settling and compaction of olivine in basaltic magmas an experimental study on the time scales of cumulate formation a series of centrifuge assisted settling experiments of 30 vol olivine in 70 vol basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates the settling experiments were performed in a centrifuging piston cylinder at 200 1 500g 1 270 1 280 c and 0 8 1 1 gpa on previously annealed and texturally equilibrated samples the mechanical settling of the dense olivine suspension occurs at about 1 6 the speed of simple stokes settling resulting in a sedimentation exponent n 4 1 6 in agreement with predictions from analogue systems the porosity m of the orthocumulate resulting from gravitational settling of crystals is about 54 and formation times of olivine orthocumulates result to 0 1 10 m day 1 for an initial crystal content of the melt of 1 5 and grain sizes of 2 10 mm after mechanical settling olivine grains rest on each other and further compaction occurs through pressure dissolution at grain contacts olivine reprecipitation where olivine is in contact with melt and concomitant expulsion of excess liquid from the cumulate layer with centrifugation at 400g for 50 h porosities as low as 30 3 vol were achieved the olivine content at the bottom of the gravitational cumulate is 1 m log h a t where is the density difference between crystals and melt h the crystal layer thickness a the acceleration and t the time of centrifuging compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction the compaction limit that is the lowermost porosity to be reached by this mechanism is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3 5 porosity for the experiments crystal size distribution curves and a growth exponent n of 3 1 3 indicate that diffusion controlled ostwald ripening is the dominant crystal growth mechanism the above relationship combined with a linear scaling for grain size as appropriate for reaction controlled pressure solution creep allows calculation of formation times of adcumulates if chemical compaction is dissolution reprecipitation limited then single layers of natural olivine adcumulates of m thickness with 70 75 vol olivine at the base as observed in the rhum layered intrusion would have typical formation times of 0 4 3 years for grain sizes of 2 10 mm this time scale compares favourably with characteristic cooling times of sills if a greater than 20 m thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt then compaction proceeds to the compaction limit within a few years it can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 in conclusion gravitational settling and gravitation driven chemical compaction are feasible cumulate forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

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

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

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

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

relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

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

apatite compositions from oceanic cumulates with implications for the evolution of mid ocean ridge magmatic systems apatite distributions and compositions from cumulates from the epr at hess deep the mar at the kane transform and the swir at odp hole 735b were determined to assess the variability at each setting and to evaluate the potential utility of apatite in understanding the evolution of the lower ocean crust apatite in cumulates with low p 2 o 5 contents are heterogeneously distributed along linear arrays or in tight clusters suggesting they crystallized from planar or pipe like channels of evolved liquid most of the variation in the apatite composition is in the halogen site the x f a and x cl ap are inversely correlated defining trends at near constant x oh ap with epr x oh ap 0 25 mar x oh ap 0 45 and swir x oh ap 0 65 these trends are defined both by sample averages and by the range of individual analyses in samples with large ranges in their f cl ratio these trends are interpreted to reflect the reequilibration of apatite from variably evolved liquids fluids that moved through the crystal mush comparison of the f cl oh contents of apatite with the f cl oh contents of glasses recovered from the same ridges show approximate correspondence apatite and glasses from the epr have the highest halogen contents while apatite and glasses from the mar and swir have more oh some of the higher cl contents in the apatite are interpreted to be produced by degassing of the cumulus pile while others reflect the assimilation of a seawater component we suggest that systematic analysis of apatite from oceanic cumulates might allow cumulates that crystallized at shallow depths and assimilated seawater derived components to be distinguished from those that crystallized below the level of seawater interaction

discordant bodies from olivine bearing zones iii and iv of the stillwater complex montana evidence for postcumulus fluid migration and reaction in layered intrusions mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35 olivine occur at three different levels in olivine bearing zones iii and iv of the middle banded series of the stillwater complex the discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses although the host olivine gabbros are foliated with tabular plagioclase the discordant bodies lack a discernible fabric and have blocky plagioclase average olivine in the host rocks is slightly more magnesian than that of the discordant bodies mg 75 8 0 7 versus mg 74 6 0 3 respectively but plagioclase compositions are indistinguishable an77 6 2 0 versus an76 6 4 3 average host and discordant bodies respectively whole rock major and trace element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance however bulk compositions of anorthositic cores from the discordant bodies are enriched in k na ba sr and p we conclude that the discordant bodies formed when cooler volatile fluids or fluid rich silicate liquids moved upward and encountered a hotter undersaturated solid plus liquid assemblage continued liquid fluid fluxing increased the permeability along the flow path and focused the flow allowing the original bulk compositions to be modified and leaving plagioclase rich troctolites and anorthosites the shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late stage liquid fluid flow chemical and mineralogical evidence from other parts of olivine bearing zones iii and iv suggests that the processes that formed the discordant bodies may have influenced other cumulates in fact it appears that the same processes that formed the discordant bodies operated within an anorthositic layer strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence

trace element compositions of apatite from the middle zone of the panzhihua layered intrusion sw china insights into the differentiation of a p and si rich melt abstract the panzhihua layered intrusion in the 260 ma emeishan large igneous province is composed of melagabbro and fe ti oxide ore bodies in the lower zone lz and the lower part of the middle zone mza and fe ti oxide poor leucogabbro in the upper part of the middle zone mzb and upper zone uz cumulus apatite grains occur in the 500 to 600 m thick mzb which makes up 25 30 of the 2 km thick intrusion apatite grains from the mzb show two compositional reversals in the composition of sr which divide the mzb into three sub units from the base upwards mzb1 mzb2 and mzb3 there is 1 3 vol apatite in the mzb1 and mzb2 and 2 5 vol apatite in the mzb3 both apatite and plagioclase have an overall trend of decreasing sr in each sub unit most apatite grains from the mzb1 and mzb2 have negative eu anomalies eu eu 0 70 0 98 on chondrite normalized ree plots and some at the top of the mzb2 have positive eu anomalies eu eu 1 09 1 18 whereas all grains from the mzb3 have positive eu anomalies eu eu 1 11 1 25 we consider that the panzhihua intrusion formed due to immiscibility of ferrobasaltic magmas in a large convection cell at high temperatures the immiscible fe rich melt tended to move towards the base of the chamber whereas the si rich melt moved upwards due to density differences crystallization of fe ti oxides from the fe rich melt at high temperatures may result in the enrichment of p in the residual magmas the upward moving residual p rich magmas may have mixed with si rich melt to form a p and si rich melt in the upper part of the chamber from which the mzb formed double diffusive convection circulated in the p and si rich melt to form stratified magma layers magma mixing between the stratified magma layers resulted in the compositional reversals of apatite along the boundaries negative eu anomaly of apatite in the mzb1 and mzb2 is attributed to prior crystallization of plagioclase whereas replenishment of a syenitic magma to the mzb3 may result in the positive eu anomaly of apatite in the mzb3 the immiscibility model can explain why the apatite rich mzb is above the major fe ti oxide layers in the panzhihua intrusion

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

differentiation and compaction in the skaergaard intrusion igneous differentiation processes are constrained from bulk compositions densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the layered series of the skaergaard intrusion east greenland the stratigraphic distribution of p u and rb in cumulates and residual magma modeled by rayleigh fractionation constrains the final porosities or trapped liquid contents to 30 52 in lza troctolites decreasing to 4 12 at the top of lzb olivine gabbros and remaining low 1 13 4 middle dot 6 on average in the oxide gabbros of lzc mz and uz local variations in trapped liquid content are associated with modal layering leucocratic low density rocks have higher proportions of trapped melt than adjacent melanocratic high density rocks these observations are explained by varying degrees of compaction compaction was most important after the onset of fe ti oxide crystallization because of the high densities of the crystal matrix here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick in the basal section lza the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor in the overlying section lzb the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation the modeling constrains the p2o5 content of the residual magma to 1 middle dot 7 wt at the level of apatite in suggesting that the magma contained 49 wt sio2 and followed a trend of iron enrichment compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

multiphase inclusions in plagioclase from anorthosites in the stillwater complex montana implications for the origin of the anorthosites multiphase inclusions consisting of clinopyroxene ilmenite apatite occur within cumulus plagioclase grains from anorthosites in the stillwater complex montana and in other rocks from the middle banded series of the intrusion the textures and constant modal mineralogy of the inclusions indicate that they were incorporated in the plagioclase as liquid droplets that later crystallized rather than as solid aggregates their unusual assemblage including a distinctive manganiferous ilmenite and the presence of baddeleyite zro2 indicates formation from an unusual liquid a process involving silicater liquid immiscibility is proposed whereby small globules of a liquid enriched in mg fe ca ti p ree zr and mn exsolved from the main liquid that gave rise to the anorthosites became trapped in the plagioclase and later crystallized to form the inclusions the immiscibility could have occurred locally within compositional boundaries around crystallizing plagioclase grains or it could have occurred pervasively throughout the liquid it is proposed that the two immiscible liquids were analogous n terms of their melt structures to immiscible liquid pairs reported in the literature both in experiments and in natural basalts for the previously reported pairs immiscibility is between a highly polymerized liquid typically granitic in composition and a depolymerized liquid typically ferrobasaltic in composition in the case of the anorthosites the depolymerized liquid is represented by the inclusions and the other liquid was a highly polymerized aluminosilicate melt with a high normative plagioclase content from which the bulk of the anorthosites crystallized crystallization of the anorthosites from this highly polymerized liquid accounts for various distinctive textural and chemical features of the anorthosites compared to other rocks in the stillwater complex a lack of correlation between p contents and chondrite normalized rare earth element ree ratios of plagioclase separates indicates that the amount of apatite in the inclusions is too low to affect the ree signature of the plagioclase separates nevertheless workers should use caution when attempting ree modelling studies of cumulates having low ree contents because apatite bearing inclusions can potentially cause problems

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

trace element constraints on the differentiation and crystal mush solidification in the skaergaard intrusion greenland new major and trace element analyses of plagioclase and clinopyroxene in gabbros from the skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush crystal cores show a continuous chemostratigraphic evolution in the layered series with compatible elements e g cr and ni in clinopyroxene being progressively depleted from the bottom up whereas incompatible elements e g sr ba ree in plagioclase and clinopyroxene become progressively enriched we performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization except for the upper 20 of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends at this stage of magma chamber solidification fractional crystallization becomes less efficient and is replaced by a major proportion of in situ equilibrium crystallization trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals they result from crystallization of the interstitial liquid in the liquid crystal mush however incompatible elements and especially ree show a degree of enrichment in plagioclase rims 20 ppm ce that strongly exceeds the highest concentrations observed in plagioclase cores up to 4 ppm ce at the top of the layered series such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite after the crystallization of plagioclase and clinopyroxene overgrowths diffusive re equilibration during a period of 0 1 0 25 myr significantly changed the original zoning profiles ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong ce enrichment observed in plagioclase rims

parental magmas of the nain plutonic suite anorthosites and mafic cumulates a trace element modelling approach equilibrium melt trace element contents are calculated from proterozoic nain plutonic suite nps mafic and anorthositic cumulates and from plagioclase and orthopyroxene megacrysts assumed trapped melt fractions tmf 20 generally eliminate all minor phases in most mafic cumulate rocks reducing them to mixtures of feldspar pyroxene and olivine which would represent the high temperature cumulus assemblage in anorthosites tmf 15 generally reduce the mode to a feldspar only assemblage all model melts have trace element profiles enriched in highly incompatible elements relative to normal mid ocean ridge basalt nmorb commonly with negative nb and th anomalies most mafic cumulates yield similar profiles with constant incompatible element ratios and can be linked through fractional crystallization high k la subtypes probably represent crust contaminated facies mafic cumulates are inferred to belong to a tholeiitic differentiation series variably contaminated by upper and lower crustal components and probably related to coeval tholeiitic basaltic dyke swarms and lavas in labrador model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates indicating that mafic cumulates and anorthosites did not crystallize from the same melts orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts precluding an origin from parental melts jotunites have lower k rb ba y yb and higher la ce than model residues from fractionation of anorthositic model melts suggesting they are not cosanguineous with them but provide reasonable fits to evolved mafic cumulate model melts incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites with steep y yb lu segments that suggest residual garnet in the source inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust the similarity of the highly incompatible trace elements and lile between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites the reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase and remobilization of these hybrid plagioclase rich mushes then produced the anorthosite massifs

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

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

chemical evolution of intercumulus liquid as recorded in plagioclase overgrowth rims from the skaergaard intrusion the intercumulus liquid of a crystal mush fills pore spaces and typically solidifies to form overgrowths on cumulus grains and poikilitic post cumulus minerals if the liquid is immobile solidification produces zoned intercumulus minerals as a result of progressive fractionation of the residual liquid convection within the mush results in buffering of the liquid composition and thus limits mineral zonation for fully solidified cumulates fossil changes in liquid composition or porosity are difficult to identify however detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material ti contents of plagioclase overgrowths in samples from the lowermost parts of the skaergaard intrusion show strong variations with anorthite content with decreasing xan ti concentrations first rise and then fall consistent with changing tio2 contents of the intercumulus liquid during solidification tio2 in plagioclase decreases sharply at an55 reflecting local saturation of fe ti oxides ti in clinopyroxene oikocrysts also falls rimward but zoning in faster diffusing species fe mg is limited other than slight reverse zones that may occur on the plagioclase margins xan falls continuously during crystallization the reverse zoning is interpreted as the result of compaction driven dissolution and reprecipitation of plagioclase the continual decrease in xan is exploited together with back scattered electron images of the cumulates to produce calibrated images showing regions of progressive crystallization this allows the regions crystallizing at each stage of solidification to be visualized these images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain boundary melt films this approach can provide information about the progressive reduction of porosity during cumulate solidification

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

the thickness of the crystal mush on the floor of the bushveld magma chamber the thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase typically clinopyroxene but also orthopyroxene and olivine and the first appearance or disappearance of the liquidus phase associated with the step change in median dihedral angle we determined the mush thickness in the rustenburg layered suite of the bushveld complex at clinopyroxene in in lower main zone and magnetite in in upper zone we also examined an intermittent appearance of cumulus apatite in upper zone using both the appearance and disappearance of cumulus apatite in all cases the mush thickness does not exceed 4 m these values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite mush thickness of the order of a few metres suggests that neither gravitationally driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification adcumulates in the bushveld are most likely to have formed at the top of the mush during primary crystallisation similarly it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy or that layering is formed by mechanisms other than primary accumulation

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

systematics of modal phenocryst assemblages in submarine basalts petrologic implications phenocryst assemblages in ocean ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases the variations in phase assemblages as well as most crystal liquid kd s are similar to variations equimodal trends predicted by low pressure laboratory experiments suggesting that many of these basalts have experienced varying degrees of low pressure cyrstallization prior to quenching with little sorting of crystals and liquid important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends in these basalts megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage however phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts a relation between spreading rate and phase proportions is neither supported nor refuted by the data which as yet do not adequately represent fast spreading ridges

plagioclase in the skaergaard intrusion part 1 core and rim compositions in the layered series the anorthite content of plagioclase grains xan in 12 rocks from the layered series of the skaergaard intrusion has been studied by electron microprobe typically 30 core and 70 rim analyses per thin section mean core compositions vary continuously from an66 at the base of the layered series lza to an32 30 at the top on the other hand crystal rims are of approximately constant composition an50 1 from the lza to the lower middle zone mz above the mz core and rim compositions generally overlap profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at xan 50 1 the simplest explanation for these features is that during postcumulus crystallization in the lower zone interstitial liquids passed through a density maximum this interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite

the lac des iles palladium deposit ontario canada part ii halogen variations in apatite analysis of apatite from the mine block intrusion mbi of the lac des iles igneous complex shows two pronounced trends in the halogens apatite from relatively fresh norite and melanorites from the pd sulfide zone contain up to 57 mol chlorapatite endmember with significant hydroxyapatite component in contrast in altered rock amphibolite and greenschist assemblages the chlorapatite component is typically less than 10 mol with wide variation in the f and oh endmember components the latter trend is attributed to cl loss to degassing and alteration whereas the former is attributed to cl enrichment in the ore bearing rocks it is suggested that the relatively h2o rich and intermediate cl content of the early igneous fluids degassed from the deeper levels of the mbi can explain the high pd pt and pd ir ratios of the deposit a model is presented in which disseminated pd rich sulfides are initially introduced by a high temperature magmatic fluid that also influenced crystallization to produce the gross modal variations of the igneous host rock this high temperature mineralization event was subsequently modified by the influx of late igneous and country fluids at amphibolite to greenschist conditions

rare earth elements in apatite uptake from h2o bearing phosphate fluoride melts and the role of volatile components the partitioning of rare earth elements rees between fluorapatite fap and h sub 2 o bearing phosphate fluoride melts has been studied at about 700 and 800 degrees c and 0 10 0 15 gpa ree uptake patterns i e plots of d ree fap melt are convex upwards and peak near nd for single ree substituted fap at minor 0 03 0 25 wt ree sub 2 o sub 3 abundances and binary lree hree substituted fap and hexa ree substituted fap at minor to major 0 25 7 8 wt ree sub 2 o sub 3 abundances partition coefficients for minor abundances of ree and depolymerized phosphate melts are about 5 8 and 1 for la nd and lu respectively and broadly comparable to those for early fluorapatite in the fractionation of melts of basaltic composition the ca2 site exerts marked control on the selectivity of apatite for ree because it preferentially incorporates lree and its effective size varies with substitution of the a site volatile anion component f cl oh using simple crystal chemical arguments melt or fluid normalized ree patterns are predicted to peak near nd for fluorapatite and be more lree enriched for chlorapatite these predictions are consistent with data from natural rocks and laboratory experiments the wide variation in d ree apatite melt in nature from 1 for whitlockite bearing lunar rocks to about 100 for evolved alkalic rocks ismore attributed largely to the influence of the volatile components 49 refs 8 figs 3 tabs less

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

microstructures and late stage magmatic processes in layered mafic intrusions symplectites from the sept iles intrusion quebec canada replacive symplectites vermicular intergrowths of two or more minerals are an important feature of layered igneous intrusions recording evidence of late stage reactions between interstitial liquid and crystals they are common throughout the layered series of the 564 ma sept iles layered intrusion in quebec canada and fall into three types oxy symplectites type i symplectites and type ii symplectites oxy symplectites are comprised of magnetite and orthopyroxene nucleate on olivine primocrysts and form via the reaction olivine o2 orthopyroxene magnetite type i symplectites of which there are 3 distinct categories are comprised of anorthitic plagioclase with pyroxene amphibole or olivine vermicules grow from primocryst oxide grains and replace primary plagioclase and type ii symplectites of which there are 2 distinct categories are comprised of anorthitic plagioclase with orthopyroxene amphibole vermicules grow from primocryst olivine

petrology and mineral compositions of the middle banded series of the stillwater complex montana the two olivine beating zones of the middle banded series of the stillwater complex are characterized by an increase in the number of cumulus minerals with height in each anorthosite and anorthositic troctolite dominate the lower part whereas olivine gabbro and gabbronorite form much of the upper portions electron microprobe analyses of cumulus minerals indicate little or no variation of average mineral compositions with height in addition no significant lateral variations in cumulus mineral compositions occur along 8 km of section plagioclase from throughout these zones shows the complex reverse and oscillatory zpnation patterns also seen in plagioclase from the thick 500 m anorthosites that sandwich the zones the data suggest that the entire middle banded series is genetically related and therefore requires models for the origin of the thick anorthosites to also explain the olivine bearing rocks between them however textural features such as discordant troctolites pegmatoids slump structures and variably developed mineral laminations and chemical features such as zpnation in clinopyroxene produced by intergranular exchange with orthopyroxene and large within sample variations in the mg number of olivine in lowolivine troctolites indicate that significant modification of these rocks by postcumulus processes has taken place thereby obscuring evidence of their genesis

petrologic evolution of a nipigon diabase sill ontario canada insights from compositional and textural profiles petrographic and geochemical profiles collected through a 250 m thick nipigon diabase sill near black sturgeon lake ontario constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks several nested reinjections of phenocryst bearing magma compactiondriven redistribution of mildly evolved interstitial liquids in the central portion of the sill and segregation of highly evolved interstitial liquids into micropegmatitic veins near the upper contact these processes impact not only the silicate mineralogy of the rocks but also distribution of sulfide phases both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine bearing magma plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk rock mgo content these textural and compositional reversals are attributed to reinjections of olivine bearing magma into partially solidified magma from earlier injections mixing between evolved resident magma and reinjections of less evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization in this sill which contains no significant mineralization localized minor enrichments and depletions in copper content are associated with the margins of a well constrained reinjection horizon chemical differentiation of the magma was controlled by three distinct processes flow differentiation crystal mush compaction and solidification front instability although the mineralogy of the sill is dominated by plagioclase and clinopyroxene major oxide compositional variations are largely controlled by the mechanical concentration of olivine via flow differentiation or phenocryst redistribution within the individual reinjection pulses this redistribution of phenocryst phases is the primary factor controlling the major element composition of the rocks in most of the sill in the central portion of the sill the abundances of incompatible trace elements are influenced by the compaction of partially crystalline mush and the expulsion of incompatible rich interstitial liquid this is recognized by a negative correlation between the alignment of plagioclase crystals and concentrations of incompatible trace elements such as zr finally there is a 2 5 m thick silicic segregation approximately 32 m below the upper contact whose composition is consistent with residual liquid after 65 to 85 fractional crystallization of magma with a composition equivalent to that of the marginal rocks similar veins and patches are common throughout the upper 50 m of the sill indicating that this segregation process was common and widespread during the cooling and crystallization of the sill the complexity of intrusion and differentiation in this sill despite its generally unremarkable texture and composition suggests that many diabase sills may have experienced a range of processes that have previously been recognized to impact the petrologic evolution of larger igneous systems and that these processes can be identified in minor intrusions such as sills and dikes using simple and robust analytical techniques

compaction and crystallization in magma chambers towards a model of the skaergaard intrusion the geochemical variations within the skaergaard intrusion provide an opportunity to discover what features of the intrusion can be understood using the simplest form of the equations governing the two phase flow of melt and matrix the equations governing the conservation of mass momentum and energy are first simplified by using the extended boussinesq approximation and then solved numerically to study the time dependent behaviour of a compacting solidifying layer at the base of a magma chamber when variations in the horizontal plane can be neglected the most important result is that the concept of a trapped liquid fraction which has been widely used to model the bulk composition of layered intrusions is a useful concept to describe the steady state behaviour of compacting layers this result is at first sight surprising because there is relative movement between the melt and crystals during compaction and the system is therefore open the reason why it is useful is because both the melt and the crystals are moving downwards in a frame fixed to the upper surface of the compacting layer because the mass of all elements must be conserved what goes into the top of the layer as melt and solid must come out of its bottom as a solid when the behaviour is not time dependent however when time dependent behaviour occurs the concept of a trapped liquid fraction ceases to be useful the governing equations are used to model the concentration of phosphorus uranium and rubidium in the lower part of the skaergaard intrusion where they behave incompatibly the observed behaviour requires the viscosity of the solid part of the compacting layer to have a value of 10 pa s

silicate liquid immiscibility within the crystal mush evidence from ti in plagioclase from the skaergaard intrusion a key target in the study of layered intrusions is to constrain the liquid line of descent of the magma however the evolution of the interstitial liquid is rarely considered and its liquid line of descent is often assumed to be equivalent to that of the bulk magma because of extensive sub solidus and diffusional changes that occur in slowly cooled rocks clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components this study uses the ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the skaergaard intrusion ti xan zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height demonstrating that the bulk and interstitial liquid lines of descent are not equivalent after fe ti oxides start to crystallize ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt ti however in the interstitial plagioclase within a single thin section divergent trends develop adjacent to fine grained interstitial pockets containing diverse mineral assemblages which are interpreted to represent the crystallized products of late stage immiscible liquids these trends vary systematically as a function of stratigraphic height and spatial location within the intrusion the distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion the author 2010 published by oxford university press all rights reserved

sulphur saturation in the lower and critical zones of the eastern bushveld complex abstract a suite of ultramafic and mafic rocks from the lower critical and lower portion of the main zones of the bushveld complex has been analysed for th cs zr ni cr and au by inaa and xrf spectrometry the incompatible elements th cs and zr correlate positively and show a gradual upward increase in abundance assuming constant average proportion of intercumulus material this upward increase implies that the zones of the complex studied represent crystallization of a single magma type some 3600 m thick pyroxenites dominate the lower portion of the section studied and their ni content shows an initial rapid decrease from 850 ppm in the lowermost rocks to around 500 ppm with considerable scatter this distribution is most likely to have resulted from bottom crystallization with superimposed convective overturn near the transient floor of the chamber gold abundances are generally higher in chromitites and correlate positively with ni indicating the presence of significant amounts of cumulus immiscible sulphide in the silicate rocks au does not correlate with any of the analysed elements and it is concluded that au was trapped in small quantities of immiscible sulphide which precipitated continuously during crystallization there is an upward increase in the amount of cumulus immiscible sulphide indicating a progressive increase in sulphur solubility in the magma

paleointensity record from the 2 7 ga stillwater complex montana 1 the record of geomagnetic intensity captured in the 2 7 ga stillwater complex montana usa provides a statistical description of the archean geodynamo we present results of modified thellier paleointensity experiments on 441 core specimens 114 of which pass strict reliability criteria the specimens are from 53 sites spanning most of the banded series rocks in the stillwater complex on the basis of thermochronologic and petrologic evidence we interpret the highest temperature component of remanence to be a late archean thermoremanence though the possibility remains that it is a thermochemical remanence thermal models indicate that the highest temperature magnetization component at each of the sites averages 20 200 ka of geomagnetic secular variation the suite of sites as distributed through the banded series samples a roughly a 1 ma time interval the average of the most reliable paleointensity measurements uncorrected for the effects of anisotropy or cooling rate is 38 2 11 3 t 1 remanence anisotropy cooling rate and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results a corrected average of 30 6 8 8 t is likely a more appropriate value earth s average dipole moment during the late archean 5 05 1 46 1022 am2 pmag 44 5 was well within the range of estimates from phanerozoic rocks the distribution of site mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the phanerozoic field

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

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

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

initiation and failure mechanism of base instability of excavations in clay triggered by hydraulic uplift excavations in clay overlying an aquifer may cause catastrophic basal failure due to hydraulic uplift although case histories with hydraulic uplift failures are reported worldwide from time to time the initiation and failure mechanism of the base instability are not well studied and understood to address these two issues dimensional analysis is firstly conducted to propose dimensionless groups dgs possibly relevant to this subject effects of these dgs on the initiation and failure mechanism of base instability are then investigated by carrying out a series of finite element analyses in which constitutive models and model parameters have been previously validated against centrifuge test results it is revealed that the initiation and failure mechanism of base instability due to hydraulic uplift is mainly governed by a ratio of excavation width over the thickness of soft clay inside excavation b d as excavation becomes narrower i e b d decreases the hydraulic pressure pi required to initi

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

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

petrogenesis and tectonic significance of the plagiogranites in the zhaheba ophiolite eastern junggar orogen xinjiang china abstract plagiogranites albitite and albite granite dikes lenses occur in the western section of the zhaheba ophiolite middle part of the eastern junggar orogen the zhaheba albitites 498 0 5 8 ma and albite granites 494 6 6 9 ma were roughly coeval and with distinct petrographic textures i e cumulus texture for the albitites and granitic texture for albite granites the zhaheba plagiogranites are na enriched and lile depleted resembling typical plagiogranites geochemical characteristics and cumulus texture of the albitites indicate that they may have been formed by early stage accumulation of albite whereas the albite granites may have been the products of the residual magma consolidation the end t values of the albitites and albite granites vary in the ranges of 7 2 7 7 and 6 4 7 3 respectively whereas the ehf t values vary in the ranges of 11 5 17 9 and 9 8 13 9 respectively isotopic compositions and zr hf ratios of the zhaheba plagiogranites are similar to those of typical morb implying a genetic relationship with the oceanic crust the low tio2 nb ta content lree enrichment and elevated 87sr 86sr i values of the zhaheba plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites which were related to hydrothermal alteration of gabbros in intra oceanic backarc basin u pb geochronology of the zhaheba plagiogranites indicates that it is more reasonable to connect the zhaheba armantai and karamay ophiolites to the zaysan collision zone

magnetic anomaly near the center of the vredefort structure implications for impact related magnetic signatures a strong magnetic anomaly near the center of the ancient and deeply eroded vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at 2 0 ga the rocks underlying the anomaly are archean gneisses thought to represent mid crustal depths that were uplifted to the surface during the postulated impact event measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination 25 inclination 56 k 18 16 that correspond to the paleomagnetic pole position at time of impact petrologic studies indicate that during impact large volumes of rock were heated enough to cause thermal remagnetization in the ambient field thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites

an evaluation of models of apatite compositional variability using apatite from the middle banded series of the stillwater complex montana apatite is a relatively common accessory mineral in the olivine bearing zones ob iii and ob iv of the middle banded series of the stillwater complex occurring interstitial to cumulus grains as monomineralic inclusions in cumulus grains and in polymineralic clusters which may or may not be included in the cumulus grains eighty nine of 185 samples examined were found to contain some apatite the f cl oh content of the apatite show distinct stratigraphic variations xclap mole fraction cl in apatite of interstitial apatite increases upsection in both ob iii and ob iv with average values ranging from 0 15 to 0 85 in ob iii and from 0 03 to 0 60 in ob iv xfap varies inversely with xclapwith average values ranging from 0 00 to 0 70 while xohap remains relatively constant near 0 40 or decreases slightly with height these variations are remarkable given that no appreciable stratigraphic variations in either the major or trace element compositions of any of the cumulus minerals are found in the 800 m of section that comprise ob iii and ob iv within sample variation of xclap for samples containing cl rich apatite is substantially larger up to 0 65 xclap than for samples with more f rich apatite xclap varies by approximately 0 15 although interstitial apatite is found throughout ob iii and ob iv apatite occurring as monomineralic inclusion in cumulus grains or in polymineralic clusters is almost exclusively found in samples with cl rich apatite the data are best explained by a model involving the degassing of a cl rich volatile phase from the crystallizing interstitial liquid the up section migration of this fluid resulted in the crystallization of f rich apatite in the lower portion and progressive cl enrichment in the apatite with height the presence of hornblende bearing dikes veins and pegmatoids at the level of maximum cl enrichment is consistent with a fluid migration model

processed and enhanced gravity and magnetic images over the vredefort structure and their interpretation abstract cryptoexplosion structures are polygonal features which have experienced violent disruption during their formation the vredefort structure is one of the largest known cryptoexplosion structures and the geology is well exposed over its northern half the origin of the violent disruption experienced by the vredefort structure is controversial and many aspects of its history are under intensive study since most of the southern half of the vredefort structure is blanketed by a cover of karoo permian strata geophysical data are essential in any attempt to map the pre karoo geology data sets of the regional gravity and magnetic fields over the vredefort structure have been processed and enhanced with the aid of an image processor the results are presented as a series of images each one enhancing particular attributes of the data sets an interpretation of the geophysical images is presented focusing on the regional geometry and setting of the structure striking features of the geophysical images include a simple concentric geometry and polygonal shape which reflect the distribution of dense and magnetic lithologies within and surrounding the structure the kinematics of the development of a polygonal uplift are discussed inward movement of about seven collar sectors separated by radial faults and uplift of the central sections of each of the sectors may account for the gross polygonal shape pull apart voids formed at kinks in the radial faults might account for the positions of several alkali stocks within the stratified collar of the vredefort structure the enhanced image of the gravity field highlights a se trending gravity low which extends beyond the southeastern quadrant of the vredefort structure this feature forms part of a fundamental nw se trending intracratonic lineament the vredefort axis

petrology of martian meteorite northwest africa 998 nakhlite northwest africa nwa 998 is an augite rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on mars this 456 gram partially fusion crusted meteorite consists of by volume 75 augite core composition wo39en39fs22 9 olivine fo35 7 plagioclase ab61an35 as anhedra among augite and olivine 3 5 low calcium pyroxenes pigeonite and orthopyroxene replacing or forming overgrowths on olivine and augite 1 titanomagnetite and other phases including potassium feldspar apatite pyrrhotite chalcopyrite ilmenite and fine grained mesostasis material minor secondary alteration materials include iddingsite associated with olivine probably martian calcite crack fillings and iron oxide hydroxide staining both probably terrestrial shock effects are limited to minor cataclasis and twinning in augite in comparison to other nakhlites nwa 998 contains more low calcium pyroxenes and its plagioclase crystals are blockier the large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late and post igneous history of this specimen and mineral thermometers give subsolidus temperatures near 730 c oxidation state was near that of the qfm buffer from about qfm 2 in earliest crystallization to near qfm in late crystallization and to about qfm 1 5 in some magmatic inclusions the replacement or overgrowth of olivine by pigeonite and orthopyroxene with or without titanomagnetite and the marginal replacement of augite by pigeonite are interpreted to result from late stage reactions with residual melts consistent with experimental phase equilibrium relationships apatite is concentrated in planar zones separating apatite free domains which suggests that residual magma rich in p and ree was concentrated in planar fracture zones and possibly migrated through them loss of late magma through these zones is consistent with the low bulk ree content of nwa 998 compared with the calculated ree content of its parent magma