a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

modeling compaction of olivine cumulates in the muskox intrusion we have developed numerical models for compaction of olivine cumulates by simultaneous grain boundary diffusion creep and power law creep the simulation is a coupled model of compaction porous medium flow and heat transport that includes latent heat of fusion and advective heat transport by flow of interstitial liquid it also includes precipitation from interstitial pore liquid and partial melting of cumulates caused by changes in temperature or passage of interstitial melt through regions of different temperature compositions and quantities of liquid and precipitated phases are aetermined from a fractional crystallization simulation using the computer program melts we use these models to simulate the accumulation and crystallization of the muskox intrusion simulations suggest that regardless of their initial state compaction will lithify cumulates into a cohesive mass within meters of the depositional interface upward velocity of interstitial liquid is also much too small to suspend or liquify the crystal column the simulations produced rapidly cooled marginal zones at both top and bottom of the intrusion high rates of compaction are found at the tops and bottoms of cyclic units as a result of both high temperature at the boundary and partial melting caused by upward flow of cooler interstitial liquid as it enters hotter compositionally more primitive cumulates above the cyclic unit contact in contrast upward movement of interstitial liquid into cooler cumulates contrib utes to precipitation both in the upper marginal zone and in the middle region of cyclic units the degree of compaction of cumulates is strongly a function of temperature but if temperature is sufficiently high adcumulates can form in the interior of cumulate bodies by compaction alone

influence of crude oil on the genesis of the lanjiagou porphyry molybdenum deposit western liaoning province china abstract the lanjiagou porphyry molybdenum deposit in western liaoning province china is hosted in fine grained jurassic granites la icp ms zircon u pb analyses indicate that the crystallization of the ore hosting granites took place 185 0 1 8 ma mswd 1 4 molybdenum mineralization in the deposit can be divided into three stages the stockwork quartz vein stage the planar quartz vein stage and the fissure filling quartz vein stage re os isotopic ages for the molybdenite from the stockwork quartz vein type ores yielded an isochron age of 188 8 9 9 ma mswd 3 0 while six samples from the planar quartz vein type ores yielded a similar isochron age of 185 6 1 2 ma mswd 0 5 re os isotopic ages for the molybdenite identical within error to zircon u pb isotopic ages indicate that the molybdenum mineralization is related to the host intrusions apart from primary inorganic fluid inclusions ifis a large number of primary organic fluid inclusions ofis are found in the latter two stages of vein quartz and minors found in the first stage the components and characteristics of ofis in the three stages of vein quartz differ from each other which is also true for the ifis ofis in stockwork vein quartz are characterized by halite bearing inclusions and organic liquids in the inclusions are brown and do not fluoresce under ultraviolet uv light homogenization temperatures t h for the primary ifis coeval with ofis of this stage ranges from 300 c to 450 c while the salinity varies from 10 to 53 wt nacl equiv in planar vein quartz ofis are predominately two phased liquid and gas and salt daughter minerals halite are absent organic liquids are light brown to colorless and show blue fluorescence under uv light the t h range for the ifis of this stage is 250 360 c and the salinity range is 3 17 wt nacl equiv finally ofis in fissure filling vein quartz are marked by liquid gas inclusions organic liquids are generally colorless and show yellow fluorescence under uv light the t h range for the primary ifis is 180 240 c and the salinity range is 4 11 wt nacl equiv organic geochemical analyses indicate that organic matter in the lanjiagou deposit was derived from mature crude oil we suggest that large volumes of crude oil bearing non magmatic fluids were flushed into the lanjiagou porphyry hydrothermal system during all phases of ore formation and likely played important roles in mineralization

postcumulus processes in layered intrusions during the postcumulus stage of solidification in layered intrusions fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks one mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir this process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions but could not form pure monomineralic rocks intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment dense magma emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers reactions between melt and matrix may lead to changes in mineral compositions mineral textures and whole rock isotope compositions another important mechanism for forming adcumulate rocks is compaction in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled for cumulate layers from 10 to 1000 metres in thickness compaction can reduce porosities to very low values 1 and form monomineralic rocks the characteristic time scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion during compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix both compaction and compositional convection can be interrupted by solidification in the pore spaces compositional convection will only occur if the rayleigh number is larger than 40 if the residual melt becomes lower in density and the convective velocity exceeds the solidification velocity measured by the rate of crystal accumulation in the chamber orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection

the structure of three dimensional convection beneath oceanic spreading centres summary buoyant flow beneath oceanic spreading centres can be driven by density gradients due to thermal expansion and compositional gradients arising from the extraction of melt numerical experiments are used to examine the pattern of mantle flow and melting produced by these two sources of buoyancy compositional buoyancy leads to three dimensional flow beneath the spreading axis with along axis variations in upwelling and melt production thermal buoyancy drives axis parallel circulation and convective rolls aligned perpendicular to the spreading axis that originate at a distance off axis controlled by the spreading rate and mantle viscosity hie distance from the axis to the onset of observed sea floor gravity lineations may show a similar dependence on spreading rate compositional density stratification suppresses the formation of thermally driven rolls near the axis and when it is large causes the thermal rolls to become unstable leading to time dependent behaviour the amount of crust produced is less sensitive to spreading rate than in passive flow models however it has a maximum at intermediate spreading rates due to the presence of thermally driven upwelling in the melting region along axis variations in upwelling and melt production suggest that buoyant flow may have an important effect on the segmentation of spreading centres particularly at slow spreading rates

a reevaluation of crystal size distributions in chromite cumulates although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling it has not been demonstrated that crystal size distributions csds are consistent with such a process plots of crystal size fractions per unit volume vs crystal size for chromite grains from the stillwater complex show a log linear distribution with negative slope at larger sizes and a concave down distribution at smaller sizes the log linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas however the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions an interpretation consistent with textural and volume balance evidence the csd trends imply that cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains similarities between the stillwater data and csd trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

bubble migration in a compacting crystal liquid mush recent theoretical models have suggested that bubbles are unlikely to undergo significant migration in a compaction crystal mush by capillary invasion while the system remains partly molten to test this experiments of bubble migration during compaction in a crystal liquid mush were modeled using deformable foam crystals in corn syrup in a volumetric burette compacted with rods of varying weights a bubble source was provided by sodium bicarbonate alka seltzer large bubbles several crystal sizes are pinched by the compacting matrix and become overpressured and deformed as the bubbles experience a load change from hydrostatic to lithostatic once they begin to move they move much faster than the compaction driven liquid bubbles that are about the same size as the crystals but larger than the narrower pore throats move by deformation or breaking into smaller bubbles as they are forced through pore restrictions bubbles that are less than the typical pore diameter generally move with the liquid the liquid bubble mixture behaves as a single phase with a lower density than the bubble free liquid and as a consequence it rises faster than bubble free liquid and allows for faster compaction the overpressure required to force a bubble through the matrix max grain size 5 mm is modest about 5 and it is estimated that for a grain size of 1 mm the required overpressure would be about 25 using apatite distribution in a stillwater olivine gabbro as an analog for bubble nucleation and growth it is suggested that relatively large bubbles initially nucleate and grow in liquid rich channels that develop late in the compaction history overpressure from compaction allows bubbles to rise higher into hotter parts of the crystal pile where they redissolve and increase the volatile content of the liquid over what it would have without the bubble migration leading to progressively earlier vapor saturation during crystallization of the interstitial liquid bubbles can also move rapidly by surfing on porosity waves that can develop in a compacting mush

geochemical evidence for low magma supply and inactive propagation at the galapagos 93 25 w overlapping spreading center 1 major and trace element concentrations sr nd and pb isotopic ratios and plagioclase and olivine compositions of samples from 92 35 w to 94 10 w on the galapagos spreading center gsc constrain magmatic processes associated with the 93 25 w overlapping spreading center osc significant variations in parental magma compositions and extent of low pressure fractionation over short along axis distances suggest small discontinuous or poorly mixed magma chambers mineralogical petrological and geochemical data indicate this osc is characterized by low overall melt supply regional along axis geochemical gradients along the western gsc e g in 206pb 204pb nb zr and la 8 sm 8 are interrupted by increased variability within the osc consistent with less efficient mixing of melts in this region despite a long term history of westward propagation bathymetric and geochemical data are inconsistent with recent propagation of this offset the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone the eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements one outside and one within the overlap zone the overlapping portion of the western limb also has low ratios of these elements these patterns are consistent with previously depleted mantle being preferentially melted beneath this offset

ages and geodynamic settings of xilamulun mo cu metallogenic belt in the northern part of the north china craton abstract the xilamulun belt along the northern part of the north china craton is located in eastern segment of the central asian orogenic belt and has great economic potential for mo cu mineralization more than ten medium to large ore deposits have been discovered in this region in the recent years the major types of mineralization type include porphyry chehugou mo cu kulitou mo cu xiaodonggou mo and jiguanshan mo quartz vein nianzigou mo xinjing mo epithermal hongshanzi mo u and alteration assemblage liulingou mo the timing of mineralization was previously thought to be yanshanian 208 290 ma however indosinian 260 208 ma ages for intrusions and mineralization have been recognized in recent years based on geochronologic data and regional geological evidence it is suggested that the mineralization in the xilamulun belt was formed during multiple events the mineralization processes are related to a post collisional extension stage 258 210 ma with the generation of the porphyry molybdenum copper deposit a tectonic stress transformation from ns to ew 185 150 ma that gave rise to vein or porphyry molybdenum deposit and a lithospheric thinning stage 140 110 ma with porphyry molybdenum deposit

buoyant melting instabilities beneath extending lithosphere 1 numerical models buoyant decompression melting instabilities in regions of partially molten upper mantle have been proposed to be an important process that might account for some characteristics of intraplate volcanism on earth and other terrestrial planets the instability is driven by variations in the melting rate within a partially molten layer whenever a relative decrease in density accompanies decompression melting of ascending mantle here the development of buoyant decompression melting instabilities in a plane layer of passively upwelling and partially melting mantle beneath diffusely extending lithosphere is studied using numerical convection models covering a wide range of physical parameters we find that the occurrence and nature of these instabilities in such a scenario is strongly affected by the rate of extension and melt percolation as well as depth distribution of solid density variations arising from melt depletion in some cases instabilities do not occur during extension but only develop after extension has slowed or stopped completely this behavior creates two pulses of magma generation due to passive upwelling accompanying extension followed by the subsequent instability and is favored by a faster rate of extension higher mantle viscosity higher rate of melt percolation and smaller amount of solid residuum depletion derived buoyancy larger degrees of solid density changes accompanying melt depletion can enhance the instability of partially molten mantle during extension but decrease the cumulative volume of generated melt this kind of behavior modifies the conventional expectation of spatially and temporally correlated volcanism and extension and may lend insight into the observed increase in localized volcanic activity following miocene basin and range extension in the western united states

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

grain scale permeabilities of synthetic quartzite with volumetrically minor phlogopite corundum or aluminosilicate abstract in order to examine grain scale permeabilities in materials that are microstructurally similar to those of the lower crust and upper mantle we produced porous synthetic quartzites with small fractions of an additional minor mineral powdered quartz was mixed with minor amounts of either corundum or phlogopite and run in ag capsules at 800 850 c and 1 4 gpa for 5 to 10 days the resulting quartzites generally exhibited microstructural equilibrium and contained phlogopite corundum or aluminosilicate the minor mineral in all runs typically resided along quartz grain boundaries at low volume fractions x but increasingly occurred as porous clusters within corner pores at higher x in all polished sections the smallest quartz bound pores appeared free of the minor mineral pore geometry of all materials differed from that observed in pure synthetic quartzite pores exhibited an increased number of facets a small decrease in the dihedral angle and partial filling of larger pores by the minor phase measurements of permeability k were both higher and lower than those of monomineralic quartzite with similar with k decreasing as a function of x this decrease is likely the result of pore throat reduction either through fluid redistribution or obstruction of fluid pathways

rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the cuillin centre isle of skye the outer bytownite gabbros of the cuillin centre on the isle of skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse grained massive gabbro and finer grained laminated gabbro we infer that the massive layers formed from repeated influxes of plagioclase phyric magmas which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor the phenocrysts accumulated rapidly trapping large amounts of pore liquid in the cumulus pile during the intervals between influxes laminated gabbro layers formed by crystallization of the resident magma these layers accumulated more slowly by crystal settling or in situ growth allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization the resulting fractions of trapped pore liquid were510 in laminated gabbros and 20 40 in massive gabbros high porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below among layers whole rock sr and nd isotope ratios reveal variable degrees of contamination by the archean crust beneath skye in the lower part of the section influxes of plagioclase phyric magma are associated with a shift to more primitive mineral compositions lower sr sr and higher nd nd indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber in accordance with this interpretation massive layers have lower sr sr than laminated layers consistent with derivation of the plagioclase phenocrysts from a less contaminated source in the upper part of the section the shift toward more primitive and less contaminated compositions is reversed suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion these observations and inferences coupled with published evidence for other isotopic reversals support a model for growth of the cuillin centre by frequent replenishment of a small volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the archean crust periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas these cycles were superimposed on an overall trend toward less contamination through time consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the hebridean magmatic province matured

electromagnetic constraints on a melt region beneath the central mariana back arc spreading ridge 1 an electrical resistivity profile across the central mariana subduction system shows high resistivity in the upper mantle beneath the back arc spreading ridge where melt might be expected to exist although seismic data are equivocal on the extent of a possible melt region the question arises as to why a 2 d magnetotelluric mt survey apparently failed to image any melt we have run forward models and inversions that test possible 3 d melt geometries that are consistent with the mt data and results of other studies from the region and that we use to place upper bounds on the possible extent of 3 d melt region beneath the spreading center our study suggests that the largest melt region that was not directly imaged by the 2 d mt data but that is compatible with the observations as well as the likely effects of melt focusing has a 3 d shape on a ridge segment scale focused toward the spreading center and a resistivity of 100 m that corresponds to 0 1 1 interconnected silicate melt embedded in a background resistivity of 500 m in contrast to the superfast spreading southern east pacific rise the 3 d melt region suggests that buoyant mantle upwelling on a ridge segment scale is the dominant process beneath the slow spreading central mariana back arc a final test considers whether the inability to image a 3 d melt region was a result of the 2 d survey geometry the result reveals that the 2 d transect completed is useful to elucidate a broad range of 3 d melt bodies

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

compaction of igneous cumulates when initially deposited mafic igneous cumulates consist of a solid matrix of mineral phases plus 50 60 interstitial liquid due to its greater density the matrix compacts squeezing the interstitial liquid upward a numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant compaction occurs in one of two distinct models for rapid deposition the system is permeability controlled with compaction occurring only at the base of the cumulate pile for slowly deposited cumulates an additional compaction zone that is partially viscosity controlled develops at the top of the pile if the extent of compaction and thickness of this zone are known for a real system the model can be used to estimate the permeability and effective viscosity of the compacting cumulates such calculations yield a permeability of 1 5 times 10 3 a 2 f 3 1 f 2 for olivine cumulates in the muskox intrusion reliable viscosity estimates await an empirical law relating ma

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

improved global maps and 54 year history of wind work on ocean inertial motions 1 the global distribution and 54 year time dependence of the energy flux from the wind to near inertial motions is computed by driving a slab mixed layer model with ncep ncar reanalysis winds improving upon previous estimates alford 2001 watanabe and hibiya 2002 the slab model is solved spectrally with frequency dependent damping the resulting solutions are more physically sensible than the previous and more skillful at high latitudes where the inertial frequency approaches the 4 daily sampling of the reanalysis winds this enables alford s calculation whose domain was limited to 50 to be extended to the poles the high latitude reliability is demonstrated by direct comparison with a high resolution regional model remo in the ne atlantic the total power input 0 47 tw has increased by 25 since 1948 paralleling observed increases in extratropical cyclone frequency and intensity if believable the trend may have important consequences for modulation of the meridional overturning circulation

the generation and composition of partial melts in the earth s mantle abstract a set of equations is presented which combines the constraints of fluid dynamics and multicomponent phase equilibrium to provide a unified description of partial melting in the earth s mantle the equations are applied to a one dimensional model for pressure release melting of a simplified mantle material which contains only two chemical components exhibiting either a complete solid solution or b a binary eutectic in both cases melting occurs over a range of depths the unmelted crystalline residue matrix is modeled as a saturated porous medium through which the melt can migrate because of its differential buoyancy since melt interacts continuously with the matrix during ascent melting occurs by equilibrium rather than fractional fusion this equilibrium fusion is not the same as batch fusion however since material elements are quickly dispersed by migration of melt relative to the matrix to a first approximation the temperature profiles adiabats in the partially molten zone are independent of melt migration the slope of the adiabats varies in inverse proportion to the number of degrees of freedom which characterizes the melting melting of a complete solid solution occurs along a wet adiabat whose slope is controlled by absorption of latent heat melting of a eutectic system occurs along a steeper univariant adiabat until one solid phase is exhausted and subsequently along a wet adiabat the velocity of melt migration can exceed the mantle upwelling velocity by an order of magnitude or more the volume fraction of melt present is always less than the fraction of the material which has melted and is unlikely to exceed a few percent for a wide range of initial conditions melting of a eutectic system produces erupted melts having constant major element composition and widely varying trace element composition this result may provide a partial explanation for the characteristic major and lil element patterns observed in morb liquid compositional paths during pressure release melting cannot be determined from isobaric sections of phase diagrams and can only be calculated by solving the energy equation subject to the appropriate depth dependent phase equilibrium constraints

compaction of density stratified cumulates effect on trapped liquid distribution igneous rocks formed by the accumulation of crystals from a cooling magma body compose a significant fraction of both the terrestrial oceanic crust and the lunar crust and mantle here we report the results of numerical simulations of compaction of accumulating crystals for which the bulk density is not constant the simulations are designed to provide insight into the behavior of interstitial liquid during compaction when a sharp drop in the density of the cumulus minerals is encountered e g at the ultramafic mafic boundary in a layered intrusion the simulations provide an explanation for trapped liquid distributions associated with sharp mineral density transitions density transition in the mineral assemblage is a feature common to many layered intrusions and the oceanic crust this process does not require external mechanisms such as injections of new and different magmas and has important implications for the formation of pt ores found at ultramafic mafic transitions the model also has implic

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

buoyancy effects on mantle flow under mid ocean ridges results of a series of two dimensional numerical experiments of mantle flow melting and melt migration under a spreading center are reported the model predicts the distribution of melt in the subridge mantle the width over which most melt is delivered to the crust and the thickness of crust the sources of buoyancy considered are thermal expansion compositional variation caused by melt extraction and the phase change of solid to melt we infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 pa s for a lower average viscosity thermal convection causes rapid cooling of a large region under a ridge raising the viscosity results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges we assume that the viscosity is proportional to exp 0 where is melt fraction we parameterize the permeability in terms of the reference velocity for percolation of the melt vr where the relative velocity of melt to solid is vr analytic approximations are used to extrapolate the model results to large values of permeability and small values of 0 if vr is less than 1 m yr then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers for vr greater than about 100 m yr so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the east pacific rise estimated from seismic surface waves buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion for most melt to be added to the crust within a few kilometers of a fast spreading center requires that 0 be less than 0 015 if vr equals 1 m yr and less than 0 003 if vr is 100 m yr

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

chemical transects across intra oceanic arcs implications for the tectonic setting of ophiolites systematic spatial variations in ti y and zr are found in volcanic sequences sampled as transects from back arc through arc to trench within the western pacific intra oceanic arc systems these correlations occur regardless of eruption age reflecting the retention of relative positions between individual tectonic units concentrations of these incompatible elements are taken to reflect the nature of the underlying mantle at a particular trench volcano distance during the subduction process volcanics become more depleted trenchwards implying that progressively shallower levels melts were extracted from more refractory or harzburgitic mantle a model is proposed whereby a subduction system is superimposed on a compositionally stratified mantle which is a product of an earlier spreading event on the basis of this evidence the trench distance variation model is applied to the troodos ophiolite complex similar spatial changes in lava chemistry are found across the ophiolite providing evidence for the relative position of the troodos trench and direction of subduction beneath the troodos basin these findings have implications for studies in other orogenic terrains where structure and exposure permit their application

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

thickening of the oceanic crust with age the most widely accepted models for ocean crust formation and composition fail to predict the thickening of the oceanic crust with age yet such thickening has been described in the pacific ocean for years to reconcile this apparent long standing conflict we have reexamined the evidence for the thickening of the pacific crust by using a statistical treatment of a large number of seismic profiles and by doing a detailed reanalysis of several profiles fanning a transect across the southern pacific ocean from the statistical studies we find that crust younger than 30 ma has a mean thickness of 5 67 km whereas crust between 30 and 100 ma has a mean thickness of 6 01 km this 0 34 km difference although statistically significant is far less than that reported in several previous studies our results from the south pacific transect suggest that changes in the crustal thickness are not systematic and from both studies it appears that crustal thickening is not particularly important the small thickening that we do observe is probably the result of isolated processes that are not active under the oceanic crust as a whole as a result arguments favoring a large component of serpentinite in the crust cannot be based upon evidence for crustal thickening beneath the pacific ocean

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

the role of mantle depletion and melt retention buoyancy in spreading center segmentation abstract numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center buoyancy results from the depletion of residual mantle in iron relative to magnesium as melt is extracted mantle depletion buoyancy and from the presence of low density melt melt retention buoyancy 3 d buoyant mantle flow arises spontaneously from an initially 2 d solution for low mantle viscosity and low spreading rate at high viscosity and high spreading rate initially 2 d solutions remain 2 d this may explain the fundamentally different structure of fast and slow spreading centers in a uniform viscosity halfspace the along axis wavelength of 3 d buoyant upwelling scales with the maximum depth of melting the only length scale in this system for reasonable maximum depths of melting 60 90 km along axis wavelengths of 200 300 km are preferred longer than the 50 100 km segmentation length of slow spreading centers in a viscosity layered halfspace the thickness of the asthenosphere introduces another length scale a wavelength of segmentation comparable to the asthenosphere thickness also develops in our numerical experiments this suggests the possibility that a wavelength corresponding to the spacing between gravity lows may be controlled by the asthenosphere thickness while the spacing of major fracture zones corresponds to the longer wavelength 3 times the maximum depth of melting intrinsic to the melting region the along axis structure of 3 d flow varies from narrow focused upwellings at low spreading rates and mantle viscosities to broad regions of upwelling at high spreading rates and mantle viscosity to allow an along axis crustal thickness variation no larger than that which is observed 3 4 km the highly focused upwelling and crustal production predicted at slow spreading rates requires appreciable along axis transport of melt

an alternative dynamic model of the axial topographic high at fast spreading ridges recent models that attempt to explain the formation of an axial topographic high at fast spreading ridges invoke the presence of a narrow low density low viscosity melt conduit to buoyantly support the high mantle up welling beneath mid ocean ridges must be highly focused to create the narrow melt zone required by these buoyancy models we show that an alternative model in which extensional stresses in the upper crust are relieved by diking predicts uplift at the ridge and can match the shape and magnitude of the axial high in this model the extensional stresses deepen toward the ridge axis creating dynamic moments within the crust that are balanced by the axial topography along axis variations in the height and width of the axial high may be caused by changes in the depth and width of the axial magma chamber or flexural rigidity of the lithosphere this model is similar to dynamic models of median valley formation at slow spreading ridges in that the axial topography is uncompensated and supported by extensional stresses in the lithosphere and asthenosphere in addition to being able to explain the axial topography observed at fast spreading ridges this model is consistent with either broad passive or narrow dynamic upwelling at mid ocean ridges the change from median valley to axial high is caused by differences in strength of the axial lithosphere and rate of thickening of the plate

quantitative modeling of compaction in the holyoke flood basalt flow hartford basin connecticut compaction in the holyoke flood basalt flow is modeled using a quantitative phase equilibrium routine based on the melts algorithm that is coupled with compaction driven heat and mass transport equations for a crystal liquid mush that is cooling from both the top and the bottom the compaction is qualitatively different in the upper and lower parts of the flow the solid fraction in the lower part increases as a result of concurrent compaction and solidification in contrast cooling from the upper boundary results in an increase in viscosity and a decrease in permeability toward the top this causes the velocity of the solid matrix to increase downward and the upper part to undergo dilation of the solid matrix the result is an s shaped profile for compatible elements and an inverse profile for incompatible elements the region of modeled maximum dilation matches the location of segregation sheets in the holyoke flow discrepancy between observed deformation as calculated from textural deformation of plagioclase chain networks versus compositional profiles suggest some crystal accumulation and or compaction prior to the formation of the plagioclase chains

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

viscosity of partial melts in the upper mantle the viscosity and density of melts of composition close to primary olivine tholeiite and alkali olivine basalt have been measured at pressures between 0 7 and 1 5 gpa and at temperatures between 1300 and 1475 c using the falling floating sphere method the viscosity of the olivine tholeiite melts decreases only slightly from 1 atm to 1 0 gpa whereas that of the alkali olivine basalt melts decreases by a factor of about 2 in the same pressure range and they become very close to one another about 2 0 pa s at 1 0 1 25 gpa at 1400 c the activation energy for viscous flow of the olivine tholeiite melt is about 250 kj mol at 1 0 gpa the viscosity of partial melts melts formed by partial melting in the upper mantle can be estimated based on the viscosities of the melts and the activation energy for viscous flow obtained by the present experiments the viscosity of the partial melts along the anhydrous solidus of peridotite decreases from about 2 5 pa s at 0 8 gpa 1220 c to only about 0 2 pa s at 3 5 gpa 1580 c thus the partial melts must be extremely fluid at depths greater than 100 km even under anhydrous conditions such partial melts might not ascend because of their high density and play an important role in the movement of the lithosphere the viscosities of the partial melts in the upper mantle may decrease with increasing depth until completion of the transformation of si to six fold coordination the viscosities of magmas and the diffusivity of oxygen in them have an inverse relation which is approximated by the eyring s equation the diffusivity of oxygen in the partial melts estimated from this relation and the viscosity increases from 10 7 cm2 s at 0 8 gpa to 10 5 cm2 s at 3 5 gpa along the anhydrous solidus of peridotite homogenization of partial melts or magmas would be greatly enhanced with increasing depth down to the viscosity minimum

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

vertical structure of the head of a cylindrical mantle plume from a model of two phase flow with melting summary within the mushroom shaped head of a cylindrical mantle plume melting occurs the melt segregates from the matrix and the matrix deforms and spreads laterally these processes have been studied with a model of two phase flow with melting after characteristics of the solution near the axis of symmetry of the plume were found a set of asymptotic relations for the variables along the symmetry axis was derived from mckenzie s equations for conservation of mass momentum and energy of a two phase system the distribution of porosity along the plume axis and the vertical and radial segregation velocity of the melt in the vicinity of the axis of symmetry were obtained as functions of depth our analytic results show that within the head of a cylindrical mantle plume the contribution of the deformation of the matrix to the total non hydrostatic pressure gradient cannot be neglected and melt convergence or divergence is controlled by the radial scale of the upward velocity profile at the depth of the beginning of melting

geological characteristics and geodynamic background of molybdenum copper deposits along yanshan liaoning metallogenic belt on northern margin of north china block the northern margin of the north china craton is one of the most important polymetallic mo cu fe pb zn au ag ore forming areas in china the yanshan liaoning molybdenum copper metallogenic belt is located in this area and controlled jointly by ew ne and nne trending faults mineralization exhibits zoning distribution with molybdenum deposits in western liaoning whereas copper molybdenum deposits in northern hebei these deposits are associated with intermediate acid granites in time and space the molybdenum copper deposits are usually distributed along the endo or exo contact zone of the granite porphyry and belong to porphyry porphyry skarn and skarn types sulfur and strontium isotope analyses show that ore forming materials and ore forming fluids were derived from the syntectic type granites of the lower continental crust molybdenum deposits are associated with mesozoic si and k rich granite porphyry whereas copper deposits are related to granites with relatively low acid alkaline contents based on metallogenic chronology the authors hold that there existed two pulses of large scale mineralization 180 ma and 140 ma in the yanshan liaoning metallogenic belt whose metallogenic geodynamic settings were the post collisional orogeny of northern china block and siberian block and the late variation stage of the tectonic regime in eastern china respectively

genesis and tectonic setting of the giant diyanqin amu porphyry mo deposit in great hingan range ne china constraints from u pb and re os geochronology and hf isotopic geochemistry abstract the great hingan range is located in the eastern sector of the central asia orogenic belt and is the second important mo province in china next to the qinling dabie orogenic belt previous studies concluded that all the mo deposits mo only or mo dominated polymetallic in the area were formed in the mesozoic and related either to the syn to post collisional setting following the closure of the paleo asia ocean or to the westward subduction of the pacific plate however the fluids forming the giant diyanqin amu porphyry mo deposit in the area disagree with the features of collision type mo deposits but accord with those of the subduction or continental arc type the mo mineralization is hosted in the late jurassic volcanic rocks and genetically related to the buried porphyritic granite and aplitic granite two porphyritic granite samples yield la icp ms zircon weighted mean 206 pb 238 u ages of 158 7 0 8 ma mswd 0 64 2 and 158 0 0 9 ma mswd 0 38 2 respectively the aplitic granite yields a zircon u pb weighted mean 206 pb 238 u age of 156 9 1 1 ma mswd 0 48 2 slightly younger than the porphyritic granite seven molybdenite samples from the ores yield re os isotope ages of 156 1 2 2 to 158 1 4 3 ma these ages constrain that the diyanqin amu mo system was formed in the period of 159 156 ma the granites at the deposit have high contents of sio 2 k 2 o and al 2 o 3 and low contents of tio 2 mgo and cao showing a peraluminous high k calc alkaline affinity these granites and the host volcanic rocks are characterized by enrichment of k rb u pb th and lree and depletion of nb ta ti and hree with negative eu anomaly which are similar to magmatic rocks in the andean arc zircon grains from the granites show positive e hf t values of 4 2 9 0 with t dm2 hf ages of 0 63 0 94 ga these geochemical data imply that the granites at diyanqin amu are highly fractionated i type formed in a continental arc generated by the southeastward subduction of the mongol okhotsk oceanic plate

the permeability controlled accumulation of primary magma the initial accumulation of primary magma occurs just after the mantle has become permeable the accumulation is caused by the compaction of the residuum which either may be controlled by the rate of creep or by the rate of flow of the interstitial melt experimental results suggest that the rate of compaction is controlled by the permeability and a model for the accumulation process is worked out on this basis the compaction causes the formation of a lower compaction boundary and an upper layer of melt the ascending mantle of plumes and convection currents will form layers of melt situated 20 100 m apart the type of partial melting for this accumulation is critical melting

the role of lithospheric phase transitions on seafloor flattening at old ages abstract uplift of old ocean seafloor relative to the familiar t 1 2 curve is generally presumed to be a consequence of an additional heat source at the base of the oceanic lithosphere several geodynamical mechanisms have been invoked to explain this behavior we have taken a petrological rather than geodynamical approach and considered whether the causes of the flattening can be found within the conductive portion of the lithosphere where viscous creep is insignificant accordingly self consistent calculations of the phase assemblages of several candidate mantle compositions have been performed subject to the constraint of experimentally determined calorimetric measurements we have found that in general for temperatures below around 800 c and pressures greater than 10 kbar the phase change from spinel peridotite to garnet peridotite has a significantly negative clapeyron slope which could cause some amount of uplift of old seafloor the transition for representative thermal profiles should be depressed from about 35 km at 40 ma to 45 km at 160 ma since spinel peridotite is of lower density the net effect is to raise the seafloor topography the extent of the uplift which depends primarily on the al 2 o 3 content of the peridotite and on the effective thermal diffusivity of the lithosphere should be on the order of 140 220 m

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

evidence from the oman ophiolite for sudden stress changes during melt injection at oceanic spreading centres the system of dikes in the uppermost mantle below oceanic spreading centres bears witness to the processes involved in the supply of melt to the ridge such systems cannot be studied on currently active ridges but can be seen in ophiolites in particular the oman ophiolite allows us to examine segments of a fast spreading ridge1 here we present a statistical analysis of the orientation of intrusions in the uppermost mantle section of the oman ophiolite spreading centres which points to the existence of two systems dikes whose azimuth is parallel to the sheeted dikes which we thus consider as related to the extensional lithospheric stress field and sills roughly parallel to the moho2 branching contemporaneous dikes and sills are frequently observed we suggest that these relations can be explained by repeated sudden changes of the dominant stress field from lithospheric to asthenospheric such changes could result from the episodic relaxation of lithospheric stress in response to tension fracturing of the lithospheric lid related to sudden melt surges this episodic behaviour is recorded in the sheeted dike complex with each dike each about one metre wide representing one of these fractures

asymmetric electrical structure in the mantle beneath the east pacific rise at 17 s the magnetotelluric component of the mantle electromagnetic and tomography melt experiment measured the electrical resistivity structure of the mantle beneath the fast spreading southern east pacific rise epr the data reveal an asymmetric resistivity structure with lower resistivity to the west of the ridge the uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles mantle resistivities to the west of the ridge are consistent with a low melt fraction about 1 to 2 percent interconnected melt distributed over a broad region and extending to depths of about 150 kilometers the asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

using apatite to dispel the trapped liquid concept and to understand the loss of interstitial liquid by compaction in mafic cumulates an example from the stillwater complex montana the size paragenetic setting distribution and composition of apatite grains in 16 thin sections from one sample of an olivine gabbro from the stillwater complex montana were documented to determine whether they grew from trapped interstitial liquid or in an open system the grains show extreme variability in cross sectional area ranging from 300 000 m2 the apatite grains are not associated with quartz fe ti oxides or evolved overgrowths on cumulus grains as expected for the crystallization of a trapped liquid which suggests that they grew in an open system the grains are very irregularly distributed with a single cluster comprising almost 25 of the cumulative apatite cross sectional area from all 16 slides the spatial patterns of the apatite indicate that the interstitial liquid moved in tubular channels during the final stages of compaction the trace element composition of liquids calculated from apatite and silicate mineral compositions are similar indicating that the apatite crystallized from liquids that were not dramatically enriched in incompatible elements we conclude that the interstitial liquid never actually became trapped that liquid was lost in an open system to essentially zero porosity and that the model of cumulates as a combination of cumulus grains and trapped liquid should be discarded

the cumulate paradigm reconsidered the cumulate terminology outlined by wager and coworkers provided a logical system for describing the textures of the rocks of layered intrusions but recent work shows that this model fails to accommodate all the processes responsible for the petrographic features of coarse grained rocks textures of adcumulates and orthocumulates in the skaergaard intrusion cannot be correlated with concentrations of incompatible elements representing trapped liquid compaction and open system percolative processes have altered their textures modes and even bulk compositions although a rock may be a cumulate in the absence of other evidence it can be misleading to refer to it in terms of the cumulate nomenclature

the origin of laminated and massive anorthosite sept iles layered intrusion quebec canada well laminated plagioclase rich rocks anorthosite occur as layers in mineralogically similar but massive rocks in the sept iles intrusion several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer there in the absence of other events crystallization produced the massive anorthosite the laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density currents such currents would have caused the zone of simple shear of the boundary layer to migrate outwards into the previously stationary part this simple shear would then have rotated the existing plagioclase crystals as well as crystals that nucleated at that time towards the shear plane some rocks show evidence of several periods of enhanced flow velocities the production of the lamination may have weakened the crystal mush and enabled slumping to occur producing folds and contorted layers finally compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal pile earlier formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more complex rocks including igneous breccias

a re os study of molybdenites from the lanjiagou mo deposit of north china craton and its geological significance article i nfo the lanjiagou mo deposit is located in the eastern part of the north china craton rhenium and osmium isotopes in molybdenites from the lanjiagou porphyry mo deposit have been used to determine the timing of mineralization molybdenite was analyzed mainly from granite porphyry which is characterized by moderate to strong silicification rhenium concentrations in molybdenite samples are between 33 and 48 g g analysis of eleven molybdenite samples yields an isochron age of 181 6 6 5 ma 2 based on the geological history and spatio temporal distribution of the granitoids it is proposed that the mo deposits in the eastern part of the north china craton were related to the subduction of the paleo pacific plate during jurassic time 2009 published by elsevier b v on behalf of international association for gondwana research

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

reply to comment by f feser et al on improved global maps and 54 year history of wind work on ocean inertial motions 1 in their comment feser et al 2003 argue that the ncep reanalysis surface winds used by alford 2003 henceforth a03 are artificially high at high frequencies due to temporal aliasing this possibility is investigated here by direct comparison with buoy winds in the northeastern atlantic as in a previous more extensive buoy comparison in the pacific alford 2001 henceforth a01 no evidence for aliasing of ncep winds is found

halogen geochemistry of the great dyke zimbabwe apatite from the great dyke of zimbabwe is relatively rich in the hydroxy fluorapatite end members the mole fraction of fluorapatite increases from approximately 40 in cumulates of the ultramafic sequence to over 60 in a sample near the top of the exposed mafic sequence the chlorapatite component decreases from a typical high of 10 20 mole in the ultramafic sequence to about 1 in the uppermost part of the mafic sequence however within sample variation may be as great as the entire stratigraphic variation halogen contents of marginal samples generally are similar to axial samples but tend not to have as high cl concentration and tend to oh enrichment biotite compositions approach hydroxyl end member compositions and apatite biotite oh f exchange geothermometers give an average closure temperature of 564 c apatite from the umvimeela dyke an unlayered dike that parallels the great dyke over much of its length contains less cl than is seen in the ultramafic sequence cumulates of the great dyke while the overall stratigraphic trend is characterized by a decrease in the cl f ratio with stratigraphic height within the p1 unit at the top of the ultramafic sequence there is a positive correlation between cl and other incompatible elements such as na and ce the apparent contradiction between the general stratigraphic trend of decreasing cl f ratio with fractionation and the apparent increase in cl and other incompatible elements seen in the p1 unit can be explained by assuming that the great dyke magma chamber was degassing near its top where confining pressure was lowest and cl was preferentially lost to a separating volatile rich fluid as cumulates formed on the floor they entrapped liquid that was increasingly depleted in cl at the higher stratigraphic levels however at any given stratigraphic interval either local fluid enrichment or the eventual crystallization of halogen bearing minerals that incorporate the smaller f ion in preference to the larger cl ion led to a local increase in the cl f ratio

molybdenite re os age h o c s pb isotopes and fluid inclusion study of the caosiyao porphyry mo deposit in inner mongolia china abstract the giant caosiyao porphyry mo deposit located in the northern margin of the north china craton is a newly discovered mo deposit in the yan liao molybdenum belt orebodies of the deposit are mainly hosted in metamorphic rocks of the mesoarchean jining group with a small amount of orebodies in the late jurassic syenogranite porphyry the ore forming process of the deposit can be divided into three stages an early quartz molybdenite magnetite pyrite stage a middle quartz molybdenite pyrite pyrrhotite chalcopyrite stage and a late quartz carbonate pyrite fluorite pyrrhotite stage molybdenite re os dating indicates that the caosiyao deposit formed at ca 150 ma four types of fluid inclusions fis have been distinguished in quartz phenocryst and various quartz veins including liquid rich gas rich h2o co2 and daughter mineral bearing inclusions the fis in the quartz phenocrysts of the syenogranite porphyry contain liquid rich gas rich h2o co2 and daughter mineral bearing types their homogenization temperatures and salinities vary from 385 c to 550 c and 6 5 wt to 65 0 wt nacl eqv respectively the fis in the quartz of the early stage are also mainly liquid rich gas rich h2o co2 and daughter mineral bearing types their homogenization temperatures and salinities vary from 243 c to 401 c and 6 0 wt to 39 8 wt nacl eqv respectively the fis in the quartz of the middle stage are mainly liquid rich h2o co2 and daughter mineral bearing types their homogenization temperatures and salinities vary from 208 c to 336 c and 1 2 wt to 34 1 wt nacl eqv respectively the fis in the quartz of the late stage are only liquid rich type their homogenization temperatures and salinities vary from 124 c to 196 c and 3 9 wt to 13 2 wt nacl eqv respectively the ore forming fluids of the caosiyao deposit are characterized by moderate high temperature and great fluctuating salinity belonging to an h2o nacl kcl mgcl2 co2 ch4 system the 18oh2o and dh2o values of the early and middle stages vary from 3 5 to 7 4 and 96 to 77 respectively the 18oh2o and dh2o values of the late stage vary from 2 9 to 0 6 and 112 to 77 respectively indicating that the ore forming fluids of the early and middle stages mainly consist of magmatic water and that the fluid in late stage is a mixture of magmatic and meteoric water the 13c values vary from 16 5 to 8 8 for the early and middle stages and 11 4 to 5 3 for the late stage suggesting that the carbon of the early and middle stages consists of a mixture of organic and magmatic sources and that the carbon in the late stage was mainly from a magmatic source the 34s values range from 4 0 to 6 1 with an average of 4 8 the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of the metallic minerals are in the ranges of 15 962 17 832 15 200 15 511 and 35 918 37 927 respectively both s and pb isotopic systems indicate that the ore forming metals came primarily from mesozoic intermediate felsic magma the multi stage boiling or immiscibility of the ore forming fluids was the dominant mechanism for the deposition of ore forming materials

the mo deposits of northeast china a powerful indicator of tectonic settings and associated evolutionary trends abstract northeast ne china lies in the eastern sector of the central asian orogenic belt caob that connects with the circum pacific orogenic belt in the east and accommodates multistage magmatism crustal growth and mineralization in this region 69 mo only or mo dominated and 9 cu mo deposits have been discovered including 65 deposits ranking medium size 10 kt mo metal or larger in tonnage and containing a total resource of 10 5 mt mo metal six giant and seventeen large deposits have total reserves of 6 7 and 3 1 mt mo respectively these deposits occur in the areas surrounding the songliao basin including the northern margin of the north china craton the great hingan range and the ji hei fold belt and have been formed during paleozoic and mesozoic tectono magmatic events all the mo only or mo dominated deposits were formed in the mesozoic postdating the closure of the paleo asia ocean and in a series of pulses around 250 200 ma 200 160 ma 160 130 and

platinum solubility and partitioning in a felsic melt vapor brine assemblage abstract the solubility and partitioning of pt in a s free vapor brine rhyolite melt pt metal assemblage has been quantified at 800 c f o 2 nno and pressures of 100 and 140 mpa vapor and brine were sampled at run conditions by trapping these phases as glass hosted fluid inclusions as the melt cooled through the glass transition temperature the vapor and brine were in equilibrium with the melt at the time of trapping and thus represent fluids which were sampled at the termination of each experimental run the microthermometrically determined salinities of vapor and brine are 2 and 63 wt nacl eq and 9 and 43 wt nacl eq at 100 and 140 mpa respectively platinum solubilities in vapor brine and glass i e quenched melt were quantified by using laser ablation inductively coupled plasma mass spectrometry la icp ms equilibrium is discussed with reference to the major and trace element concentrations of glass hosted fluid inclusions as well as the silicate melt over run times that varied from 110 to 377 h at 140 mpa and 159 to 564 h at 100 mpa platinum solubility values 1 in h2o saturated felsic melt are 0 28 0 13 g g and 0 38 0 06 g g at 140 and 100 mpa respectively platinum solubility values 1 x at 140 and 100 mpa respectively in aqueous vapor are 0 91 0 29 g g and 0 37 0 17 g g and in are brine 16 10 g g and 3 3 1 0 g g the measured solubility data were used to calculate nernst type partition coefficients for pt between vapor melt brine melt and vapor brine the partition coefficient values 1 x for vapor melt brine melt and vapor brine at 140 mpa are 2 9 1 0 67 27 and 0 13 0 05 and at 100 mpa are 1 0 0 2 6 8 2 4 and 0 15 0 05 the partitioning data were used to model the pt scavenging capacity of vapor and brine during the crystallization driven degassing i e second boiling of a felsic silicate melt over a depth range i e 3 6 km consistent with the evolution of magmatic hydrothermal ore deposits model calculations suggest that aqueous vapor and brine can scavenge sufficient quantities of pt and by analogy other platinum group elements pge to produce economically important pge rich magmatic hydrothermal ore deposits in earth s upper continental crust

re os ages for molybdenum mineralization in the fengning region of northern hebei province china new constraints on the timing of mineralization and geodynamic setting abstract a recently recognized molybdenum mo metallogenic belt is present within and adjacent to the northern part of the north china craton ncc more than 20 mo deposits are present in the belt including the sadaigoumen and dacaoping porphyry deposits located in the fengning region of the northern part of hebei province the sadaigoumen deposit has a re os molybdenite weighted mean age of 236 5 2 2 ma mswd 1 4 n 6 which is more reliable than existing dates and is interpreted as the precise age for formation of the deposit the dacaoping mo deposit is about 100 million years younger with a re os molybdenite isochron age of 140 1 3 4 ma 2 mswd 0 26 n 5 which is within error of the weighted mean age of 139 4 0 9 ma the ages of the two deposits show that there are at least two episodes of mo porphyry formation in the fengning region in combination with the regional geological evolution of this part of the craton margin we propose that the triassic mo event at sadaigoumen is associated with a collisional event during the closure of the ancient asian ocean whereas the early cretaceous mo event at dacaoping is associated with lithospheric thinning of the ncc

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

the seismic moment budget of slowly spreading ridges mid ocean ridges with well developed median valleys are commonly the sites of large normal faulting earthquakes1 2 a recent global study3 6 of the centroid depths and source mechanisms of such earthquakes has yielded two important conclusions first that all the largest mid ocean ridge earthquakes occur beneath the inner floor of the median valley and second that the maximum thickness of the seismogenic layer increases with decreasing spreading rate these results lend support to the hypothesis that the median valley arises from the necking of a strong brittle lithosphere under horizontal extension7 9 here we compare the global rate of seismic moment release along slowly spreading mid ocean ridges with the rate predicted if widening of the median valley occurs entirely by seismogenic extension of the brittle lithosphere beneath the inner floor we find that earthquakes account for no more than about 10 20 of the rate of plate separation across the central median valley

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data