seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

upper mantle electrical resistivity structure beneath the central mariana subduction system this paper reports on a magnetotelluric mt survey across the central mariana subduction system providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back arc spreading ridge after calculation of mt response functions and their correction for topographic distortion two dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab the resultant isotropic electrical resistivity structure contains several key features there is an uppermost resistive layer with a thickness of up to 150 km beneath the pacific ocean basin 80 100 km beneath the mariana trough and 60 km beneath the parece vela basin along with a conductive mantle beneath the resistive layer a resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge there is no evidence for a conductive feature beneath the back arc spreading center sensitivity tests were applied to these features through inversion of synthetic data the uppermost resistive layer is the cool dry residual from the plate accretion process its thickness beneath the pacific ocean basin is controlled mainly by temperature whereas the roughly constant thickness beneath the mariana trough and beneath the parece vela basin regardless of seafloor age is controlled by composition the conductive mantle beneath the uppermost resistive layer requires hydration of olivine and or melting of the mantle the resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three dimensional and of limited size in contrast the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab the resistive region beneath the back arc spreading center can be explained by dry mantle with typical temperatures suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge evidence for electrical anisotropy in the central mariana upper mantle is weak

3 d electrical resistivity structure based on geomagnetic transfer functions exploring the features of arc magmatism beneath kyushu southwest japan arc our 3 d electrical resistivity model clearly detects particular subsurface features for magmatism associated with subduction of the philippine sea plate psp in three regions a southern and a northern volcanic region and a non volcanic region on the island of kyushu we apply 3 d inversion analyses for geomagnetic transfer function data of a short period band in combination with results of a previous 3 d model that was determined using network magnetotelluric response function data of a longer period band as an initial model in the present inversion to improve resolution at shallow depths specifically a two stage inversion is used instead of a joint inversion in contrast to the previous model the presented model clearly reveals a conductive block on the back arc side of kirishima volcano at shallow depths of 50 km the block is associated with hydrothermal fluids and hydrothermal alteration zones related to the formation of epithermal gold deposits a second feature revealed by the model is another conductive block regarded as upwelling fluids extending from the upper surface of the psp in the mantle under kirishima volcano in the southern volcanic region third a resistive crustal layer which confines the conductive block in the mantle is distributed beneath the non volcanic region fourth our model reveals a significant resistive block which extends below the continental moho at the fore arc side of the volcanic front and extends into the non volcanic region in central kyushu

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

the importance of vein selvaging in controlling the intensity and character of subsurface alteration in hydrothermal systems the identity and abundance of secondary minerals produced when a hydrothermal fluid initially in equilibrium with its host formation convects or is otherwise propelled down a temperature gradient toward the surface are calculated with and without boiling the calculations show that the mass of secondary minerals produced by a unit throughput of hydrothermal water is surprisingly dependent on selvaging mineral precipitation is over 1 000 times greater when chemical contact is maintained with the host formation than when the upwelling fluid is chemically isolated from the host rock by selvages this means for example that a selvaged vein with only one tenth the hydrothermal mineral alteration of a nonselvaged vein formed in a similar environment may nevertheless have transmitted over 100 times more hydrothermal fluid and precipitated 100 times the mass of ore minerals these conclusions are for end member equilibrium models the magnitude of the effect suggests however that selvage formation will remain an important consideration in cases between the two end member extremes and for cases subject to kinetic control

connectivity of molten fe alloy in peridotite based on in situ electrical conductivity measurements implications for core formation in terrestrial planets abstract the connectivity of molten fe s in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 gpa starting materials were powdered mixtures of peridotite klb 1 with various amounts 0 3 6 13 19 24 vol of the 1 gpa eutectic composition in the fe fes binary system at temperatures above the eutectic point in the fe fes system 980 c and below the solidus of klb1 1200 c molten fe s in a solid silicate matrix interconnects when the volume fraction is over 5 conductivity temperature paths indicate that in the presence of partial silicate melting the connectivity of molten fe s in a peridotite matrix is inhibited based on observations of retrieved samples the percolation threshold of fe s melts in the presence of low to moderate degrees of silicate melt is estimated at 13 2 vol these results indicate that if the volume fraction of fe alloy in a planetesimal was initially greater than 5 and if early heating by decay of radionuclides raised the temperature of the interior above the fe alloy melting point initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix these conditions were likely met by many terrestrial objects in the early solar nebula efficient removal of residual fe alloy 5 vol from silicate requires high degree melting of silicate so that metal can segregate as droplets giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high degrees of melting alternatively if initial metal segregation were delayed until a planetary object grew to large size 1000 km in diameter release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

stability and instability of quiescently active volcanoes the case of masaya nicaragua quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity i present a model of coupled conduit convection and foam accumulation to explain degassing behavior of masaya an active volcano in nicaragua that is currently emitting large amounts of gas but not erupting gas rich magma is transported through a conduit 2 6 m in radius and then released into a shallow reservoir the magma is degassed in the reservoir and forms a foam 1 3 m thick at the top with bubbles 20 60 m in diameter the foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface if the foam layer is destabilized the volcano can erupt explosively the most likely cause of foam destabilization is a large injection of volatile rich magma from deeper levels into the shallow conduit reservoir system thereby increasing magma fluxes and gas fluxes exsolving large gas bubbles and reducing surface tension of the magma

the high pressure phase of lawsonite a single crystal study of a key mantle hydrous phase lawsonite caal2si2o7 oh 2 h2o is an important water carrier in subducting oceanic crusts and the primary hydrous phase in basalt at depths greater than 80 km we have conducted high pressure synchrotron single crystal x ray diffraction experiments on natural lawsonite at room temperature up to 10 0 gpa to study its high pressure polymorphism we find that lawsonite remains orthorhombic with cmcm symmetry up to 9 3 gpa and shows nearly isotropic compression above 9 3 gpa lawsonite becomes monoclinic with p21 m symmetry across the phase transition the ca polyhedron becomes markedly distorted and the average positions of the h2o molecules and hydroxyls change the changes observed in the h atom positions under compression are different than the low temperature changes in this material we resolve for the first time the h bonding configuration of the high pressure monoclinic phase of lawsonite a bond valence approach is deployed to determine that the phase transition from orthorhombic to monoclinic is primarily driven by the si2o7 groups and in particular their bridging oxygen atoms o1 the changes in the structure strongly indicate that entropy increases across the symmetry lowering transition and hence that the slope of the phase transition is negative monoclinic lawsonite is thus stable under the pressure and temperature conditions that exist in the earth and is likely to be a major water carrier in colder deep subducted slabs monoclinic lawsonite also likely has enhanced electrical conductivity along its c axis due to its dynamically disordered hydrogen atoms

lawsonite composition and zoning as tracers of subduction processes a global review abstract lawsonite is an abundant hydrous mineral in oceanic crust sediments and metasomatic rocks at depths of 45 to 300 km in most subduction zones but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism owing to the significance of lawsonite for water and element cycling in subduction zones occurrences of fresh lawsonite in blueschist and more rarely eclogite provide important opportunities to determine lawsonite composition zoning and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation in this review we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet omphacite in the rock matrix as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite facies rocks that lack matrix lawsonite as lawsonite blueschist is much more common than lawsonite eclogite we survey the composition of lawsonite in representative localities of blueschist including blueschist associated with eclogite lawsonite bearing epidote bearing and blueschist not associated with eclogite at current exposure levels included in this review are metabasaltic rocks silica and carbonate rich metasedimentary rocks metasomatic rocks and lawsonite rich veins this dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation and specifically of fluid rock interaction with implications for element cycling in subduction zones furthermore most exhumed lawsonite eclogite records slab surface conditions that correspond to the location where the slab mantle interface transitions from decoupled to coupled and therefore provides key insights into the thermal history and dynamics of subduction zones

conceptual and numerical models of ring fault formation abstract most ring faults of collapse calderas are primarily shear fractures the initiation and development of which depends on the state of stress in the host rock the state of stress in a volcano is controlled by the loading conditions such as magma chamber geometry and pressure but also by the mechanical properties of its rock units and structures such as existing contacts faults and joints ring fault formation is thus essentially a problem in rock physics field observations show that some ring faults are dip slip whereas others are partly faults shear fractures and partly ring dykes extension fractures although slip on existing ring faults is much more common in basaltic edifices shield volcanoes than in true composite volcanoes in both types of volcanoes most caldera unrest periods do not result in ring fault slip here i present new conceptual and numerical models of caldera formation in volcanoes with shallow spherical circular or oblate ellipsoidal sill like magma chambers in the layered models the host rock above the chamber is composed of 30 comparatively thin layers with stiffnesses young s moduli alternating between 1 gpa and 100 gpa the chamber itself is located in a single thick layer the crustal segment hosting the chamber is either 20 km or 40 km wide but has a constant thickness of 20 km the loading conditions considered are 1 a crustal segment subject to 5 mpa tension 2 crustal segment subject to excess magmatic pressure of 10 mpa at the bottom doming of the volcanic field containing the chamber 3 a combination of tension and doming and 4 chamber subject to underpressure negative excess pressure of 5 mpa the main results are as follows 1 excess pressure and underpressure in a chamber normally favour dyke injection rather than ring fault formation 2 for doming or tension a spherical magma chamber favours dyke injection except when the layer hosting the chamber is very soft 10 gpa or one with recent dyke injections in which case the surface stress field favours ring fault formation 3 for a sill like chamber in a 20 km wide crustal segment a ring fault can be generated by either tension or tension and doming for a 40 km wide segment doming alone is sufficient to generate a ring fault 4 since individual layers in a volcano may develop different local stresses stress field homogenisation through all the layers between the chamber and the surface is a necessary condition for ring fault formation 5 because the mechanical properties of the layers that constitute basaltic edifices are more uniform than those that constitute true composite volcanoes it follows that stress field homogenisation and thus ring fault formation or slip is more commonly reached in basaltic edifices than in composite volcanoes 6 both for basaltic edifices and composite volcanoes the stress fields most likely to initiate ring faults are those generated around sill like chambers subject to tension doming or both

electrical structure of the central cascadia subduction zone the emslab lincoln line revisited abstract the emslab experiment was an ambitious onshore offshore magnetotelluric mt transect of the cascadia subduction zone when completed 1985 1988 it was the largest experiment of its kind modeling and inversion capabilities at the time were however not sufficiently sophisticated to handle a fully regularized inversion of the data including the seafloor data and bathymetric constraints with the main final model presented based on trial and error forward modeling of the responses moreover new data collected as part of the earthscope usarray program are of higher quality due to improvements in instrument technology and augment the original emslab data set presenting an opportunity to revisit the structure in this part of the subduction system we have integrated the original wide band mt data as well as several long period stations from the original emslab data set and invert these in conjunction with emslab seafloor responses and new earthscope data on land this new composite data set has been analyzed in several ways within a two dimensional geometry in which conductivity is assumed to be invariant along a strike direction roughly coincident with that of the subduction zone we have solved for fully smooth regularized models as well as solutions that allow discontinuities in conductivity along the top surface of the descending slab finally we have tested specific features in the emslab model notably a moderately shallow 30 km depth forearc conductor a feature similar to this shallow conductor is a consistent and required feature in our new inversion models but the new models highlight the connection between the slab and what is interpreted to be an accumulation of aqueous fluids in the deep crust the depth 40 km at which the conductor intersects the slab suggests that the fluids are released by the transition of hydrous basalt to eclogite at upper greenschist facies and higher metamorphic grade the nose of the mantle wedge has a conductivity consistent with a dry peridotite composition and thermal models of the system at a depth of around 80 km the mantle intersecting the slab shows a slight increase in conductivity this increase is not sufficient to require the presence of melt but a conductor indicative of melt can be inserted into the model at this depth without compromising the fit

imaging the source region of cascadia tremor and intermediate depth earthquakes the subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface trigger earthquakes and regulate transient phenomena such as episodic tremor and slip ets it is not clear how dehydration can separately induce earthquakes within the subducting plate and ets partly because few data exist on their relationship to subduction zone structures we present results of a seismic experiment in the washington cascades united states that images a region producing both earthquake types migration of scattered teleseis mic waves provides images of low velocity subducting crust at depths 40 45 km with sharp boundaries above and below it the sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40 45 km depth regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is 95 km deep but ets only exists where the sharp upper boundary occurs the ets location supports models of slow slip that require near lithostatic fluid pressure whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration very low shear stresses on the plate boundary may limit seismicity to ets and similar phenomena

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

electrical conductivity of tremolite under high temperature and pressure implications for the high conductivity anomalies in the earth and venus we measured the electrical conductivity of tremolite over a range of pressures 1 0 1 5 and 2 0 gpa and temperatures 648 1373 k at temperatures lower than 1173 k the electrical conductivity of tremolite was 0 001 s m but once the dehydroxylation reaction took place at 1173 k we observed a significant increase in the electrical conductivity the electrical conductivity continued to increase until it reached its maximum value of 1 s m at a temperature of 1373 k which we attributed to dehydroxylation in the tremolite samples the electrical behavior was altered by the physical breakdown of the tremolite structure rather than the oxidation process the decomposition of tremolite resulted in the creation of diopside enstatite and quartz to explain some anomalous conductivity measurements we propose that iron free amphibole experiences a dehydroxylation mechanism that differs from the dehydroxylation mechanism observed in iron bearing amphiboles our conductivity analyses indicate that the observed increase in the electrical conductivity in deep subduction regions at depths of 180 km is driven by the dehydroxylation of tremolite and by the mixing of lherzolite and tremolite in the upper mantle using our data and previously published temperature gradient data we were able to extrapolate reasonable electrical conductivity values for the surface and interior of venus

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

hydration reduced lattice thermal conductivity of olivine in earth s upper mantle earth s water cycle enables the incorporation of water hydration in mantle minerals that can influence the physical properties of the mantle lattice thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs however the effect of hydration on lattice thermal conductivity remains poorly understood and has often been assumed to be negligible here we have precisely measured the lattice thermal conductivity of hydrous san carlos olivine mg0 9fe0 1 2sio4 fo90 up to 15 gigapascals using an ultrafast optical pump probe technique the thermal conductivity of hydrous fo90 with 7 000 wt ppm water is significantly suppressed at pressures above 5 gigapascals and is approximately 2 times smaller than the nominally anhydrous fo90 at mantle transition zone pressures demonstrating the critical influence of hydration on the lattice thermal conductivity of olivine in this region modeling the thermal structure of a subducting slab with our results shows that the hydration reduced thermal conductivity in hydrated oceanic crust further decreases the temperature at the cold dry center of the subducting slab therefore the olivine wadsleyite transformation rate in the slab with hydrated oceanic crust is much slower than that with dry oceanic crust after the slab sinks into the transition zone extending the metastable olivine to a greater depth the hydration reduced thermal conductivity could enable hydrous minerals to survive in deeper mantle and enhance water transportation to the transition zone

electrical investigation of metal olivine systems and application to the deep interior of mercury we report electrical conductivity measurements on metal olivine systems at about 5 and 6 gpa and up to 1 675 c in order to investigate the electrical properties of core mantle boundary cmb systems electrical experiments were conducted in the multianvil apparatus using the impedance spectroscopy technique the samples are composed of one metal layer fe fes fesi2 or fe ni s si and one polycrystalline olivine layer with the metal olivine ratio ranging from 1 0 7 to 1 9 2 for all samples we observe that the bulk electrical conductivity increases with temperature from 10 2 5 to 101 8 s m which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions in some experiments a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase both the metal olivine ratio and the metal phase geometry control the electrical conductivity of the two layer samples by combining electrical results textural analyses of the samples and previous studies of the structure and composition of mercury s interior we propose an electrical profile of the deep interior of the planet that accounts for a layered cmb outer core structure the electrical model agrees with existing conductivity estimates of mercury s lower mantle and cmb using magnetic observations and thermodynamic calculations and thus supports the hypothesis of a layered cmb outermost core structure in the present day interior of mercury we propose that the layered cmb outer core structure is possibly electrically insulating which may influence the planet s structure and cooling history

mercury s thermal evolution controlled by an insulating liquid outermost core abstract the weak intrinsic magnetic field of mercury is intimately tied to the structure and cooling history of its metallic core recent constraints about the planet s internal structure suggest the presence of a fes layer overlying a silicon bearing core we performed 4 electrode resistivity experiments on core analogues up to 10 gpa and over wide temperature ranges in order to investigate the insulating properties of core materials our results show that the fes layer is liquid and insulating and that the electrical resistivity of a miscible fe si s core is comparable to the one of an immiscible fe s fe si core the difference in electrical resistivity between the fes rich layer and the underlying fe si s core is at least 1 log unit at pressure and temperature conditions relevant to mercury s interior estimates of the lower bound of thermal conductivity for fes and fe si s materials are calculated using the wiedemann franz law a thick 40 km fes rich shell is expected to maintain high temperatures across the core and if temperature in this layer departs from an adiabat then this might affect the core cooling rate the presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in mercury and is likely to impact the generation and sustainability of a magnetic field

the spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at miyakejima volcano japan basaltic feeder systems exposed in the caldera wall of miyakejima volcano are classified into three groups 1 effusive feeders 2 moderately explosive feeders and 3 highly explosive feeders the surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them ranging from non explosive lava effusions to violent explosive eruptions despite the apparent lack of influence of external water effusive feeders are filled with coherent non fragmented intrusive rock indicating no significant fragmentation in the feeder system the other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part their uppermost parts show an upward flaring shape the transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma the depth of the coherent pyroclastic transition is deeper 20 150 m in highly explosive feeders than in the moderately explosive feeders 20 m and coincides with the depth at which the system flares upwards presence of lithic fragments derived from the host rock in the products of the highly explosive feeder systems indicates the removal of the wall rock by explosive activity this observation suggests that the fragmentation of rising magma promoted the enlargement of the feeder systems to form their upward flaring shapes by mechanical erosion and wall collapse

subsurface cyclic magma sill expansions in the 2000 miyakejima volcano eruption possibility of two phase flow oscillation 1 step like tilt changes were observed at intervals of dozens of hours during the caldera formation stage occurred during the 2000 miyakejima volcano eruption our analysis suggests that these tilt steps were caused by the cyclic expansion of a subsurface sill like magma plumbing system trending from se to nw of the volcano in the same period a caldera gradually grew at the summit area to a very large 1 6 km in diameter and 450 m in depth the source mechanism of these tilt steps is fairly well explained by a two phase flow instability called a pressure drop oscillation this model indicates that the gas phase capacity in the upper part of the conduit acts as a pump that controls the flux of two phase magma in the sill like reservoir caldera formation as well as magma uplift and descent in the conduit may reflect temporal changes in the intervals between tilt steps which are themselves proportional to the volume of this gas capacity

an active ring fault detected at tendurek volcano by using insar 1 although ring faults are present at many ancient deeply eroded volcanoes they have been detected at only very few modern volcanic centers at the so far little studied tendurek volcano in eastern turkey we generated an ascending and a descending insar time series of its surface displacement field for the period from 2003 to 2010 we detected a large 105 km2 region that underwent subsidence at the rate of 1 cm yr during this period source modeling results show that the observed signal fits best to simulations of a near horizontal contracting sill located at around 4 5 km below the volcano summit intriguingly the residual displacement velocity field contains a steep gradient that systematically follows a system of arcuate fractures visible on the volcano s midflanks rapideye satellite optical images show that this fracture system has deflected holocene lava flows thus indicating its presence for at least several millennia we interpret the arcuate fracture system as the surface expression of an inherited ring fault that has been slowly reactivated during the detected recent subsidence these results show that volcano ring faults may not only slip rapidly during eruptive or intrusive phases but also slowly during dormant phases

the november 2002 eruption at piton de la fournaise volcano la reunion island ground deformation seismicity and pit crater collapse an eruption on the eastern flank of piton de la fournaise volcano started on 16 november 2002 after 10 months of quiescence after a relatively constant level of activity during the first 13 days of the eruption lava discharge volcanic tremor and seismicity increased from 29 november to 3 december lava effusion suddenly ceased on 3 december while shallow earthquakes beneath the dolomieu summit crater were still recorded at a rate of about one per minute this unusual activity continued and increased in intensity over the next three weeks ending with the formation of a pit crater within dolomieu based on ground deformation measured by rapid static and continuous gps and an extensometer seismic data and lava effusion patterns the eruptive period is divided into five stages 1 slow summit inflation and sporadic seismicity 2 rapid summit inflation and a short seismic crisis 3 rapid flank inflation onset of summit deflation sporadic seismicity accompanied by stable effusion 4 flank inflation coupled with summit deflation intense seismicity and increased lava effusion and finally 5 little deflation intense shallow seismicity and the end of lava effusion we propose a model in which the pre intrusive inflation of stage 1 in the months preceding the eruption was caused by a magma body located near sea level the magma reservoir was the source of an intrusion rising under the summit during stage 2 in stage 3 the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano causing lava effusion pressure decrease in the magmatic plumbing system followed resulting in upward migration of a collapse front forming a subterranean column of debris by faulting and stoping this caused intense shallow seismicity increase in discharge of lava and volcanic tremor at the lateral vent in stage 4 and eventually the formation of a pit crater in stage 5

zoning in the carboniferous lower permian cracow epithermal vein system central queensland australia four epithermal vein deposits i e dawn central extended rose s pride and klondyke in the cracow gold field central queensland were investigated in terms of paragenesis mineralogy vein textures fluid inclusions and stable isotopes the cracow epithermal field is confined to an area approximately 6 by 5 kilometers all the deposits are hosted by the massive camboon andesite of upper carboniferous to lower permian age occur as open space vein fillings and have similar paragenesis however significant variations in mineralogy textures of quartz and adularia and fluid geochemistry were found for a main mineralisation stage stage ii of each individual deposits at rose s pride and klondyke basemetal sulphides are virtually absent but significant amounts of calcite and quartz with minor adularia are widely distributed replacement textures are distinct and mineralisation temperature is less than 220 c and salinity less than 0 2 wt the 18o values of quartz and calcite range from 2 65 to 2 06 and from 6 66 to 6 34 respectively and calculated 18oh2o value is about 17 which represents a nearly unshifted palaeo meteoric water gold mineralisation is best developed at central extended among the studied deposits where patches rich in electrum are often observed in polished thin sections and where gold grades exceeding 10 g t are frequently indicated by assays base metal sulphides are only present locally and rarely exceed 5 volume percent of the vein samples quartz is the dominant gangue mineral but significant amounts of rhombic adularia and chlorite are widely distributed various primary and recrystallisation textures possibly inherited from silica gel are well developed and widespread at individual sites where crustiform bands developed from both walls of a fissure temperatures could drop sharply from 275 c to less than 220 c the ore forming fluid at central extended compared with that at rose s pride and klondyke was isotopically shifted from meteoric water with 18oh2o value of 13 5 calculated in equilibrium with quartz 18o values of 3 09 to 1 44 the orebodies at dawn are rich in base metal sulphides which are commonly coarse grained and form up to 20 volume percent of the vein materials quartz is the predominant gangue mineral and commonly shows a coarse comb texture the ore forming fluid was 275 10 c and low salinity 0 4 to 0 7 wt the 18o values of quartz range from 3 97 to 3 22 and calculated 18oh2o value is about 12 indicating large isotopic shifts from palaeo meteoric water a depth zoning in typical boiling epithermal systems corresponding to different fluid compositions wall rock permeability and boiling behaviors was invoked to explain different characteristics of these selected epithermal veins

searching for patterns in caldera unrest the ultimate goal of volcanology is forecasting eruptions this task is particularly challenging at calderas where unrest is frequent affects wider areas and its evidence is often masked by the activity of hydrothermal systems a recent study has compiled a database on caldera unrest derived from seismicity geodetic gravity and geochemical monitoring data at calderas worldwide from 1988 to 2014 here we exploit this database searching for the most recurring features of unrest and in turn its possible dynamics in particular we focus on a the duration of unrest at calderas b recurring patterns in unrest c unrest episodes culminating in eruptions including time or size predictability and a multivariate regression analysis our analysis indicates that pre eruptive unrest is shorter than non eruptive unrest particularly with open or semi open calderas calderas with mafic or mixed composition of past eruptive products or unrest driven by mafic magma conversely lack of data on pre eruptive unrest driven by felsic magma and or at felsic or plugged calderas prevents an analysis of these specific subsets in addition 72 of pre eruptive unrest lasts 10 months and shows high seismicity and degassing the remaining 28 a is essentially aseismic in calderas with open conduit 17 or b lasts between 10 and 18 months with seismicity and degassing constituting a longer duration tail of the pre eruptive unrest with seismicity and degassing 11 surface deformation is not always reliable to characterize pre eruptive unrest our analysis suggests that magma may withstand only a limited period of eruptability before becoming stored in the upper crust

geochemistry and u pb dating of felsic volcanic rocks in the riotinto nerva unit iberian pyrite belt spain crustal thinning progressive crustal melting and massive sulphide genesis abstract we present new geochemical sm nd and u pb data on the felsic volcanic rocks containing the world class volcanic hosted massive sulphide deposits in the riotinto nerva unit iberian pyrite belt spain three new u pb ages from older plagioclase quartz phyric dacites and plagioclase phyric rhyolites to youngest plagioclase quartz phyric rhyolites indicate a time span for felsic volcanism ranging from 351 5 0 4 to 345 7 0 6 ma the youngest felsic rocks exhibit lower e nd as well as contrasting ti sr zr hf and eu eu values we interpret that in the riotinto nerva unit crustal melting successively affected shallower more evolved horizons in the crust progressive crustal melting is consistent with current interpretations of the iberian pyrite belt in terms of a late devonian early carboniferous transtensional setting coupled with underplating by basic magma we suggest that low e nd evolved crustal magmatism could be used as a proxy in studies of the genesis of these and possibly other phanerozoic massive sulphide deposits supplementary material details of the analytical methods and whole rock data for basalts are available at http www geolsoc org uk sup18452

h2o emission rate by the volcanic plume during the 2000 2002 miyakejima volcanic activity 1 extremely intense degassing at miyakejima volcano japan was observed after the 2000 eruption the h2o emission rate was estimated using a record of infrared thermal images at 400 kton day in november 2000 by april 2002 h2o emission rate had decreased exponentially to 100 kton day the h2o emission rate varied in parallel with the so2 emission rate except for the early stage in september 2000 the ratio of h2o to so2 emission rate after november 2000 remained constant at about 10 which is consistent with the h2o so2 ratio in the magma the large rate of h2o and so2 emission requires efficient degassing at shallow depth the agreement between the gas composition of the plume and that of dissolved volatiles in melt inclusions indicates h2o discharged from the vent was mostly magmatic in origin after november 2000

adularia in epithermal veins queensland morphology structural state and origin four types of adularia i e sub rhombic rhombic tabular and pseudo acicular are recognised from examination of samples from ten epithermal vein deposits and prospects in queensland based on morphology of the individual crystals further investigation of the structural state of adularia reveals that each group has some specific features in terms of the degree of al si disordering which can be related to various crystallisation conditions and the thermal history sub rhombic adularia is commonly 2 4 mm in size and subhedral with more or less rhombic outlines and has a relatively ordered al si distribution 2t1 0 84 reflecting slow crystallisation conditions adularia of this type in association with coarse grained quartz is predominant at deep levels of epithermal systems where boiling is initiated in an environment of low permeability and the fluid is slightly supersaturated with respect to adularia and quartz tabular adularia characterised by its lath shape and disordered structure 2t1 values ranging from 0 64 to 0 74 is likely to have formed when the fluid moves up to a more permeable environment and starts boiling violently relatively high temperatures and rapidly changing conditions account for its special morphology and disordered structure rhombic adularia showing very small crystal size 0 2 mm with euhedral rhombic form has an intermediate degree of al si ordering pseudo acicular adularia is interpreted as pseudomorphs after carbonate and its high al si ordered structure is attributed to the presence of a carbonate precursor these two types of adularia commonly occur within crustiform and colloform bands in association with high grade ore and chalcedony or fine grained quartz which often displays various recrystallisation textures it is most likely that adularias of these two types are formed when extensive boiling is protracted microprobe analyses indicate the composition of all adularia types close to pure kalsi3o8 sericite and or carbonate altered adularias consistently display more ordered structures suggesting that the post crystallisation thermal regime affects the structural state of altered adularia even at temperatures as low as in epithermal environments

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

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

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

sedimentary and volcano tectonic processes in the british paleocene igneous province a review research on the british paleocene igneous province bpip has historically focused on the emplacement chemistry and chronology of its elaborate central intrusive complexes and lava fields however the bpip has also been dramatically shaped by numerous erosion sedimentation and volcano tectonic events the significance of which becomes ever clearer as localities in the bpip are re investigated and our understanding of volcano sedimentary processes advances the resultant deposits provide important palaeo environmental palaeo geographical and stratigraphical information and highlight the wide range of processes and events that occur in ancient volcanic settings such as the bpip in this paper we review the sedimentary and volcano tectonic processes that can be distinguished in the bpip and conceptualize them within a generalized framework model we identify and describe the sedimentary responses to four broadly chronological stages in the history of the bpip volcanoes 1 the development of the lava fields 2 early intrusion induced uplift 3 caldera collapse and 4 post volcano denudation and exhumation of central complexes we highlight and illustrate the range of sedimentary processes that were active in the bpip these operated on and helped shape a dynamic landscape of uplands and lowlands of alluvial fans braided rivers lakes and swamps and of volcanoes torn apart by catastrophic mass wasting events and or caldera collapse

geochemical and nd isotopic signature of felsic volcanic rocks as a proxy of volcanic hosted massive sulphide deposits in the iberian pyrite belt sw spain the paymogo volcano sedimentary alignment abstract the iberian pyrite belt ipb located in the south portuguese zone of the iberian massif spain is a world class vhms province containing numerous huge mineral deposits hosted by bimodal volcanic sequences the province can be subdivided in several domains which differ from each other both in the nature and relative abundance of their volcanic sequences and in the distribution of associated vhms deposits in spite of its geological and economic interest information available for the northern part of the province is still relatively scarce we present a new dataset of geochemical and nd isotopic analyses on the paymogo volcano sedimentary alignment pvsa an area that is considered to be representative of this northern segment of the ipb volcanic sequences in the pvsa contain very scarce mafic rocks in contrast felsic rocks are abundant and have been classified in two groups i the paymogo rhyolitic sequence hosting most of the vhms deposits and ii a dacitic sequence although approximately coeval these groups of felsic rocks clearly differ in geochemical features and nd signature in view of these differences it is suggested that felsic rocks were produced from different mid crustal domains during a major event of partial melting related to a regional late devonian early carboniferous transtensional event it is further suggested that some volcanic rocks previously described as andesites are better interpreted as dacites the age 347 3 0 8 ma geochemical features zr

water and the viscosity of andesite melts the viscosity of a synthetic andesite like melt has been measured between 1010 and 1014 p for water contents in the range 0 3 5 wt the very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision after a steep viscosity decrease of 5 orders of magnitude for 1 wt h2o an additional 2 5 wt h2o causes a further viscosity decrease of only 2 orders of magnitude these viscosity decreases are qualitatively similar to those observed previously for more silica rich compositions the new data join smoothly with available high temperature measurements made at high pressures on water bearing andesite melts because the intrinsic effects of pressure are as small for water bearing as for water free samples the depressing effect of water on the viscosity of natural andesite melts can be estimated changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity although quantitative applications are not yet possible the configurational entropy theory accounts qualitatively for these features

a volcanotectonic survey of ascraeus mons mars 1 ascraeus mons is one of the largest volcanoes on mars it is replete with well preserved features that can be used to understand its volcanotectonic evolution previous studies of this volcano focused on specific features and were limited by the quality and coverage of contemporary data our objective is to review and enhance the existing developmental model for ascraeus by considering all endogenic surface features on the volcano we surveyed the volcano s caldera complex flank terraces pit structures sinuous rilles arcuate grabens and small vents we report the spatial and temporal distributions of these features appraise their proposed formation mechanisms in light of our mapping results and propose a detailed geological history for ascraeus mons an initial shield building phase was followed by the formation of a summit caldera complex and small parasitic cones while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks an eruptive hiatus followed ending with the construction of expansive rift aprons to the northeast and southwest against later extensive flank resurfacing in the late amazonian continued flexure formed arcuate grabens concentric to the edifice localized eruption and surface flow of a fluid agent lava and or water from within the volcano then produced a population of rilles on the lower flanks finally in a change of flank tectonic regime from compression to extension pit crater chains and troughs developed on the main shield and rift aprons eventually coalescing to form large embayments at the northeast and southwest base of the volcano

caldera collapse during the 2000 eruption of miyakejima volcano japan a collapsed caldera 1 6 km in diameter and 450 m in depth was formed at the summit of miyakejima volcano during the 2000 eruption the collapsed caldera appeared on 8 july with a minor phreatic eruption 12 days after seismic activity and magma intrusion occurred northwest of the volcano growth of the caldera took from 8 july to the middle of august with seismic swarms associated with the continuous intrusion of magma northwest of the volcano the growth rate of the caldera was about 1 4 107 m3 day and the final volume of the collapsed caldera was about 6 108 m3 major phreatomagmatic eruptions produced a total of about 1 6 1010 kg 1 1 107 m3 of volcanic ash after caldera growth the caldera structure and the nature of the eruptive materials of the first collapse on 8 july suggest that the surface subsidence was caused by the upward migration of a steam filled cavity with stoping of the roof rock above the magma reservoir the diameter of the stoping column was estimated to be 600 700 m from circumferential faults that developed in the caldera floor and the collapse of the caldera wall enlarged the diameter of the caldera to 1 6 km the total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1 6 2 1 km

electrical conductivity of lawsonite and dehydrating fluids at high pressures and temperatures lawsonite is a calcium aluminum bearing hydrous silicate mineral with caal2si2o7 oh 2 h2o stoichiometry it is thermodynamically stable in the hydrated oceanic crust low velocity anomalies observed in the cold subducted slabs have been related to the unusual shear wave velocities of lawsonite eclogite however electrical conductivity of lawsonite at high pressure and temperature remains unknown in this study we measured the electrical conductivity of lawsonite at 7 gpa and temperatures ranging from 298 k 1320 k at 1173 k the electrical conductivity of lawsonite is around 10 1 s m a sharp increase of electrical conductivity is observed at temperatures exceeding the dehydration 1258 k the high electrical conductivity up to 101 s m observed in our experiments is due to the presence of highly conductive fluid and could explain the low resistivity observed at 150 250 km depths in subduction zone settings such as ne japan northern and central chile

chemical mass transfer modelling of ore forming hydrothermal systems current practise and problems abstract chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid rock reactions in natural hydrothermal systems based on thermodynamic data for minerals and fluid species quantitative simulations of hydrothermal wall rock alteration and ore mineral deposition can be used to develop and test geological models for ore formation which are a basis for mineral exploration and resource assessment in practice the application of thermodynamic mass transfer modelling to specific ore forming systems faces two main problems even for the limiting assumption of local equilibration between fluids and minerals the first problem relates to the geometry of fluid rock interaction where the scheme of the model must be adjusted to the geological question of interest one dimensional reactor models defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock do not necessarily provide a more realistic description of chemical mass transfer than simple titration models without temporal and spatial dimension because in most ore depositing systems the fluids are partially channelized by structures like veins or faults the efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand and wall rock reaction promoting ore mineral precipitation on the other an example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates how ore deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two dimensional multibox model the second problem relates to the extent and quality of thermodynamic data explicit consideration of fe bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid silicate sulphide reactions associated with hydrothermal ore formation although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits aqueous species in high temperature aqueous brines silicate oxide carbonate minerals including fe mg silicates and sulphur bearing minerals including sulphides and sulphates discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the salton sea geothermal system suggest that the redox scale for rock forming silicates may be inconsistent with that of sulphides and aqueous s c o h species this analysis is preliminary but illustrates the importance of field testing of thermodynamic data particularly for hydrothermal conditions between 150 and 450 c which lie between the high temperatures of reliable equilibrium experiments among rock forming minerals 500 c and the low temperatures of many experiments defining the thermodynamics of aqueous species mostly

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

structural and isotopic constraints on fluid flow regimes and fluid pathways during upper crustal deformation an example from the taemas area of the lachlan orogen se australia 1 structural and stable isotope studies of calcite veins and host rocks in a 1000 m thick devonian limestone sequence in the taemas area of the lachlan orogen in se australia indicate that externally derived fluids migrated through the sequence over a protracted interval during folding and associated reverse faulting at temperatures of 200 c the evolution of fluid pathways was governed by growth of fold related and fault related fracture networks at transiently supralithostatic fluid pressures the internal structures of veins indicate growth during repeated cycles of permeability enhancement and sealing systematic increase in 18o of vein calcite upward through the limestone sequence and the presence of an o isotope alteration front in the vein system is related to buffering of fluid compositions by fluid rock reaction during upward migration of overpressured low 18o fluids repeated switching of fractures between high permeability and low permeability states promoted episodic flow and changes in flow paths during progressive deformation a lack of significant migration of the isotopic front during the flow history is consistent with fluid rock interaction being associated with numerous separate pulses of fluid ascending through the percolation network irregular variations in fluid 18o with time during vein growth are interpreted to be driven by repeated changes in reactive path lengths reaction rates and flow rates in a discontinuous flow regime systematic upward changes in vein 18o over an area of 20 km2 requires that on a time averaged basis most of the vein system was hydraulically well connected to the external fluid reservoir and that growth of the fracture network occurred by fluid driven invasion percolation processes repeated failure and sealing events in the fracture system are interpreted to have been driven by episodic migration of fluid pressure waves up through the fracture network possibly in response to deeper level fault rupture events repeatedly breaching an overpressured fluid reservoir

the magmatic plumbing of the submarine hachijo nw volcanic chain hachijojima japan long distance magma transport 1 recent geophysical observations on basaltic composite volcanoes in izu bonin arc reveal the process of long distance lateral magma transport within arc crust to clarify a long distance magma transport system of the basaltic arc volcano from geological and petrological aspects we investigated 20 km long submarine volcanic chains hachijo nw chain and hachijo kojima chain and cones on the northeastern slope ne edifices as well as subaerial satellite cones nested hachijo nishiyama volcano in the northern izu arc front basalts from hachijo nw chain have more primitive composition than those from other edifices the composition of the hachijo nw chain basalts is controlled by fractional crystallization while plagioclase accumulation occurred in ne edifices and subaerial satellite cones trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the volcano this leads us to consider that magma was transported long distances between the nishiyama volcano and the hachijo nw chain primitive magma was laterally transported nnw for at least 20 km in the middle to lower crust 10 20 km deep from nishiyama volcano with only minimal crustal level modifications and formed hachijo nw chain on the other hand magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath nishiyama volcano seems to have been transported in a short distance 5 km and formed ne trending edifices and subaerial satellite cones the long distance magma transport seems to be controlled by a regional extensional stress regime while short distance transport may be controlled by local stress regime affected by load of main volcanic edifice

electrical conductivity of majorite garnet and its implications for electrical structure in the mantle transition zone abstract electrical conductivities of majorite garnet with compositions of pyrolite minus olivine pyrolite majorite and mid ocean ridge basalt morb majorite were measured under physical conditions of the mantle transition zone 18 and 23 gpa and temperatures up to 2000 k in a kawai type multi anvil apparatus the samples with morb composition are mainly composed of majorite which has higher fe and al contents and contain a small amount of stishovite the conductivity of the morb majorite is more than twice higher than those of the pyrolite majorite at the same temperature the activation energies of these majorites are both 1 4 ev at temperature of 1000 1600 k suggesting that the dominant mechanism of charge transportation is fe 2 fe 3 hopping small polaron conduction at higher temperatures 1600 k corresponding to temperature conditions of the transition zone conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction the pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite respectively and will not largely change the conductivity depth profile predicted for the dry mantle transition zone the laboratory based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi global conductivity depth profile obtained from electromagnetic study beneath pacific on the other hand the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of morb majorite is significantly higher than those of wadsleyite and ringwoodite conductivity values of morb majorite agree with those of the stagnant slab beneath the northeastern china

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs