basal platinum group element mineralization in the federov pansky layered mafic intrusion kola peninsula russia the 2501 to 2446 ma federov pansky layered mafic intrusion in the central part of the kola peninsula russia is situated along the northern contact of the early proterozoic imandra varzuga rift and archean granite gneiss the federov pansky intrusion has eight major zones from bottom to top 1 marginal zone mafic schists 50 to 100 m 2 taxitic gabbronorite zone bpca 150 to 300 m 3 norite zone bcpa pbc 200 m 4 main gabbronorite zone pacb 1 000 m 5 lower layered horizon zone bc pc pbac 100 m 6 gabbro zone pacb pbac 1 500 m 7 upper layered horizon zone pocab 250 m 8 upper gabbro zone pacb 400 m the taxitic gabbronorite zone is restricted to the federov and east lastjavr blocks this zone is distinctive due to its pegmatitic taxitic textures and abundant norite xenoliths concentrated near its upper contact with the norite zone chemically the taxitic gabbronorite is more feldspathic and iron enriched than the overlying mgo and cr 2 o 3 rich norite zone and norite xenoliths the zone hosts cu ni pge mineralization with disseminated sulfides which is continuous for over 2 5 km along strike and 150 to 200 m downdip 98 percent of the sulfides consist of varying amounts of pyrrhotite chalcopyrite and pentlandite the mineralization is divided into an upper mineralized section with two higher grade lenses a and b and the restricted lower mineralized section with six narrower lenses the lower mineralized section is less continuous and less uniform in terms of cu ni pge distribution the norite xenoliths and blocks occurring within the taxitic gabbronorite zone are practically barren of sulfide and can dilute higher pge and base metal grades in the gabbronorite matrix average total pge grades in rocks do not exceed a few grams per tonne the mineralization is palladium rich with a pd pt ratio from 4 22 to 4 34 rh and au grades are 0 1 to 0 5 g t os and ir grades are the most common pge bearing minerals are merenskyite moncheite kotulskite sobolevskite and michenerite less common are sperrylite and vysotskite braggite the cu ni pge sulfides are primarily magmatic in origin this is supported by s isotope compositions with a narrow range of 34 s of 0 2 to 1 4 percent and s se ratios of 1 050 to 3 300 a three stage process is postulated for the genesis of the federov mineralization this involves the intrusion of a more fractionated and sulfur saturated gabbronorite magma and its brecciation and mixing with a residual primitive magma forming immiscible sulfides that scavenged cu ni pge the federov resembles other contact type mineralization such as in the portimo intrusion in finland and the lac des iles and east bull lake intrusions in ontario canada

an approximation for the xu white velocity model in 1995 s xu and r e white described a method for estimating compressional and shear wave velocities of shaley sandstones from porosity and shale content their model was able to predict the effect of increasing clay content on compressional wave velocity observed in laboratory measurements a key step in the xu white method estimates dry rock bulk and shear moduli for the sand shale mixture this step is performed numerically by applying the differential effective medium method to the kuster toksoz equations for ellipsoidal pores this step is computationally intensive using reasonable assumptions about dry rock elastic properties this step can be replaced with a set of approximations for dry rock bulk and shear moduli numerical experiments show an extremely close match between velocities obtained with these approximations and velocities computed with the differential effective medium method these approximations simplify the application of the xu white method and make the method computationally more efficient they also provide insight into the xu white method for example these approximations show how the xu white model is related to the critical porosity model

strongly negative δ34s values associated with secondary pyrite above and below the j m reef stillwater complex montana abstract previous sulfur isotope studies of the stillwater complex and in particular the j m reef have noted the generally restricted range of 34s values in the range of 0 2 5 e g zientek and ripley 1990 ripley et al 2017 detailed studies of high s zones in both the hanging wall of the reef package which contains the j m reef and the footwall within the reef package but below the mineralized j m reef indicate the presence of significant quantities of secondary pyrite the pyrite rich intervals are characterized by 34s values that strongly differ from those of the j m reef in the range of 0 2 to 17 4 the pyrite rich rocks above and below the j m reef are both serpentinized lherzolites although the j m reef is locally strongly altered to clinozoisite epidote rich assemblages only the sepentinized lherzolites show anomalously low 34s values the highly negative 34s values of the pyrite rich assemblages result from oxidation processes and s isotope fractionation between reduced and oxidized s species one possible process involves the production of a mixed sulfate sulfide fluid with pyrite produced from h2s with low 34s values a second alternative is that infiltration of an oxidized fluid and interaction with sulfide bearing rocks of the complex produced a sulfate rich fluid that then underwent partial reduction to sulfide via reaction with reduced fe bearing minerals strongly negative 34s values would characterize secondary pyrite produced as a result of either process due to the large s isotope fractionation between oxidized and reduced s species at hydrothermal temperatures of 400 c or below the varying degrees of alteration and restricted pyrite rich intervals with highly negative s isotope signatures suggest that fluid infiltration was not pervasive secondary pyrite that was produced as a result of redox driven reactions is preserved only in olivine rich lithologies characterized by a high degree of serpentinization

bayesian inversion for effective pore fluid bulk modulus based on fluid matrix decoupled amplitude variation with offset approximation fluid indicators estimated from seismic dataplay important roles in reservoir characterization and prospect identification traditionally there are a variety of fluid indicators proposed but they are very likely to provide ambiguous results for fluid identification due to the fact that their sensitivity is dependent upon the mixed effect of pore fluid and rock porosity to raise the sensitivity of fluid indication we used the effective porefluid bulk modulus as a fluid indicator starting with the poroelastic amplitudevariation with offset avo theory and the corresponding rock physics model with the homogeneous sorting trend we derived a new avo approximation that allowed us toestimatethe effective pore fluidbulkmodulusina direct fashion the inversion for the fluid indicator is formulated inbayesian framework withthe cauchydistributionasa prior constraint we tested the method on synthetic data and analyzed the feasibility and stability of the inversion a field dataexampleshowsthatthe effective pore fluidbulkmodulus can reduce the ambiguity caused by the rock porosity and improve the quality of fluid discrimination in a clastic reservoir further research needs to be done on the reservoirs that do not fit the rock physics model without a sorting trend

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

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

linking dynamic elastic parameters to static state of stress toward an integrated approach to subsurface stress analysis abstract stress is the most important parameter to understand basin dynamics and the evolution of hydrocarbon systems the state of stress can be quantified by numerical geo mechanical modelling techniques these techniques require static elastic parameters of the rocks as input while tectonic and gravitational forces are given as explicit boundary conditions to compute the local state of stress at different scales we developed a technique to determine the density and elastic constants at seismic frequencies using full zoeppritz inversion on angle dependent seismic reflection data the dynamic elastic parameters as obtained from seismic data differ from their static equivalents which are necessary to determine the static state of stress the dynamic elastic parameters are related to their static equivalents through experimentally obtained relations in these rock physics experiments the static and dynamic elastic parameters are measured simultaneously during different external loading conditions the experiments used here are all carried out in a tri axial pressure machine under equal axial stresses then pre stack seismic data analysis in combination with the relation between the static and dynamic elastic parameters from the rock physics experiments provides the input parameters for geo mechanical modelling

surface electric variations induced by deep hydraulic stimulation an example from the soultz hdr site 1 we carried out monitoring of surface electric potential variations at the soultz hot dry rock site france during a stimulation experiment where 23 000 m3 of water were injected in granite at 5 km depth we have observed a long period 9 10 days potential variation with a maximum amplitude of 5 mv thanks to the conductive well casing the start of this anomalous potential coincides with that of the water injection and its magnitude increases as more water is injected before returning toward its original value after shut in we interpret this anomalous potential as an electrokinetic effect which is caused by the injection of water circulating in deep fractures we modeled the pressure distribution during the stimulation and obtained the surface sp using electrokinetic coupling relationships we found that an in situ coupling coefficient of 1 mv mpa at the low end of laboratory values gives a good fit to the data

rhenium osmium and samarium neodymium isotopic systematics of the stillwater complex isotopic data for the stillwater complex montana which formed about 2700 ma million years ago were obtained to evaluate the role of magma mixing in the formation of strategic platinum group element pge ore deposits neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas ultramafic affinity u type magmas had initial e nd of 0 8 to 3 2 and a chondritic initial 187 os 186 os ratio of 0 88 whereas anorthositic affinity a type magmas had e nd of 0 7 to 1 7 and an initial 187 os 186 os ratio of 1 13 these data suggest that u type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas a type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust the nd and os isotopic data also suggest that os and probably the other pges in ore horizons such as the j m reef was derived from a type magmas the nd and os isotopic heterogeneity observed in rocks below the j m reef also suggests that a type magmas were injected into the stillwater u type magma chamber at several stages during the development of the ultramafic series

gassmann s fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld someareundergoing enhanced oil recovery most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones but applying relations developed for sandstones to carbonates is problematic at best we measure in the laboratory nine carbonate samples from the same reservoir at seismic 3 3000 hz and ultrasonic 0 8 mhz frequencies samples are measured dry humidified and saturated with liquid butane and brine our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation however we explore the applicability of gassmann s theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion andgassmann stheoryassumptions forourcarbonatesetat high differential pressures and seismic frequencies the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by gassmann s relation we also explore in detail some of the assumptions of gassmann s relation especiallyrock framesensitivitytofluidsaturation our carbonate samples show rock shear modulus change from dry to brine saturation conditions and we investigate several rock fluid mechanisms responsible for this change to our knowledge these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange

occurrence and petrological significance of graphite in the upper critical zone western bushveld complex south africa abstract graphite occurs as a major rock forming constituent in pyroxenitic pegmatites near the platiniferous merensky reef in the western bushveld it is associated with amphibole biotite low k phyllosilicates chlorite sulphides and platinum group minerals rhass irass locally rocks with up to 80 graphite occur chlorine is a significant constituent in both hydrous silicates 0 1 0 3 cl and graphite 0 2 1 9 magnetite and quartz also occur with the above association this facilitates estimation oft 500 600 c andfo2 10 21 to 10 23 bar during graphite deposition which took place from c o h s fluids at an oxygen fugacity in the vicinity of qmf in equilibrium with maximum h2o mole fraction the latter accounts for the widespread association of graphite with hydrous silicates there is evidence for buffering offo2 of the melt by fluid phase this process may be more widespread than hitherto assumed c o h s fluids are considered instrumental not only in the formation of graphite rich pegmatites and associated mineralization but also in the genesis of pothole depressions and in the general development of stratiform pegmatites such as the merensky reef in layered igneous complexes

investigations of the stillwater complex part v apatites as indicators of evolving fluid composition variations in the f cl and oh contents of apatite are not constrained by crystal chemical factors in contrast to micas and amphiboles and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks microprobe analysis of intercumulus apatites from the stillwater complex reveal that cl rich apatites typically containing 6 0 wt cl occur throughout the lower 1 3 of the complex excluding the basal series a change from cl rich to more f rich apatite occurs within olivine bearing zone i ob i of the banded series the host zone of the platiniferous j m reef although apatite compositions are somewhat variable above the j m reef more f rich apatites predominante and typically contain 1 2 wt f and 3 0 wt cl the most f rich apatites occur in the uppermost exposed cumulates pristine apatites from coeval sills and dikes from below the complex and from the basal series are similarly f rich in all apatites the cl and f contents are lower in rocks affected by later metamorphic fluids rare earth element ree concentrations in chlorapatites show a marked peak in the olivine rich rocks of the j m reef and contain up to 2 wt ce2o3 la2o3 the trend of first increasing then decreasing cl f ratios with stratigraphic height is modeled by a vapor driven zone refining process occurring within the cumulate pile causing cl enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of cl from the magma as crystallization proceeded the abrupt change from cl rich to more f rich apatites within ob i is interpreted as the result of a breakdown of the cl rich zone refining front and mixing with cl poor supernatant melt any high temperature fluids that exsolved and circulated through the lower 1 3 of the complex must have been enriched in cl and could have transported ree and trace metals

magmatic unconformities and stratigraphic relations in the peridotite zone stillwater complex montana new stratigraphic relations between cumulate layers and units are documented in the peridotite zone of the stillwater complex a late archean mafic to ultramafic layered intrusion exposed along the northern edge of the beartooth uplift montana first low angle unconformities occur between all types of successive stratigraphic units and at all scales within the peridotite zone second new stratigraphic units and divisions based on the unconformities and type of cumulate layering within the peridotite zone are proposed and are correlated with published stratigraphic columns the stratigraphic units identified on the basis of rock type s and contacts with adjacent units from smallest to largest are cyclic units multicyclic units megacyclic groups and subzones the three larger units are usually bounded by low angle magmatic unconformities the three subzones and five of the six megacyclic groups identified in the peridotite zone can be traced over a distance of 17 7 mi 28 3 km the stratigraphic relations in the peridotite zone indicate that low angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common the relations suggest an extremely dynamic and rapidly changing environment during formation of the peridotite zone overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the peridotite zone from more primitive stratigraphic units dominated by olivine and olivine bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite olivine cumulates several major magmatic or possibly structural events appear to have affected the peridotite zone during its formation as indicated by thickness variation patterns in megacyclic groups and subzones the thickest massive chromite layers in the peridotite zone are associated with two of the major magmatic events

linearized avo and poroelasticity the technique of amplitude variation with offset avo allows geoscientists to extract fluid and lithology information from the analysis of prestack seismic amplitudes various avo parameterizations exist all of which involve the sum of three weighted elastic constant terms in present day avo approaches the weighting terms involve either knowledge of the incidence angle only or knowledge of both the incidence angle and the in situ vp vs ratio we have used the theory of poroelasticity to derive a generalized avo approximation that provides the estimation of fluid rigidity and density parameters we have combined two previously independent avo formulations thus reducing instead of adding to the total number of formulations this new approach requires knowledge of a third parameter to compute the weights the dry rock vp vs ratio we have derived a new equation and applied it to model and real data sets the new formulation has allowed us to estimate fluid properties of the reservoir in a more dire

olivine rich units in the fongen hyllingen intrusion norway implications for magma chamber processes abstract the fongen hyllingen intrusion fhi is considered to have crystallised from stratified magma residing in a bowl shaped magma chamber seven olivine rich units representing the most primitive cumulates in the central part of the intrusion are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor based on phase relationships the crystallisation order is explained in terms of magma mixing and fractional crystallisation repeated influxes of small volumes of dense primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber this was due to the small volume of replenishing magma the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced it is estimated that the chamber floor sloped as little as 1 2 but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks

waveform joint inversion of seismograms and electrograms for moment tensor characterization of fracking events electromagnetic signals have been observed in association with fracking experiments in the laboratory and in the field we have developed a seismoelectric forward modeling approach to produce synthetic seismograms and electrograms generated by fracking events using the finite element method with perfect matched layer boundary conditions the poroelastodynamic equations are solved in the frequency domain using a formulation based on the solid phase displacement and the pore pressure these results are used to compute the electrical field disturbances of electrokinetic nature three types of electrical signals are generated type i disturbance is associated with the seismic source itself type ii disturbance corresponds to seismoelectric conversions and type iii corresponds to coseismic signals this model is applied to simulate the seismic and electrical signals corresponding to the occurrence of a fracking event in a two layers system we perform a stochastic joint inversion of the seismograms and electrograms using the adaptive metropolis algorithm ama to obtain the posterior probability density functions of the parameters characterizing the seismic source assuming that the velocity model is perfectly known the joint waveform inversion is performed on synthetic noise free data and the ama algorithm is successful in retrieving the true values of the unknown parameters the proposed approach is then tested on the same synthetic data after being contaminated with 15 random noise with respect to the maximum amplitude of the signals the model parameters are better determined for the joint inversion of seismic and electrical data by comparison with the inversion of the seismic time series alone we also propose a deterministic tomographic algorithm that is successful in locating the in situ source current density distribution for types i and ii anomalies from the electrical data alone

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

compositional and lithological controls on the pge bearing sulphide zones in the selukwe subchamber great dyke a combined equilibrium rayleigh fractionation model the platinum group element pge bearing main and lower introduction sulphide zones of the selukwe subchamber of the great dyke are an increasing number of mafic layered intrusions are made up of a series of subzones within which the ratios pd pt are reported to have distributions of platinum group elements effectively constant whereas these ratios vary significantly between pge au and base metals characterized by vertically the subzones fractionation of pd with respect to pt varies by a offset profiles offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients the link with sulphide is great dyke prendergast keays 1989 wilson et al indisputable and the control is likely to have been the degree of 1989 naldrett wilson 1990 wilson tredoux oversaturation of pge micro nuggets in the magma the apparent 1990 the phenomenon has since been recognized in partition coefficients for pt and pd between silicate and sulphide the munni munni intrusion of western australia barnes liquid are dependent on the degree of oversaturation and thereby et al 1990 the skaergaard intrusion bird et al 1991 exhibit spurious correlation with the pge content of the sulphide and the rincon del tigre complex of eastern bolivia modelling replicates the pt and pd distribution and ratios only by prendergast 2000 pge are also postulated to be transdramatically changing the effective partition coefficients pyroxene ported in fluids emanating from expulsion of intercumulus compositions including tio2 are shown to be strongly dependent liquid from the crystal pile and there is strong evidence on the incompatible element content of the whole rock and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the pge of layered intrusions such as the skaergaard intrusion subzones the overall control is rayleigh fractionation but constancy andersen et al 1998 and in the stillwater complex of the ratio pd pt and the initial pyroxene composition before re boudreau mccullum 1986 1992 in contrast the equilibration with trapped liquid within the subzones is indicative mineralized zones in the great dyke and munni munni of equilibrium crystallization this layered structure may have been intrusion are located in pyroxenite and primary magderived from liquid layers in the magma chamber matic processes have been invoked to explain the min

metal redistribution during fluid exsolution and migration in the middle banded series of the stillwater complex montana abstract olivine bearing zones iii and iv ob iii and iv of the stillwater complex mt preserve evidence of fluid migration particularly in the variation of apatite compositions this feature coupled with the lack of evidence of the addition of substantial amounts of primitive magma allows these zones to be used to test the importance of volatile fluids in redistributing metals the concentrations of cu au ag and as all show marked increases at and or below the levels inferred to represent the top of the fluid migration front in the crystal pile however no significant sulfide enrichment is associated with the maximum metal enrichments the au concentration peak is much sharper and offset stratigraphically above that of cu and corresponds with the maximum cl content of interstitial apatite these concentration profiles are modeled numerically assuming fluid transport and chromatographic concentration of the metals mass balance calculations indicate that cumulates significantly below the level of the inferred fluid saturation front lost cu while those at or immediately below gained cu we conclude that cl rich fluids redistributed the background metal concentrations although no dramatic enrichments are observed enrichment factors range from two to three the results demonstrate the potential importance of fluids in redistributing and concentrating metals in layered intrusions

separation of sulfide melt droplets in sulfur saturated silicate liquids this experimental study explores the controls of run duration oxygen fugacity silicate melt structure and sulfide melt composition on the size of sulfide melt droplets that generate in sulfur saturated silicate liquids the sizes of the formed sulfide melt droplets influence the physical behavior of the separate sulfide phases during magma evolution within the earth s mantle and crust the relative enrichments of rh pt and pd in the primitive mantle melts and of os ir and ru in mantle residue mirror the physical and geochemical behavior of sulfide droplets that are finely dispersed in silicate melts and get entrained by ascending primitive mantle melts hypotheses that are based on kinetic constraints to model the formation of the magmatic sulfide deposits of kambalda and sudbury can be reinforced by the expected physical behavior of separate sulfide phases within the parental magmas of the magmatic deposits while formation scenarios of the deposits of bushveld and skaergaard remain ambiguous when combining kinetic constraints with expected physical behavior of separate sulfide phases within the parental magmas

stillwater anorthosites a lunar analog samples of anorthosite from stillwater complex units an i and an ii terminology of mccallum et al 1980 were analyzed by instrumental neutron activation analysis inaa for feo cao na2o ree and other trace elements no compositional trend with stratigraphic position was observed mean concentrations for most elements are about the same for three traverses two for an ii but cao feo and sr concentrations are slightly different for the three traverses the elements are not normally or lognormally distributed among samples from a single traverse strong interelement correlations can be understood in terms of sample mineralogy little evidence for compositional evolution of minerals during precipitation of the entire middle banded zone is apparent as in lunar anorthosites the component of trapped residual liquid is relatively insignificant less than 5 in addition to plagioclase which makes up on average 90 of the anorthosites the rocks contain interstitial pyroxene augite and inverted pigeonite and in some cases quartz titano magnetite sulfides and rare apatite and allanite there is an inverse relationship between the amounts of pyroxene and quartz general compositional trends exemplified by la sc systematics indicate that the bulk of the poikilitic pyroxene appears to be cumulus adcumulus presumably formed at the crystallization front from a magma saturated in plagioclase and pyroxene a small fraction of the pyroxene and plagioclase plus all the quartz and other trace minerals formed from the minor trapped liquid component the irregular distribution of pyroxene oikocrysts indicates that interstitial melt migrated substantially prior to final consolidation of the rock

genesis of compositional characteristics of stillwater an i and an ii thick anorthosite units a single 10 m 10 m outcrop of stillwater anorthosite unit an ii was mapped for pyroxene mode on a decimeter scale and samples from each range of pyroxene proportion were analyzed for major and trace elements distinct one to several square meter regions of different pyroxene mode were observed samples from regions with 10 pyroxene showed strong elemental correlations and anticorrelations with mgo concentrations that can be interpreted in terms of the following scheme for development of the stillwater anorthosites aggregates containing 50 to 80 cumulus plagioclase average 67 formed stable frameworks in a crystal mush of complexly zoned plagioclase the interstices among the plagioclase grains in these frameworks filled in part with pyroxene and plagioclase that were produced in equilibrium with the bulk melt and in part with pyroxene and plagioclase derived by crystallization of trapped intercumulus liquid the proportions of these three types of materials were mutually constrained in a manner that gave rise to lines instead of fields on two element variation diagrams samples richest in cumulus plagioclase are also richest in intercumulus plagioclase and pyroxene regions of the outcrop with 10 original pyroxene underwent substantial alteration by hydrous fluids that produced variations in fe2o3 tio2 loi and most incompatible trace elements

origin of anorthosite and magnetitite layers in the bushveld complex constrained by major element compositions of plagioclase the bushveld complex a layered mafic intrusion in south africa shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite in a continuous borehole core drilled through the uppermost 2 8k m of the intrusion typical rocks range upwards from troctolite through gabbronorite and ferrogabbronorite to ferrodiorite with extreme examples of anorthosite magnetitite and feldspathic pyroxenite the an content of plagioclase has previously been determined for 420 samples and decreases upward from an78 to an36 with six minor slow reversals variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole rocks forty five anorthosite layers have been identified ranging from 1 to 23 m thick none of these layers is associated with the abovementioned reversals in an content in plagioclase and nearly all have leucocratic rocks below and above with more than the likely cotectic proportions of plagioclase these observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation rhythmically pulsed crystallization possibly associated with pressure changes followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations twenty layers of magnetitite have been identified there is a significant reversal in an content in the overlying plagioclase compared with the underlying sample across only one such layer again this observation challenges hypotheses that such layers result from magma addition but is consistent with a pressure change hypothesis for triggering magnetite crystallization the upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions demonstrating that there is no downward crystallizing roof facies furthermore this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions hence we find no evidence supporting flotation or prolonged suspension of plagioclase

the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion a reply to the comment by rais latypov on wall et al j petrology 59 153 190 2018

wireline shear and avo modeling application to avo investigations of the tertiary u k central north sea numerous examples exist in the literature of the successes and failures of the application of amplitude variation with offset avo gas effect theory in hydrocarbon exploration the acquisition of wireline and vsp shear data is considered critical information for accurate calibration quantification modeling and successful application of avo analysis techniques in a particular geologic environment wireline compressional wireline dipole shear well vertical seismic profiling vsp and surface seismic data are used to investigate the avo behavior of the major lithologic boundaries and hydrocarbon bearing zones within the study area lithologic boundaries produce significant avo anomalies that may be easily confused with class iii avo gas effect in addition the principal economic target oil bearing sands does not appear to produce avo gas effect anomalies although a class iii avo gas effect response is observed when as little as 2 m of free gas is present at the top of the reservoir confirmatio

horizontal and vertical variations in noble metal distribution in the great dyke of zimbabwe a model for the origin of the pge mineralization by fractional segregation of sulfide abstract a study based largely on diamond drilling undertaken specifically for research purposes has shown that in the darwendale subchamber of the great dyke of zimbabwe there are three principal sulfide zones that are located in subunits 1a and 1b of cyclic unit 1 sulfide zone 1 the main sulfide zone msz is subdivided into three subzones the base of the msz occurs at the top of the main bronzitite layer of cyclic unit 1 extending into the overlying websterite the noble metals are concentrated at the bases of each sulfide zone and subzone and decrease in concentration rapidly upwards whereas variations in the concentrations of base metals such as cu correlate well with variations in the amounts of sulfide across each zone when calculated in terms of their concentration in 100 sulfides it is found that pd decreases upwards most rapidly followed by pt and then au with the cu concentration remaining essentially constant the decrease of the noble metals is interpreted in terms of simple rayleigh fractionation as the consequence of the removal of sulfide the observed correlation between the tops and bottoms of sulfide zones with small variations in the mg number of bronzite is taken to indicate that in zones 2 and 3 sulfide segregation started as a result of crystallization increasing the s content of the resident magma in the chamber until it reached that of sulfide saturation coupled with fresh inputs of sulfide unsaturated more primitive magma causing the resident magma to become unsaturated again this recharge by primitive magma also raised the pge content of the sulfides segregating from the trapped intercumulus liquid indicating that the resident magma was becoming recharged in pge in this way subunits i and ii of sulfide zone 1 are thought to contain too much sulfide to be explicable in this way and it is suggested that mixing with a plagioclase saturated magma possibly generated around the flanks of the intrusion was responsible for these subunits the sulfides of each of the sulfide zones are more concentrated at the margin than at the axis of the great dyke but have a lower tenor of noble metals it is proposed that the axially located sulfides were the consequence of magma sulfide equilibration at a higher r factor magma sulfide ratio than those at the margin the magma sulfide partition coefficients required to explain the percentage difference in the tenors of the different noble metals between axis and margin in this way are the same as those required to explain the vertical variations in noble metal tenor in terms of the fractional segregation model

fluid substitution estimating changes in vp without knowing vs two methods are presented for estimating the change of seismic p wave velocity that accompanies pore fluid changes in a rock in the common situation when s wave velocity is unknown in contrast gassmann s relation operates on the rock bulk modulus which can only be calculated when both vp and vs are measured the first method operates directly on the p wave modulus and is equivalent to replacing the bulk moduli of the rock and mineral in gassmann s relation with the corresponding p wave moduli the second method uses a graphical construction to estimate the decomposition of the measured p wave modulus into bulk and shear moduli which then allows the conventional gassmann s formula to be used when applied to a large set of sandstone data the predictions of both methods computed with vp only are within a few percent of the gassmann s relation using both vp and vs

the significance of magmatic erosion for bifurcation of ug1 chromitite layers in the bushveld complex abstract one of the most puzzling features of the ug1 chromitite layers in the famous exposures at dwars river eastern bushveld complex is the bifurcation i e convergence and divergence of layers along strike that isolate lenses of anorthosite the bifurcations have been variously interpreted as resulting from 1 the intermittent accumulation of plagioclase on the chamber floor as lenses terminated by crystallization of continuous chromitite layers the depositional model 2 late stage injections of chromite mush or chromite saturated melt along anastomosing fractures that dismembered semi consolidated plagioclase cumulates the intrusive model 3 post depositional deformation of alternating plagioclase and chromite cumulates resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally the deformational model none of these hypotheses account satisfactorily for the following field observations a wavy and scalloped contacts between anorthosite and chromitite layers b abrupt lateral terminations of thin anorthosite layers within chromitite c in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite many of these inclusions are contiguous with footwall and hanging wall cumulates d transported anorthosite fragments enclosed by chromitite e disrupted anorthosite and chromitite layers overlain by planar chromitite f protrusions of chromitite into underlying anorthosite g merging of chromitite layers around anorthosite domes we propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo chemical erosion of the temporary floor of the chamber the melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers resulting in lens like remnants of anorthosite resting on continuous layers of chromitite on cooling the magma crystallized chromite on the irregular chamber floor draping the remnants of anorthosite and merging with pre existing chromitite layers excavated by erosion with further cooling the magma crystallized chromite bearing anorthosite emplacement of multiple pulses of magma led to repetition of this sequence of events resulting in a complex package of anorthosite lenses and bifurcating chromitite layers this hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the bushveld complex

joint pp and ps ava seismic inversion using exact zoeppritz equations the zoeppritz equations describe the relationship between seismic properties that are useful for the interpretation of lithology and fluid properties various approximations to the zoeppritz equations are often used in linear amplitude variation with offset and amplitude variation with angle inversions assuming lowcontrast boundaries these approximations restrict the inversion method to not too large angle cases and reduce the inversion accuracy therefore it is necessary to develop joint pp and ps prestack inversion methods using the exact zoeppritz equations we have developed a method for nonlinear joint prestack inversion using the exact zoeppritz equations we established an objective function to combine pp and ps information based on a least squares approach and weusedthetaylorexpansionmethod to derive a model updating formula for the inversion we also used a mean shift method to improve the accuracy of inversion results we validated our inversion using synthetic and field seismic data the model for calculating synthetic data contained high contrast interfaces to match the reservoir layers which included for example coal seams and unconsolidated sandstone the outputs of the inversion were the elastic parameters p and s wave velocities as well as density rather than the changes in elastic parameters we found that the nonlinear joint prestack inversion achieved more accurate results than the linear joint prestack inversion regardless of incidence angle size furthermore if the prestack data were of high enough quality it was possible to identify thin layers from the inversion result

crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion canada the sept iles layered intrusion quebec canada 564 ma is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro and an upper border series with anorthosite capped by cupolas of a type granite chilled margin compositions suggest a ferrobasaltic parental magma close to that of the skaergaard intrusion but much richer in iron and titanium samples from drill cores and surface sampling of the 4 7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 fe ti oxide layers cm to m thick and many anorthositic autolithic blocks the sequence of crystallization in the layered series is plagioclase an72 34 and olivine fo75 21 followed by magnetite and ilmenite then ca rich pyroxene and finally apatite an olivine gap is observed between fo66 and fo59 the saturation of fe ti oxides before ca rich pyroxene is interpreted to be the result of the high feot and tio2 contents and the low cao content of the parental magma contamination by old continental crust has occurred during crystallization of the layered series as indicated by sr isotopic compositions 87sr 86sr564 0 70360 0 70497 the differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases cr content of magnetite and lower sr isotope ratios these reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma the author 2010 published by oxford university press all rights reserved for permissions please e mail journals permissions oxfordjournals org

strontium isotope evidence for preserved density stratification in the main zone of the bushveld complex south africa the bushveld complex formed from two isotopically distinct magmas magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates rocks with ratios of 0 7065 lie below the lower 2 2 km of the main zone ratio 0 7085 whereas those with ratios of 0 7073 lie above it this unit represents a liquid layer intruded into the density stratified magma pile near the base of the chamber the merensky reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals

accurate jacobian matrix using the exact zoeppritz equations and effects on the inversion of reservoir properties in porous media the analysis of amplitude variation with offset avo plays a significant role in fluid detection and lithology discrimination in hydrocarbon reservoirs the zoeppritz equations are part of the basic theory of avo analysis which describes the relationship between seismic reflection and transmission coefficients and elastic rock properties e g p and s wave velocities and density currently most avo inversion methods are based on approximations of the exact zoeppritz equations which not only limit the accuracy of avo inversion but also restrict its application to wide angle seismic reflection data in addition the most difficult part of linear avo inversion obtaining an accurate jacobian matrix partial derivatives of reflection coefficients with respect to inverted parameters based on our previous study on the accurate gradient calculation of seismic reflection coefficients for the inversion of rock properties we further combine the exact zoeppritz equations with biot gassmann equations to compute the gradients of seismic reflection coefficients with solid density and reservoir properties e g porosity water gas oil saturations in porous media in this paper the partial derivative expressions of the zoeppritz matrix elements with respect to solid density and reservoir properties are simplified to simple algebraic equations which are readily calculated by comparing reflection coefficients and partial derivative curves with those obtained by classic shuey and aki richards approximations we show that our proposed method can be used to accurately obtain reservoir properties in avo inversion

the lac des iles complex ontario petrology and platinum group elements mineralization in an archean mafic intrusion the lac des iles complex is a late archean mafic to ultramafic complex that is host to 20 4 106 t pt pd and au mineralization with an average grade of 6 34 g t platinum group elements pge the 30 km2 complex located in the wabigoon subprovince consists of several coalescing mafic to ultramafic intrusive centers the complex is emplaced into early gneissic tonalite and is contemporaneous with late granitoids the magmatic evolution of the complex reflects the emplacement of multiple pulses of previously fractionated magma some of which underwent subsequent in situ fractionation distinct magma sequences are recognized on the basis of intrusive relationships differences in the cumulus minerals crystallization order and trace element chemistry these sequences are 1 hornblende gabbro 2 gabbro and gabbronorite and 3 ultramafic where there is evidence of intrusive relations the most primitive sequences are emplaced late and occur toward the north end of the complex microprobe analyses in

correlation between chromite composition and pge mineralization in the critical zone of the western bushveld complex detailed mineralogical investigations of chromite in the lower and critical zones in the northwestern sector of the bushveld complex have revealed significant compositional variations with regard to modal proportions host rock lithology and stratigraphic height superimposed on these variations are long range systematic trends in the composition of chromite in the massive layers these long range trends are closely linked with the evolution of the silicate cumulates the massive chromitite layers are divided into two types type 1 comprises the chromitites hosted entirely within ultramafic cumulates while type 2 chromitites are within cyclic units in which plagioclase cumulates occur the types are also distinguishable by their respective contents of platinum group elements pges and distribution patterns thereof viz the ratios between ru os ir and pt pd rh or relative element proportions both of which display a systematic change with height in accordance with chromite composition the relation between silicate geochemistry chromite composition and pge tenor leads to the development of a model explaining the formation of pge mineralized sulphide poor chromitite layers in the critical zone of the bushveld complex

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

electric potential source localization reveals a borehole leak during hydraulic fracturing abstracta laboratory experiment was performed to see if passively recorded electric signals can be inverted to retrieve the position of fluid leakages along a well during an attempt to hydraulically fracture a porous block in the laboratory the cubic block was instrumented with 32 nonpolarizing sintered ag agcl electrodes during the test several events were detected corresponding to fluid leakoff along the seal of the well each event showed a quick burst in the electric field followed by an exponential type relaxation of the potential distribution over time the occurrence of these electric events was always correlated with a burst in the acoustic emissions and a change in the fluid pressure these self potential data were inverted in two steps 1 using a deterministic least square algorithm with focusing to retrieve the position of the source current density in the block for a given snapshot in the electric potential distribution and 2 using a genetic algorithm to refine the position of the sou

seismic reflection amplitudes seismic amplitude variations with offset contain information about the elastic parameters prestack amplitude analysis seeks to extract this information by using the variations of the reflection coefficients as functions of angle of incidence normally an approximate formula is used for the reflection coefficients and variations with offset of the geometrical spreading and the anelastic attenuation are often ignored using angle of incidence as the dependent variable is also computationally inefficient since the data are recorded as a function of offset improved approximations have been derived for the elastic reflection and transmission coefficients the geometrical spreading and the complex travel time including anelastic attenuation for a 1 d medium these approximations are combined to produce seismic reflection amplitudes p wave s wave or converted wave as a taylor series in the offset coordinate the coefficients of the taylor series are computed directly from the parameters of the medium without using the ray parameter for primary reflected p waves dynamic ray tracing has been used to compute the offset variations of the transmission coefficients the reflection coefficient the geometrical spreading and the anelastic attenuation the offset variation of the transmission factor is small while the variations in the geometrical spreading absorption and reflection coefficient are all significant the new approximations have been used for seismic modelling without ray tracing the amplitude was approximated by a fourth order polynomial in offset the traveltime by the normal square root approximation and the absorption factor by a similar expression this approximate modelling was compared to dynamic ray tracing and the results are the same for zero offset and very close for offsets less than the reflector depth

layering and precious metals mineralization in the rincon del tigre complex eastern bolivia geologic investigations supporting recent technically successful platinum exploration of the rincon del tigre complex a ca 4 6 km thick layered sill have provided significant new understandings of the layering development and precious metals mineralization of one of south america s largest mafic ultramafic intrusions three macrocyclic units are recognized from the base up basal unit unit 1 and unit 2 each most likely the product of a major replenishment with mafic magma of one compositional type the ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite granular harzburgite olivine bronzitite each a few meters thick with occasional development of bronzitite at the top mafic rocks absent from unit 1 and present only locally in the basal unit form a well developed norite gabbro magnetite gabbro sequence in the upper half of unit 2 associated with the base of the magnetite gabbro is the precious metals zone a persistent zone of low grade subeconomic sulfide and precious metals mineralization 80 to 185 m thick the precious metals zone comprises an upper cu sulfide rich portion its base the sulfide phase boundary about 23 to 70 m above the magnetite phase boundary and a lower precious metals rich portion the lower part straddling the magnetite phase boundary the individual precious metals are concentrated in separate sulfide poor subzones each up to many tens of meters thick in the stratigraphic order rh pt pd au with peak offsets at the same scale modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic precious metals layering of the sulfides in the precious metals zone all at three different superimposed scales from about 3 m to several tens of meters a possible interpretive framework for the cryptic layering and perhaps for much of the other layering including the meter scale layering of the ultramafic rocks is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and in the magnetite gabbro by repeated convective overturns caused by the onset of magnetite precipitation the characteristics of the precious metals zone fit an orthomagmatic model the composition stratigraphic relations and modal correlation with magnetite of the sulfides all imply that s saturation was induced by loss of feo to magnetite precipitation this took place after prolonged closed system fractionation following the last major magma injection at the base of unit 2 during which crystallization of olivine pyroxene plagioclase and finally magnetite precipitation led to overall s cu and precious metals enrichment and ni and finally fe depletion of the magma the development of the mineralization and its complex layering was controlled by sulfide segregation rayleigh fractionation the different but generally very high partition coefficients dsilicate sulfide for each precious metal and repeated convective overturns at three different superimposed scales the decoupled metals distribution is analogous and similar in origin to that of the main sulfide zone of the great dyke zimbabwe and of the main sulfide layer offset portion of the munni munni complex western australia but the stratigraphic order of metal enrichment peaks is different pt pd au vs pd pt au this suggests a reversal in the relative magnitudes of the partition coefficients dpt and dpd that was possibly brought about by changes in the temperature and o s and fe activities of the magmas between the middle and late stages of crystallization constancy of d values during fractionation of mafic magma should not be assumed the precious metals zone is one of several skaergaard type strata bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions and the origin of the skaergaard mineralized zone itself including the potentially economic platinova reefs can be explained in terms of the precious metals zone model presented here the very low grade great thickness and wide separation of individual metal subzones in the precious metals zone are similar to those of the subeconomic axial facies of the main sulfide zone in the wider parts of the great dyke cf the economic more compact marginal facies on the basis of the lateral and vertical thermal model of the great dyke magma chamber the subeconomic status of the precious metals zone may be in large part a function of the specific fluid dynamics of the rincon del tigre magma chamber

self potential observations during hydraulic fracturing 1 the self potential sp response during hydraulic fracturing of intact sierra granite was investigated in the laboratory excellent correlation of pressure drop and sp suggests that the sp response is created primarily by electrokinetic coupling for low pressures the variation of sp with pressure drop is linear indicating a constant coupling coefficient cc of 200 mv mpa however for pressure drops 2 mpa the magnitude of the cc increases by 80 in an exponential trend this increasing cc is related to increasing permeability at high pore pressures caused by dilatancy of microcracks and is explained by a decrease in the hydraulic tortuosity resistivity measurements reveal a decrease of 2 prior to hydraulic fracturing and a decrease of similar to 35 after fracturing an asymmetric spatial sp response created by injectate diffusion into dilatant zones is observed prior to hydraulic fracturing and in most cases this sp variation revealed the impending crack geometry seconds before failure at rupture injectate rushes into the new fracture area where the zeta potential is different than in the rock porosity and an anomalous sp spike is observed after fracturing the spatial sp distribution reveals the direction of fracture propagation finally during tensile cracking in a point load device with no water flow a sp spike is observed that is caused by contact electrification however the time constant of this event is much less than that for transients observed during hydraulic fracturing suggesting that sp created solely from material fracture does not contribute to the sp response during hydraulic fracturing

the plagioclase magma density paradox re examined and the crystallization of proterozoic anorthosites intermediate composition plagioclase an40 60 is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dens residual liquid thus plagioclase should have a tendency to float in these magmatic systems there is however little direct evidence for plagioclase flotation cumulates either in layered intrusions or in proterozoic anorthosite complexes the layered series of the poe mountain anorthosite southeast wyoming contains numerous anorthosite leucogabbro blocks that constrain density relations during differentiation all blocks are more mafic than their hosting anorthositic cumulates their plagioclase compositions are more calcic and each block is in strong sr isotopic disequilibrium with its host cumulate associated structures disrupted and deformed layering indicate that 1 a floor was present during crystallization and that plagioclase was accumulating and or crystallizing on the floor 2 compositional layering and plagioclase lamination formed directly at the magma crystal pile interface and 3 the upper portions of the crystal pile contained significant amounts of interstitial melt liquid densities are calculated for proposed high al olivine gabbroic parental magmas and fe enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure oxygen fugacity p2o5 estimated volatile contents and variable temperatures of crystallization for all reasonable conditions calculated block densities are greater than those of the associated melt the liquid densities however are greater than those for an40 60 plagioclase which cannot have settled to the floor plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ a sloping floor possibly produced by diapiric ascent of relatively light plagioclase rich cumulates is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many proterozoic anorthosites and layered intrusions

vnir spectral variability of the igneous stratified stillwater complex a tool to map lunar highlands lunar highlands are plagioclase rich terrains produced by crystal floating in a magma ocean system lunar samples revealed the presence of anorthositic plagioclase 90 samples from the highlands associated to more mafic rocks recently remote sensing data permit mapping those terrains with high spatial and spectral resolution allowing detection of plagioclase and mafic crystal field c f absorptions in this paper we have studied bidirectional spectral characteristics in the visible near infrared vnir of rocks from the stillwater complex a cumulitic igneous stratified complex with composition varying from mafic to sialic e g pyroxenite anorthosite we investigated both slabs and powders of these rocks to give indication of the spectral variability of rock analogs of lunar crust from a mineralogical point of view samples have been spectrally separated in four main groups considering the different c f absorption association reflectance and spectral shape for both slab and powder spectra more spectral details can be obtained from the analysis of powder spectra than from the slab spectra the composition of rocks can be addressed by studying spectral parameters such as the position and the intensity of the absorption e g band center and band depth the analysis of our plagioclase pyroxene bearing samples indicates that mafic composition can be clearly obtained for samples characterized by one pyroxene phase even for few amounts of pyroxene from powder spectra on the other hand slab spectra show clear pyroxene absorptions only for rocks with mafic abundance at least 20 the intensity of the mafic absorptions of these samples shows a linear trend with respect to the abundance of pyroxenes orthopyroxene clinopyroxene for samples with ferrosilite amount less than ca 25 considering all pyroxene bearing samples the band depth of slab spectra are linearly related to the volumetric distribution of ferrous iron in pyroxenes

concentrations of rare earth elements in silicate rocks of the lower critical and main zones of the bushveld complex concentrations of rare earth elements in the lower critical and main zones of the bushveld complex at union section show distinct variation with stratigraphic height in that lower and lower critical zone cumulates are lree and th enriched ce sm 10 25 ce smn 3 65 th smn 6 17 over main zone cumulates ce sm 4 10 ce smn approximately 2 th smn 1 57 the upper critical zone constitutes a transitional interval in terms of ree concentrations ce sm 9 17 this pattern is broadly coupled albeit in a contraposed manner to that in initial sr isotopic ratio and confirms earlier studies that the bushveld complex crystallized from at least two compositionally distinct parental magmas mixing between the two magmas occurred as early as in the lower zone and is sensitively reflected by variation in ce sm th sm modal plagioclase and chromite while ree contents of the lower and critical zones are approximately 15 those of the corresponding b1 and b2 marginal rocks the main zone cumulates have similar ree contents to the b3 marginal rocks this suggests that the main zone rocks either crystallized from a highly viscous crystal mush without appreciable fractionation or from magma strongly ree enriched over the marginal rocks which would imply that the main zone is not represented in the suite of marginal rocks

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

hydraulic stimulation of geothermal reservoirs fluid flow electric potential and microseismicity relationships summary we have observed surface electric potential variations of several mv during a reservoir stimulation experiment run in 2000 at the soultz sous forets france hot dry rock site that we have interpreted as deep electrokinetic processes using data from another experiment run in 2003 at the same site we show here that the electrically conducting steel casing makes possible the measurement of surface sp anomalies even 5 km above the injection zone but obliterates any information about the geometry of the electrokinetic sources we also observed a slow temporal sp decay after the end of the that we interpret as large fluid flow persisting long after shut in at the edge of the stimulated area this persisting deep fluid flow explains why large induced microseismic events are recorded in this area long after the end of the stimulation experiment as fluid overpressures diffuse far away from the injection area this interpretation is consistent with the diffusive pattern of seismicity analysis of shapiro et al

platinum group element distribution in chromite ores from ophiolite complexes implications for their exploration abstract compilation of some new data on ophiolites for greece and yugoslavia and published data from previous studies indicate that platinum group element pge and gold concentrations in chromite ores are generally low ranging from less than 100 ppb to a few hundred ppb however samples from several ophiolite complexes exhibit an enrichment of a few ppm a only in os ir and ru b only in pt and or pd or c in all pge this enrichment up to 10s ppm is mainly related with chromitites hosted in supra moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local independent of the chromitite major element composition and the chromite potential of the ophiolite complexes the contents of pge combined with less chalcophile elements ni co cu the ratios of incompatible compatible elements and pge patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas although they have a similar major element composition thus they can be used for a stratigraphic orientation in the mantle sequence and therefore for exploration targets moreover pge data offer valuable information for the evaluation of the chromite potential in ophiolite complexes the most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type the other type being only in small proportion of limited compositional variation in both major elements and pge low ratios of pd ir while pge enriched chromitites in the mantle sequence are only occasionally present in contrast ophiolites which contain both high cr and al chromitites and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores although they are related with a ssz environment

ultramafic rocks in the centre of the vredefort structure south africa geochemical affinity to bushveld rocks abstract the geochemistry and mineral chemistry of ultramafic rocks in the centre of the vredefort structure have been reinterpreted a detailed examination of previously published major element trace and ree analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material a strong geochemical affinity to ultramafic rocks from the bushveld complex is evident the magmatic nature of the ultramafic rocks in the centre of the vredefort structure has implications for the discussion of the metamorphic history of the vredefprt structure which shows evidence of unusually high and as yet not satisfactorily explained geothermal gradients during peak metamorphism

stochastic inversion of facies from seismic data based on sequential simulations and probability perturbation method abstractwe presented a new methodology for seismic reservoir characterization that combined advanced geostatistical methods with traditional geophysical models to provide fine scale reservoir models of facies and reservoir properties such as porosity and net to gross the methodology we proposed was a stochastic inversion where we simultaneously obtained earth models of facies rock properties and elastic attributes it is based on an iterative process where we generated a set of models of reservoir properties by using sequential simulations calculated the corresponding elastic attributes through rock physics relations computed synthetic seismograms and finally compared these synthetic results with the real seismic amplitudes the optimization is a stochastic technique the probability perturbation method that perturbs the probability distribution of the initial realization and allows obtaining a facies model consistent with all available data through a relatively small number of iterations the pr

geophysical characterization of the northwest geysers geothermal field california abstract the clear lake volcanic field in northern california is the youngest and northern most part of a long lived volcanic system that has produced recent 10 ka eruptions adjacent to the clear lake volcanic field is the world s largest energy producing geothermal field the geysers the hottest part of the geyser s geothermal field is in the northwest where temperatures reach 400 c at 3 km depth low permeability high thermal gradients and low steam saturation prescribed development of an enhanced geothermal system egs in the northwest geysers to increase energy producing capacity though the northwest geysers is known to be the hottest part of the field geophysical methods have failed to adequately image any inferred heat source this project aims to image the heat source of the northwest geysers using newly collected gravity and magnetotelluric mt measurements gravity data were jointly modeled with existing magnetic data along a two dimensional profile aligned with an existing geologic cross section the key feature of the potential field model is a low density low susceptibility body at 5 km depth bmsl under the egs mt data were modeled in three dimensions to characterize subsurface resistivity structure where the upper 3 km of the resistivity model agrees well with existing data lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data below 3 km depth bmsl the resistivity model images a possible young intrusion under the egs a possible zone of partial melt

the influence of country rock assimilation and silicate to sulfide ratios r factor on the genesis of the dunka road cu ni platinum group element deposit duluth complex minnesota the dunka road deposit is one of several cu ni platinum group element pge sulfide occurrences found along the northwestern margin of the duluth complex where the host troctolitic rocks are in contact with metasedimentary rocks of the animikie group magma contamination through assimilation of sulfidic argillaceous country rocks is generally recognized as having played a key role in the genesis of the mineralization three main types of disseminated sulfide mineralization have been identified within the dunka road deposit i norite hosted sulfides ii troctolite hosted sulfides and iii pge rich sulfide horizons the norite hosted sulfides are found either adjacent to country rock xenoliths or near the basal contact the troctolite hosted sulfides form the bulk of the deposit and occur throughout the lower 250 m of the intrusion the pge rich sulfide horizons are typically localized directly beneath ultramafic layers the composition of the different types of sulfide occurrences is modelled using cuipd ratios it is shown that each type results from the interplay of two main parameters namely the degree of magma contamination and the silicate magma to sulfide melt ratio r factor the norite hosted sulfides formed at low r factors and high degrees of contamination as expressed by their pge depleted nature low seis ratios and elevated content in pyrrhotite and arsenide minerals the troctolite hosted sulfides formed at moderate r factors and small degrees of contamination as shown by their moderate pge content and mantle like seis ratios finally the pge rich sulfide horizons are modelled using elevated r factors from an uncontaminated parental magma which is substantiated by their elevated noble metal content and seis ratios and low pyrrhotite to precious metal sulfide ratio le gite de dunka road est un parmi plusieurs gites de sulfures de cu ni elements du groupe du platine egp trouves le long de la bordure nord ouest du complexe de duluth ou les roches hotes troctolitiques sont en contact avec les roches metasedimentaires du groupe d animikie il est generalement reconnu que la contamination du magma par l assimilation de roches argileuses sulfurees encaissantes a joue un role capital dans la genese de la mineralisation trois types principaux de mineralisation de sulfures dissemines ont ete identifies dans le gite de dunka road i sulfures dans la norite hote ii sulfures dans la troctolite hote et iii horizons de sulfures enrichis en egp les sulfures dans la norite hote apparaissent soit dans les zones adjacentes aux xenolites encaissants ou pres du contact basal les sulfures dans la troctolite hote forment le plus gros du gisement et ils apparaissent dans les 250m inferieurs de l intrusion la localisation typique des horizons de sulfures enrichis en egp se trouve directement sous les couches ultramafiques les rapports cu pd ont ete utilises pour elaborer un modele de la composition des differents types de mineralisation de sulfures il est demontre que chaque type resulte de l interaction de deux parametres dominants a savoir le degre de contamination du magma et le rapport du magma silicate au liquide sulfure facteur r les sulfures dans la norite hote ont ete formes sous des conditions de faibles facteurs r et de fortes contaminations comme en temoignent la teneur appauvrie en egp les faibles rapports se s et les concentrations elevees de la pyrrhotite et des mineraux arsenieux les sulfures dans la troctolite hote ont ete formes sous des conditions de facteurs r moderes et de faibles contaminations comme le revelent leur teneur modere en egp et les rapports se s analogues a ceux du manteau finalement les horizons de sulfures enrichis en egp sont modelises en utilisant des facteurs r eleves et un magma parental non contamine ce qui est appuye par leur forte teneur en elements nobles et les rapports se s eleves et par le rapport faible de la pyrrhotite sur les sulfures de metaux precieux

strontium isotopic evidence against magma addition in the upper zone of the bushveld complex abstract sr isotopic data from the main and upper zones of the bushveld complex show that the evolution of the upper zone started with a large influx of magma close to the level of the pyroxenite marker a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites whole rock samples which span the complete stratigraphic succession ca 2100 m above this layer fall on a single rb sr isochron 2066 58ma and hence have a common initial ratio of 0 7073 1 this ratio is significantly lower than those of the main zone ca 0 7085 below the level of the pyroxenite marker the entire upper zone crystallized from a mixed magma which was thoroughly blended before crystallization this magma had an isotopic ratio intermediate between that of the main zone and the added magma which had an initial ratio of ca 0 7067 further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma which is considered improbable hence the layering and mineralogical diversity of the upper zone was produced by internal processes and not produced by magma influxes during crystallization the lithological compositional and isotopic changes at the pyroxenite marker and the petrological coherence of all rocks above this horizon support the placing of the upper zone boundary at this point in the stratigraphy

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

pge rich podiform chromitites in the al ays ophiolite complex saudi arabia an example of critical mantle melting to extract and concentrate pge the al ays ophiolite complex in saudi arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements pge in podiform chromitite with maximum values of 2 570 ppb pt 6 870 ppb pd 840 ppb rh 5 800 ppb ru 6 200 ppb ir and 3 300 ppb os smooth chondrite normalized pge profiles indicate igneous pge ratios these suggest that in situ alteration of the pgm caused only minor mobility of pge during secondary modification of the mineralogy thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration three main types of pge mineralization are observed that are defined by their relative abundances of individual pge type 1 has ru both pt and pd with negative slope chondrite normalized profiles type 2 has ru either pt or pd pt pd ir ratios of 1 to 5 and convex upward chondrite normalized profiles type 3 has ru either pt or pd pt pd ir ratios of 5 to 60 positive slope chondrite normalized profiles and is associated with elevated cu and ni concentrations the unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition for example cr2o3 varies from 39 to 69 wt percent pge mineralization types 1 2 and 3 are related to the composition of the chromite type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent cr2o3 type 2 occurs in chromitite having a range of 53 to 61 wt percent cr2o3 and type 3 occurs in chromitite having a range of 39 to 51 wt percent cr2o3 the pge form a great variety of platinum group minerals pgm and they differ among the three types of pge mineralization type 1 is characterized by euhedral os ir and ru ipge alloys and laurite both commonly enclosed in chromite as well as members of the irarsite hollingworthite solid solution series and pt ipge bearing pgm both commonly interstitial to the chromite grains type 2 pge enrichment is characterized by ipge pt and rh bearing pgm type 3 pge enrichment hosts predominantly pd and pt bearing pgm associated with ni and cu bearing minerals where exposed to the serpentinization process the pgm are altered to alloys arsenides antimonides and oxides that form irregular shapes or may form pseudomorphs of former pgm they are commonly associated with ni and cu bearing minerals including ruthanian pentlandite millerite arsenides and pge bearing awaruite mantle melting and subsequent crystallization were at an optimum to concentrate pge in the al ays ophiolite complex crystallization of ipge commonly prior to chromite crystallization and latterly with some pt and rh occurred across the range of chromitite composition crystallization of pd with some remaining pt occurred during sulfur saturation in chromitite formed from a more evolved magma we propose that this crystallization was from a magma that was enriched in pge because the degree of mantle melting was just sufficient to extract the pge but not dilute them in a melt that includes further mantle melting this feature is likely to be common to other pge rich ophiolite complexes such as in shetland in the united kingdom leka in norway thetford in canada pindos in greece tropoja in albania and in new caledonia if equilibrium partial melting had continued in al ays then the magma would have been diluted by subsequent pge poor melt this would have prevented sulfur saturation until much higher in the sequence producing pt and pd bearing base metal sulfides in the crustal wehrlite and gabbro as has occurred in the cyprus and oman ophiolites

interpretation of avo anomalies we investigate the effects of changes in rock and fluid properties on amplitude variation with offset avo responses in the slope intercept domain reflections from wet sands and shales fall on or near a trend that we call the fluid line reflectionsfromthetopofsandscontaininggasorlight hydrocarbonsfallonatrendapproximatelyparalleltothefluidline reflectionsfromthebaseofgassandsfallonaparallel trendontheopposingsideofthefluidline thepolaritystandard of the seismic data dictates whether these reflections from the top of hydrocarbon bearing sands are below or above the fluid line typically rock properties of sands and shales differ and therefore reflections from sand shale interfaces are also displaced from the fluid line the distance of these trends from the fluid line depends upon the contrast of the ratio of p wave velocity vp and s wave velocity vs this ratio is a function of pore fluid compressibility and implies that distance from the fluid line increases with increasing compressibility reflections from wet sands are closer to the fluid line than hydrocarbon related reflections porosity changes affect acoustic impedance but do not significantly impactthevp vscontrast asaresult porositychangesmove theavo response along trends approximately parallel to the fluidline theseobservationsareusefulforinterpretingavo anomaliesintermsoffluids lithology andporosity

reservoir monitoring by a 4 d electrical technique an advanced geoelectrical technique for imaging potential fractures has been developed by engineering geophysics laboratory of kyushu university the method fluid flow tomography fft has been applied to monitor fluid flow behaviors during massive water steam injection operations through a borehole fluid flow behaviors in potential fractures could be continuously traced and visualized as a function of time by the fft system potential fractures can be evaluated by 3 d inversion of self potential sp anomalies based on a potential theory similar to the hypocenter of the earthquakes the fft method has been applied to determine the location of fractures in a geothermal area in japan

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time

re os isotopic systematics in chromitites from the stillwater complex montana usa abstract new re os isotopic data on chromitites of the stillwater complex demonstrate isotopic equilibrium between cumulate chromite and whole rock initial osmium isotopic ratios for the chromitites chosen for their freshness are consistent with derivation from a mantle derived magma that suffered little or no interaction with the continental crust prior to crystallization molybdenite separated from a sample of the g chromitite yields a re os age of 2740 ma indistinguishable from the age of the intrusion the presence of molybdenite documents rhenium and probably osmium mobilization by hydrothermal fluids that permeated the intrusion shortly after crystallization initial osmium isotopic variability observed in chromitites and other rocks from the stillwater complex could result from interaction with these fluids in this context there is no compelling reason to call on assimilation of crust by mantle derived magma to explain the osmium or neodymium isotopic variability although osmium isotopic systematics have been affected by hydrothermal processes re os results demonstrate that more than 95 of the osmium and by inference other pges in the stillwater complex derive from the mantle

origin of howardites diogenites and eucrites a mass balance constraint abstract howardite meteorites are regolith breccias impact produced mixtures that consist of two main rock types basalts that strongly resemble eucrite meteorites and pyroxene cumulates that strongly resemble diogenite meteorites there are basically two petrogenetic models for the noncumulate basaltic parts of howardites 1 1 genesis as residual liquids of the same magma s that had earlier crystallized the pyroxene cumulates and 2 2 genesis as primary unmodified partial melts from the interior of the parent asteroid a mass balance constraint based on al indicates that unless the ratio degree of melting during genesis of melts parental to pyroxene cumulates degree of melting during genesis of primary basalts was on average greater than about 2 8 a scenario that implies an implausible bimodality of degree of melting on the howardite parent body an average howardite must contain more residual liquid basalt than primary basalt if as seems likely eucrites per se are identical to the howardite basaltic components and diogenites per se are the pyroxene cumulate components then more than half of the eucrites probably formed from residual liquids of magmas that had earlier produced the diogenites considering all the potential complexities of petrpgenesis on the howardite parent body neither the partial melting model nor the fractional crystallization residual liquid model is likely correct for all noncumulate eucrites