changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

self destructive sulfide segregation systems and the formation of high grade magmatic ore deposits the metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals some high grade magmatic ni cu pge deposits and many magmatic platinum group element pge ni cu deposits demand very high magma sulfide ratios that seem improbable in the light of physical and kinetic constraints recent models for some high grade deposits invoke multistage upgrading processes in which an early formed sulfide liquid reacts with multiple later batches of silicate magma quantitative models of this open system process demonstrate that it is indeed more efficient than a closed system however it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids further increasing metal concentrations in the remaining sulfide liquids this combined process is termed multistage dissolution upgrading and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude furthermore if sulfide liquids are extensively dissolved during enrichment their base metal concentrations will generally stabilize at a limiting value whereas their pge concentrations will generally increase without limit this divergence could account for unusually high pge concentrations and high pge base metal ratios in many pge dominated deposits the multistage dissolution upgrading model is tested using reasonable degrees of dissolution in the context of natural deposits in the noril sk area and the bushveld intrusion the models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits thus dynamic sulfide forming magmatic systems may be intrinsically self destructive but this apparently undesirable attribute could play an important role in forming high grade deposits however dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches in this case no sulfide deposit would remain but metal depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits

osmium isotopes and crustal sources for platinum group mineralization in the bushveld complex south africa the bushveld complex in south africa is the largest layered mafic intrusion in the world and a major producer of platinum group elements pges economic mineralization is found in the merensky reef and the ug2 and ug1 chromitites and includes osmium bearing laurite ru ir os s 2 the laurite bearing samples have 187 os 186 os ratios of 1 28 1 60 which are more radiogenic than predicted for mantle derived 2 0 ga os 0 9 and indicate a system wide assimilation of crustal os the ug1 chromitite is stratigraphically the lowest and the least radiogenic 1 28 whereas the merensky reef is the highest and most radiogenic 1 41 1 60 hart and kinloch 1989 radiogenic os from crustal assimilation in mantle derived magmas or from hydrothermal fluids can account for these high isotopic ratios assimilation models require that os and by inference other pges are carried in al rich tholeiitic a magmas rather than high mg ultramafic u magmas pyritic black shales in contact with the intrusion may be high in radiogenic os and less than 1 shale assimilation in an a magma can explain the observed ratios late stage hydrothermal fluids driven by the heat of the intrusion could also have carried os and other pges from adjacent rocks into specific layers of the bushveld the os isotope data add to mounting evidence that pges in layered mafic intrusions may be derived from crustal sources and can no longer be considered as solely a product of mantle derived magmatic processes

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

pge enrichment in chromitite layers and the merensky reef of the western bushveld complex a re os and rb sr isotope study abstract four poikilitic pyroxenites of the bastard unit all being relatively low in osmium concentrations define a re os isochron age of 2043 11 ma with initial 187os 188os 0 151 which is in accordance with the normally cited crystallization age of the bushveld complex the isochron fit demonstrates an excellent radiogenic os isotopic homogeneity of this portion of the complex at the time of crystallization and excludes any later disturbance of the re os isotope system initial 187os 188os ratios of platinum group element pge enriched sulfides and whole rocks of the directly underlying merensky reef are highly radiogenic and variable with values between 0 168 and 0 181 separates of chromite grains and interstitial phases of critical zone chromitite layers show initial 187os 188os signatures ranging from the near chondritic mantle values of lower group chromitites i e 0 120 to 0 137 for middle group chromitites and to 0 150 for the upper group 2 chromitite layer osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold showing the presence of highly pge enriched interstitial sulfides equal or very similar initial 187os 188os signatures of the interstitial minerals and the cumulus chromite grains together with their coupled increasing and decreasing trends in this ratio clearly demonstrate that interstitial sulfides and associated cumulus chromite of the different chromitite layers scavenged their osmium from the same or similar sources therefore a pge enrichment of the interstitial sulfides by a primary magmatic process such as precipitation of immiscible sulfide droplets during magma mixing appears more likely than a scenario involving fluid or melt transport of pge into these layers such a primary magmatic pge enrichment process is also preferred for the formation of the merensky reef

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

halogen variations in the paleoproterozoic layered mafic ultramafic intrusions of east kimberley western australia implications for platinum group element mineralization a number of paleoproterozoic layered mafic ultramafic intrusions in the central part of the halls creek orogen of east kimberley western australia have been explored for platinum group elements chromium nickel copper cobalt and gold here we report on the halogen geochemistry of apatite and biotite in a number of these intrusions interstitial apatite is ubiquitous in these intrusions and in most samples tends to be relatively enriched in f and oh end members and relatively poor in cl 20 mole fluorapatite occurs in the more evolved igneous rocks and in samples from the margins of the intrusions that apparently have been contaminated by metamorphic country rock cl f ratios generally increase with bulk rock molar mg mg fe ratios as observed in other intrusions only a few samples show cl enrichment as high as that seen in the stillwater and bushveld complexes beneath the major stratabound platinum group element deposits the most cl rich compositions observed occur in the upper part of the springvale intrusion where it is associated with troctolite and in a single sample from the mcintosh intrusion for the former intrusion it is suggested that volatiles migrating out of the lower part of the mafic stratigraphy stabilized olivine at the expense of pyroxene associated biotite tends to contain little no more than 10 mole cl or f it is concluded that the east kimberley intrusions contained a low to moderate volatile component that during the combined processes of crystallization degassing and fractionation of interstitial halogen bearing minerals produced a late mobile interstitial silicate liquid or volatile rich fluid phase of variable cl f content that in turn caused most of the observed variations within any given intrusion the exceptions include the margins of the intrusions that appear to have been affected by country rocks either during emplacement assimilation or during later metamorphism apatite s generally low cl f ratio the lack of primary amphibole and the high background sulfur concentrations of the east kimberley intrusions suggest that these magmas were relatively volatile poor the development of high grade platinum group element enriched horizons by late stage hydrothermal processes that could have mobilized significant amounts of the platinum group elements and sulfur is considered to be unlikely

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

re and os distribution and os isotope composition of the platreef at the sandsloot mogolakwena mine bushveld complex south africa abstract rhenium and os concentrations and os isotopic compositions were determined for a suite of previously characterized samples from a traverse across the platreef at the sandsloot mogolakwena mine of the northern limb of the bushveld complex the samples comprise five pyroxenites and five parapyroxenites contact rocks representing variable mixtures of calc silicate country rocks and magmatic rocks the isotopic data from the pyroxenites define a re os regression line with an age 2011 51 ma that agrees within uncertainty with the accepted age for the bushveld complex whereas the parapyroxenite data scatter about this trend the initial 187os 188os ratios of the pyroxenites 0 1820 0 1848 are similar to but slightly more radiogenic than previously published values for the merensky reef our preferred interpretation is that this section of the platreef formed from a well mixed crustally contaminated magma genetically related to that which gave rise to the merensky reef the platreef parental magma may also have experienced minor additional contamination by wall rock interaction during emplacement

hydrogen isotope composition of the merensky reef and related rocks atok section bushveld complex hydrogen isotope compositions of rocks from the bushveld complex at the atok mine have been determined to evaluate the relative importance of hydromagmatic and subsolidus hydrothermal processes for defining the geochemical characteristics of the merensky reef and adjacent rocks the rocks include mineralized pyroxenite and pegmatoid of the merensky reef as well as underlying and overlying pyroxenite norite and anorthosite cumulates from the merensky and bastard cyclic units of the upper critical zone the samples were chosen specifically because of the absence of deuteric alteration phases the water contents of all rocks including the pegmatoid are less than 0 26 wt percent and typically in the range 0 05 to 0 15 wt percent the major hydrous phase observable optically is phlogopite however there is not enough phlogopite in most samples to account for even these low water contents additional water is believed to be present either in solution in pyroxene or in submicroscopic phlogopite lamellae within pyroxene the upper critical zone rocks have relatively homogeneous hydrogen isotope compositions having delta d smow values in the range of 93 to 55 per mil there is a small but systematic difference in composition between pegmatoid and pyroxenite of the merensky reef avg delta d 65ppm and of the generally more leucocratic rocks elsewhere in the section avg delta d 79ppm within each lithology there is no relationship between isotopic composition and water content and the data cannot be rationalized in terms of simple mixing of light magmatic water with heavier meteoric crustal water the narrow range of the d h ratio of all the rocks suggests that their isotopic signatures were established under similar conditions this and the fact that different lithologies exhibit different isotopic compositions but possess the same mineralogy the rocks differ from each other mainly in modal abundances of the major phases are used to argue that their isotopic compositions could not have been imposed by subsolidus processes this contention is supported by the composition of merensky reef phlogopite the halogen contents of phlogopite do not vary with the crystal chemical factors known to influence their substitutional behavior phlogopite composition was in part controlled by its equilibration with an intergranular fluid of variable composition implying that there was no significant flow to homoginize pore fluid during subsolidus cooling in the massive rocks subsolidus circulation was therefore limited to large through going fractures the geochemical features of the merensky reef and adjacent rocks at atok were established during late stage solidification the hydrogen isotope data are consistent with a model involving addition of hydrous vapor to the merensky protoreef horizon this vapor originated from crystallization of the intercumulus melt below the reef and may also have included a small portion of water from the country rocks

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

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

experimental study of a kiglapait marginal rock and implications for trace element partitioning in layered intrusions abstract experiments on a chilled marginal rock from the kiglapait intrusion labrador have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of kiglapait lower zone lz cumulates at 0 1 mpa and f o 2 close to fmq the liquidus is bracketed between 1225 and 1212 c and the only liquidus phases are plagioclase an 73 74 and olivine fo 82 85 in contrast lz troctolites contain fo 67 5 olivine and an 63 4 plagioclase plagioclase melt partition coefficients d s for li ti rb sr y zr ba la and eu determined by ion microprobe analysis of run products are similar to those for other basaltic systems but differ systematically from apparent d s calculated from bulk analyses of lz plagioclases and modelled liquid compositions obtained by summation the discrepancies in d s and phase compositions are inconsistent with diffusion controlled boundary layer growth crystallisation of trapped intercumulus liquid is quantitatively plausible however a the volume of liquid required to reconcile experimental and model d s 20 1 is larger than that estimated from observed proportions of intercumulus phases 11 2 and b this mechanism alone cannot reduce x an in lz plagioclases due to very slow al si interdiffusion the discrepancy between experiment and model is best resolved if the parental melt to lz cumulates was more evolved higher fe mg na ca ree and alkalis lower sr than the summed bulk composition in accord with a recent revision of the kiglapait fe mg based on olivine compositions

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

infiltration metasomatism in layered intrusions an example from the stillwater complex montana abstract the contact between anorthosite zone ii an ii and the overlying troctolite of olivine bearing zone v ob v of the stillwater complex is characterized by a 5 m thick adcumulate anorthosite this adcumulate anorthosite separates rocks of an ii characterized by the apparent crystallization sequence plagioclase followed by orthopyroxene clinopyroxene quartz fe ti oxide assemblages from those of ob v where the apparent crystallization sequence is olivine plagioclase followed by orthopyroxene the grain size and compositional zoning of plagioclase in this transition zone is similar to that in the overlying troctolite and distinct from the coarser complexly zoned plagioclase that makes up the underlying anorthosite in contrast the minor interstitial pyroxene in the transition zone compositionally overlaps that in the underlying anorthosite and both have a lower molar mg mg fe ratio than the pyroxene of the troctolite these relationships suggest that the intercumulus liquids of an ii and ob v were compositionally distinct model phase diagram considerations suggest that infiltration of any plagioclase saturated liquids into a troctolite assemblage will result in the assimilation of olivine and depending on the actual composition of the anorthositic liquid will also either assimilate or crystallize plagioclase as well hence the monomineralic anorthosite zone at the top of an ii can be attributed to infiltration of interstitial liquids from an ii that were displaced upward into the troctolite during compaction of an ii this reaction can be either exothermic or endothermic depending on the actual composition of the infiltrating liquid and the bulk solids composition numerical models of the thermal effects of infiltration suggest that heat of reaction can significantly affect the liquid solid ratio away from the infiltration boundary

hornblende and phlogopite bearing gabbroic xenoliths from volcan san pedro 36 s chilean andes evidence for melt and fluid migration and reactions in subduction related plutons

origin of phlogopite orthopyroxene inclusions in chromites from the merensky reef of the bushveld complex south africa about 30 of the chromite grains of variable sizes in a chromitite seam at the base of the merensky reef of the bushveld complex on the farm vlakfontein contain abundant composite mineral inclusions the inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite they vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains na phlogopite and orthopyroxene are most abundant in the inclusions edenitic hornblende k phlogopite oligoclase and quartz are less abundant cl rich apatite rutile zircon and chalcopyrite are present at trace levels na phlogopite is unique to the inclusions it has not been found elsewhere in the bushveld complex other minerals in the inclusions are also present in the matrix of the chromitite seam but their compositions are different the mg mg fe2 ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix k phlogopite in the inclusions contains more na than in the matrix the average compositions of the inclusions are characterized by high mgo 26 wt na2o 2 4 wt and h2o 2 6 wt and low cao 1 1 wt and feo 4 4 wt the 18o value of the trapped melt estimated by analysis of inclusion rich and inclusion poor chromites is 7 this value is consistent with the previous estimates for the bushveld magma and with the 18o values of silicate minerals throughout the reef the textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile rich melts during chromite crystallization the volatile rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and na k rich fluids expelled from the underlying crystal pile the addition of fluid to the magma is thought to have caused dissolution of orthpyroxene leaving the system saturated only in chromite both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution most of the cr and al in the inclusions was contributed through wall dissolution of the host chromite dissolution of minor rutile trapped along with orthopyroxene provided most of the ti in the inclusions the na and k rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile rich melts

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

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

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

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

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

the mechanics of continental extension and sedimentary basin formation a simple shear pure shear flexural cantilever model abstract deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins major basement faults are generally planar and extend down to 10 15 km depth corresponding to the base of the seismogenic layer beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle a mathematical model is described of the geometric thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle during faulting upper crustal footwall and hanging wall blocks behave as two mutually self supporting flexural cantilevers their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse for extension on multiple faults interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics the flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing horizontal displacement and polarity the effects of fault spacing fault polarity and the amount of extension as well as erosion of footwall uplift on crustal structure and basin geometry during both syn rift and post rift stages of basin evolution are explored models of passive rifting involving lithosphere extension in response to the build up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere appear to conform to the main geological observations the role of active rifting driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

behavior of a loosely compacted unsaturated volcanic soil in this paper the stress strain relationsihp and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil fill were studied by conducting three series of triaxial stress path tests 1 consolidated undrained on the saturated fill 2 constant water content and 3 a reducing suction under constant deviator stress on the unsaturated fill the last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes respectively it was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear for isotropically consolidated unsaturated specimens sheared under a constant water content a hardening stress strain and a nonlinear shear strength suction relationship are observed at relatively high suctions both angle of friction and apparent cohesion appear to be independent of suction volumetric contraction during shear is observed in this series of tests on the other hand anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases this observed volumetric behavior unlike the shear strength is stress path dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils

multiple merensky reef of the bushveld complex south africa the merensky reef mr of the bushveld complex is known for containing the lion s share of the world s reserves of platinum group elements pge it is commonly described as a single relatively thin layer or package of chromite and sulphide bearing orthopyroxenite hosted by almost barren norite and or anorthosite here we present a drill core section that contains however four separate mr orthopyroxenite layers hosted by mottled anorthosites each of these orthopyroxenites is distinguished through occurrence absence of marginal chromitite seams grain size petrographic composition and pge abundance we interpret them as multiple undercutting mr that occurs as sill like apophyses extending laterally from a pothole margin into the footwall the undercutting mr is attributed to in situ crystallization within sheet like melt filled cavities at potholes margins which are produced through thermal and or chemical erosion of the footwall rocks by replenishing magmas this finding indicates that the mr forming event involved several distinct pulses of replenishing magmas whose crystallization products were mostly eroded away in the stratiform portions but remained locally preserved in apophyses within footwall rocks the mineralized mr formed a few meters below a temporary chamber floor is indicative of strong thermal compositional convection in the basal layer of the bushveld s chamber the convection was able to deliver large volumes of fresh magma for the scavenging of pge by sulphide and chromite growing within the sheet like cavities of the mr potholes

trace elements in the merensky reef and adjacent norites bushveld complex south africa trace elements were analysed in rocks and minerals from three sections across the merensky reef in the rustenburg platinum mine in the bushveld complex of south africa whole rocks and separated minerals were analysed by inductively coupled plasma mass spectrometer icp ms and in situ analyses were carried out by ion microprobe and by laser source icp ms merensky reef pyroxenites contain extremely high concentrations of a wide range of trace elements these include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements for major elements and compatible trace elements the measured concentrations in cumulus phases and the bulk rock compositions are similar for highly incompatible elements however concentrations in bulk rocks are far higher than those measured in the cumulus phases in situ analyses of plagioclase have far lower concentrations of th zr and rare earth elements than icp ms analyses of bulk separates of plagioclase a difference that is attributed to the presence of trace element rich accessory phases in the bulk mineral separates we used these data to calculate the trace element composition of the magmas parental to the merensky unit and adjacent norites we argue that there is no reason to assume that the amount of trapped liquid in the merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites we propose that the bushveld complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks the magma that gave rise to the merensky unit was the more contaminated and unusually rich in incompatible trace elements and when it entered the main bushveld chamber it precipitated the unusual phases that characterize the merensky reef the hybrid magma segregated sulphides or platinum group element rich phases during the course of the contamination in the lower chamber these phases accumulated following irruption into the main bushveld chamber to form the merensky ore deposits

the distribution of base metals and platinum group elements in magnetititeand its host rocks in the rio jacare intrusion northeastern brazil anomalously high pt and pd values have been found in three magnetite bodiesin the rio jacare intrusion of northeastern brazil the intrusion hosting thesemagnetite bodies consists predominantly of pyroxenite and gabbro one magnetitebody occurs in the lower zone and two in the upper zone of the intrusion thesebodies contain approximately 0 04 percent ni 0 1 percent cu 0 18 percent s 1ppb ir 3 ppb rh 160 ppb pt 120 ppb pd and 37 ppb au they are much richer inplatinum group elements pge than the surrounding silicate rocks and there aresignificant correlations among all of the pge and between pge and ni howeverthe correlations between pge and au cu and s are much weaker than correlationsbetween au cu and s in the magnetite bodies palladium rich minerals especially bismuthides andantimonides are the most abundant platinum group minerals pgm in most casesthese occur with interstitial silicates or within silicate inclusions inmagnetite and ilmenite grains and are associated with co bearing pentlandite andin a few cases with co ni sulfarsenides and arsenides sperrylite ptas2 is the most abundant pt mineral and is associated with silicates interstitial tomagnetite and ilmenite grains and sometimes with co ni arsenides at sites wherethe igneous mafic minerals have been altered to amphiboles sperrylite may bealtered to pt fe alloys other alloys present include pd sn cu pt cu pt ni and pt au it is suggested that ni and pge were concentrated in the magnetite bodies bythe coprecipitation of a small quantity of sulfide with the magnetite these pge bearingbase metal sulfides subsequently exsolved pgm the association of pd mineralswith base metal sulfides and the small variation in the pt pd ratio ca 1 4 suggests that the pge have not been extensively remobilized in the magnetitite in contrast the strong correlation between s cu and au suggests that inaddition to the redistribution of s it is likely that cu and au wereremobilized it is not possible to say whether the redistribution of sulfur wasdue to late magmatic fluids dissolving s or the later metamorphic events the association of pge enrichment with magnetite layers in the rio jacareintrusion contrasts with that of the bushveld stillwater great dyke and munnimunni complexes in these complexes pge enriched layers or reefs are found inthe lower third of the complexes and the oxide associated with the reefs ischromite magnetite bearing layers which form from an evolved magma in theupper parts of the intrusions are generally barren of pge because at the timeof magnetite crystallization the pge had already precipitated either insulfides or pgm however in a number of intrusions e g rincon del tigre skaergaard stella and rio jacare the upper magnetite bearing portion of theintrusion shows pge enrichment this enrichment is rarely associated withvisible sulfides but suggests a possible new target for pge exploration

variation in the composition of apatite through the merensky cyclic unit in the western bushveld complex microprobe analysis of interstitial apatite from a 100 m section through the merensky reef of the bushveld complex reveals that this stratigraphic section marks a transition from cl rich to more f rich compositions in the critical and main zones respectively apatite compositions across the section vary by more than 80 mole percent in the chlorapatite component the trend of decreasing cl concentration with stratigraphic height is not regular and cl contents in apatites from some samples may vary as much as 50 mole percent la 2 o 3 and ce 2 o 3 tend to be enriched in the more melanocratic orthocumulate parts of the section sro concentrations of apatite show an increase with stratigraphic position a trend which is opposite that observed in coexisting cumulus plagioclase the data are consistent with mineralogic and textural evidence for an influx of cl rich vapor into the cumulus section which has affected the cl f variations through the lower portion of the bastard unit this influx occurred above the crystallization temperature of apatite 1 000 degrees 1 050 degrees c it is suggested that much of the modal variation seen in the section may be the result of shifts in liquidus field boundaries caused by gain or loss of volatile components from the melt

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

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

reactive flow as dominant evolution process in the lowermost oceanic crust evidence from olivine of the pineto ophiolite corsica the jurassic pineto ophiolite from corsica exposes a 1 km thick troctolite olivine gabbro sequence interpreted to represent a lowermost sector of the gabbroic oceanic crust from a ultra slow spreading system to constrain the petrogenesis of the olivine gabbros minor and trace element analyses of olivine forsterite 84 82 mol were carried out olivine from the olivine gabbros is depleted in incompatible trace elements sc v ti y zr and heavy rare earth elements with respect to olivines from associated troctolites depleted incompatible element compositions are also shown by olivine forsterite 86 mol from a clinopyroxene rich troctolite the incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization we propose that melts migrating through an olivine plagioclase crystal mush chemically evolved by reaction with the existing minerals changing in composition as it flowed upward the melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene generating olivine gabbro bodies within a troctolite matrix reactive flow was the major evolution process active in the 1 km crustal transect exposed at the pineto ophiolite producing lithological variations classically attributed to fractional crystallization processes

character of the ug2 chromitite and host rocks and petrogenesis of its pegmatoidal footwall northeastern bushveld complex the ug2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the middelpunt mine northeast bushveld complex in this region the ug2 chromitite and its coarse grained footwall rocks are underlain and overlain by plagioclase pyroxenite the ug2 chromitite itself is a massive 70 cm thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral immediately beneath the chromitite is the so called mixed layer a previously undescribed but distinct mappable layer 50 to 70 cm thick consisting of chromite bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters the mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite the contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters based on observations of drill core and outcrop the mixed layer appears to exist throughout the central sector of the eastern bushveld and may be ubiquitous throughout the entire complex the mixed layer usually rests directly on the ug2 pegmatoid which is a well known and widespread pegmatoidal plagioclase pyroxenite the pegmatoid is much coarser grained than the underlying pyroxenite but the two lithologic units are similar in terms of mineral modes and compositions the contact between them is irregular at scales of decimeters to meters with vertical fingers of pegmatoid extending into the pyroxenite the mixed layer and ug2 pegmatoid contain rare lenses of massive anorthosite encased by continuous centimeter thick rims of chromitite a model is proposed to account for the footwall sequence of the ug2 chromitite in this model the ug2 pegmatoid formed when hydrous interstitial melt percolated up through the partially molten crystal pile accumulated beneath the preexisting compacted massive chromitite and metasomatized and partially melted the pyroxenite protolith which upon cooling recrystallized to a coarse grained rock on the basis of published experiments illustrating the effect of water on melting in mafic systems it is shown how the hydrous melt was able to penetrate the pyroxene bearing assemblages below the chromitite but unable to partially melt the chromitite itself which in consequence remained relatively impermeable the anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting with the anorthosite representing the crystalline residue the mixed layer formed by gradual collapse of the base of the ug2 chromitite into the less dense relatively fluid footwall the mixed layer and ug2 pegmatoid are diagnostic of the ug2 chromitite and the three lithologic units should be considered a single petrologic unit