a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

high precision dating of the kalkarindji large igneous province australia and synchrony with the early middle cambrian stage 4 5 extinction the voluminous kalkarindji flood basalts erupted in australia during the cambrian and covered 2 x 10 6 km 2 new u pb and ar 40 ar 39 age data from intrusive rocks and lava flows yielded statistically indistinguishable ages at ca 511 ma suggesting a relatively brief emplacement for this province a zircon age of 510 7 0 6 ma shows that this province is temporally indistinguishable at the few hundred thousand year level from the early middle cambrian stage 4 5 boundary age of 510 1 ma which marks the first severe extinction of the phanerozoic and an extended marine anoxia period sulfur concentration measurements ranging from 50 to 1900 mu g g and fractal analysis of extensive explosive volcanic breccias suggest that blasts and phreatomagmatic explosions have contributed to injection of large amounts of sulfur into the stratosphere in addition magma intrusions in oil gas and sulfate deposits may have generated significant emission of ch4 and so2 which along with volcanic gases would have combined to cause an oscillation of the climate and led to the cambrian extinction less

evolution of crystallizing interstitial liquid in an arc related cumulate determined by la icp ms mapping of a large amphibole oikocryst high resolution chemical mapping of a large single oikocrystic introduction amphibole grain from the eriksberg gabbro by laser ablation the most commonly studied cumulus rocks are those inductively coupled plasma mass spectrometry la icp ms formed by crystallization of relatively dry basaltic liquids documents the progressive evolution of interstitial liquid in a hydrous e g parsons 1986 cawthorn 1996 these include most basaltic system the amphibole has a nearly constant mg number layered intrusions ophiolites oceanic cumulates and and only minor variations in most major elements in part because most cumulates recovered from the moon and in metof the resorption of olivine buffering the liquid composition however eorite collections studies of these cumulates have innon buffered trace elements including sc v nb zr th u and cluded efforts to understand how the crystals are the rare earth elements ree show at least an order of magnitude segregated from the liquid cumulus processes as well variation for example v varies from 1000 ppm zr as how the resulting pile of crystals plus liquid evolves from 4 3 to 1000 ppm nb from 0 14 to 12 3 ppm and the during slow cooling postcumulus processes by comree patterns range from bowed up with a negative eu anomaly parison very little effort has been directed toward underto bowed down with a positive eu anomaly all within a single standing these processes in the crystallization of relatively crystal the distribution of amphibole compositions indicates that hydrous basaltic liquids such as those associated with arc the interstitial liquid was not uniformly distributed as crystallization magmatism the presence of a significant amount of proceeded rather the compositional variations reflect progressively water in the liquid is especially important for the postmore channelized flow of interstitial liquid during compaction when cumulus evolution of a crystal pile as it will stabilize the interstitial liquid evolved so that the crystallizing amphibole had hydrous minerals and cause vapor saturation while a 300 ppm v vapor saturation was reached and the behavior of the significant amount of interstitial liquid remains incompatible trace elements changed markedly the final amphibole to the importance of fractional crystallization in procrystallize is enriched in soluble incompatible elements and depleted ducing compositional variability in crystallizing silicate in zr and nb a relationship that is consistent with crystallization liquids has been well established since bowen s exfrom a fluid where compaction returns such evolved liquid or fluid perimental work bowen 1928 during perfect fractional to an overlying magma reservoir resulting fractionation trends will crystallization the solids are isolated from the liquid before the liquid evolves appreciably and thus they proshow the relative depletions in ti nb and zr seen in arc related vide an instantaneous record of the liquid evolution basalts incorporation of liquid in a crystal pile can change both the composition of the solids and the evolution of the supernatant liquid initially the parental liquid if the

magmatic metasomatism and formation of the merensky reef bushveld complex the rare earth element ree contents of pyroxenes and other minerals from the merensky reef and stratigraphically adjacent rocks of the atok section bushveld complex have been determined with the ion microprobe merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the ree than their cumulus counterparts in rocks several meters below the reef chondrite normalized merensky clinopyroxene ce contents vary from 10 to 90 for ce and from 4 to 17 for yb they also possess deep negative eu anomalies the eu anomalics being deeper for crystals having high ree contents and relatively shallow for pyroxenes with low ree contents similar compositional characteristics are displayed by cl rich apatite which is an accessory phase in the rocks interstitial pyroxene in cumulates above and below the reef also tends to have elevated ree contents and in general is not in equilibrium with coexisting cumulus minerals the melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts that from which merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in ree than any normal tholeiitic or alkalic basalt despite their highly evolved nature in terms of the ree the merensky reef pyroxenes are not evolved in terms of major elements the decoupling of incompatible trace and major elements is best explained by a metasomatic process it is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile the fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed

the lower zone critical zone transition of the bushveld complex a quantitative textural study the lower zone critical zone boundary of the bushveld complex is an intrusion wide major stratigraphic transition from ultramafic harzburgite and pyroxenite in the lower zone to increasingly plagioclase richpyroxenitesandnoritesinthecriticalzone quantitative textural and compositional data for 29 samples through this transition show the following lower zone orthopyroxene grains are larger have higher aspect ratios are better foliated and have a lower trapped liquid component than those of the critical zone the larger grain size of the lower zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the critical zone although all rocks show differing amountsoffoliation mineral lineations are weak to absent these data are consistent with significant compaction driven recrystallization in the study section numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the lower zone critical zone transition may have formed plagioclase is rare in the lower zone because compaction removes interstitial liquid before it reaches plagioclasesaturation however asthecrystalpilegrows plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites producing more abundant but still modest amounts of plagioclase characteristic of the lower critical zone it is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization primary magmatic textures are not preserved

crystallization history of enriched shergottites from fe and mg isotope fractionation in olivine megacrysts abstract martian meteorites are the only samples available from the surface of mars among them olivine phyric shergottites are basalts containing large zoned olivine crystals with highly magnesian cores fo 70 85 and rims richer in fe fo 45 60 the northwest africa 1068 meteorite is one of the most primitive enriched shergottites high initial 87 sr 86 sr and low initial e 143 nd it contains olivine crystals as magnesian as fo 77 and is a major source of information to constrain the composition of the parental melt the composition and depth of the mantle source and the cooling and crystallization history of one of the younger magmatic events on mars 180 ma in this study fe mg isotope profiles analyzed in situ by femtosecond laser ablation mc icp ms are combined with compositional profiles of major and trace elements in olivine megacrysts the cores of olivine megacrysts are enriched in light fe isotopes 56 fe irmm 14 0 6 to 0 9 and heavy mg isotopes 26 mg dsm 3 0 0 2 relative to megacryst rims and to the bulk martian isotopic composition 56 fe 0 0 05 26 mg 0 27 0 04 the flat forsterite profiles of megacryst cores associated with anti correlated fractionation of fe mg isotopes indicate that these elements have been rehomogenized by diffusion at high temperature we present a 1 d model of simultaneous diffusion and crystal growth that reproduces the observed element and isotope profiles the simulation results suggest that the cooling rate during megacryst core crystallization was slow 43 21 c year and consistent with pooling in a deep crustal magma chamber the megacryst rims then crystallized 1 2 orders of magnitude faster during magma transport toward the shallower site of final emplacement megacryst cores had a forsterite content 3 2 1 5 mol higher than their current composition and some were in equilibrium with the whole rock composition of nwa 1068 fo 80 1 5 nwa 1068 composition is thus close to a primary melt i e in equilibrium with the mantle from which other enriched shergottites derived

high latitude eruptions cast shadow over the african monsoon and the flow of the nile 1 nile river records indicate very low flow following the 1783 1784 laki volcanic eruption as well as after other high latitude volcanic eruptions as shown by climate model simulations of the laki eruption significant cooling 1 to 3 c of the northern hemisphere land masses during the boreal summer of 1783 resulted in a strong dynamical effect of weakening the african and indian monsoon circulations with precipitation anomalies of 1 to 3 mm day over the sahel of africa thus producing the low nile flow future high latitude eruptions would significantly impact the food and water supplies in these areas using observations of the flow of the nile river this new understanding is used to support a date of 939 for the beginning of the eruption of the eldgja volcano in iceland the largest high latitude eruption of the past 1500 years

orthopyroxene oikocrysts in the mg1 chromitite layer of the bushveld complex implications for cumulate formation and recrystallisation two typical mineral textures of the mg 1 chromitite of the bushveld complex south africa were observed one characterised by abundant orthopyroxene oikocrysts and the other by coarse grained granular chromitite with only minor amounts of interstitial material oikocrysts form elongate clusters of several crystals aligned parallel to the layering and typically have subhedral almost chromite free core zones containing remnants of olivine the core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts chromite grains show no preferred crystal orientation whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation oikocryst core zones have lower mg and higher concentrations of incompatible trace elements compared to their poikilitic aureoles core zones are relatively enriched in ree compared to a postulated parental magma b1 and did not crystallise in equilibrium with the surrounding minerals whereas the composition of the poikilitic orthopyroxene is consistent with growth from the b1 magma these observations cannot be explained by the classic cumulus and post cumulus models of oikocryst formation instead we suggest that the oikocryst core zones in the mg1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards percolating melt enriched in incompatible trace elements poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of b1 composition near the magma crystal mush interface finally adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

analyzing three component 3d vertical seismic profiling data a three component 3d vertical seismic profile vsp was acquired over the blackfoot oil field in alberta canada the vsp survey was recorded simultaneously with a surface seismic program the objectives of the vsp were to develop recording logistics data handling and processing procedures and to determine if the 3d vsp volumes could image the glauconitic sand reservoir of the blackfoot field dynamite shots from the surface seismic survey which fell within a 2200 m offset from the recording well were used in the vsp analysis the shots were recorded by a string of three component borehole receivers that was moved seven times resulting in a receiver depth range of 400 to 910 m the borehole data were processed using basic vsp processing techniques that included hodogram analysis wavefield separation using median filters and vsp deconvolution the final p p and p s image volumes were obtained by vsp common depth point and vsp common conversion point stacking the upgoing wavefields followed by f xy de

using tracer tests to estimate vertical recharge and evaluate influencing factors for irrigated agricultural systems accurate estimation of groundwater recharge is critical for semi arid regions especially in places like the north china plain ncp where recharge from irrigation and intermittent precipitation events comprises the largest portion of recharge tracer tests were used to estimate potential recharge beneath agricultural systems irrigated by groundwater and to help delineate factors that influence recharge a bromide solution was applied below root regions to trace infiltration in the vadose zone beneath irrigated agricultural fields and non irrigated woodlands at both piedmont plain shijiazhuang and alluvial and lacustrine plain areas hengshui in the ncp the tracer tests lasted for more than 2 years and were conducted at 37 subsites grouped in sets of two to four at 12 regionally distributed sites for the piedmont plain sites the potential recharge rate ranged between 37 466 mm a 6 27 of precipitation plus irrigation p i beneath wheat maize 110 564 mm a 12 52 of p i beneath orchard and 7 10 mm a 1 2 of p i beneath woodlands for the alluvial and lacustrine plain sites the potential recharge rate ranged between 14 177 mm a 2 20 of p i beneath wheat maize 6 57 mm a 0 5 5 of p i beneath orchard 87 279 mm a 10 31 of p i beneath cotton and 6 44 mm a 1 8 of p i beneath woodlands the potential recharge was impacted by various external factors like lithology crops and irrigation when an irrigation controlled experiment was conducted in the same field with the same crop cultivation the results revealed that larger irrigation quantities led to larger potential recharge rates

impacts of thickening unsaturated zone on groundwater recharge in the north china plain summary unsustainable groundwater development shown by rapid groundwater depletion in the north china plain ncp underscores the need to quantify spatiotemporal variability in groundwater recharge for improved management of the resource the objective of this study was to assess spatiotemporal variability in recharge in response to thickening of the unsaturated zone in the ncp recharge was estimated by linking a soil water balance swb model on the basis of monthly meteorological data irrigation applications and soil moisture monitoring data 1993 2008 to the water table using a deep unsaturated zone flow model the dynamic bottom boundary water table position was provided by the saturated zone flow component which simulates regional pumping the model results clearly indicate the effects of unsaturated zone thickening on both temporal distribution and magnitude of recharge smoothing temporal variability in recharge and increasing unsaturated storage and lag time between percolation and recharge the thickening unsaturated zone can result in average recharge reduction of up to 70 in loam soils with water table declines 30 m declining groundwater levels with irrigation sourced by groundwater converts percolation to unsaturated zone storage averaging 14 mm equivalent water depth per year in mostly loam soil over the study period accounting for 30 of the saturated groundwater storage depletion this study demonstrates that in thickening unsaturated zones modeling approaches that directly equate deep drainage with recharge will overestimate the amount and underestimate the time lag between percolation and recharge emphasizing the importance of more realistic simulation of the continuity of unsaturated and saturated storage to provide more reliable estimates of spatiotemporal variability in recharge

repeatability issues of 3 d vsp data increased repeatability is recognized as one major issue for improving the time lapse seismic technology as a reservoir management tool a 3 d vertical seismic profiling vsp data set acquired over a period of two days is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning it is found that for a frequency range between 3 5 and 50 hz the difference root mean square rms level between two recorded traces belonging to two different shots is about 8 this fact shows that there is a potential for acquiring very accurate time lapse seismic data by using a permanently installed downhole geophone array repeatability variation with increasing shot separation distances is analyzed showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases horizontal geophone components show approximately the same degree of repeatability compared to the vertical component but horizontal geophone data is slightly more sensitive to positioning errors the results show that repeated 3 d vsp surveys preferably using permanently installed geophone arrays might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

interpreting channel sands with 3c 3d seismic data a 3c 3d seismic survey was acquired over the blackfoot field near strathmore alberta canada in 1995 the survey sponsored by a group of exploration companies was planned and conducted by the crewes project department of geology and geophysics the university of calgary and boyd exploration consultants simultaneously with the surface data acquisition a five level 3 c downhole tool from western atlas international was deployed in a well and a 3c 3d vsp was recorded

experimental study on the effects of h 2 o upon crystallization in the lower and critical zones of the bushveld complex with an emphasis on chromitite formation mafic ultramafic layered rocks of the lower and critical zones of the bushveld complex are believed to have formed from a parental magma usually labelled b1 with sio2 content at 55 wt mgo at about 12 wt and cr2o3 concentration at 0 15 wt previous experimental study at 300 mpa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 c and the mineral was joined by plagioclase and clinopyroxene at and below 1166 c the objective of this study was to investigate the effects of variable additions of h2o 2 4 and 6 wt in starting compositions on liquidus and sub liquidus phase equilibria main attention was given to the solubility of cr spinel and cr distribution between melt and crystalline phases six experiments were carried out in the internally heated pressure vessel ihpv at 300 mpa and in the temperature interval 1100 1300 c redox conditions were not controlled and thermal dissociation of the h2o component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products the effective values of fo2 in experiments were calculated using spinmelt 2 0 software bases on the regression of experimental data on the spinel melt equilibrium according to the calculations h2o additions in isothermal series of experiments increased fo2 by 0 8 1 6 log units from the level in a nominally dry run overall the experiments showed that h2o additions suppressed crystallization of silicates shifted olivine orthopyroxene peritectic equilibrium towards higher olivine stability olivine first appears in h2o bearing compositions at 1175 c and increased the proportion of magnetite component in spinel solid solution the addition of 4 wt h2o to the starting composition at 1200 c resulted in complete melting of orthopyroxene which left cr spinel as the only liquidus phase equal amount of h2o at 1125 c leads to complete melting of plagioclase orthopyroxene and clinopyroxene which leaves only olivine and spinel stable and increases the liquid mass fraction from 0 62 in the nominally dry compositions to 0 9 the results of this study appear to support the idea that chromitite layers in the critical zone of the bushveld complex were formed by episodes of hydration melting of silicate cumulates

a late archaean radiating dyke swarm as possible clue to the origin of the bushveld complex the origin of the bushveld complex south africa is elusive identification and dating of a radial swarm of volcanic dykes suggests that a mantle plume affected the region but was active 600 million years before the bushveld complex formed implying that the bushveld magmas could be an indirect consequence of the plume

a juvenile oceanic island arc origin for the archean ca 2 97 ga fiskenaesset anorthosite complex southwestern greenland evidence from oxygen isotopes abstract the archean ca 2 97 ga fiskenaesset layered intrusion southwestern greenland consists of an association of anorthosite leucogabbro gabbro hornblendite pyroxenite peridotite and dunite the intrusion is characterized by well preserved igneous layering cumulate texture and primary igneous minerals including olivine pyroxene plagioclase hornblende and chromite we use new whole rock n 36 and mineral n 32 oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the fiskenaesset complex at majorqap q va to revisit geodynamic and petrogenetic hypotheses proposed for the origin of archean terranes the fiskenaesset complex has modern mantle like whole rock o isotope compositions o 18 5 8 0 5 average o 18 values increase from peridotite o 18 5 0 through hornblendite o 18 5 7 gabbro o 18 5 8 pyroxene hornblendite o 18 6 0 and leucogabbro o 18 6 3 to anorthosite o 18 6 3 these whole rock isotopic compositions reflect the approximate modal abundances of olivine average o 18 4 9 hornblende average o 18 5 7 clinopyroxene average o 18 6 4 and plagioclase average o 18 6 4 in each rock type as a consequence of mineral fractionation in the magma chamber s field relationships and the absence of crustal contamination suggest that the fiskenaesset complex formed in an oceanic setting subduction zone like whole rock trace element signatures and mantle like o 18 and initial e nd values are consistent with formation of these rocks in a juvenile oceanic island arc setting field and geochemical data from the fiskenaesset region and adjacent terranes suggest that the origin of archean crust in southwestern greenland is consistent with phanerozoic like plate tectonic processes rather than density driven sinking delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre existing continental crust mantle like o 18 values in the ca 2 97 ga fiskenaesset complex are inconsistent with substantial recycling of continental crust indicating that the fiskenaesset rocks originated distal from continental sources

heterogeneously hydrated mantle beneath the late archean yilgarn craton abstract archean mafic ultramafic melts crystallized as layered intrusions in the upper crust and extruded as komatiitic flows are primary probes of upper mantle chemistry however the message from their primary chemical composition can be compromised by different modes of contamination contaminants are typically cryptic in terms of their geochemical and isotopic signals but may be related to metasomatised mantle sources ambient crustal assimilation or subduction related inputs in this work we present critical evidence for both dry and wet archean mantle sources for two juxtaposed layered intrusions in the australian yilgarn craton the ca 2813 ma windimurra and ca 2800 ma narndee igneous complexes in western australia are two adjacent layered intrusions and would be expected to derive via similar mantle sections a key difference in their chemistry is the presence of crystal bound water in the narndee igneous complex represented primarily by abundant hornblende such a primary hydrous phase is notably absent in the windimurra igneous complex new 40ar 39ar plateau ages for fresh narndee hornblende weighted mean 2805 14 ma mswd 1 8 probability 0 18 agrees with the published u pb age of 2800 6 ma for the complex and is consistent with a magmatic origin for this phase zircon hf and whole rock hf and nd isotopes for the narndee igneous complex indicate only minor crustal contamination in agreement with h and o isotope values in amphibole and o isotope values in rare zircon crystals plagioclase and pyroxene within both complexes these findings illustrate a fast temporal transition in proximal bodies from anhydrous to hydrous mantle sources with very minor crustal contamination these large layered mafic ultramafic intrusions are igneous bodies with a primitive chemical bulk composition that requires large degrees of mantle melting this has been attributed by many workers to mantle plume activity yet not without dispute as subduction related flux melting may also generate large melt fractions we conclude that the source of the magmatic water at narndee is the mantle which in conjunction with its absence in the adjacent windimurra igneous complex argues for a heterogeneous hydration of mantle source regions under the yilgarn craton in the mesoarchean

field study of spatial variability in unsaturated flow beneath and adjacent to playas we quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the southern high plains united states to resolve issues related to where and how water moves through the unsaturated zone this is the first study in which the data density 39 boreholes and the variety of techniques used physical chemical and isotopic were sufficient to quantify spatial variability in unsaturated flow water contents water potentials and tritium eoneentrations were much higher and chloride eoneentrations were much lower beneath playas than in interplaya settings which indicated that playas focus recharge these results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr 1 beneath playas and were 0 1 mm yr 1 during the past 2000 5000 years beneath natural interplaya areas not subjected to ponding to evaluate the apparent inconsistency between high recharge rates and thick clay layers beneath playas we applied bromide and fd c blue dye to evaluate flow processes these applied tracer experiments showed preferential flow along roots and desiccation cracks through structured clays in the shallow subsurface in playas

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

rapid map and inversion of p sv waves multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional p wave sections and to find and use rock properties associated with shear waves e g dohr 1985 danbom and dominico 1986 mode converted p sv waves hold a special interest for several reasons they are generated by conventional p wave sources and have only a one way travel path as a shear wave through the typically low velocity and attenuative near surface for a given frequency they will have a shorter wavelength than the original p wave and thus offer higher spatial resolution this has been observed in several vertical seismic profiling vsp cases e g geis et al 1990 however for surface seismic data converted waves are often found to be of lower frequency than p p waves e g eaton et al 1991

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

longitudinal and lateral dispersion in an unsaturated field soil lateral and longitudinal dispersion was quantified in a field soil under water unsaturated conditions the relatively mobile dye tracer brilliant blue fcf was applied as a line source and leached into the soil at two different rates of infiltration 4 and 24 mm d 1 respectively the resulting tracer plume was photographically recorded at vertical soil profiles excavated perpendicularly to the line source after 50 100 and 200 mm of cumulative infiltration an image analysis technique was used to determine two dimensional concentration distributions from the photographs average horizontal and vertical concentration distributions were analyzed using the two dimensional advection dispersipn equation model parameters were fitted to optimize the agreement between measured and modeled averaged concentration profiles in both horizontal as well as vertical directions dispersivities showed a dependency on flow rates and amount of cumulative infiltration but this dependency appeared to be related to the degree of irregularities of observed flow patterns large dispersivities were associated with higher degree of irregularities in the flow patterns and vice versa layer boundaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large this study demonstrates 1 that more than just the vertical concentration profiles are needed to define the transport regime under unsaturated conditions and 2 that even subtle layer boundaries affect the lateral mixing regime and exert a marked influence on the transport in the main flow direction

preferential flow in a sandy vadose zone 1 field observation abstract field experiments of water movement in the lower portion of an unsaturated zone i e vadose zone are far more difficult to conduct than those in the root zone thus our concept of the transport mechanism in the vadose zone is still primitive and mainly an extrapolation of our understanding of transport in the root zone recent results from field experiments conducted at the hancock agricultural research station indicated that water and solute applied uniformly to soil surfaces did not flow through the entire vadose zone by using dye tracing and profile excavation field experiments were conducted during the 1987 growing season to examine water flow patterns in the unsaturated zone of the central sand area of wisconsin csaw the results confirmed that preferential flow paths constitute the dominant flow pattern at this location water flowing through the root zone was funneled into concentrated flow paths that occupied only a small portion of the soil matrix in the vadose zone yet accounted for most of the transport the observed phenomena of preferential flow in a sandy vadose zone are discussed in this paper the mechanism and implications of preferential flow are presented in a companion paper

rapid vsp cdp mapping of 3 d vsp data vertical seismic profiling common depth point vsp cdp mapping with rapid ray tracing in a horizontally layered velocity model is used to create 3 d image volumes using blackfoot and oseberg 3 d vertical seismic profiling vsp data the ray tracing algorithm uses fermat s principle and is specially programmed for the layered model the algorithm is about ten times faster than either a 3 d vsp cdp mapping program with an eikonal traveltime computation method or a 3 d vsp kirchhoff migration program the mapping method automatically separates the image zone from the nonimage zone within the 3 d image volume the oseberg data example shows that the lateral extent of the image zone created by the 3d vsp cdp mapping is larger than that created by 3 d vsp kirchhoff migration the same sample result also provides high frequency events at target zones we include an analysis of the imaging error induced from using a horizontally layered model for the oseberg data indicating that the method is reliable in the presence of gently dipping structure

global dispersal of pb by large volume silicic eruptions in the parana etendeka large igneous province volcanism from large igneous provinces is often inferred to have had a significant global environmental impact but demonstrating synchronicity between the volcanism and sedimentary records of environmental change is not straightforward several studies use sediment pb isotopes to fingerprint the timing of input of volcanic pb from a particular large igneous province published data on ocean drilling program site 1149b sediments in the pacific ocean associated with the valanginian 13 c excursion that has been linked to the parana etendeka volcanism in central south america and namibia require input of radiogenic pb the pb isotope analyses of parana etendeka lavas show that this radiogenic pb is uniquely associated with the large volume silicic eruptions rather than the effusive basaltic lavas this strengthens the case for a temporal link between the parana etendeka volcanism and the valanginian 13 c excursion although basaltic magmatism dominates the province 95 there is no evidence for pb associated with volatile metal degassing from such eruptions in the site 1149b sediments on the other side of the globe suggesting that eruption columns produced by the basaltic eruptions did not reach the stratosphere instead the widespread global dispersal of pb is apparently linked to the ability of large volume 1000 km 3 silicic eruptions to loft aerosols into the upper stratosphere

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

parsimonious migration of 3 c 3d vsp data conventional kirchhoff migration can be made much faster by a priori calculation of wave arrival directions and tracing rays back only along the measured directions this is known as parsimonious migration for surface seismic data where acquisition with 2d arrays is common the wave arrival directions can be measured by using local slant stacking to calculate the two horizontal slowness components the vertical slowness is then deduced by using the dispersion relation for vertical seismic profile vsp data where multiline acquisition is not possible only the vertical component of the slowness vector can be measured which is insufficient to constrain the wave arrival direction we overcome this limitation for three component vsp data by polarization analysis of the incident waves polar and azimuth anglesare calculated at the receivers by estimating the data covariance matrix and the vertical slowness assuming isotropic media and linear polarization of both p and s waves for p waves the data co

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

petrogenesis of the ultramafic zone of the stillwater complex in north america constraints from mineral chemistry and stable isotopes of li and o to investigate the petrogenesis of cyclic units in layered intrusions we examined chromitite dunite poikilitic harzburgite and bronzitite from the ultramafic zone of the stillwater complex and measured stable isotopes of li and o in their major minerals the li isotopes in olivine range from 4 to 26 in 7li with uniform li contents of 1 3 ppm whereas orthopyroxene and clinopyroxene have li contents of 0 5 5 ppm and 4 8 ppm and 7li ranges of 13 to 7 and 14 to 6 respectively the 18o values vary from 4 91 to 5 72 in olivine from 5 11 to 5 87 in orthopyroxene and from 4 64 to 5 86 in clinopyroxene for a given sample olivine displays more variable and higher 7li but lower 18o values than orthopyroxene indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene the general li and o isotopic compositions are interpreted as the result of re equilibration between interstitial liquids from which pyroxenes crystallized and cumulus minerals the inter mineral and inter sample isotopic variations correlate with mineral assemblages crystal sizes and major and trace element compositions revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids which collected on chromite surfaces and were later released from chromite seams played an additional critical medium of chemical exchange between minerals in the chromitites

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

electrochemical processes in a crystal mush cyclic units in the upper critical zone of the bushveld complex south africa the upper critical zone ucz of the bushveld igneous complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order from bottom to top pyroxenite norite anorthosite electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the ug2 chromitite and the merensky reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral melt evolution the anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates here chromite compositions change abruptly from high and constant mg mg fe 2 and fe 2 fe 3 ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers furthermore the composition of disseminated chromites varies depending on the host silicate assemblage for example in ti v and zn contents importantly the abrupt change in chromite composition across the chromitite silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re equilibration with intercumulus liquid among those features is the decoupling of alkalis from other incompatible lithophile elements in comparison with cumulus plagioclase intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible li k and rb strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers to explain these features we propose a new model of post cumulus recrystallization which intensifies the modal layering in the crystal liquid mush producing the observed sequence of nearly monomineralic layers of chromitite pyroxenite and anorthosite that define the cyclic units the crucial element of this model is the establishment of redox potential gradients at contacts between chromite rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt because basaltic melts are ionic electrolytes with na as the main charge carrier the redox potential gradient induces electrochemical migration of na and other alkali ions selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions which may eventually result in the formation of sharply divided monomineralic layers the observed variations in ferric ferrous iron ratios in chromite from the cyclic units and fe distribution in plagioclase imply a redox gradient of

estimating groundwater recharge in hebei plain china under varying land use practices using tritium and bromide tracers summary tritium and bromide were used as applied tracers to determine groundwater recharge in hebei plain north china to evaluate the impacts of different soil types land use irrigation and crop cultivation practice on recharge additional objectives were to evaluate temporal variability of recharge and the effect on results of the particular tracer used thirty nine profiles at representative locations were chosen for investigation average recharge rates and recharge coefficient determined by tritium and bromide tracing for different sites were 0 00 1 05 mm d and 0 0 42 5 respectively the results showed relative recharge rates for the following paired influences items within each pair are listed with the influence producing greater recharge first flood irrigated cropland and non irrigated non cultivation land flood irrigation 0 42 0 58 mm d and sprinkling irrigation 0 17 0 23 mm d no stalk mulch 0 56 0 80 mm d and stalk mulch 0 44 0 60 mm d vegetable e g chinese cabbage and garlic 0 70 mm d and wheat maize 0 38 mm d peanut 0 51 mm d and peach 0 43 mm d the results also showed greater recharge for the first year of tracer travel than for the second because total precipitation and irrigation were greater in the first year than in the second this may reflect temporal variability of recharge the method may not be applicable where the water table is shallow less than 3 m a comparison of the near ideal tritium tracer with the more common but less ideal bromide showed that bromide moved approximately 23 faster than tritiated water perhaps because of anion exclusion

macrorhythmically layered gabbronorites in the windimurra gabbroid complex western australia abstract plagioclase orthopyroxene augite cumulates gabbronorites in the 3 5 km thick wagoo hills section show strong rhythmic modal layering over about 200 m of section layers of mafic gabbronorite mgn 1 10 m thick alternate with leucogabbronorite lgn 10 5 m thick with sharp contacts mgn are adcumulates with 65 70 plagioclase cores an68 reverse zoning 20 orthopyroxene mg69 15 augite mg78 and whole rock mg number 65 in contrast lgn are mesocumulates more orthocumulate character with 80 90 plagioclase cores an75 normal zoning 6 augite mg71 4 orthopyroxene mg62 and whole rock compositions have mg numbers ca 54 and higher concentrations of incompatible elements e g k p and lower concentrations of compatible elements and pge than in mgn lgn contain minor interstitial postcumulus quartz hornblende fe ti oxides apatite and 5 25 hydrous alteration products unlike mgn the cause of the presumed cumulus stage differences in pyroxene plagioclase modal ratio noncotectic proportions in lgn and sharp mgn lgn mgn contacts is not clear but could result from rhythmic variations in supercooling and nucleation the postcumulus differences postcumulus mineralogy normal zoned plagioclase more fe rich pyroxenes and higher incompatibles in lgn are interpreted in terms of an in situ crystallisation compositional convection model cumulus mineral assemblages in mgn have a higher fractionation density and their in situ crystallisation released a buoyant residual liquid which was continuously displaced by fresher magma resulting in only 0 5 residual liquid trapped with the mush adcumulate formation in contrast in situ crystallisation of a plagioclase rich cumulus assemblage lower fractionation density in lgn resulted in a denser stagnant residual liquid 10 15 of which was trapped in the crystal mush producing normal zoning in plagioclase 6 8 atom fe shift in pyroxene compositions and higher incompatible element concentrations and h 2 o the latter resulting in selective alteration of lgn as a result mgn had higher solidus t and acted like seals or aquicludes whereas lgn had lower solidus t and were sinks or aquifers for incompatibles and volatiles

platinum group element mineralization in the critical zone of the western bushveld complex i sulfide poor chromitites below the ug 2 primary stratiform platinum group element pge mineralizations associated with chromitites in the critical zone of the bushveld complex south africa are confined to cumulate layers that are regionally persistent over tens of kilometers four categories ofchromitite are recognized layers at the bases of cycles in the lower critical zone type i layers at the bases of cycles in the upper critical zone type ii thin layers in the intermediate parts of cycles type iii and stringers associated with orthopyroxene pegmatoids type iv these latter may not be of primary magmatic origin nevertheless probably each type of chromitite contains anomalous concentrations of pge chromitites at the bases of cycles comprising the lower group lg middle group mg and upper group ug are further categorized into types ia lg 1 lg 4 ib lg 5 mg 1 iia mg 2 ug 1 and iib ug 2 and above on the basis of lithostratigraphy chromite and pge chemistry and sulfide content type iib chromitites contain significant quantities of sulfides and are not investigated here new whole rock and electron microprobe chemical analyses of the sulfide poor type ia ib and iia chromitites cu contents 1 8 low pge contents 2 type ib chromitites occur in pyroxenite cycles and form an intermediate group these regular variations with increased height are interpreted as a differentiation trend pge are concentrated in sulfide poor chromitites as a result of chromitite control a process not fully understood but thought to involve a combination of direct nucleation of platinum group minerals and localized s saturation it is the crystallization of copious quantities of chromite from hybridized melts close to the crystal liquid interface that triggers this process but magma mixing essential to drive the melt into the chromite field is the overall cause replenishment of the magma chamber was from a single magma batch because the chromitites below the ug 2 are not associated with major isotopic inflections huge volumes of dense hot magma were fed as basal flows along the crystal liquid interface the magma chamber was compartmentalized at an early stage possibly in response to basement tectonics and lateral mixing over distances of 150 km is an alternative mechanism for achieving sufficiently high r values repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height type ia chromitites are ascribed to replenishment and mixing with the relatively primitive lower zone residue type ib crystallized as a result of mixing with lower critical zone resident liquid and type iia due to mixing with more evolved upper critical zone resident liquid differentiation of pge with height is a critical part of the process which culminates in the occurrence of higher grade pge mineralization in sulfide bearing chromitites ug 2 merensky reef in the uppermost part of the upper critical zone

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

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

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

an origin for harrisitic and granular olivine in the rum layered suite nw scotland a crystal size distribution study dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low results from crystal size distribution csd measurements on harrisitic olivines from rum scotland combined with estimated olivine growth rates suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long may have exceptionally short crystal growth times several hours to several hundreds of days this together with very low calculated nucleation densities for harrisitic olivine supports the interpretation of harrisite being a disequilibrium texture developed in response to supersaturation of the magma in olivine we propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the rum magma chamber floor beneath cooler resident magma it is envisaged that the picrite sheets were largely free of suspended olivine crystals coupled with the olivine enriched composition of the melt and the increasing cooling rate this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei the enhanced supersaturation caused rapid growth of olivine once nucleation began with skeletal and dendritic shapes it is suggested that the observed interlayered sequences of harrisite and cumulus peridotite found throughout the rum layered suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling

platinum group element geochemistry of the lower and middle group chromitites of the eastern bushveld complex the results of neutron activation analyses for the six platinum group elements and au show that the lower group and middle group chromitites are enriched in them over the silicate rich host rocks the platinum group elements and gold contents increase up the sequence and the middle group chromitites are richer than the lower group chromitites in these elements of the two platinum group element groups pt pd rh and ru os ir the former occurs in greater abundance in the middle group chromitites whereas the ru os ir group only slightly enriches them the lower group chromitites are enriched in ru and depleted in ir os compared to the middle group chromitites this trend is probably due to upward cryptic variation in laurite the most likely mineral host for these elements in the bushveld complex chromitites there is high interelement correlation between the various platinum group elements in the pt pd rh and the ru os ir groups chromitites with more than 80 vol percent chromite are depleted in the pt pd rh group due to the effect of postcumulus events there is generally low correlation between ni and the platinum group elements plus gold and none with cu

pge tenor and metal ratios within and below the merensky reef bushveld complex implications for its genesis the merensky reef is the consequence of several major influxes of magma into the bushveld magma chamber that gave rise to at least two cyclic units including the pre merensky and merensky cyclic units analysis of the s ni cu platinum group element pge and au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the reef to close to the ug 2 horizon of the western bushveld complex provide important petrogenetic constraints on the origin of the reef particular attention is paid to variations in the calculated pge tenors and cu pd ratios of sulfides both vertically and laterally all of the profiles show a decrease in pt tenor and concomitant increase in cu pd ratio upward across the pyroxenite of the merensky unit and laterally away from certain centres that are interpreted as feeder zones the data also document that more mafic horizons within the merensky to ug 2 interval of the southwestern bushveld are characterized by much lower cu pd 100 400 than the ambient ratio 4000 this is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there the favoured interpretation for the merensky reef is that the bushveld chamber s was occupied by a resident magma crystallizing orthopyroxene and plagioclase pre merensky and subsequently merensky magmas entered the chamber s intermittently from localized feeder structures mixing or mingling with the resident magma and giving rise to the more mafic cumulates at specific horizons including the merensky reef the mixing or mingling caused sulfide immiscibility in these cases as the new magma pulses flowed laterally away from the entry zones they deposited sulfide becoming depleted in pge on coming to rest the magma continued to segregate sulfide and crystallize pyroxene so that the sulfides became progressively depleted in pge and acquired a higher cu pd upward through the pyroxenite this model requires that the merensky magmas could dissolve up to two orders of magnitude more pge than current measurements on dry diopside anorthite melts have indicated it is suggested that very pge rich magmas can develop as a result of sulfide unsaturated magma reacting with early formed sulfide in a staging chamber dissolving fes and enriching the remaining sulfide in pge later magmas equilibrating with this enriched sulfide will themselves become highly enriched in pge

oxygen isotope systematics of the banda arc low delta o 18 despite involvement of subducted continental material in magma genesis abstract this study reports new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the banda arc eastern indonesia the melt oxygen isotope values 18omelt were calculated from phenocryst 18o data using theoretical and empirical mineral melt fractionation factors the 18omelt of individual volcanic centers within the arc varies between 5 57 and 6 54 except for serua 18omelt 6 13 7 48 and ambon 18omelt 8 12 8 38 these 18omelt values are up to 2 lower than new and previously published oxygen isotope data obtained on whole rock powders by conventional methods we attribute this discrepancy to post emplacement low temperature alteration and or to a systematic deviation of the bulk analysis sediment 18owr calculated from the 18o carbonate and silica fractions both measured conventionally range between 12 9 and 24 2 the low 18omelt values excluding serua and ambon overlap with the mantle range and are in agreement with simple two component source mixing models that predict 1 5 addition of subducted continental material to a depleted morb type source in the sub arc mantle this percentage is consistent with previous models based on sr nd pb th he hf isotope data however correlations between incompatible trace element ratios and oxygen isotope systematics requires involvement of partial melts derived from subducted continental material as the major slab component rather than bulk addition the contribution of hydrous fluids from both subducted altered oceanic crust and continental material is probably of minor importance magma mantle wedge interaction models could account for the observed low 18o signatures but predicted effects are difficult to distinguish from models without mantle wedge interaction assimilation of arc crust material is thought to be important for the high 18omelt values of serua and ambon afc modelling suggests up to 20 and 80 assimilation at serua and ambon respectively inclusions of meta sedimentary material and whole rock sr nd isotopes point to assimilation processes at nila but this probably had little effect on the 18o of phenocrysts which record original source values according to radiogenic isotope data magma sources in the banda arc are the most heavily influenced by fluxes of subducted continental material among currently active oceanic island arcs hence the results of this study suggest that high 18o 6 5 in arc lavas are difficult to reconcile with addition of subducted components to magma sources but must reflect assimilation of arc crustal material

chloride hydrochemical and isotope methods of groundwater recharge estimation in eastern mediterranean areas a case study in jordan jordan is classified as an arid to semi arid country with a population according to 1999 estimates of 4 8 millions inhabitants and a growth rate of 3 4 efficient use of jordan s scarce water is becoming increasingly important as the urban population grows this study was carried out within the framework of the joint european research project groundwater recharge in the eastern mediterranean and describes a combined methodology for groundwater recharge estimation in jordan the chloride method as well as isotopic and hydrochemical approaches recharge estimations using the chloride method range from 14 mm year 1 mean annual precipitation of 500 mm for a shallow and stony soil to values of 3 7 mm year 1 for a thick desert soil mean annual precipitation of 100 mm and values of well below 1 mm year 1 for thick alluvial deposits mean annual rainfall of 250 mm isotopically most of the groundwater in the hammad basin east jordan falls below the global meteoric water line and far away from the mediterranean meteoric water line suggesting that the waters are ancient and were recharged in a climate different than mediterranean tritium levels in the groundwater of the hammad basin are less than the detection limit 1 3 tu however three samples in east hammad where the aquifer is unconfined present tritium values between 1 and 4 tu copyright 2007 john wiley sons ltd

platinum group minerals in the middle group of chromitite layers at marikana western bushveld complex indications for collection mechanisms and postmagmatic modification the platinum group minerals in a drill core taken through the middle group of chromitite layers in the critical zone at marikana in the western bushveld complex were found to consist mainly of laurite as inclusions in chromite grains the platinum group minerals containing pt pd and rh are concentrated in the intercumulus silicates and frequently associated with base metal sulphides up to about 20 of all platinum group minerals in the investigated chromitite layers contain sub stantial amounts of as the base metal sulphides are strongly modified in the postmagmatic stage which led to a significant loss of fe and s in this way concentrating cu ni and the platinum group elements by factors of up to 10 interaction between chromite and base metal sulphides cannot account for all the fe lost in chromite poor samples and the importance of additional processes is indicated inclusions in chromite and orthopyroxene indicate the formation of discrete platinum group minerals and as rich phases before the

textural immaturity of cumulates as an indicator of magma chamber processes infiltration and crystal accumulation in the rum eastern layered intrusion textural maturity describes the extent to which rocks have approached textural equilibrium for igneous rocks this is a function of the time integrated thermal history in the sub solidus which for cumulate rocks generally translates to the cooling history via the rate of solidification of the accumulating crystal mush the most straightforward way to quantify the textural maturity of orthocumulates sensu lato is to measure the angle subtended at the corners of interstitial grains texturally mature cumulates have angle populations with median values of c 120 whereas texturally immature cumulates have lower medians inherited by the interstitial phase which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology textural observations of allivalites from the rum layered suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient relatively rapid solidification rates perhaps a result of the arrival of cold crystal laden bodies of fluid on the chamber floor or late stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons these two processes have distinct textural and compositional signatures

data acquisition with a 12 level 48 channel borehole seismic system cgg began design and development of its sst500 seismic string tool for data acquisition up to 500 hz in the late 1980s when it became apparent that the evolution of borehole seismic technology was being hindered by the relatively high cost of data acquisition which in turn had consequences on the quality of results

recharge to groundwater in the turi basin northern chile an evaluation based on tritium and chloride mass balance techniques summary the turi basin contains an unconfined phreatic aquifer within the toconce formation volcano sedimentary sequence that passes eastwards under the quaternary volcanic chain and outcrops in the altiplano the phreatic aquifer is floored by impermeable welded miocene ignimbrites precipitation captured by the volcanic edifices infiltrates through high permeability soils to recharge the phreatic aquifer the hydrochemistry of the groundwater in the phreatic aquifer shows little evidence of mixing with geothermal water and therefore approximates to a closed system tritium values demonstrate that recharge is occurring under current climatic conditions and takes around 40 years to pass 10 km from recharge areas on the slopes of the volcanoes and the altiplano to the center of the basin the chloride mass balance technique has been used to quantify precipitation recharge at 15 500 m3 d 1 this is significantly less than the flow through the phreatic aquifer 58 000 86 000 m3 d 1 calculated from its physical properties and the difference is interpreted to be due to subsurface inflow through the toconce formation from the altiplano the results of this study have wider application throughout the andean hydrological environment of northern chile

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

uniformly mantle like δ18o in zircons from oceanic plagiogranites and gabbros lower ocean crust is primarily gabbroic although 1 2 felsic igneous rocks that are referred to collectively as plagiogranites occur locally recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater derived fluids and thus they may indicate early high temperature hydrothermal fluid circulation to explore seawater rock interaction prior to and during the genesis of plagiogranite and other late stage magmas oxygen isotope ratios preserved in igneous zircon have been measured by ion microprobe a total of 197 zircons from 43 plagiogranite evolved gabbro and hydrothermally altered fault rock samples have been analyzed samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near the mid atlantic and southwest indian ridges with the exception of rare distinctively luminescent rims all zircons from ocean crust record remarkably uniform 18o with an average value of 5 2 0 5 2sd the average 18o zrc would be in magmatic equilibrium with unaltered morb 18o wr 5 6 5 7 and is consistent with the previously determined value for equilibrium with the mantle the narrow range of measured 18o values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts if plagiogranite forms by hydrous partial melting the uniform mantle like 18o zrc requires melting and zircon crystallization prior to significant amounts of water rock interactions that alter the protolith 18o zircons from ocean crust have been proposed as a tectonic analog for 3 9 ga detrital zircons from the earliest hadean earth by multiple workers however zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments many of the 3 9 ga zircons have mildly elevated 18o 6 0 7 5 but such values have not been identified in any zircons from the large sample suite examined here the difference in 18o in combination with newly acquired lithium concentrations and published trace element data clearly shows that the 3 9 ga detrital zircons did not originate by processes analogous to those in modern mid ocean ridge settings

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

inversion of p and sv waves from multicomponent offset vertical seismic profiles downgoing waves in multicomponent vsp experiments are used to obtain seismic p and s wave velocities as a function of depth and angle of incidence if p and sv waveforms do not overlap in time at the depth of interest local velocities of the medium are obtained by separate analysis of these events the apparent velocity of the event p or sv is computed from the moveout across several neighboring depth locations the angle of incidence of the same event is computed from the particle motion hodogram within an appropriately chosen time window then the local medium velocity p wave or s wave depending on the chosen event is given by the apparent velocity multiplied by the cosine of the angle of incidence layer interfaces with reasonably sharp velocity contrasts are efficient p wave to sv wave converters even at moderate angles of incidence in offset vsp experiments converted sv waves are generated with varying strengths at practically all depths consequently the converted sv waveforms partially o

crystal settling and convection in the shiant isles main sill the 168 m thick shiant isles main sill is a composite body dominated by an early 24 m thick picrite sill formed by the intrusion of a highly olivine phyric magma and a later 135 m thick intrusion of olivine phyric magma that split the earlier picrite into a 22 m thick lower part and a 2 m thick upper part forming the picrodolerite crinanite unit pcu the high crystal load in the early picrite prevented effective settling of the olivine crystals which retain their initial stratigraphic distribution in contrast the position of the most evolved rocks of the pcu at a level 80 of its total height point to significant accumulation of crystals on the floor as evident by the high olivine mode at the base of the pcu crystal accumulation on the pcu floor occurred in two stages during the first most of the crystal load settled to the floor to form a modally and size sorted accumulation dominated by olivine leaving only the very smallest olivine grains still in suspension the second stage is recorded by the coarsening upwards of individual olivine grains in the picrodolerite and their amalgamation into clusters which become both larger and better sintered with increasing stratigraphic height large clusters of olivine are present at the roof forming a foreshortened mirror image of the coarsening upwards component of the floor accumulation the coarsening upwards sequence records the growth of olivine crystals while in suspension in a convecting magma and their aggregation into clusters followed by settling over a prolonged period with limited trapping at the roof as olivine was progressively lost from the convecting magma crystal accumulation on the contemporaneous floor of the pcu was increasingly dominated by plagioclase most likely forming clusters and aggregates with augite and olivine both of which form large poikilitic grains in the crinanite while the pcu is unusual in being underlain by an earlier still hot intrusion that would have enhanced any driving force for convection we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies 100 m thickness

a new chloride leaching approach to the estimation of diffuse recharge following a change in land use abstract a new approach has been developed to estimate the increase in groundwater recharge following land use modification the approach uses the degree of leaching of chloride to quantify soilwater drainage below the root zone which ultimately leads to groundwater recharge it is more general than similar previously reported techniques for analysing transient chloride profiles to infer recharge rates and hence has wider application we have applied the technique to a field situation in southern australia where clearing of native vegetation for agricultural production leads to large increases in groundwater recharge the examples serve to demonstrate the technique and some of the practical difficulties in the application of solutes techniques to recharge estimation

tracing infiltration and recharge using stable isotope in taihang mt north china the groundwater in headwater region is an important recharge source for the adjacent mountain front plain in order to reveal the relationship among precipitation soil water and groundwater from june to september in 2004 stable isotopes deuterium and oxygen 18 in precipitation and soil waters at the depths of 10 20 30 50 70 90 and 110 cm were analyzed at two sites covered by black locust robinia pseudoacia l site a and grass predominated by themeda triandra t japonica willd tanaka and bothriochloa ischaemum b ischaemum l keng site b in an experimental catchment at taihang mt north china respectively the 18o of precipitation in daily rain events shows large variations 13 3 to 4 3 with a mean of 8 1 the 18o and d of soil waters along profiles in two sites suggest that the influence of canopy cover was just up to 10 cm in top soil water the soil water moved over the zero flux plane at 70 cm in depth is expected to escape the evaporative effect at the end of september in both sites the results show that the stable isotope instead of tritium as tradition can be used to trace the soil water behaviors based on the movement of isotopic peak along the vertical profiles in this semi arid and semi humid mountainous region the infiltration depths of soil water in taihang mt are 12 and 10 mm day from june to september in 2004 in site a and site b respectively tracing by stable isotope recharge fluxes of soil water to local groundwater are of 3 8 and 3 2 mm day in site a and site b respectively the results provide desirable information for assessment of local groundwater resources

converted wave seismic exploration applications converted seismic waves specifically downgoing p waves that convert on reflection to upcoming s waves are increasingly being used to explore for subsurface targets rapid advancements in both land and marine multicomponent acquisition and processing techniques have led to numerous applications for p s surveys uses that have arisen include structural imaging e g seeing through gas bearing sediments improved fault definition enhanced near surface resolution lithologic estimation e g sand versus shale content porosity anisotropy analysis e g fracture density and orientation subsurface fluid description and reservoir monitoring further applications of p s data and analysis of other more complicated converted modes are developing

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

multivariate geochemical classification of chromitite layers in the bushveld complex south africa abstract the bushveld complex the largest layered mafic ultramafic intrusion worldwide is host of numerous laterally continuous and chemically similar chromitite layers based on their stratigraphic position the layers are subdivided into a lower middle and upper group lg mg and ug within these groups the layers are numbered successively from the base to the top of each group attempts of discriminating between single layers based on their composition have failed mainly due to the significant overlap of compositional fields e g of chromitite mineral assemblages and chromite mineral chemistry between neighboured layers in this contribution a tailored and easy to use multivariate classification scheme for the chromitite layers is proposed based on a comprehensive classification routine for the lg and mg chromitites this routine allows a clear attribution with known uncertainty of eight distinct chromitite layers the study was carried out at the thaba mine a chromite mine located on the western limb of the bushveld complex the classification is based on a large geochemical database n 1205 from thaba mine it comprises of a hierarchical discrimination approach relying on linear discriminant analysis and involves five distinct steps using default homogeneous prior probabilities classification results are excellent for the first discrimination steps lgs vs mgs 97 lg 6 vs lg 6a 94 and very good for the following steps mg 1 2 vs mg 3 4 86 mg 1 vs mg 2 92 mg 3 vs mg 4 93 mg 4 vs mg 4z 97 mg 4 vs mg 4a 88 the classification scheme was tested using the same sample set as a training set with unknown composition overall classification results for unknown samples belonging to one of the layers are 81 hence the classification scheme is at least valid for the thaba mine the approach may however be extended across the entire bushveld provided that an appropriate geochemical data set is available

quantifying the building blocks of igneous rocks are clustered crystal frameworks the foundation most phenocryst populations in volcanic rocks and those preserved in shallow level igneous intrusions are clustered variously referred to as clots clumps or glomerocrysts these clusters of crystals are the building blocks that accumulate to form the high porosity touching crystal frameworks from which igneous cumulates form examination of touching crystal frameworks in olivine komatiite cumulates and experimental charges and plagioclase dominant crystal populations holyoke flood basalt connecticut usa reveal complex high porosity clustered crystal arrangements olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10 10 mm s to less than 3 years for a growth rate of 5 x 10 10 mm s based on crystal size distributions the origin of crystal clusters is likely to involve either or a combination of heterogeneous nucleation remobilization of cumulate mushes or crystals sticking together during settling and or flow the spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non touching crystal populations and further improve our understanding of crystal packing arrangements and cluster size distributions

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

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

pyroxene oikocrysts in troctolitic cumulates evidence for supercooled crystallisation and postcumulus modification studies of pyroxene oikocrysts in plagioclase olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock plagioclases within oikocrysts are very elongate with a high surface area show extensive mutual attachment at high angles resembling skeletal growth and tend to be randomly orientated some are rounded and embayed are reversely zoned and more sodic by an 5 10 than some of the plagioclase in the host rock plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock in contrast plagioclases in the host rock are discrete coarser grained less elongate strongly aligned and locally polygonised

converted wave seismic exploration methods multicomponent seismic recording measurement with vertical and horizontal component geophones and possibly a hydrophone or microphone captures the seismic wavefield more completely than conventional single element techniques in the last several years multicomponent surveying has developed rapidly allowing creation of converted wave or p s images these make use of downgoing p waves that convert on reflection at their deepest point of penetration to upcoming s waves survey design for acquiring p s data is similar to that for p waves but must take into account subsurface vp vs values and the asymmetric p s ray path p s surveys use conventional sources but require several times more recording channels per receiving location some special processes for p s analysis include anisotropic rotations s wave receiver statics asymmetric and anisotropic binning nonhyperbolic velocity analysis and nmo correction p s to p p time transformation p s dip moveout prestack migration with two velocities and wa