentration of loosely bound ti3 vacancy pairs which enhance the mobility of other cations this behaviour is likely to be present in natural systems where trivalent e g fe3 al3 cr3 and divalent cations diffuse together consideration should be given to diffusive interference by highly charged fast moving cations when extracting time scales from frozen diffusion profiles

accuracy of timescales retrieved from diffusion modeling in olivine a 3d perspective diffusion modeling in olivine is a useful tool to resolve the timescales of various magmatic processes practical olivine geospeedometry applications employ 1d chemical transects across sections that are randomly sampled from a given 3d crystal population but the accuracy and precision with which timescales can be retrieved from this procedure are not well constrained here we use numerical 3d diffusion models of fe mg to evaluate and quantify the uncertainties associated with their 1d counterparts the 3d diffusion models were built using both simple and realistic olivine morphologies and incorporate diffusion anisotropy as well as different zoning styles the 3d model crystals were sectioned along ideal or random planes which were used to perform 1d models and timescale comparisons results show that the timescales retrieved from 1d profiles are highly inaccurate and can vary by factors of 0 1 25 if diffusion anisotropy is not taken into account even when anisotropy is corrected for timescales can still vary between 0 2 10 times the true 3d diffusion time due to crystal shape and sectioning effects simple grain selection procedures are described to reduce the misfit between calculated and actual diffusion times and achieve an accuracy and precision of 5 and 15 25 relative respectively provided that the grains are carefully selected about 20 concentration profiles and associated 1d models suffice to achieve this accuracy

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

the rate of oceanic detachment faulting at atlantis bank sw indian ridge abstract the rates of slip on oceanic detachment faults and how those rates compare to sea floor spreading rates constitute fundamental data required to constrain how oceanic core complexes form and their role during crustal accretion we combine sea surface magnetic data with the magnetic polarity of shallow core samples and pb u shrimp ages of igneous zircon to determine the time averaged half spreading rate during oceanic detachment faulting at atlantis bank 100 km south of the ultraslow spreading southwest indian ridge swir the pb u zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 myr creating and exposing a 1 5 km thick layer of gabbro for 35 km parallel to spreading we use bivariate linear regression of distance age data and forward modeling of magnetic anomaly data to calculate a half spreading rate during detachment faulting of 14 1 1 8 1 5 km myr 95 confidence limits when integrated with regional constraints on spreading history we note that detachment faulting coincided with a short lived regional increase in the full spreading rate along the swir and for the ridge segment containing atlantis bank spreading was highly asymmetric with 80 of plate motion accommodated by detachment faulting consequently the detachment fault effectively formed the plate boundary at the surface in this spreading segment highly asymmetric spreading was confined to the spreading segment containing atlantis bank and to the duration of detachment faulting so the ridge segment containing atlantis bank migrated northward relative to its symmetrically spreading eastern neighbour such that the intervening non transform discontinuity shortened we suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting an inference supported by more poorly constrained half spreading rates determined at several other oceanic core complexes

unstable behaviour of sand and its implication for slope instability results of some constant shear drained csd tests conducted on both loose and dense sand are presented using the critical state line and a modified state parameter a new framework for analysing the instability of granular soil slopes is proposed the test data were examined and interpreted using the new framework instability lines for sand with different void ratios were established within this framework the conditions for the occurrence of instability in both contractive and dilative granular soil slopes under various shear stress levels were examined using the proposed framework key words deformation laboratory tests liquefaction sands shear strength slope stability

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

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

a revised subduction inception model to explain the late cretaceous double vergent orogen in the precollisional western tethys evidence from the northern apennines the meso cenozoic alpine belts of the mediterranean area are characterized by complex architectures result of a complex subduction and collision evolution that preserve also a legacy of the rifting related configuration of the continental margins the northern apennines is a segment of these belts originated during closure of the ligure piemontese ocean and collision between the europe and adria plates the different configuration of the adria and europe margins inherited from asymmetric rifting is recorded in the ligurian units that preserve incorporation into the subduction factory of fragments of the oceanic domain internal ligurian units and portions of the ocean continent transition zone octz toward adria external ligurian units we provide here unpublished data on the stratigraphy and sedimentology of these units together with a review of what is already established in literature both datasets combined testify that at 80 m a an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm the continental crust and the subcontinental mantle we propose that closure of the ligure piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned adria margin and developed a double vergent prism by accreting oceanic material and continental extensional allochthons from the octz we believe the revised site of subduction initiation and the pre collisional architecture inherited from the rifting and spreading phases allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre collisional evolution of the apennines

drilling constraints on lithospheric accretion and evolution at atlantis massif mid atlantic ridge 30 n expeditions 304 and 305 of the integrated ocean drilling program cored and logged a 1 4 km section of the domal core of atlantis massif postdrilling research results summarized here constrain the structure and lithology of the central dome of this oceanic core complex the dominantly gabbroic sequence recovered contrasts with predrilling predictions application of the ground truth in subsequent geophysical processing has produced self consistent models for the central dome the presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100 220 kyr and not as a single magma pulse isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth below 800 m alteration is restricted to narrow zones surrounding faults veins igneous contacts and to an interval of locally intense serpentinization in olivine rich troctolite hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range deformation of the sequence was remarkably localized despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation presumably during unroofing via detachment faulting both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent southern ridge of the massif there the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70 of recovered samples we develop a working model of the evolution of atlantis massif over the past 2 myr outlining several stages that could explain the observed similarities and differences between the central dome and the southern ridge

extremely asymmetric magmatic accretion of oceanic crust at the ends of slow spreading ridge segments we report the results of a deep towed magnetic survey of part of a mid atlantic ridge spreading segment analysis of the magnetic reversals indicates that for the past 0 7 m y magmatic accretion at the end of the segment has been effectively one sided with new crust being added only to the outside corner of the ridge offset eastern flank and not to the inside corner western flank spreading on the inside corner was accommodated by significant displacement on a single large fault the area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process in the short term asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner western edge of the old one asymmetric spreading is unlikely to be sustainable as a steady state process the termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner eastern plate thus isolating the old axial volcanic ridge on the inside corner plate or by simply arresting movement on the large fault and reverting to symmetric spreading at the axial volcanic ridge highly asymmetric accretion may be a common process at slow spreading segments particularly near discontinuities this asymmetry cannot be maintained for long periods and may be directly linked to intervals of spreading by tectonic extension

extended planar defects and the rapid incorporation of ti 4 into olivine the formation of extended planar defects in minerals such as olivine is related to high point defect concentration and can be driven by large gradients in chemical potential where the energy of the system is lowered by the ordering of defects along specific planes in the crystal the presence of extended defects has the potential to create the apparently anomalous ionic diffu sion in olivine as reported recently spandler and o neill in contrib mineral petrol 159 6 791 818 2010 high resolution transmission electron microscopy and energy filtered imaging were done using experimental samples designed to examine the impact of atio2 and fo2 on the potential to form such defects in ferromagnesian olivine doped basalt 5 wt tio2 olivine reaction couple experiments were run at 1 atm and 1 310 and 1 410 c for 50 h at various fo2 ranging from 10 2 below to 10 2 above the quartz fayalite magnetite buffer our results show that extended planar defects in olivine parallel to 101 ol and occurring in ordered clusters with a prolate spheroid geometry 5 25 nm across and extending up to 150 nm into the olivine are present near the olivine glass inter faces in all of our experimental high tio2 basalt olivine samples increased ti content in the olivine is associated with the defects ordering of ti 4 and octahedral site vacancies leads to a two or three layer superstructure in the olivine defect nucleation and growth is driven by the large tio2 chemical potential gradient across the phase boundary at the start of the experiments which provides access to microstructures not otherwise present

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

timescales of mixing and mobilisation in the bishop tuff magma body perspectives from diffusion chronometry we present two feldspar thermometry and diffusion chronometry from sanidine orthopyroxene and quartz from multiple samples of the bishop tuff california to constrain the temperature stratification within the pre eruptive magma body and the timescales of magma mixing prior to its evacuation two feldspar thermometry yields estimates that agree well with previous fe ti oxide thermometry and gives a 80 c temperature difference between the earlier and later erupted regions of the magma chamber using the thermometry results we model diffusion of ti in quartz and ba and sr in sanidine as well as fe mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases diffusion profiles of ti in quartz and fe mg in orthopyroxene both yield timescales of 150 years for the formation of overgrowth rims in contrast both ba and sr diffusion in sanidine yield nominal timescales 1 2 orders of magnitude longer than these two methods the main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for ba and sr diffusion modelling i e growth zoning exists utilising the divergent diffusion behaviour of ba and sr we place constraints on the initial width of the interface and can refine our initial conditions considerably bringing ba and sr data into alignment and yielding timescales closer to 500 years the majority of which are then within uncertainty of timescales modelled from ti diffusion in quartz care must be thus taken when using ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the bishop tuff magma chamber at least 500 years prior to eruption timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1 2 orders of magnitude or that the minerals concerned did not experience a common history despite being extracted from the same single pumice clasts introduction of the magma initiating crystallisation of the contrasting rims on sanidine quartz orthopyroxene and zircon was prolonged and may be a marker of other processes that initiated the bishop tuff eruption rather than the trigger itself

constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the oman ophiolite we present major and trace element as well as sr nd and hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the oman ophiolite the rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt sio2 i e from gabbros to tonalites the samples are grouped into a ti rich and relatively light rare earth element lree enriched p1 group ce yb n 0 7 resembling the early v1 lavas and a ti poor and lree depleted p2 group ce yb n 0 7 resembling the late stage v2 lavas based on the geochemical differences and in agreement with previous structural and petrographic models we define phase 1 p1 and phase 2 p2 plutonic rocks felsic magmas in both groups formed by extensive fractional crystallization of olivine clinopyroxene plagioclase apatite and ti magnetite from mafic melts the incompatible element compositions of p1 rocks overlap with those from mid ocean ridges but have higher ba nb and th nb trending towards the p2 rock compositions and indicating an influence of a subducting slab the p2 rocks formed from a more depleted mantle source but show a more pronounced slab signature these rocks also occur in the southern blocks with the exception of the tayin block of the oman ophiolite implying that the entire ophiolite formed above a subducting slab initial nd and hf isotope compositions suggest an indian morb type mantle source for the oman ophiolite magmas isotope compositions and high th nb in some p2 rocks indicate mixing of a melt from subducted sediment into this mantle

formation of fast spreading lower oceanic crust as revealed by a new mg ree coupled geospeedometer abstract a new geospeedometer is developed based on the differential closures of mg and rare earth element ree bulk diffusion between coexisting plagioclase and clinopyroxene by coupling the two elements with distinct bulk closure temperatures this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples as the existing mg exchange thermometer was calibrated for a narrow temperature range and strongly relies on model dependent silica activities a new thermometer is developed using literature experimental data when the bulk closure temperatures of mg and ree are determined respectively using this new mg exchange thermometer and the existing ree exchange thermometer this speedometer can be implemented for a wide range of compositions mineral modes and grain sizes applications of this new geospeedometer to oceanic gabbros from the fast spreading east pacific rise at hess deep reveal that the lower oceanic crust crystallized at temperatures of 998 1353 c with cooling rates of 0 003 10 2 c yr stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies together with existing petrological geochemical and geophysical evidence results from this new speedometry suggest that the lower crust formation at fast spreading mid ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal scale hydrothermal circulations the replenished melts become chemically and thermally evolved accumulate as small magma bodies at various depths feed the shallow axial magma chamber and may also escape from the mush zone to generate off axial magma lenses

oceanic detachment faults generate compression in extension in extensional geologic systems such as mid ocean ridges deformation is typically accommodated by slip on normal faults where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion however at slowly spreading mid ocean ridges where the tectonic plates move apart at rates 244 000 events detected along 25 km of the ridge axis to depths of 10 km below seafloor surprisingly the majority of these were reverse faulting events restricted to depths of 3 7 km below seafloor these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events this deformation pattern is consistent with flexural models of plate bending during lithospheric accretion our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor these compressive stresses trigger reverse faulting even though the detachment itself is an extensional system

a geophysical study of oceanic core complexes and surrounding terrain mid atlantic ridge 13 n 14 n 1 we describe a geophysical study of oceanic core complexes occ and surrounding seafloor on the mid atlantic ridge at 13 n 14 n and off axis to 1 9 myr data include a detailed deep towed side scan sonar magnetic field and bathymetry survey supplemented by concurrent sea surface bathymetry magnetic field and gravity measurements using side scan and bathymetry we infer areas and relative ages of seafloor volcanism revealing a complex pattern of melt accretion across the median valley including close to its walls we estimate tectonic and magmatic extension throughout the area and find that average tectonic extension since chron 2 on plates containing occs is up to three times that on their conjugates deep towed magnetic data reveal asymmetric spreading faster on occ containing plates and crustal magnetization that is highly heterogeneous on a scale of 5 km suggesting that exhumed domes of occs have highly variable lithologies perhaps comprising both serpentinized peridotite and gabbro improved fits to magnetic data are provided by models incorporating 45 of occ footwall rotation an axial zone of normal magnetization of presumed brunhes epoch has highly variable width and amplitude with parts of the ridge axis displaying very low or apparently reversed magnetization gravity requires that occs have dense cores capped by lower density zones several kilometers thick gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of occ formation and show that waxing magmatism may terminate occs

a detrital record of lower oceanic crust exhumation within a miocene slow spreading ridge macquarie island southern ocean uplift exhumation and denudation of the lower oceanic crust are recorded by sedimentary rocks of macquarie island 54 30 s 158 54 e which were deposited within the slow spreading proto macquarie spreading ridge between ca 9 and 12 ma measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid ocean ridge basalt e morb with thin intercalations of gabbroic sedimentary lithofacies basaltic detritus has zeolite to lower greenschist metamorphic grades typical of the upper oceanic crust and gabbroic detritus has upper greenschist to amphibolite metamorphic grades typical of the lower oceanic crust breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading related faults sedimentary structures detrital clinopyroxene major and trace element geochemistry and 206 pb 238 u zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a paleogene aged tholeiitic morb source detrital zircon populations of ca 27 and ca 33 ma correspond to oceanic magnetic anomalies 8o and 13o respectively and exclude ca 8 5 ma gabbroic rocks of macquarie island as a potential source geodynamic reconstructions show that anomaly 8o crust from the southeast indian ridge was juxtaposed against the active proto macquarie spreading ridge when sedimentary rocks of macquarie island were deposited and was a likely source for the gabbroic lithofacies the proto macquarie spreading ridge and southeast indian ridge were connected by the jurru long offset transform which has undergone significant transpression since 27 ma this transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged paleogene source crust which provided the source for macquarie island9s gabbroic sedimentary lithofacies

parental magmas of the nain plutonic suite anorthosites and mafic cumulates a trace element modelling approach equilibrium melt trace element contents are calculated from proterozoic nain plutonic suite nps mafic and anorthositic cumulates and from plagioclase and orthopyroxene megacrysts assumed trapped melt fractions tmf 20 generally eliminate all minor phases in most mafic cumulate rocks reducing them to mixtures of feldspar pyroxene and olivine which would represent the high temperature cumulus assemblage in anorthosites tmf 15 generally reduce the mode to a feldspar only assemblage all model melts have trace element profiles enriched in highly incompatible elements relative to normal mid ocean ridge basalt nmorb commonly with negative nb and th anomalies most mafic cumulates yield similar profiles with constant incompatible element ratios and can be linked through fractional crystallization high k la subtypes probably represent crust contaminated facies mafic cumulates are inferred to belong to a tholeiitic differentiation series variably contaminated by upper and lower crustal components and probably related to coeval tholeiitic basaltic dyke swarms and lavas in labrador model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates indicating that mafic cumulates and anorthosites did not crystallize from the same melts orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts precluding an origin from parental melts jotunites have lower k rb ba y yb and higher la ce than model residues from fractionation of anorthositic model melts suggesting they are not cosanguineous with them but provide reasonable fits to evolved mafic cumulate model melts incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites with steep y yb lu segments that suggest residual garnet in the source inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust the similarity of the highly incompatible trace elements and lile between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites the reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase and remobilization of these hybrid plagioclase rich mushes then produced the anorthosite massifs

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

evidence for the protracted construction of slow spread oceanic crust by small magmatic injections abstract gabbroic cumulates drilled south of the kane transform fault on the slow spread mid atlantic ridge preserve up to three discrete magnetization components here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust by comparing the paleomagnetic data with mineral compositions and based on thermal models of local reheating we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter scale sills some of these magmas were crystal laden and the subsequent expulsion of interstitial liquid from them produced cumulus sills these small scale magmatic injections took place over at least 210 000 years and at distances of 3 km from the ridge axis and may have formed much of the lower crust this model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow spread ridges

baddeleyite zro2 and zircon zrsio4 from anorthositic rocks of the laramie anorthosite complex wyoming petrologic consequences and u pb ages

3 d p wave velocity structure of oceanic core complexes at 13 n on the mid atlantic ridge the mid atlantic ridge at 13 n is regarded as a type locality for oceanic core complexes occs as it contains within 70 km along the spreading axis four that are at different stages of their life cycle the wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of occ development here we describe the results of p wave seismic tomographic modelling within a 60 60 km footprint containing several occs the ridge axis and both flanks which determines occ crustal structure detachment geometry and occ interconnectivity along axis a grid of wide angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean bottom seismographs was deployed approximately 130 000 first arrival travel times together with sparse moho reflections have been modelled constraining the crust and uppermost mantle to a depth of 10 km below sea level depth slices through this 3 d model reveal several independent structures each with a higher p wave velocity vp than its surrounds at the seafloor these features correspond to the occs adjacent to the axial valley walls at 13 20 n and 13 30 n and off axis at 13 25 n these high vp features display dipping trends into the deeper crust consistent with the surface expression of each occ s detachment implying that rocks of the mid to lower crust and uppermost mantle within the footwall are juxtaposed against lower vp material in the hanging wall the neovolcanic zone of the ridge axis has systematically lower vp than the surrounding crust at all depths and is wider between occs on average throughout the 13 n region the crust is 6 km thick however beneath a deep lava floored basin between axial occs the crust is thinner and is more characteristically oceanic in layering and velocity depth structure thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation while modelling of the sparse moho reflections suggests the crust mantle boundary is a transition zone throughout most of the 13 n segment our results support a model in which occs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each occ suggesting a geometry and mechanism of faulting that is more complicated than previously thought the steepness of the northern flank of the 13 20 n detachment suggests that it represents a transfer zone between different faulting regimes to the south and north we propose that individual detachments may not be linked along axis and that occs act as transfer zones linking areas of normal spreading and detachment faulting along ridge variation in magma supply influences the nature of this detachment faulting consequently not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive but also how when and where new occs are created

constraints on crustal structure of adjacent occs and segment boundaries at 13 n on the mid atlantic ridge the 13 n segment of the mid atlantic ridge is an example of a morphologically well studied slow spreading ridge segment populated with oceanic core complexes occs in this paper we present the results of a 200 km long 2d seismic and gravity transect through this segment the bounding fracture zones to the south and the ridge discontinuity to the north we use this transect to consider the two end member models of occ evolution in which one referred to as the segment scale model implies they are interconnected with their detachments being part of a single segment long feature and the other the localised model that each occ is structurally isolated we show using the 7 5 km s 1 velocity contour as the base of crust marker that the crust is consistently relatively thin ridge parallel at 5 km thick on average and that beneath the occs the moho marks the top of a velocity gradient transition into the mantle rather than a distinct velocity discontinuity although each occ is not traversed in an identical structural location they show a different crustal velocity density structure with depth with along axis variations in this structure mirrored by the bathymetric deeps between them older occs have a contrasting velocity depth signature to the currently active 13 20 n occ the 13 20 n occ is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours while the 13 30 n occ has an apparently thinner crust our combined p wave seismic travel time tomography and gravity forward modelling suggests that the occs of the 13 n segment are not interconnected at depth to the north of the 13 30 n occ our modelling also suggests that the crust is being magmatically refreshed or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge the profile also crosses the marathon and mercurius fracture zones that mark the southern limit of the 13 n segment and the southern ridge transform intersection outside corner along profile marathon fracture zone offsets younger 1 my vs 8 my oceanic crust than mercurius fracture zone 8 my vs 11 my when considered in combination both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of marathon fracture zone coupled with a region of low density that most likely reflects serpentinisation of the uppermost mantle in addition the crust captured between fracture zones appears relatively rotated about an e w axis and uplifted to the north with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre both the outside corner and the crust bounded by fracture zones have velocity depth characteristics similar to that of the 13 n segment occs rather than normally accreted oceanic crust particularly in the upper to middle crust overall our results support the localised model of occ evolution and suggest that fracture zones do not become locked immediately on transform to fracture transition as current models dictate

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

zircon dating of oceanic crustal accretion most of earth s present day crust formed at mid ocean ridges high precision uranium lead dating of zircons in gabbros from the vema fracture zone on the mid atlantic ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery combined with results from previous dating studies this finding suggests that two distinct modes of crustal accretion occur along slow spreading ridges individual samples record a zircon date range of 90 000 to 235 000 years which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

fossil oceanic core complexes recognized in the blueschist metaophiolites of western alps and corsica tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks by contrast tothe complete mantle crustal sections of semail type ophiolite sheets tethyan ophiolites are characterized by asmaller amount of mafic rocks gabbros and basalts by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle derived peridotites and associatedgabbroic intrusions therefore they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean continent transition at distal passive margins in the very first models offormation of the tethyan ophiolites in the years 1980 the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present dayoceans in particular at that time little work had focused on the distribution origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics here we reconsider the geology of sometypical metaophiolites from the western alps and corsica and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the tethyan ophiolites in general revisitedexamples include a set of representative metaophiolites from the blueschists units of the western alps queyrasregion and from alpine corsica golo valley field relationships between the ophiolitic basement andthe metasedimentary metavolcanic oceanic cover are described outlining a typical character of the tethyanophiolite lithological associations jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle gabbo basement in a way strictly similar to what is observed in the non metamorphicappennine ophiolites or chenaillet massif this confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction this juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of oceanic core complexes occs at the axis of slow spreading ridges this fundamental plate tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries in addition fieldrelationships between the discontinuous basaltic formations and the ultramafic mafic basement indicate that thistectonic stage is followed or not by a volcanic stage we discuss this issue in the light of available field constraints

seismic evidence for large scale compositional heterogeneity of oceanic core complexes long lived detachment faults at mid ocean ridges exhume deep seated rocks to form oceanic core complexes occs using large offset 6 km multichannel seismic data we have derived two dimensional seismic tomography models for three of the best developed occs on the mid atlantic ridge our results show that large lateral variations in p wave velocity occur within the upper 0 5 1 7 km of the lithosphere we observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology and we use these correlations to show that gabbros are heterogeneously distributed as large tens to 100 km2 bodies within serpentinized peridotites neither the gabbros nor the serpentinites show any systematic distribution with respect to along isochron position within the enclosing spreading segment indicating that melt extraction from the mantle is not necessarily focused at segment centers as has been commonly inferred in the spreading direction gabbros are consistently present toward the terminations of the detachment faults this suggests enhanced magmatism during the late stage of occ formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults and it therefore may explain eventual abandonment of detachments that form occs detailed seismic studies of the kind described here when constrained by seafloor morphology and geological samples can distinguish between major lithological units such as volcanics gabbros and serpentinized peridotites at lateral scales of a few kilometers thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced

the liquefaction of sands a collapse surface approach recent large scale slides occurring during the hydraulic placement of an artificial island berm in the beaufort sea resulted from the liquefaction of the berm sand subsequent laboratory tests and

effect of latent heat of freezing on crustal generation at low spreading rates lithospheric structure changes at low spreading rates 40 mm a 1 such that steady state molten rock cannot exist above the normal moho depth of 6 km dredge statistics of recovered lithologies indicate that the percentage of basalt is significantly decreased compared to faster ridges and that the mode of crust generation is variable one important effect on lithospheric structure is the latent heat released when basaltic magma freezes within the mantle instead of being transported all the way to the surface using thermal models we show that freezing of melt at mantle depths buffers temperature due to the latent heat of crystallization two quasi stable seafloor spreading patterns are imposed on the model 1 upwelling of magma along a narrow axial zone and 2 pure shear extension over a broad zone the variability in crustal structure predicted by these models explains variability observed in dredge lithology statistics at eight slow to ultraslow spreading ridge segments for example the gakkel eastern volcanic zone and sparsely magmatic zone are both spreading at 12 mm a 1 but the former is composed of 7 peridotite whereas the latter is 46 peridotite this difference can be explained by a change from a narrow axis regime with 2 km thick crust to a wide axis regime with a crust mantle mix overall dredge statistics and thermal models suggest that some but not all latent heat of ascending magmas is released at mantle depths and that various ridge axial morphologies can occur

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

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

fuji dome a large detachment fault near 64 e on the very slow spreading southwest indian ridge a continuous domed detachment surface fuji dome has been imaged on the very slow spreading southwest indian ridge using deep towed side scan sonar and has been investigated by manned submersible and sea surface geophysics the dome is morphologically similar to other oceanic detachments core complexes or mega mullions in addition to bathymetric mullions observed in ship borne bathymetry finer scale spreading parallel striations were imaged with the side scan on the detachment surface metabasalt crops out near the termination probably as part of a thin fault sliver gabbro and troctolite probably crop out near the summit of the dome the rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite most of the detachment surface dips toward the ridge axis at 10 20 but near the breakaway it is strongly rotated outward and dips away from the axis at up to 40 normal undeformed volcanic seafloor crops out adjacent to the detachment modeling of sea surface magnetic data suggest the detachment was active from 1 95 ma for about 1 ma during a period of reduced and asymmetric magmatic accretion modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the dome and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the dome

mg in plagioclase experimental calibration of a new geothermometer and diffusion coefficients abstract the temperature sensitive exchange of mg between plagioclase pl and clinopyroxene cpx has been studied experimentally accounting for different anorthite contents in plagioclase x an and various silica activities a sio 2 in the system the partitioning of mg between plagioclase and clinopyroxene was determined over a temperature range of 1100 1200 c using plagioclase single crystals of different compositions x an 0 5 0 8 surrounded by different clinopyroxene bearing matrix powders to account for different silica activities from 0 55 to 1 0 the experimental design also allows the diffusivity d mg pl of mg in plagioclase under these conditions to be determined both k mg pl cpx defined as k mg pl cpx c mg pl c mg cpx and d mg pl decrease with temperature and increase with a sio 2 isothermal data for different x an in plagioclase show a linear increase of ln k mg pl cpx with increasing x an but d mg pl appears to be insensitive to x an the partitioning data allow a new geothermometer to be calibrated which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist t k 9219 2034 x an ln k mg pl cpx 1 6 ln a sio 2 application of this geothermometer to experimental data from this study reproduces the experimental temperatures within 20 c diffusion of mg in plagioclase is described by d mg pl m 2 s 1 1 25 10 4 m 2 s 1 exp 320 924 j mol 1 rt a sio 2 2 6

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

plate tectonics piecing together rifts earth s crust is formed where tectonic plates rift apart and upwelling magma solidifies disparate observations from rifts beneath the oceans and on land provide insights into the dynamics of rifting and opportunities for synthesis

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

the cooling history and the depth of detachment faulting at the atlantis massif oceanic core complex 1 oceanic core complexes occs are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults we combine previously reported u pb zircon crystallization ages with u th he zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the atlantis massif occ 30 n mar and help establish cooling rates as well as depths of detachment faulting and gabbro emplacement we present nine new u th he zircon ages for samples from iodp hole u1309d ranging from 40 to 1415 m below seafloor these data paired with u pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at atlantis massif as 2895 1276 1162 c myr 1 from 780 c to 250 c the lower 600 m of the borehole cooled more slowly at mean rates of 500 125 102 c myr 1 from 780 c to present day temperatures rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of 300 c myr 1 possibly due to hydrothermal circulation to 4 km depth through the detachment fault zone assuming a fault slip rate of 20 mm yr from u pb zircon ages of surface samples and a rolling hinge model for the sub surface fault geometry we predict that the 780 c isotherm lies at 7 km below the axial valley floor likely corresponding both to the depth at which the semi brittle detachment fault roots and the probable upper limit of significant gabbro emplacement

detachment shear zone of the atlantis massif core complex mid atlantic ridge 30 n near bottom investigations of the cross section of the atlantis massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex integrated direct observations sampling photogeology and imaging define a mylonitic low angle detachment shear zone dsz along the crest of the massif the shear zone may project beneath the nearby corrugated upper surface of the massif the dsz and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme localized tectonic extension during seafloor spreading over the past 2 m y the dsz is characterized by strongly foliated to mylonitic serpentinites and talc amphibole schists it is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction the dsz foliation arches over the top of the massif in a convex upward trajectory mimicking the morphology of the top of the massif kinematic indicators show consistent top to east toward the mar axis tectonic transport directions foliated dsz rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop scale foliation the dsz and underlying basement rocks are cut by discrete anastomosing normal slip shear zones widely spaced steeply dipping normal faults cut all the older structures and localize serpentinization driven hydrothermal outflow at the lost city hydrothermal field a thin few meters sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the dsz and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present gently dipping orientation during lateral spreading and uplift

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

phase relations and liquid lines of descent in hydrous ferrobasalt implications for the skaergaard intrusion and columbia river flood basalts crystallization experiments using a hydrous ferrobasalt as starting material conducted at 200mpa 940 12008c at a wide range of water activities 0 1 1 and redox conditions qfm 3 to qfm 4 where qfm is the quartz fayalite magnetite oxygen buffer show that h2o influences significantly the differentiation history of ferrobasaltic magmas a combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical fe rich tholeiitic system under both dry and hydrous conditions the addition of h2o decreases liquidus temperatures and changes significantly the proportions temperature range and sequence of crystallizing mineral phases the dry liquidus is at about 11708c whereas the liquidus for h2o saturated melts is at 10608c the main phases crystallizing from h2o bearing ferrobasalt at the investigated conditions are olivine ol clinopyroxene cpx plagioclase pl magnetite mt hematite hm ilmenite ilm and amphibole am the phase assemblage is similar to that of the dry system except for the presence of hm at extremely oxidizing conditions and am at low temperatures 59508c and h2o saturated conditions the important observation made in this study is that the stability of fe ti oxides and in particular mt as well as the simultaneous coprecipitation of mtand ilm are almost independent of the activity of h2o ah2o in the system whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing ah2o the stabilities of oxides are controlled mainly by the redox conditions prevailing in the system the most pronounced effect of ah2o on the liquidus temperatures of silicates is observed for pl which shows a considerable delay in crystallization with progressive magma differentiation early crystallization of fe ti oxides in h2o bearing ferrobasaltic compositions precludes any significant fe enrichment during differentiation as fe enrichment is a characteristic feature of the skaergaard intrusion it implies that the skaergaard parental magma did not contain considerable amounts of water on the other hand our experiments indicate that the differentiation of some ferrobasaltic series from the columbia river flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles 0 5 3 wt h2o

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

theoretical investigation of the undrained stability of shallow submerged slopes the problem of the criteria of stability of the equilibrium state of an infinitely long slope with constant inclination is tackled and the conditions of spontaneous collapse of the slope are proposed an experimental programme is described to define the stability locus for loose hostun rf sand the validity of the instability line concept introduced by lade for isotropically consolidated virgin specimens is corroborated it is further shown that this instability line can still be defined when specimens are anisotropically consolidated although then the slope of the line is greater it is shown that an instability line cannot be defined for specimens pre loaded in drained conditions and subsequently reloaded undrained as the level at which peak occurs in undrained conditions is dependent on the pre loading history the results are examined in the light of elastoplastic strain hardening theory by using a model to allow for mixed isotropic kinematic hardening all the experimental phenomena observed can

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

evolution of the southwest indian ridge from 55 45 e to 62 e changes in plate boundary geometry since 26 ma from 55 45 e to 58 45 e and from 60 30 e to 62 00 e the ultraslow spreading southwest indian ridge swir consists of magmatic spreading segments separated by oblique amagmatic spreading segments transform faults and nontransform discontinuities off axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the swir individual magmatic segments show varying magnitudes and directions of asymmetric spreading which requires that the shape of the plate boundary has changed significantly over time in particular since 26 ma the atlantis ii transform fault grew by 90 km to reach 199 km while a 45 km long transform fault at 56 30 e shrank to become an 11 km offset nontransform discontinuity conversely an oblique amagmatic segment at the center of a first order spreading segment shows little change in orientation with time these changes are consistent with the clockwise rotation of two 450 km wide first order spreading segments between the gallieni and melville transform faults 52 60 e to become more orthogonal to spreading we suggest that suborthogonal first order spreading segments reflect a stable configuration for mid ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge axis that can more easily accommodate seafloor spreading

fluid evolution in an oceanic core complex a fluid inclusion study from iodp hole u1309 d atlantis massif 30 n mid atlantic ridge in the detachment mode of slow seafloor spreading convex upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor large long lived hydrothermal systems such as tag are situated off axis and may be controlled by fluid flow up a detachment fault with the source of magmatic heat being as deep as 7 kmbsf the consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated microthermometry on fluid inclusions trapped in diabase gabbro and trondjhemite recovered at the atlantis massif oceanic core complex 30 n mid atlantic ridge reveals evidence for magmatic exsolution phase separation and mixing between hydrothermal fluids and previously phase separated fluids four types of fluid inclusions were identified ranging in salinity from 1 4 to 35 wt nacl although the most common inclusions have salinities close to seawater 3 4 wt nacl homogenization temperatures range from 160 to 400 c with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low salinity inclusions trapped in quartz veins the fluid history of the atlantis massif is interpreted in the context of published thermochronometric data from the massif and a comparison with the inferred circulation pattern beneath the tag hydrothermal field to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by iodp hole u1309d fluids have been trapped

age of eclogites formed by the subduction of the mesoarchaean oceanic crust salma belomorian eclogite province eastern fennoscandian shield russia a synthesis abstract competing evolutionary models and age of eclogite facies metamorphism mesoarchaean neoarchaean or palaeoproterozoic of the subducted mesoarchaean oceanic crust salma association belomorian eclogite province are discussed on a basis of systematic analysis of previously known and newly obtained data four main types of zircons were distinguished in eclogites porous crystals with numerous inclusions from eclogite metagabbro wide rimmed zircons with relict porous cores similar to previous type separated from garnetites round oval zircons from eclogite metagabbronorite that are characteristic for granulite facies rocks and zircons with euhedral oscillatory zoning cores and oval grains that are characteristic for the eclogite facies pillow basalts regular changes in ree patterns and in crystallization recrystallization temperatures of certain domains of the porous zircons display sequence of magmatic and metamorphic events the 2 9 ga domains retain magmatic type ree patterns low and medium temperature inclusions of prenite pumpelliite albite actinolite chlorite diaspore and saponite in garnet and abundant microinclusions of the prenite pumpelliite and greenschist facies in zircons with lree mree enrichment indicate hydrothermal metamorphism in the spreading ridge and ocean floor at 2 9 2 82 ga disappearance of ce positive anomaly from ree pattern in zircon change negative to positive eu anomaly and lree mree enrichment caused by plagioclase removal and replacement of rutile with sphene evidence eclogite facies metamorphism linked with subduction at 2 82 2 78 ga temperatures in the 700 900 c range of the round oval zircons from eclogite metagabbronorite records the neoarchaean granulite facies overprint at 2 77 2 70 ga series of the high temperature palaeoprpoterozoic events was terminated by 2 1 1 7 ga event marked by the rims with lowest ree that frame all types of zircons change from positive to negative eu anomaly retrieval of negative ce anomaly indicate the presence of plagioclase reduction type of fluids and low water activity characteristic of high temperature metamorphism under stretching condition and mantle plume activity the deep reworking of the sm nd isotope system in the belomorian tectonic province at 1 9 ga including the salma eclogite association was caused by the enormous crustal heating that spread from the lapland granulite belt southward radiogenic 176hf enrichment of 1 9 ga zircon indicates recrystallization of a long existed garnet with release of significant amount of 176hf

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

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

slow cooling of the lowermost oceanic crust at the fast spreading east pacific rise the cooling history of the lowermost ocean crust formed at the east pacific rise and exposed at the hess deep rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by integrated ocean drilling program expedition 345 the mg in plagioclase and ca in olivine geospeedometers constrain the initial subsolidus cooling rates down to closure temperatures in the range of 900 750 c these independent geospeedometers yield similar slow cooling rates 0 005 0 0001 c yr 1 with a mean value of 0 0011 c yr 1 making these results robust despite the uncertainties inherent in the methods the slow cooling rates derived using natural samples provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling the cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex this implies that lower crust formed at fast spreading rates remains hot in the near axis environment indicating limited hydrothermal fluid ingress and reaction

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

retrieving timescales of oceanic crustal evolution at oceanic core complexes insights from diffusion modelling of geochemical profiles in olivine abstract the building of oceanic crust at oceanic core complexes occ has been described as a complex process involving multiple intrusions of magma over a protracted period of time the migration of primitive magmas i e mid ocean ridge basalts morbs can lead to melt rock interactions during reactive porous flow processes through the lithosphere the timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained diffusion modelling has been widely used to retrieve timescales of magmatic processes in this study we use diffusion models to constrain i the minimum timescales of melt rock interactions and ii the cooling rates of the gabbroic sequence forming the oceanic crust at an occ we chose samples of the most primitive olivine rich troctolites from the gabbroic sequence sampled in iodp hole u1309d atlantis massif occ mid atlantic ridge 30 n olivine rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust and thus allowed to understand the dynamics of mantle assimilation and the formation of slow spreading oceanic crust at the atlantis massif occ olivines in olivine rich troctolites represent relicts of pre existing mantle olivine while clinopyroxenes and plagioclases are crystallized during reactive percolation olivine chemical compositions show that olivine rich troctolites inherit chemical heterogeneity from the mantle precursor flat geochemical profiles in olivine indicate complete chemical re equilibration of olivine crystals with the locally modified percolating melt exception is made for most ca profiles that show lower ca contents at the olivine rim compared to the relative crystal core as the result of subsolidus cooling three dimensional 3d diffusion models at magmatic conditions t 1210 1300 c and p 2 kbar reveal that complete chemical re equilibration of 4 mm size mantle derived olivine with percolating morb type melts can be attained within durations of less than 300 yr the ca in olivine geospeedometer reveal that cooling rates from 1200 c to 1050 c are constant downhole and on average 0 004 c yr they are comparable with lower temperature cooling rates 850 c 250 c estimated at the atlantis massif occ the minimum timescales from 3d models point to rather fast re equilibration of olivine the downhole chemical heterogeneity inherited from the precursor mantle coupled with the timescales of diffusive re equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time frame of a single melt input our cooling data are consistent with building of the oceanic crust at occs controlled by continuous uplift in turn governed by long lived detachment faults the latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros and the cooling history of the oceanic crust at occ from magmatic conditions to hydrothermalism

diffusion in solid silicates a tool to track timescales of processes comes of age understanding how processes occurring on a wide range of temporal and spatial scales combine to produce a stable dynamic earth is a major goal of the earth scientist determining durations of processes is a key step toward attaining that goal records of incomplete diffusive equilibration preserved in minerals are uniquely suited for the purpose of unraveling timescales of a variety of processes compositional zoning in minerals is like the tracks of a cd that can be decoded with suitable technology this review discusses the causes for the limited use of this tool until recently and how these hindrances are being overcome examples are presented to illustrate that diffusion modeling can clock processes that last from only a few days to those that last over tens of millions of years recorded in rocks that range in age from current volcanic eruptions to condensates from the early solar nebula

regional scale modelling of shallow landslides with different initiation mechanisms sliding versus liquefaction abstract rainfall induced landslides can result from different soil failure mechanisms each leading to slope instabilities characterized by varying times and depths of occurrence this paper discusses the performance of a physically based model for the spatially distributed analysis of landslide susceptibility the model allows the simultaneous assessment of two failure modes frictional slips and liquefaction induced flowslides the underlying mechanical formulation relies on stability principles for unsaturated soils while infiltration is treated as a transient process described by the richards equation the model has been applied to a series of landslides occurred in 1998 in campania italy using laboratory data to constrain the input parameters the analyses provide spatial and temporal patterns of landslide initiation consistent with field evidences in that they capture most of the affected landslide source areas and predict a temporal evolution of instabilities in agreement with reported failure times similar to available regional inventories the computed fraction of flowslides corresponds to 75 of the total unstable area while the resulting ratio between success and error indices is twice as large as those reported in other studies for the same site parametric analyses conducted by varying the hydraulic conductivity ks have pointed out a strong interplay between the dynamics of infiltration the mode of slope instability and the depth of failure in particular it has been found that low values of ks promote shallow flowslides due to moisture perching and high saturation at the surface while larger values of ks favor deeper infiltration fronts and frictional slips initiating at higher suction the comparison between these scenarios showed that simulations based on calibrated hydraulic properties encompass the widest distribution of failure depths thus reproducing more accurately the typical landslides of the site

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

geologic implications of seafloor character and carbonate lithification imaged on the domal core of atlantis massif abstract we document the seafloor character on atlantis massif an ocean core complex located at 30 n on the mid atlantic ridge with an emphasis on the distribution of carbonate features seafloor imagery near bottom backscatter and bathymetry were analyzed on the central dome and the western shoulder of the exposed footwall to the detachment and on the eastern block a hanging wall to the fault we merged argo ii still images to produce photo mosaics and evaluated these together with video imagery acoustic reflectivity and basic rock composition the seafloor was classified as unconsolidated sediment lithified carbonate crust consolidated carbonate cap exposed basement or rubble and the spatial distribution of each type was assessed unconsolidated sediment exposed basement and rubble were documented in all three regions studied lithified carbonate crust was also present on the western shoulder and eastern central dome consolidated carbonate cap was found on the eastern block the formation of the carbonate rock is interpreted to reflect precipitation and or sediment cementation via fluids derived from serpentinization both processes occur at the nearby lost city hydrothermal field the newly documented locations of seafloor carbonate lithification therefore mark pathways of past possibly recent fluid flux from subsurface water rock reaction zones and represent an additional constituent of the carbon cycling hosted by oceanic lithosphere

hydro mechanical modelling of landslides with a material instability criterion the petacciato landslide that occurred between 1906 and 1966 along a stretch of the adriatic coast italy and the trevoux landslide that appeared in 1983 in france are analysed using hill s material sufficient stability condition which is based on the sign of the second order work local and global sufficient stability criteria allow detecting and describing the instability phenomena respectively at the material point level and for the whole domain by considering the non associated plasol elasto plastic constitutive model and the second order work criterion it is shown that unstable stress strain states can be reached in plane strain conditions strictly inside the plastic limit criterion the two landslides that occurred in petacciato and trevoux are then analysed by modelling unsaturated hydro mechanical coupling the analysis is carried out using the lagamine finite element code this original approach to slope stability analysis makes it possible successfully to exhibit the landslide mechanism o

soil stability and flow slides in unsaturated shallow slopes can saturation events trigger liquefaction processes this paper illustrates an application of the theory of material stability to the analysis of unsaturated slopes the main goal is to contribute to the understanding of rainfall induced flow slides

stacked gabbro units and intervening mantle a detailed look at a section of iodp leg 305 hole u1309d hole u1309d integrated ocean drilling program iodp legs 304 305 penetrated 1415 m into the seafloor of the atlantis massif an oceanic core complex at 30 n mid atlantic ridge more than 96 of all recovered rocks are gabbroic on the basis of a mineral chemical overview we suggest that between 800 and 1100 m below sea floor mbsf a magmatic unit occurs ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part below 1235 mbsf massive gabbronorites oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit the focus here is on the zone where both units interact and screens totaling 50 m of a microstructurally distinct olivine rich troctolite occur we argue that the olivine rich troctolite is a former mantle rock which was converted to a crust mantle transition zone dunite at the base of the upper magmatic unit later as melts derived from the lower magmatic unit percolated through it it was equilibrated to a more evolved chemistry and transformed to a fine grained olivine rich troctolite our main arguments against a possible cumulate nature of the olivine rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine rich troctolite its distinctly fine grained microstructure the high cr content of cpx and its ni rich olivine composition the high nio for a given mg mg fe in the olivine rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt evidence for the percolation of evolved melts through the olivine rich troctolites are ti rich interstitial pyroxenes and as inclusions in cr spinel highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter scale noritic veins the percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust mantle transition zone

residence time and crystallization history of nickeliferous olivine phenocrysts from the northern yatsugatake volcanoes central japan application of a growth and diffusion model in the system mg fe ni abstract the chemical zoning of nickeliferous olivine phenocrysts in calc alkaline andesites from the northern yatsugatake volcanoes central japan is examined by a growth and diffusion model in the system mg fe ni nickeliferous olivine phenocrysts are included in the high alumina basaltic end member magma which forms the mixed calc alkaline andesites in the northern yatsugatake volcanoes the olivine phenocrysts can be divided into three types on the basis of chemical composition zoning profile and occurrence type 1 olivine is rich in ca and mg and the zoning profile of type 1 forms a slightly more fe rich trend than the maximum fractionation trend in the nio versus forsterite content fo diagram the most mg and ni rich and mn poor core of type 1 fo 91 5 mol nio 0 40 wt mno 0 13 wt can be in equilibrium with peridotite in the upper mantle by contrast type 2 olivine is ca poor and rich in fe and ni compared to type 1 the zoning patterns of type 2 are more variable the nio content in fe rich type 2 phenocrysts decreases rapidly from core to rim within a narrow range of fo the maximum nio content of type 2 is 0 53 wt the transitional type olivine has an intermediate composition between type 1 and type 2 and a similar zoning profile as that of type 1 type 1 olivine mainly occurs as isolated phenocrysts while fe rich type 2 occurs as crystal clots the growth and diffusion calculation in the system mg fe ni revealed that the composition of normally zoned olivine is enriched in fe by diffusion without a marked depression in the ni content due to a larger fe mg interdiffusion coefficient than the ni tracer diffusion coefficient in olivine the zoning patterns of both type 1 and type 2 can be reproduced by calculation the residence time of type 1 olivine in the magma is estimated to be less than 10 0 5 1 y at 1150 1050 c whereas the residence time of fe enriched type 2 olivine is from ten to one hundred times longer than that of type 1 the long residence time and occurrence as crystal clots suggest that the fe enriched type 2 olivine phenocrysts were captured and crystallized in the crystal mush at the roof and bottom of a magma chamber

building of the deepest crust at a fossil slow spreading centre pineto gabbroic sequence alpine jurassic ophiolites this work presents new field and petrological data on a poorly known lower crustal section from the alpine jurassic ophiolites the pineto gabbroic sequence from corsica france the pineto gabbroic sequence is estimated to be 1 5 km thick and mainly consists of clinopyroxene rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gab bros in its deeper sector the sequence also encloses olivine rich troctolite and mantle peridotite bodies at dif ferent stratigraphic heights the composition and the lith ological variability of the pineto gabbroic sequence recall those of the lower crustal sections at slow and ultra slow spreading ridges the gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros which approximately constitute 2 3 of the section in particular the lower sector of the pineto gab broic sequence shows the existence of large scale frag ments of the deepest oceanic crust displaying a highly primitive bulk composition the mineral chemical varia tions document that the origin and the evolution of the pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation the clinopyroxenes from the olivine gabbros and the olivine rich troctolites also record the infiltration of olivine dissolving cr2o3 rich melts that presumably formed within the mantle into replacive dunite bodies cooling rates of the troctolites and the olivine gabbros were evaluated using the ca in olivine geospee dometer we obtained high and nearly constant values of 2 2 to 1 7 c year log units which were correlated with the building of the pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold litho spheric mantle

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids