spine growth and seismogenic faulting at mt unzen japan the concluding episode of activity during the recent eruption of mt unzen october 1994 to february 1995 was characterized by incremental spine extrusion accompanied by seismicity analysis of the seismic record reveals the occurrence of two dominant long period event families associated with a repeating nondestructive source mechanism which we attribute to magma failure and fault controlled ascent we obtain constraints on the slip rate and distance of faulting events within these families that analysis is complemented by an experimental thermomechanical investigation of fault friction in mt unzen dacitic dome rock using a rotary shear apparatus at variable slip rates and normal stresses a power density threshold is found at 0 3 mw m 2 above which frictional melt forms and controls the shear resistance to slip inducing a deviation from byerlee s frictional law homogenized experimentally generated pseudotachylytes have a similar final chemistry thickness and crystal content facilitating the construction of a rheological model for particle suspensions this is compared to the viscosity constrained from the experimental data to assess the viscous control on fault dynamics the onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event this combination of experimental data viscosity modeling and seismic analysis offers a new description of material response during conduit plug flow and spine growth showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock this model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes

global positioning system measurements of ground deformation caused by magma intrusion and lava discharge the 1990 1995 eruption at unzendake volcano kyushu japan abstract global positioning system gps measurements made around unzendake volcano kyushu japan since january 1991 have detected ground deformation caused by magma intrusion and lava discharge in the intermittent phreatic and phreatomagmatic eruption stage the ground was inflating after growth of the lava dome and following frequent pyroclastic flows at unzendake volcano the ground began deflating these ground deformations are explained by the inflation and deflation of a mogi s source model a point source model located about 6 km west of the active crater at a depth of 11 km at an aseismic region the observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation the horizontal displacements at the reference gps station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other gps baseline systems these observations validate our estimates for the pressure source based on the relation between the deformation volume of the ground surface and the discharged volume of the lava it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater magma is estimated to be supplied to the reservoir at an average rate of 1 1 105 m3 day magma intrusion began in december 1989 at the latest and continued for 1 9 103 days

the identification and significance of pure sediment derived granites the characterization of the geochemical reservoirs of the earth s continental crust including the determination of representative upper and lower crustal compositions underpins our understanding of crustal evolution the classic i and s type granite classification has often been invoked to distinguish between melts derived from igneous protoliths and those derived from the melting of a sedimentary source recent geochemical studies suggest that most granites even those cited as typical examples of s type show evidence for a mixture of mantle and upper crustal sources thereby implying that granite formation is evidence for overall crustal growth we have examined the source of leucogranite bodies in one of the world s youngest collisional orogens using novel zircon techniques that can resolve the presence of even minor mantle contributions 232 zircons from 12 granites from the bhutan himalaya were analysed by in situ techniques for o hf and u pb isotopic signatures in combination with data from the granite host rocks our data show that the himalayan leucogranites were derived solely from metamorphosed crustal sediments and do not record any mantle contribution this finding is consistent with the time lag between crustal thickening and widespread crustal melting and the heat producing capacities of the pelitic source rocks we conclude that himalayan leucogranites provide a more suitable type locality for s type granites than the lachlan area in south east australia where the term was first defined the himalayan leucogranites therefore provide evidence that syn orogenic melting during collisional events does not necessarily result in crustal growth importantly crustal growth models should not always assume that crustal growth is achieved during collisional orogenesis

molybdenum isotopic composition of modern and carboniferous carbonates we investigate the redox sensitive isotope system of molybdenum mo in marine carbonates to evaluate their potential as archive of the mo isotopic composition of coeval seawater we present mo isotope data delta 98 95 mo of modern skeletal and non skeletal carbonates as well as a variety of precipitates from the mid and late carboniferous the external reproducibility is determined by repeated analyses of two commercially available carbonate standards the resulting uncertainty of the low concentration samples is 0 1 parts per thousand 2 sigma analysis of modern ooid sands from the bahamas shows a consistently heavy mo isotopic composition delta 98 95 mo between 2 and 2 2 parts per thousand approaching modern mean seawater values delta 98 95 mo 2 3 parts per thousand 0 1 parts per thousand 2 sigma this suggests that isotope fractionation during mo uptake into non skeletal carbonate precipitates is small in contrast modern skeletal carbonates show variable isotopic compositions 0 1 to 2 2 parts per thousand which suggests a biologically controlled fractionation process the varying mo signatures found in carboniferous cement phases point to a strong response to local changes in fluid composition from which they precipitated overall we recognized three important factors to cause an offset relative to ocean water mo derived from skeletal components input of detrital mo and admixture of light hydroxide derived mo via diagenetic fluids all of these factors cause a lighter mo isotopic composition relative to seawater due to the apparent small isotope fractionation during mo uptake into non skeletal carbonates their delta 98 95 mo closely reflects the ambient fluid composition from these results we conclude that carbonates represent a promising new tool to characterize the water mass mo isotopic composition of marine paleo environments

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

near surface models in saudi arabia a single layer model of the near surface throughout the kingdom of saudi arabia is available while this simple model suffices for most areas and large subsurface structures it fails in situations where the surface topography is complex the base of weathering is below the datum or where the time structural closure is less than the uncertainty in the static correction in such cases multiple layered models that incorporate velocities derived from analysis of first arrivals picked from seismic shot records have proved to be successful in defining the lateral heterogeneity of the near surface the additional velocity information obtained from this first arrival analysis direct as well as refracted arrivals vastly improves the velocity depth model of the near surface regardless of the topography static corrections computed from these detailed near surface velocity models have significantly enhanced subsurface image focusing thereby reducing the uncertainty in the closure of target structures other non seismic methods have been used either to confirm qualitatively or to enhance the layer models previously mentioned gravity data may be particularly useful in sandy areas to confirm general structure while geostatistical modelling of vibrator base plate attributes has yielded information that enhances the velocity field in the global context exploration targets of the oil and gas industry are seeking smaller and lower relief time structures thus near surface models will need to enhance and integrate these methods particularly in areas where the assumption of flat lying near surface layers cannot be met

study of effect of magma pocket on mixing of two magmas with different viscosities and densities by analogue experiments article i nfo we conducted analogue experiments to examine the effect of a magma pocket on magma mixing processes at both high and low reynolds numbers our idea is that when mafic and silicic magmas are simultaneously injected into a magma pocket gravitationally unstable layers may be formed at both high and low reynolds numbers under certain conditions the rayleigh taylor instability occurs and mixing of the two magmas is promoted in the pocket even at a low reynolds number this shows that gravitational force overcomes viscous force and the annular flow of the magmas in the pocket is disrupted if the viscous force is large the annular flow is maintained and it passes through the pocket at a critical condition two liquids with different densities and viscosities become unstable while mixing in the pocket this condition is represented by a dimensionless parameter p p u g ab where is the viscosity u is the flow velocity g is the acceleration due to gravity is the density difference between the two liquids and a and b are the base dimensions of the apparatus when pb0 1 the two liquids are gravitationally unstable and they mix in the pocket however when pn0 1 the annular flow of the two liquids is maintained and they do not mix with each other the critical condition is also dependent on the flux of the liquids and the height of the pocket and is applicable to the present experimental conditions the presence of the magma pocket is one of the factors responsible for the disruption of the annular flow leading to the mixing of the two magmas we conclude that magmas can mix thoroughly if they flow through several magma pockets

ground viscosity and stiffness measurements for near surface seismic velocity in land surveys the weathering layer can often distort the seismic signal due to it passing through rapid velocity and density changes dispersion scattering and inelastic absorption in a simple spring dashpot model for the earth response an equivalent medium groups these complex phenomena into two parameters only these are called ground viscosity and ground stiffness the most recent controllers for vibrators can estimate both parameters to validate these measurements saudi aramco conducted an experiment measuring ground viscosity and stiffness from two different vibrator control systems over an area of varying terrain conditions including unconsolidated sand and limestone outcrop the two systems measured different values but detected similar trends that correlated well with weathering conditions and surface geology e g lower viscosity values on the outcrop than on the sand the ratio of ground viscosity to ground stiffness can approximate the shallow s wave velocity which we converted into p wave velocity through calibration with sparse uphole data static corrections incorporating this velocity information somewhat improved the focusing of seismic time sections this new approach does not require additional acquisition efforts and can model shallow complex formations in arid areas where classical static methods often fail

the effects of topography on magma chamber deformation models application to mt etna and radar interferometry we have used a three dimensional elastic finite element model to examine the effects of topography on the surface deformation predicted by models of magma chamber deflation we used the topography of mt etna to control the geometry of our model and compared the finite element results to those predicted by an analytical solution for a pressurized sphere in an elastic half space topography has a significant effect on the predicted surface deformation for both displacement profiles and synthetic interferograms not only are the predicted displacement magnitudes significantly different but also the map view patterns of displacement it is possible to match the predicted displacement magnitudes fairly well by adjusting the elevation of a reference surface however the horizontal pattern of deformation is still significantly different thus inversions based on constant elevation reference surfaces may not properly estimate the horizontal position of a magma chamber we have investigated an approach where the elevation of the reference surface varies for each computation point corresponding to topography for vertical displacements and tilts this method provides a good fit to the finite element results and thus may form the basis for an inversion scheme for radial displacements a constant reference elevation provides a better fit to the numerical results

topographic amplification of tectonic displacement implications for geodetic measurement of strain changes correlations of level changes with topography demand an assessment of the effect of an irregular free boundary on surface deformation this is examined through a plane strain elastic model with topography of small slope subjected to a change in the far field horizontal stress to leading order vertical surface displacements due to the topographic perturbation are proportional to the local relief elevationdependent uplift results from a compressional change and downdrop results from a tensional change the model further predicts that the ratio of elevation change to elevation is proportional to and of the same order of magnitude as the regional strain horizontal strains are locally perturbed by topography as well with the magnitude scaling with the local slope the predicted localization of level changes is very small in aseismic regions and cannot contribute significantly to measured correlations a test case in southern california bears this out with strains of order 10 6 accompanied by elevation change to elevation ratios of order 10 4 releveling following the nankaido tonankai earthquakes which induced large coseismic and postseismic strains reveals scattered examples of elevation dependent level changes however when compared to modeled strains the correlations are again at least an order of magnitude larger than the localization effect predicted by the elastic model although the topographic perturbation of vertical displacements appears to be unmeasurably small local variations in horizontal strain or borehole dilatation across steep relief may be discernible with current technology

the molybdenum isotope system as a tracer of slab input in subduction zones an example from martinique lesser antilles arc molybdenum isotopes are fractionated by earth surface processes and may provide a tracer for the recycling of crustal material into the mantle here we examined the mo isotope composition of arc lavas from martinique in the lesser antilles arc along with cretaceous and cenozoic deep sea drilling project sediments representing potential sedimentary inputs into the subduction zone mo stable isotope composition defined as 98mo in deviation from the nist 3134 standard in lavas older than 7 million years ma exhibits a narrow range similar to and slightly higher than morb whereas those younger than 7 ma show a much greater range and extend to unusually low 98mo values sediments from dsdp leg 78a site 543 have uniformly low 98mo values whereas leg 14 site 144 contains both sediments with isotopically light mo and mo enriched black shales with isotopically heavy mo when coupled with published radiogenic isotope data mo isotope systematics of the lavas can be explained through binary mixing between a morb like end member and different sedimentary compositions identified in the dsdp cores the lavas older than 7 ma were influenced by incorporation of isotopically heavy black shales into the mantle wedge the younger lavas are the product of mixing isotopically light sedimentary material into the mantle wedge the change in mo isotope composition of the lavas at 7 ma is interpreted to reflect the removal of the cretaceous black shale component due to the arrival of younger ocean crust where the age equivalent cretaceous sediments were deposited in shallower oxic waters isotopic fractionation of mo during its removal from the slab is not required to explain the observed systematics in this system

an accurate and efficient method for including the effects of topography in three dimensional elastic models of ground deformation with applications to radar interferometry topography has a large effect on the results predicted by elastic surface deformation models in regions of significant relief in some cases topography may have more influence on the predicted deformation field than do model source parameters we have developed an approximate analytical technique for including topographic effects that retains most of the computational simplicity of elastic half space models while providing an accurate representation of topographic effects we use a series expansion of the elastic half space solution with a small slope approximation yielding a set of higher order corrections the integrated effect of these corrections is evaluated using fourier methods we investigate the effectiveness of our method by comparing predicted results for a tilted triaxial ellipsoid with those predicted by finite element models the resulting displacements and displacement gradients are in good agreement with finite element results both for relatively smooth topography synthetically generated and for the greater relief in the vicinity of mount etna volcano we then compare the results of our method with those predicted by traditional elastic half space models using different reference elevations and with a previously proposed method of estimating topographic effects our new method provides a significantly better fit to the finite element results than do the other methods our method is able to accurately portray both the number and the horizontal pattern of fringes in a synthetic interferogram when compared with finite element results in particular the method accurately represents the broadening of fringes that is observed in regions of high relief as well as reproducing the location of the fringe center and the topographically induced deviation of the fringes from a regular pattern elastic half space models typically show a fringe center that is displaced with respect to the finite element results indicating that parameter inversions based on such a model would provide incorrect estimates of the horizontal source position the lack of topographically generated fringe distortions in elastic half space results would also likely lead to inaccuracies in the predicted magnitude and orientation of proposed deformation sources our new technique over comes these limitations and should be useful when evaluating either traditional geodetic results or the greater areal coverage provided by interferometric observations

evidence for mo isotope fractionation in the solar nebula and during planetary differentiation abstract mass dependent mo isotope fractionation has been investigated for a wide range of meteorites including chondrites enstatite ordinary and carbonaceous chondrites iron meteorites and achondrites eucrites angrites and martian meteorites as well as for lunar and terrestrial samples magmatic iron meteorites together with enstatite ordinary and most carbonaceous chondrites define a common 98 95 mo value of 0 16 0 02 relative to the nist srm 3134 mo standard which is interpreted to reflect the mo isotope composition of bulk planetary bodies in the inner solar system heavy mo isotope compositions for iab iron meteorites most likely reflect impact induced evaporative losses of mo from these meteorites carbonaceous chondrites define an inverse correlation between 98 95 mo and metal content and a positive correlation between 98 95 mo and matrix abundance these correlations are mainly defined by cm and ck chondrites and may reflect the heterogeneous distribution of an isotopically light metal and or an isotopically heavy matrix component in the formation region of carbonaceous chondrites alternatively the elevated 98 95 mo of the cm and ck chondrites could result from the loss of volatile isotopically light mo oxides that formed under oxidized conditions typical for the formation of these chondrites the mo isotope compositions of samples derived from the silicate portion of differentiated planetary bodies are heavy compared to the mean composition of chondrites and iron meteorites this difference is qualitatively consistent with experimental evidence for mo isotope fractionation between metal and silicate the common 98 95 mo values of 0 05 0 03 of lunar samples derived from different geochemical reservoirs indicate the absence of significant mo isotope fractionation by silicate differentiation or impact metamorphism volatilization on the moon the most straightforward interpretation of the mo isotope composition of the lunar mantle corresponds to the formation of a lunar core at a metal silicate equilibration temperature of 1800 200 c the investigated martian meteorites angrites and eucrites exhibit more variable mo isotope compositions which for several samples extend to values above the maximum 98 95 mo 0 14 that can be associated with core formation for these samples post core formation processes such as partial melting metamorphism and in the case of meteorite finds terrestrial weathering must have resulted in mo isotope fractionation estimates of the metal silicate equilibration temperatures for mars 2490 770 c and the angrite parent body 1790 230 c are thus more uncertain than that derived for the moon although the mo isotope composition of the bulk silicate earth has not been determined as part of this study a value of 0 16 98 95 mo 0 can be predicted based on the chondrite and iron meteorite data and by assuming a reasonable temperature range for core formation in the earth this estimate is in agreement with four analyzed basalt standards 0 10 0 10 improved application of mass dependent mo isotope fractionation to investigate core formation most of all requires an improved understanding of potential mo isotope fractionation during processes not related to metal silicate differentiation

deflation of mount etna monitored by spaceborne radar interferometry ground based measurements of volcano deformation can be used to assess eruptive hazard but require the costly and often hazardous installation and maintenance of an instrument network here we show that spaceborne radar interferometry which has already shown its utility in mapping earthquake related deformation1 can be used to monitor long term volcano deformation two families of synthetic aperture radar images acquired from ascending and descending orbits by the satellite ers 1 and looking at mount etna from opposite sides cover the time period from 17 may 1992 to 24 october 1993 and include the second half of etna s most recent eruption despite artefacts of the interferometric technique we can observe a volcano wide deflation which is an expected consequence of the eruption but which had not previously been appreciated we quantify it using a simple model based on the change of pressure in a sphere located in an elastic half space the modelled deformation increases linearly with time until the end of the eruption our results show that it will be possible to use this technique to detect the inflation of volcanic edifices that usually precedes eruptions

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

contrasting molybdenum cycling and isotopic properties in euxinic versus non euxinic sediments and sedimentary rocks refining the paleoproxies abstract the redox sensitive transition element molybdenum is present in the earth s crust at trace concentrations but is abundant in seawater marine sediments and ancient sedimentary rocks these occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of mo its isotopic composition and the covariation of mo with total organic carbon toc in ancient black shales however these proxies have almost exclusively targeted the identification and interpretation of euxinic environments where bottom waters are both anoxic and sulfidic here we present a discussion of the mo geochemistry of non euxinic sediments where hydrogen sulfide is sometimes present yet always restricted to pore waters we propose a new paleoredox application that uses mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters under ideal conditions it is also possible to infer the presence of mn oxides recycling in non euxinic paleoenvironments based on the mo isotope composition of ancient black shales providing a critical constraint on bottom water oxygenation

geostatistical integration of near surface geophysical data accurate statics calculation and near surface related noise removal require a detailed knowledge of the near surface velocity field conventional seismic surveys currently are not designed to provide this information and 3d high resolution reflection refraction acquisition is not feasible for large survey areas satellite images and vibrator plate attributes are dense low cost data which can be used in spatially extrapolating velocities from sparse uphole data by geostatistics we tested this approach in two different areas of saudi arabia and found that the optimal recipe depends on the local geology

joint seismological magnetotelluric investigation of shallow and implosive non dc and dc earthquakes beneath the gravitationally unstable heisei shinzan lava dome unzen volcano japan abstract shallow implosive non double couple ndc earthquakes have been detected beneath unzen volcano japan by a routinely operated seismometer network here we analyze the waveforms from 14 temporary seismic stations around mt fugen fugendake and heisei shinzan lava dome which formed during the 1990 1995 eruptions to further investigate this shallow seismicity we also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity we find that 1 implosive ndc earthquakes occur at the craters that were active during the 1990 1991 eruptions which are currently buried by the approximately 200 m thick heisei shinzan lava dome 2 the detected ndc earthquakes generally possess downward p wave arrivals although upward p wave arrivals are observed at a few seismic stations 3 the tensile axes of the ndc earthquakes show various slip directions 4 repeated ndc earthquakes are in some cases detected within 20 s of the initial event and 5 a high resistivity zone exists beneath the ndc earthquakes we conclude that the implosive ndc earthquakes were generated by the collapse of small randomly oriented vapor filled voids at the two buried craters with active volcanic conduits still present beneath these craters the shallow dc earthquakes consist of two clusters that are located 2 3 km below the ndc earthquakes the focal mechanisms of the shallower cluster approximately 2 km depth indicate various fault mechanism whereas those of the deeper cluster approximately 3 km depth indicate strike slip faulting the top of the shallower cluster corresponds to the base of the conductive zone which suggests that the dc earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer

dynamic inversion of ground deformation data for modelling volcanic sources etna 1991 93 the 1991 93 etna eruption that represented the most important recent eruptive event both in terms of duration 472 days and total volume of erupted lava ca 250 106 m caused marked ground deformation measured by using different geodetic techniques such as edm gps levelling and tiltmetry an inversion termed dynamic because it took into account the sequence of events considering both the separate and cumulative effects of two sources a shallow tensile crack and a deeper ellipsoidal deflating source was performed it was based on 163 data collected by the different techniques plus 2 geometrical conditions for constraining the geometry of the crack on the surface the final solution furnished a good fit for all measurements defining a shallow tensile crack located inside the volcanic edifice which represents the effect of the intrusion and a depressuring body with centre at ca 3000 m b s l which could represent the centre of a generalized and more complex deflation of the volcanic edifice during the course of the eruption

the impact of igneous bedrock weathering on the mo isotopic composition of stream waters natural samples and laboratory experiments abstract river waters have been shown to be systematically enriched in the heavy molybdenum mo isotopes when compared to typical granites and basalts which generally possess mo isotopic compositions 98 95 mo of around 0 this inconsistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine 98 95 mo signatures incongruent dissolution of primary bedrock however may be an important process by which the anomalous mo signatures of the river dissolved load are produced this study therefore investigates the effect of igneous crustal rock weathering on the aquatic 98 95 mo signal by comparing stream water and bedrock mo isotope data to results of bulk rock leach experiments for this purpose stream water and bedrock orthogneiss granite basalt as well as soil and vegetation samples were collected in a small catchment in the french massif central in accordance with the results of earlier studies on riverine mo both streams are isotopically heavier 98 95 mo 0 5 1 1 than the typical granites and basalts the excellent agreement of these data with those of mo released during experimental leaching of the basalt bedrock 0 6 1 0 identifies a predominance of basalt weathering over the stream water mo geochemistry while other processes i e soil formation secondary mineral precipitation and adsorption are subordinate in this catchment given that the basalt bulk rock 98 95 mo reflects a value typical for crustal magmatic rocks ca 0 1 mo isotope fractionation during the incongruent dissolution of basalt can explain the observed isotopically heavy aquatic mo signatures laser ablation analyses demonstrate that the volumetrically minor magmatic sulfides can be highly enriched in mo and mass balance calculations identify the sulfide melt inclusions as the principal mo source for the leach solutions these data suggest that the magmatic sulfides possess a distinctly heavier 98 95 mo signature than the coexisting silicate melt in this case mo would behave like fe by showing a detectable isotope fractionation at magmatic temperatures incongruent crustal bedrock weathering may thus cause a preferential release of heavy mo isotopes this effect however is highly dependent on the primary bedrock mineralogy consequently the average continental runoff may have been significantly affected by incongruent weathering during periods when the earth system was exceptionally far from steady state e g large glaciations with enhanced physical weathering or large subaerial basalt eruptions such as the deccan and the siberian plateau

reconstruction of early cambrian ocean chemistry from mo isotopes the neoproterozoic cambrian transition was a key time interval in the history of the earth especially for variations in oceanic and atmospheric chemical composition however two conflicting views exist concerning the nature of ocean chemistry across the precambrian cambrian boundary abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late ediacaran while other studies provide seemingly conflicting evidence for anoxic deep waters with ferruginous conditions fe ii enriched persisting into the cambrian here two early cambrian sedimentary platform and shelf slope sections in south china were investigated to trace early cambrian ocean chemistry from mo isotopes the results reveal that early cambrian sediments deposited under oxic to anoxic euxinic conditions have 98 95mo values ranging from 0 28 to 2 29 which suggests that early cambrian seawater may have had 98 95mo values of at least 2 29 similar to modern oceans the heaviest and relatively homogeneous 98 95mo values were recorded in siltstone samples formed under completely oxic conditions which is considered that mn oxide free shuttling was responsible for such heavy 98 95mo value further combined with fe species data and the accumulation extent of mo and u the variation of 98 95mo values in the two studied sections demonstrate a redox stratified ocean with completely oxic shallow water and predominantly anoxic even euxinic deeper water having developed early on which eventually became completely oxygenated this suggests that oceanic circulation at the time became reorganized and such changes in oceanic chemistry may have been responsible for triggering the cambrian explosion of biological diversity

a mainly crustal origin for tonalitic granitoid rocks superior province canada implications for late archean tectonomagmatic processes the central wabigoon subprovince of the superior province like introduction most plutonic domains within archean cratons is dominated by over the last decade information fundamental to undergranitoid rocks of the tonalite trondhjemite granodiorite ttg standing and resolving many questions regarding arseries heterogeneous 2 74 throughout archean time documentation of alterative ga tonalite gneiss crust by younger tonalite magmas was probably petrogenetic models for archean ttg or ttd suites is an important process unlike the model of ttgs representing direct of particular relevance to archean tectonic processes see partial melts of subducting slabs in an arc setting this model de wit 1998 hamilton 1998 smithies 2000 as well requires no direct tie to subduction careful re evaluation of the as processes of archean continental crustal growth and ttg classification is required for it can mistakingly pigeon hole recycling e g ireland et al 1994 de wit 1998 hamilton temporally and genetically unrelated rocks and perhaps assume an 1998 henry et al 1998 2000 nutman et al 1999 incorrect petrogenetic or tectonic model an important role of crustal as in most archean shield terrains plutonic domains recycling processes in central wabigoon tonalite petrogenesis is in of the western superior province typically consist mainly keeping with evidence that supports substantial rates of continental of ttg or ttd compositions but also include more recycling as far back as the earliest archean mafic and more evolved granitoid compositions thurs

highly fractionated himalayan leucogranites and associated rare metal mineralization abstract the formation of the himalaya was associated with the exhumation of high grade metamorphosed rocks of the higher himalayan sequence hhs complex which underwent amphibolite granulite to eclogite facies metamorphism and anatexis occurring along the himalayan mountain crest in the south and the lhagoi kangri mountain in the north the himalayan leucogranites contain varying proportions of biotite

an integrated method for resolving the seismic complex near surface problem we describe an integrated method for solving the complex near surface problem in land seismic imaging this solution is based on an imaging approach and is obtained without deriving a complex near surface velocity model we start by obtaining from the data the kinematics of the one way focusing operators i e time reversed green s functions that describe propagation between the acquisition surface and a chosen datum reflector using the common focus point technology the conventional statics solutions obtained from prior information about the near surface are integrated in the initial estimates of the focusing operators the focusing operators are updated iteratively until the imaging principle of equal traveltime is fulfilled for each subsurface gridpoint of the datum reflector therefore the seismic data is left intact without any application of time shifts which makes this method an uncommitted statics solution the focusing operators can be used directly for wave equation redatuming to the respective reflector or for prestack imaging if determined for multiple reflecting boundaries the underlying velocity model is determined by tomographic inversion of the focusing operators while also integrating any hard prior information e g well information this velocity model can be used to perform prestack depth imaging or to calculate the depth of the new datum level we demonstrate this approach on 2d seismic data acquired in saudi arabia in an area characterized by rugged topography and complex near surface geology

molybdenum and boron isotopic compositions of porphyry cu mineralization related adakitic rocks in central eastern china new insights into their petrogenesis and crust mantle interaction

petrogenesis of synorogenic high temperature leucogranites damara orogen namibia constraints from u pb monazite ages and nd sr and pb isotopes highlights geochemical data from high t leucogranites imply pure crustal melting new u pb monazite ages constrain intrusion time close to peak metamorphism updated sr nd pb isotope data imply metasedimentary sources two suites of leucogranites were emplaced at 508 5 9 ma in the okombahe district of the damara belt namibia synchronous with the peak of regional high temperature metamorphism the sr 87sr 86srinit 0 707 to 0 711 nd endinit 4 5 to 6 6 and pb isotopic 206pb 204pb 18 51 19 13 207pb 204pb 15 63 15 69 208pb 204pb 38 08 38 66 compositions indicate that these peraluminous s type granites were derived from mid to lower crustal rocks which are slightly different to the metapelitic rocks into which they intruded since the leucogranites are unfractionated and show no evidence for assimilation or contamination they constrain the temperature and pressure conditions of their formation calculated zr and lree saturation temperatures of ca 850 c indicate high temperature crustal melts high rb sr and low sr ba ratios are consistent with biotite dehydration melting of pelitic source rocks qz ab or systematics reveal that melting and segregation for the least fractionated samples occurred at ca 7 kbar corresponding to a mid crustal level of ca 26 km however there is no evidence for a mantle component that could have served as a local heat source for crustal melting therefore the hot felsic magmas that formed close to the time of peak metamorphism are the result of long lasting high temperature regional metamorphic conditions and intra crustal collision

the double spike toolbox the double spike technique is a well established method for correcting for instrumental mass fractionation in mass spectrometry the precision of the technique is controlled by the choices of double spike composition and the proportions in which the double spike and sample are mixed to make these choices easier we provide software the double spike toolbox for calculating optimal double spikes which are chosen purely on the basis of minimising error propagation in addition we provide cocktail lists of optimal double spikes for all 33 elements that have 4 or more naturally occurring isotopes using some sensible default parameters as examples we discuss the application of the software to fe pb and ca isotopes

intermittent generation of mafic enclaves in the 1991 1995 dacite of unzen volcano recorded in mineral chemistry mafic enclaves in the 1991 1995 dacite of unzen volcano show chemical and textural variability such as bulk sio2 contents ranging from 52 to 62 wt and fine to coarse grained microlite textures in this paper we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types type i mafic enclaves contain high mg plagioclase and low cl hornblende as microlites whereas type iii enclaves include low mg plagioclase and high cl hornblende type ii enclaves have an intermediate mineral chemistry type i mafic enclaves tend to show a finer grained matrix have slightly higher bulk rock sio2 contents 56 60 wt when compared with the type iii mafic enclaves sio2 53 59 wt but the overall bulk enclave compositions are within the trend of the basalt dacite eruptive products of quaternary monogenetic volcanoes around unzen volcano the origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion controlled re equilibration of minerals in a low temperature mushy dacitic magma reservoir mafic enclaves with a long residence time in the dacitic magma reservoir whose constituent minerals were annealed at low temperature to be in equililbrium with the rhyolitic melt represent type iii enclaves in contrast type i mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high temperature mineral chemistry taking temperature ca ca na ratio of plagioclase and water activity of the hydrous unzen magma into account the mg contents of plagioclase indicate that plagioclase microlites in type iii enclaves initially crystallized at high temperature and were subsequently re equilibrated at low temperature conditions compositional profiles of mg in plagioclase suggest that older mafic enclaves type iii had a residence time of 100 years at 800 c in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991 1995 unzen eruption presence of different types of mafic enclaves suggests that the 1991 1995 dacite of unzen volcano tapped mushy magma reservoir intermittently replenished by high temperature mafic magmas

absence of molybdenum isotope fractionation during magmatic differentiation at hekla volcano iceland this study investigates the behaviour of molybdenum mo isotopes during magmatic differentiation molybdenum isotope compositions as well as concentrations of rare earth elements and a selection of trace elements have been determined for a well characterised sequence of lavas from hekla volcano iceland covering a compositional range from basalt to rhyolite 46 72wt sio2 and thought to have developed by differentiation and mixing of melts derived from a cogenetic source all samples have identical mo isotopic compositions with an average 98mo of 0 15 0 05 2 s d n 23 there is therefore no resolvable mo isotope fractionation during magmatic differentiation at hekla this finding is supported by the fact that mo remains highly incompatible in hekla lavas increasing from 1 3 to 4 6 g g from basalt to rhyolite indicating that the crystallising phases are extracting only limited amounts of mo from the magma and therefore that significant fractionation of mo isotopes is unlikely it has previously been proposed that cerium ce and mo have similar bulk distribution coefficients and are equally incompatible during mantle melting while both ce and mo remain incompatible in hekla lavas the ce mo ratio decreases from 50 to 36 during magmatic differentiation indicating that mo is more incompatible than ce comparison of mo with other incompatible trace elements indicates that mo is as incompatible as la and slightly less incompatible than k sulphur s decreases strongly from 200 to as low as 2 g g from basalt to andesite and more evolved compositions yet this has no effect on the mo isotopes therefore mo does not exhibit significant chalcophile behaviour in hekla magmas the mo isotopic signature therefore may be used as an indicator of parent magma composition and a potential discriminant of assimilation processes

high precision molybdenum isotope analysis of low mo igneous rock samples by mc icp ms abstract recent advances in analytical techniques have achieved high precision 2 0 06 determinations of mo isotopic ratios in silicate rocks but few such data are as yet available especially for low mo

on precisely modelling surface deformation due to interacting magma chambers and dykes combined data sets of insar and gps allow us to observe surface deformation in volcanic settings however at the vast majority of volcanoes a detailed 3 d structure that could guide the modelling of deformation sources is not available due to the lack of tomography studies for example therefore volcano ground deformation due to magma movement in the subsurface is commonly modelled using simple point mogi or dislocation okada sources embedded in a homogeneous isotropic and elastic half space when data sets are too complex to be explained by a single deformation source the magmatic system is often represented by a combination of these sources and their displacements fields are simply summed by doing so the assumption of homogeneity in the half space is violated and the resulting interaction between sources is neglected we have quantified the errors of such a simplification and investigated the limits in which the combination of analytical sources is justified we have calculated the vertical and horizontal displacements for analytical models with adjacent deformation sources and have tested them against the solutions of corresponding 3 d finite element models which account for the interaction between sources we have tested various double source configurations with either two spherical sources representing magma chambers or a magma chamber and an adjacent dyke modelled by a rectangular tensile dislocation or pressurized crack for a tensile okada source representing an opening dyke aligned or superposed to a mogi source magma chamber we find the discrepancies with the numerical models to be insignificant 5 per cent independently of the source separation however if amogi source is placed side by side to an okada source in the strike perpendicular direction we find the discrepancies to become significant for a source separation less than four times the radius of the magma chamber for horizontally or vertically aligned pressurized sources the discrepancies are up to 20 per cent which translates into surprisingly large errors when inverting deformation data for source parameters such as depth and volume change beyond 8 radii however we demonstrate that the summation of analytical sources represents adjacent magma chambers correctly

the molybdenum isotopic compositions of i s and a type granitic suites this study reports mo isotopic compositions for fifty two palaeozoic granitic rocks with contrasting source affinities a i and s type from the lachlan fold belt lfb and the new england batholith neb both in se australia and three compositionally zoned plutons loch doon criffell and fleet located in the southern uplands of scotland the results show relatively large variations in 98mo for igneous rocks ranging from 1 73 to 0 59 with significant overlaps between different types no relationships between 98mo and 18o or asi alumina saturation index are observed indicating that mo isotopes do not clearly distinguish igneous vs sedimentary source types instead effects of igneous processes source mixing regional geology as well as hydrothermal activity control the mo isotope compositions in these granites it is found that mo is mainly accommodated in biotite and to a lesser extent in hornblende hornblende and fe3 rich minerals may preferentially incorporate light isotopes as reflected by negative correlations between 98mo and k rb and fe2o3 there is a positive correlation between initial 87sr 86sr and 98mo in i type granitic rocks reflecting the admixing of material from isotopically distinct sources granitic rocks from scotland and australia display strikingly similar curvilinear trends in 98mo vs initial 87sr 86sr despite the differing regional geology localized hydrothermal effects can result in low 98mo in granite as seen in three samples from loch doon and criffell which have anomalously light 98mo of 1 based on this study an estimate of 98mo 0 14 0 07 95 s e for the phanerozoic upper crust is proposed this is slightly heavier than basalts indicating an isotopically light lower crust and or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and or pelagic sediment over time

when do black shales tell molybdenum isotope tales molybdenum mo isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions organic rich black shales can be used to reconstruct ancient ocean mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater comparison of 98 95 mo in two devonian shale sequences of similar age the new york oatka creek and geneseo formations reveals that this assumption cannot be applied to all organic rich shales although both sequences contain laminated intervals elevated organic carbon and enrichments of redox sensitive metals the mean 98 95 mo differs systematically between the formations by 0 59 independent paleoredox indicators reveal that portions of the oatka creek formation were deposited under pervasively euxinic anoxic and sulfidic conditions whereas conditions during deposition of the geneseo formation were intermittently euxinic to suboxic oxygen deficient but not sulfidic in the water column we infer that reconstruction of ancient ocean 98 95 mo from organic rich shales requires independent verification of persistent local euxinia with these considerations in mind our data point to 98 95 mo in the devonian oceans 0 6 lighter than in today9s oceans consistent with expanded anoxia

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

pressure sources beneath unzen volcano inferred from leveling and gps data abstract unzen volcano erupted from 1990 to 1995 and the resultant volcanic deformations were measured by gps leveling tilt meter and edm the largest vertical displacement observed was subsidence of 8 cm in the western part of shimabara peninsula numerous magma chamber models were proposed based on the geodesic data obtained however these models could not explain the volcanic deformations observed around chijiwa bay after the eruption had finished because previous studies only used data collected during the eruption we therefore re examined the magma chamber model using leveling and gps data and included measurements collected both during and after the eruption on leveling we applied the mogi model with a height correction applied to the measuring points there are four parameters for each source longitude latitude depth and volume change each source parameter was estimated using a grid search scheme we also used aic akaike information criterion to fix the number of pressure sources beneath unzen to explain the vertical and horizontal deformations during 1986 2004 the results show that there are four pressure sources under unzen volcano and that magma ascended obliquely at an angle of about 45 from beneath chijiwa bay to the crater created by this eruption after the eruption had stopped magma inflows into deep sources were identified based on our leveling survey conducted after 1996 we compare our source model with several seismic data sets before the unzen eruption an earthquake swarm occurred from 1989 to 1990 in which pressure sources were detected just below the earthquake hypocenters other seismic refraction studies support our estimates of the location of these pressure sources using volume change values we inferred that magma supply had continued into this deep source at the rate of 2 0 2 5 m 3 yr from at least 1984 until after eruption stopped

vibrator attribute leading velocity estimation accurate knowledge of the near surface velocity macromodel is vital for hydrocarbon exploration and reservoir characterization that utilize seismic data because this model is crucial for statics and depthing however determination of near surface velocities using direct measurements via uphole surveys is economically prohibitive consequently geophysicists face a problem when determining how to interpolate between sparse near surface well control when near surface velocity varies laterally with lithology that in turn does not vary equally in all directions therefore techniques are needed to estimate near surface velocity using other available data in order to minimize the associated risk resulting from incomplete knowledge of the near surface velocity model

a molybdenum isotope perspective on phanerozoic deoxygenation events the careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters a synthesis and analysis of data from two mesozoic ocean anoxic events and the palaeocene eocene thermal maximum applies these techniques to constrain past ocean deoxygenation

effects of topography on the interpretation of the deformation field of prominent volcanoes application to etna we have investigated the effects of topography on the surface deformation field of volcanoes our study provides limits to the use of classical half space models considering axisymmetrical volcanoes we show that interpreting ground surface displacements with half space models can lead to erroneous estimations of the shape of the deformation source when the average slope of the flanks of a volcano exceeds 20 tilting in the summit area is reversed to that expected for a flat surface thus neglecting topography may lead to misinterpreting an inflation of the source as a deflation comparisons of mogi s model with a three dimensional model shows that ignoring topography may lead to an overestimate of the source volume change by as much as 50 for a slope of 30 this comparison also shows that the depths calculated by using mogi s solution for prominent volcanoes should be considered as depths from the summit of the edifices finally we illustrate these topographic effects by analyzing the deformation field measured by radar interferometry at mount etna during its 1991 1993 eruption a three dimensional modeling calculation shows that the flattening of the deflation field near the volcano s summit is probably a topographic effect

displacements and tilts from dip slip faults and magma chambers beneath irregular surface topography surface displacements and tilts due to buried deformation sources are influenced by topography the leading order corrections due to topography of arbitrary profile but small slope are determined for dip slip faults edge dislocations and magma bodies lines of inflation in a two dimensional elastic medium the vertical displacement correction is simply the product of the topography and the horizontal normal strain due to the source in a flat half space the correction for a normal fault beneath an idealized basin and range topography is a slight increase in the uplift on the range side the effect of topography for a line of inflation beneath a symmetric volcano is to reduce the central uplift failure to account for topographic influences can bias estimates of source depth and geometry

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day