the skaergaard layered series part vi excluded trace elements in contrast to the smooth trends of major elements and mineral principal minerals of the skaergaard layered series my purpose here is to examine the corresponding excluded compositions the excluded trace elements in the skaergaard layered series have an irregular distribution that does not conform to the elements and their relations to progressive differentiation of the skaergaard magma the principal conclusion that normal trends of rayleigh type fractionation their concentrations are about constant or even decline through the lower and middle will emerge is that the trends of enrichment of these elements differ markedly from those of the major elezones before increasing sharply to reach maximum concentrations 100 200 m above the sandwich horizon as in the case of ments possibly because their distribution was affected by a build up and eventual exsolution of volatile comincluded elements the relative concentrations of excluded elements in coexisting phases deviate widely from those predicted by exponents in the magma it will also be shown that the observed partition coefficients differ widely from those perimentally determined partition coefficients under presumed magmatic conditions this is seen most clearly in the immiscible predicted from experimental studies several earlier studies addressed various aspects of the melanogranophyres and conjugate ferrogabbros although the major elements conform to the experimentally determined relations for skaergaard trace elements haskin and his co workers haskin haskin 1968 paster et al 1974 were the first immiscible liquids the trace elements do not they follow a totally independent trend the abrupt increase in the concentrations of to attempt to reconcile the distribution of excluded trace elements with earlier interpretations of trace element excluded elements in the upper part of the intrusion could plausibly be attributed to an addition of new magma or to a density inversion behavior during crystallization and differentiation of the layered series other previous work has been sumthat resulted in upward migration of a late liquid or fluid but these possibilities are inconsistent with the compositional and spatial marized by wager brown 1968 and more recently by mcbirney 1996 1998a relations of the upper parts of the intrusion although a late residual liquid certainly migrated upward the most likely explanation for the structural form of the intrusion and its principal units illustrated in simplified form in fig 1 have already the observed distribution of excluded elements is that the partition coefficients were altered by volatile components which gradually been described at length mcbirney 1989 1995 1996 boudreau mcbirney 1997 mcbirney nicolas 1997 increased during the early stages of crystallization then began to exsolve near the top of the middle zone sonnenthal mcbirney 1998 and need not be repeated apart from minor differences noted in the appendix the distribution and selection of samples as well as the methods by which they have been analyzed are essentially the same as those described in part v of this

postcumulus processes in layered intrusions during the postcumulus stage of solidification in layered intrusions fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks one mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir this process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions but could not form pure monomineralic rocks intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment dense magma emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers reactions between melt and matrix may lead to changes in mineral compositions mineral textures and whole rock isotope compositions another important mechanism for forming adcumulate rocks is compaction in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled for cumulate layers from 10 to 1000 metres in thickness compaction can reduce porosities to very low values 1 and form monomineralic rocks the characteristic time scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion during compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix both compaction and compositional convection can be interrupted by solidification in the pore spaces compositional convection will only occur if the rayleigh number is larger than 40 if the residual melt becomes lower in density and the convective velocity exceeds the solidification velocity measured by the rate of crystal accumulation in the chamber orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection

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

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

liquidus phase relationships on the join anorthite forsterite quartz at 10 kbar with applications to basalt petrogenesis liquidus phase relationships determined on the join caal2si2o8 anorthite mg2sio4 forsterite sio2 quartz at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel enstatite and silica fields to expand the boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the sio2 apex as pressure increases therefore simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases as has been found in other studies also increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization although anorthite is unstable in simplified peridotite above 9 kbar in the system cao mgo al2o3 sio2 it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

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

discordant iron rich ultramafic pegmatites in the bushveld complex and their relationship to iron rich intercumulus and residual liquids the iron rich ultramafic pegmatites comprise a suite of coarse grained rocks that form discordant bodies within the layered sequence of the bushveld complex these pegmatites which are considerably more abundant than is generally realized provide evidence for the differentiation of iron rich residual melts the pegmatites are composed largely of iron rich olivine and clinopyroxene together with ti magnetite and ilmenite feldspar is characteristically absent but paradoxically the pegmatites preferentially replace anorthositic cumulates two subgroups are recognized olivine clinopyroxene pegmatite and fe ti oxide pegmatite with increased stratigraphic height the pegmatites become richer in fe ti oxides thus olivine clinopyroxene pegmatite is prevalent in the upper critical and lower main zones whereas fe ti oxide pegmatite is restricted to the upper main and upper zones zoned pegmatite with a core of fe ti oxide pegmatite is transitional between the two subgroups new whole rock and electron microprobe analyses of olivine clinopyroxene pegmatite from the upper critical and lower main zones provide convincing evidence that their composition is directly related to height cryptic compositional variations are analogous to those displayed by the layered cumulates but for a given height the pegmatites are always more evolved compositions of clinopyroxene in the pegmatites reflect a near linear relationship with height whereas cumulus pyroxenes display upward iron enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid the olivine clinopyroxene pegmatite formed by magmatic replacement of earlier formed cumulates in response to infiltration of iron rich melts suitably differentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers the absence of feldspar although not fully understood is attributed to an immiscible relationship between dense iron rich melts and light silica alkali rich liquids the latter infiltrated upward to be reincorporated into the resident magma the iron rich melts however drained down into the crystallizing cumulate pile channelling along early formed fractures and joints was significant locally resulting in huge pipe like bodies of pegmatite the iron rich melts became increasingly differentiated with height partly in response to the fractional crystallization of more evolved cumulates the olivine clinopyroxene pegmatites are related to infiltration of fe ti oxide rich silicate melt whereas fe ti oxide pegmatite is ascribed to fe ti oxide liquid as originally argued by bateman 1951 the bushveld complex followed the fenner trend of almost uninterrupted iron enrichment evidence of pronounced iron enrichment is however manifested in the discordant iron rich ultramafic pegmatites several thousands of metres below the height at which iron rich cumulates are observed

skier triggering snow temperatures and the stability index for dry slab avalanche initiation for more than 30 years the quantitative method of evaluating stability e g roch 1966 fohn 1987 jamieson 1995 jamieson and johnston 1998a has been focused on calculation ofa strength to load ratio or stability index when the shear stress applied to the weak layer reaches the shear strength failure is imminent however field observations combined with experience and measurements indicate that snow slab temperatures and slab hardness can have a strong influence on dry snow slab stability in this paper we present a simple static analysis of the stability index and discuss the importance of slab temperatures and hardness and macroscopic size effects factors not contained in the stability index on snow slab stability our conclusion is that the traditional method lacks some elements which are very important in snow slab stability particularly when skier triggering is involved

fracture energy applicable to dry snow slab avalanche release 1 dry slab avalanches release by a sequence of propagating fractures first by shear fracture mode ii and mode iii in a weak layer at the base of the slab and second by tensile fracture through the crown after which release of the slab is imminent the fracture energy is the energy which must be provided to produce a unit area of fracture surface it is a key parameter in determining how and when fractures will propagate and important slab characteristics such as length and width that are necessary in order to estimate slab mass and volume in relation to destructive potential in this paper approximate estimates of fracture energy are calculated from field measurements of slab properties laboratory measurements and in situ strength tests in alpine snow estimates of fracture energy are given for both tensile fracture mode i through the crown as well as for shear fracture mode ii in fragile weak layers of slab avalanches the results when compared with other values from ice rock and engineering materials suggest that snow has the lowest values

evaluation of slope stability with respect to snowpack spatial variability the evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas the spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation since it strongly influences failure initiation and crack propagation in weak snow layers hence the determination of the link between these spatial variations and slope stability is very important in particular for avalanche public forecasting in this study a statistical mechanical model of the slab weak layer wl system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches this model accounts in particular for the spatial variations of wl shear strength and stress redistribution by elasticity of the slab we show how avalanche release probability can be computed from release depth distributions which allows us to study the influence of wl spatial variations and slab properties on slope stability the importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so called knock down effect on slope stability is revisited using this model finally critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests

joint electromagnetic and seismic inversion using structural constraints we have developed a frequency domain joint electromagnetic em and seismic inversion algorithm for reservoir evaluation and exploration applications em and seismic data are jointly inverted using a cross gradient constraint that enforces structural similarity between the conductivity image and the compressional wave p wave velocity image the inversion algorithm is based on a gauss newton optimization approach because of the ill posed nature of the inverse problem regularization is used to constrain the solution the multiplicative regularization technique selects the regularization parameters automatically improving the robustness of the algorithm a multifrequency data weighting scheme prevents the high frequency data from dominating the inversion process when the joint inversion algorithm is applied in integrating marine controlled source electromagnetic data with surface seismic data for subsea reservoir exploration applications and in integrating crosswell em and sonic data for reservoir monitoring and evaluation applications results improve significantly over those obtained from separate em or seismic inversions

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

constraints on the formation of platinum group element deposits in igneous rocks mechanisms of concentration of the platinmn group and other chalcophile elements iu several platinum group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts the similarity of the partition coefficieut d super sulfide melt silicate melt of ir to that of pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility in experiments designed to test the possibility that this fractionation was due to the presence of os ir alloy sulfide and silicate melts were equilibrated with ir metal at 1 450 degrees c 8 kbars total pressure and under sulfur and oxygen fugacity conditions appropriate for nature the ir content of the sulfide liquid was found to be or 3 7 wt percent and that of the coexisting silicate melt inferred to be or 1 000 ppb these concentrations are far in excess of the ir contents of natural sulfides or rocks and indicate that os ir alloys are not stable in the presence of sulfide or silicate melts in nature using our preferred values of d super sulfide melt silicate melt for the chalcophile elements and their estimated concentrations in the mantle limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle the ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits including the j m reef stillwater complex the ug 2 bushveld complex and the lac des isles complex are inconsistent with concentration solely by segregation of sulfide liquid in contrast the merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide stratigraphic variations in chalcophile element concentrations through the ug 2 and the main sulfide zone of the great dyke also indicate that metals must have been redistributed on at least the meter scale our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism

ten years of marine csem for hydrocarbon exploration marine controlled source electromagnetic csem surveying has been in commercial use for predrill reservoir appraisalandhydrocarbonexplorationfor10 years althougha recent decrease has occurred in the number of surveys and publications associated with this technique the method has become firmly established as an important geophysical tool intheoffshoreenvironment thisisaconsequenceoftwoimportant aspects associated with the physics of the method first it is sensitive to high electrical resistivity which although not an unambiguous indicator of hydrocarbons is an important property of economically viable reservoirs second although the method lacks the resolution of seismic wave propagation it has a much better intrinsic resolution than potential field methods such as gravity and magnetic surveying which until now have been the primary nonseismicdatasetsusedinoffshoreexploration althoughbymany measures marine csem is still in its infancy the reliability and noise floors of the instrument systems have improved significantly over the last decade and interpretation methodology has progressed from simple anomaly detection to 3d anisotropic inversion of multicomponent data using some of theworld sfastestsupercomputers researchdirectionspresently include tackling the airwave problem in shallow water by applying time domain methodology continuous profiling tools and the use of csem for reservoir monitoring during production

the skaergaard layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

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

dry snow slab shear fracture speeds 1 dry snow slab avalanches release by propagating shear fractures within thin weak layers underneath thicker stronger planar slabs in this paper measured shear fracture speeds from small scale snow slabs are compared with theoretical estimates compatible with dynamic fracture mechanics given the important physical quantities including density and elastic shear modulus it is concluded that estimated snow slab shear fracture speeds scale with the shear wave speed similar to estimates in other geophysical applications including shear fractures from earthquakes and in engineering materials however the constant of proportionality is much lower than for alpine snow than for earthquakes and engineering materials the lower constant of proportionality may indicate the possibility of important energy dissipation mechanisms in snow avalanche fracture propagation

application of the perfectly matched layer in 3 d marine controlled source electromagnetic modelling in this study the complex frequency shifted perfectly matched layer cfs pml in stretching cartesian coordinates is successfully applied to three dimensional 3d frequency domain marine controlled source electromagnetic csem field modelling the dirichlet boundary which is usually used within the traditional framework of em modeling algorithms assumes the electric or magnetic field values are zero at the boundaries this requires the boundaries be sufficiently far away from the sources in the area of interest to mitigate the boundary artifacts a large modelling area may be necessary even though cell sizes are allowed to grow toward the boundaries due to the diffusion of the electromagnetic wave propagation compared with the conventional dirichlet boundary the pml boundary is preferred as the modelling area of interest could be restricted to the target region and only a few absorbing layers surrounding can effectively depress the artificial boundary effect without losing the numerical accuracy furthermore for joint inversion of seismic and marine csem data if we used the pml for csem field simulation instead of the conventional dirichlet the modeling area for these two different geophysical data collected from the same survey area could the same which is convenient for joint inversion grid matching we apply the cfs pml boundary to 3d marine csem modelling by using the staggered finite difference sfd discretization numerical test indicates that the modeling algorithm using the cfs pml also shows good accuracy compared to the dirichlet furthermore the modeling algorithm using the cfs pml shows advantages in computational time and memory saving than that using the dirichlet boundary for the 3d example in this study the memory saving using the pml is nearly 42 and the time saving is around 48 compared to using the dirichlet

analysis of critical length measurements for dry snow slab weak layer shear fracture snow slab avalanches primarily release by propagation of shear fractures within thin weak layers under much thicker slabs in some cases the weak layer is on the order of 1 mm thick and such fractures may be considered to be a mode ii shear fracture at initiation in the cases analysed in this paper the weak layer has finite thickness and slope normal effects may be present field data from 500 snow shear fracture tests are analysed and applied to the problem of weak layer fracture the paper contains a detailed analysis using a simple analytical model to estimate the critical length prior to an unstable shear fracture the model contains the assumption of a finite fracture process zone which may be a significant fraction of the slab depth d or the critical length l for weak layer shear fracture the field results show that the l d ratio varies from 0 1 to just over 2 and the model results are close to the same range the analysis also shows that both the field and model results for l d follow a gumbel probability density function since the experimental field data contain rate dependent viscoelastic and slope normal effects it is imperative to account for these in the model and for snow slab instability evaluation when using test data detailed evaluations considering both these effects are given the applicability of the test data to avalanche release is discussed

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

the distribution and main geologic features of discordant bodies of iron rich ultramafic pegmatite in the bushveld complex transgressive bodies of iron rich ultramafic pegmatite composed predominantly of varying proportions of hortonolite clinopyroxene ilmenite and ti magnetite are abundantly distributed throughout the layered mafic sequence of the bushveld complex new information from field and underground studies together with interpretations of detailed magnetic surveys is presented along with an overview of the available literature some whole rock and mineralogical chemical data are also included within the bushveld complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized these include besides the main group of iron rich ultramafic pegmatite bodies of magnesian dunite and the well known platiniferous ultramafic or dunitic pipes transgressive often pipelike bodies composed almost entirely of fe ti oxides occur in the main and upper zones of the complex which although part of the iron rich ultramafic pegmatite suite are described separately composite bodies in which massive fe ti oxides occur as a core within a peripheral shell of silicate rich pegmatite are also recognized the major occurrences of iron rich ultramafic pegmatite are documented and depicted in a series of maps their distribution is subject to two major stratigraphic controls iron rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones it is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus ti magnetite layer in the upper zone it is also evident that these rocks become increasingly iron rich toward the top of the layered sequence so that in the upper zone fe ti oxide pegmatite is the dominant variety many bodies particularly the large ones are structurally controlled and occur in disturbed areas characterized by faulting and post bushveld dikes stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron rich ferromagnesian silicates and fe ti oxides this iron rich material is considered to represent the accumulated and trapped late stage residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large pipelike bodies of iron rich ultramafic pegmatite these bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids the authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron rich ultramafic pegmatite is an integral feature of the layered sequence

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

dry snow slab quasi brittle fracture initiation and verification from field tests 1 dry snow slab avalanches initiate from shear fracture in a thin weak layer underneath a planar slab of cohesive snow in this paper field data from more than 400 snow shear fracture tests are analysed and applied to the problem of snow slab release and snow pack instability evaluation the paper contains a new but simple analytical model to estimate the critical length for fracture the model contains the assumption of a finite fracture process zone which may be a significant fraction of the slab depth d or the critical length l for weak layer shear fracture the results suggest that the ratio l d is of order 1 for both the data and the model as previously predicted for the snow slab using a combination of experimental results from laboratory tests two sets of field data and the new model suggests that previous theoretical arguments about snow slab release over almost 30 years are congruent with the field data simple extension to explain viscoelastic temperature effects and slope normal weak layer deformation on snow slab instability evaluation is included in the analysis since the experimental field data contain viscoelastic effects and slope normal effects it is imperative to account for these in the model and for snow slab instability evaluation

probabilistic size effect law for mode ii fracture from critical lengths in snow slab avalanche weak layers from field observations dry snow slab avalanche initiation is associated with fracture within relatively thin weak layers under stronger cohesive slabs for risk based avalanche prediction it is important to understand the fracture properties of alpine snow alpine snow is a quasi brittle material with a fracture mechanical size effect on nominal shear strength meaning that strength decreases with increasing specimen size a related size effect is the critical length required for rapid propagation of a shear fracture in that case the probability of fracture increases with increasing crack length in this paper 45 sets of field measured critical lengths are presented based on 591 individual tests from analysis a probabilistic size effect law based on critical lengths is derived analogous to the deterministic size effect law for nominal shear strength related to fracture mechanics the new size effect law may be useful in applications particularly since the critical length is easily measured in the field and it is a principal component of weak layer fracture toughness

the geologic setting of the platiniferous ug 2 chromitite layer on the farm maandagshoek eastern bushveld complex the platiniferous ug 2 chromitite layer forms part of the upper chromitite group of the critical zone on the farm maandagshoek 254kt in the eastern bushveld complex the upper chromitite group is made up of four cyclic units namely ug 1 ug 2 ug 3 and ug 3a each of which consists of a basal chromitite layer with or without olivine overlain successively by melanorite and norite and terminated with anorthosite the ug 2 chromitite layer and its associated basal pegmatoid is found close to the base of the ug 2 cyclic unit the cyclic units are separated by sharp contacts and have a remarkable persistence along strike and downdip whole rock cu total ni pt and pd show an upward enrichment from the base of the ug 2 unit reaching a maximum in the main chromitite layer they are depleted in the hanging wall sequence of melanorite bulk analyses of the ug 2 chromitite and its enclosing lithologies for pt pd ru rh ir au s sulfide ni cu and co show that the platinum group elements and base metals are concentrated 1 close to the top and base of the main chromitite layer 2 in the leader layer and 3 in minor chromitite lenses and layers both in the footwall pegmatoid and the hanging wall melanorite the pt pt pd and cu ni ratios are low in the footwall pegmatoid and base of the main chromitite layer values of these ratios however increase upward with pt pt pd decreasing at the upper contact of the ug 2 layer the calculated concentration of platinum group metals in the sulfide phase in the ug 2 chromitite layer ranges from 3 000 to just under 12 000 ppm the above determination is primarily controlled by the concentration of s in the layer with the average precious metal distribution remaining relatively constant the calculated d super sulfide liquid sub silicate liquid with the parental liquid values in parentheses are pt 52 575 40 ppb pd 132 940 16 ppb rh 22 080 8 ppb ir 730 000 0 4 ppb and au 2 190 21 ppb evidence of an influx of magma into the eastern lobe of the bushveld complex at a level between the upper and middle chromitite groups is indicated by discontinuous and disturbed layers in borehole drilling on maandagshoek it is suggested that this magma batch was slightly more enriched in the platinum group elements in order to produce the increased value of these elements in the ug 2 chromitite layer it is proposed that preexisting sulfides in the ug 2 chromitite layer collected platinum group elements from upward migrating highly fractionated intercumulus fluids under highly reducing conditions during the early stages of formation of the layer the relatively large volume of sulfide initially present in the layer was later reduced by volatiles enriched in o 2 passing through the layer resorbing the sulfides and blowing off so 2 the partial resorption of the sulfides accounts for the presence of minor pt fe alloys and the abnormally high partition coefficients of the platinum group elements in the sulfide liquid of the ug 2 chromitite layer

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

platinum group element chromitite associations in the bushveld complex new and recently published platinum group element data of chromite rich layers in the bushveld complex are evaluated chondrite normalized platinum group element plots of these data show that trends for chromitite layers differ from that of the merensky reef the data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers suggesting that the conditions of formation of chromitite layers and their associated platinum group element mineralization were very uniform over large areas of the magma chamber significant differences in the proportions of platinum group elements within a particular layer are only evident on a regional scale the platinum group minerals of the middle group of chromitite layers in the marikana area of the western bushveld are mainly sulfides and arsenides laurite is the most common platinum group mineral and is mostly enclosed by chromite a variety of other platinum group minerals occur in close association with base metal sulfides extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite rich layers based on partition coefficients and the r factor

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

connectivity between the western and eastern limbs of the bushveld complex abstract the mafic layered rocks of the bushveld complex are 6 8 km thick and crop out over an area of 65 000 km 2 previous interpretations of the bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies however the mafic sequences in these arcuate western and eastern limbs are remarkably similar with at least six petrologically distinctive layers and sequences being recognisable in both limbs such similarity of sequences in two totally discrete bodies 200 300 km apart is petrologically implausible and it is suggested that they formed within a single lopolithic intrusion all previous bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km with this revised whole crustal model it becomes possible to construct a gravity model consistent with observed data which includes a 6 km thick sequence of mafic rocks connecting the western and eastern limbs of the bushveld complex the exact depth at which the mafic rocks of the bushveld complex lie in the centre of the structure cannot be constrained by the gravity data such a first order model is an approximation because there have been subsequent deformation and structural readjustments in the crust some of them probably related to the emplacement of the bushveld complex specifically the observed geometry of the rocks around the crocodile river dennilton marble hall and malope domes suggests that major upwarping of the crust occurred on a variety of scales triggered by emplacement of the bushveld complex

the distribution of some platinum group elements in the ug 2 chromitite layer of the bushveld complex samples representing consecutive 2 cm thick slabs of a drill core through the ug 2 chromitite layer were analyzed for platinum group elements the results when plotted against height reveal an interesting pattern with sequences that follow exponential curves of the type y zx k where y equals the concentration of the element in samples taken at a distance x from the base of the layer and z is a constant the data fit the curves with correlation coefficients of better than 0 90 and levels of significance less than 0 1 percent which shows that the fit cannot be due to chance but that a process following or approximating an exponential equation must have been involved the most probable such process is fractionation according to the rayleigh law it is postulated that these sequences that follow the rayleigh law formed by bottom crystallization when the magma was in motion calculations indicate that the elements supplied to the site of crystallization were withdrawn from a surprisingly small thickness of magma and that their supply was not limited by diffusion between the sequences that follow the rayleigh law are sequences that probably crystallized from stagnant magma so that the supply of elements to the site of crystallization was diffusion controlled in these sequences the ratios between the different platinum group elements change progressively it is argued that these results reveal that the platinum group minerals crystallized very early in ug 2 layer type deposits and show a pattern of element behavior conforming closely with that of a classical compatible element which has a very large bulk distribution coefficient therefore the well known processes of enrichment applicable to such elements during processes of melting and crystallization could have been operative this one example is not sufficient to disclaim the possibility that platinum group elements can also behave like incompatible elements under different conditions as is assumed by many authors in this field

useful characteristics of shallow and deep marine csem responses inferred from 3d finite difference modeling hydrocarbon reservoirs can be mapped if sufficient resistivity contrastsexistbetweenthemandtheirconfininglayers butpractical problems remain in target discrimination in deep and shallowwaters especiallyinthepresenceofheterogeneousoverburden wehavedevelopedanefficient3dstaggered gridfinite difference controlled source electromagnetic csem modeling code that enables study of the physics underlying some practical problems we undertook a comparative analysis of reservoir detection in 2000 m and 300 m deep waters using the simulated electric and magnetic field responses of a simple 3d reservoir we examined the effect of two types of near surface heterogeneitymimickingdisconnectedgascloudsand orpatchygeochemicalalterationhalosonthe3dreservoirresponse wefoundthat small scale shallowheterogeneitiescausedistortionsthatarealmost independent of the source frequency these persist at all source receiver offsets in the electric amplitude response in deep and shallow waters and phase response in shallow water they decrease in magnitude with increasing offset in deepwater phase response large scale near surface heterogeneities distort the horizontal electric field response more significantly than the small scale ones but the near surface response gets smaller in amplitude as the offset increases the distortions in shallow water are much smaller in magnitude than those for the deepwater case so that the reservoir signatures still are visible on the responseprofiles thismightbeconsideredasapositivefeaturefor shallow water inline electric field exploration the magnetic field responses for the orthogonal direction provide diagnostic target signatures that are similar to the inline electric field responsesindeepwaterbutthataredifferentinshallowwater the magnetic responses are affected by the airwave in a different manner from the electric field suggesting that combined 3d electric and magnetic field analysis might be vital for handling theairwaveproblem

the distribution of chalcophile and platinum group elements in the ug 2 chromitite layer of the bushveld complex a ug 2 chromitite sample contains two sequences in which the concentrations of cu ni s pt pd rh ru and ir conform to a process of fractionation according to the rayleigh law both of these sequences are followed by others in which the trends are almost horizontal from the sequences that conform to the rayleigh law one can calculate the relations between the possible thicknesses of the magma column that gave rise to the ug 2 layer the bulk distribution coefficients and the original concentrations of the elements involved it appears that the elements were withdrawn from a surprisingly thin layer of magma between the first and the second cycles certain elements have been depleted and others enriched which indicates a heave of new magma rather than the recirculation of old partly depleted magma by a process like convective overturn the fact that s and the chalcophile elements cu and ni were concentrated in the same way and at the same time as the platinum group elements shows that the same collector was probably involved therefore sulfide could not have acted as a collector for the platinum group elements it is thought that chromite or a precursor of chromite was involved in some way with the collection process

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

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

anticrack nucleation as triggering mechanism for snow slab avalanches snow slab avalanches are believed to begin by the gravity driven shear failure of weak layers in stratified snow the critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases however recent experiments show that the critical length of artificially introduced cracks remains constant or if anything slightly decreases with decreasing slope this surprising observation can be understood in terms of volumetric collapse of the weak layer during failure resulting in the formation and propagation of mixed mode anticracks which are driven simultaneously by slope parallel and slope normal components of gravity such fractures may propagate even if crack face friction impedes downhill sliding of the snowpack indicating a scenario in which two separate conditions have to be met for slab avalanche release

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

snow avalanche formation 1 snow avalanches are a major natural hazard endangering human life and infrastructure in mountainous areas throughout the world in many countries with seasonally snow covered mountains avalanche forecasting services reliably warn the public by issuing occurrence probabilities for a certain region however at present a single avalanche event cannot be predicted in time and space much about the release process remains unknown mainly because of the highly variable layered character of the snowpack a highly porous material that exists close to its melting point the complex interaction between terrain snowpack and meteorological conditions leading to avalanche release is commonly described as avalanche formation it is relevant to hazard mapping and essential to short term forecasting which involves weighting many contributory factors alternatively the release process can be studied and modeled this approach relies heavily on snow mechanics and snow properties including texture while the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner the knowledge of the quantitative relation between snow texture and mechanical properties is limited but promising developments are under way fracture mechanical models have been applied to explain the fracture propagation and micromechanical models including the two competing processes damage and sintering have been applied to explain snow failure there are knowledge gaps between the sequence of processes that lead to the release of the snow slab snow deformation and failure damage accumulation fracture initiation and fracture propagation simultaneously the spatial variability that affects damage fracture initiation and fracture propagation has to be considered this review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale from the size of a bond between snow grains to the size of a mountain slope will continue to be very challenging

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

dimensions of dry snow slab avalanches from field measurements 1 dry slab avalanches release by a sequence of propagating fractures in this paper i provide field measurements of the important length parameters resulting from fractures for hundreds fallen slabs including depth length and width the field data show wide variations in these parameters probability plots of all dimensions length width and depth suggest they approximately obey log normal probability density functions given slab dimensions two applications are considered on the basis of the field data these applications are 1 estimates of total fracture energy consumed around the perimeter of the slab are given where it is presumed fracture is mostly mode i and along the base weak layer of the slab where shear fracture mode ii and iii is expected for average characteristic dimensions the analysis suggests that energy consumed on the perimeter is somewhat less than in the weak layer even though fracture energy around the perimeter is expected to be higher than in the weak layer the larger area fractured in shear at the base of the slab compared to area fractured in tension around the perimeter results in a comparable amount of total energy needed around the perimeter 2 approximate estimates of avalanche mass for average characteristic dimensions based on slab depth d the only length possibly known prior to avalanching are made the mass is considered related to destructive potential and simple guidelines are given to estimate mass in relation to d with validation by considering concurrent size and estimates of d by mountain guides

fracture mechanical models of dry slab avalanche release experimental evidence shows that snow is a pressure sensitive dilatant strain softening material in slow constant rate shear deformation when strain softening initiates in a weak layer underneath a snow slab avalanche release is hypothesized to be possible with or without additional loading specifically two cases are discussed 1 a shear crack like disturbance can initiate after formation of a slip surface in the weak layer and traverse the layer by a self propagating progressive failure with or even without loading and 2 a self propagating shear instability can develop when a region of the weak layer is driven past peak shear strength by loading for these cases the implied fracture sequence fracture geometry time scale of release and temperature effects are discussed and shown to be consistent with the known facts of dry slab avalanche release

the chronology of magma influxes to the eastern compartment of the bushveld complex as exemplified by its marginal border groups the floor contact of the eastern compartment of the bushveld complex has been mapped in detail for 130 km field relations petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated these represent the 4 main magma influxes which formed the malic phase of the bushveld complex and are in chronological order 1 a contaminated magnesian basalt b1 which gave rise to a saucer shaped laminated marginal zone at the base of the complex with diverse related sills 2 a harrisitic peridotite representing quenched lower zone magma 3 a border group to the more evolved probably basaltic critical zone magma which was intruded in two phases an early quenched norite facies n being followed by a chilled bulk critical zone facies b2 which forms a skin to the whole critical zone finally 4 a border group to the main and upper zone b3 which represents an early magnesian pulse of this last immense volume of tholeiitic liquid there is also evidence for minor influxes of magma within the critical zone and main zone

cryptic variations within chromitites of the upper critical zone northwestern bushveld complex the close association of platinum group elements with chromite in the upper critical zone calls for the appraisal of structures textures and composition of chromitites in the development of genetic models cryptic variations in successive chromitite layers of the ug 1 unit are consistent with fractionation of limited melt volumes individual chromitite bands within the composite ug 2 chromitite layer exhibit distinctive compositions but all display long range compositional trends with height superimposed on short range homogeneity accessory chromites are substantially different from massive chromitites in composition reaction with residual liquids and subsolidus reactions were significant recrystallization of chromitites was ubiquitous but the importance of in situ growth is shown by relict textures long range settling adcumulus growth or deposition from density currents are processes that find little support in the data presented

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

mapping thin resistors and hydrocarbons with marine em methods insights from 1d modeling the use of marine controlled source electromagnetic em csem sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling we examine the sensitivity of the csem method to such layers with forward and inverse modeling in one and three dimensions the 3d modeling demonstrates that if both source and receivers are over a tabular 3d target 1d modeling predicts the observed response to very high accuracy experimental design can thus be based on 1d analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure modeling in three dimensions shows that the vertical electric field response is largest over the edges of a 3d target the 3d modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by csem sounding a simple air wave model energy propagating from source to receiver via the atmosphere allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range frequency and seafloor resistivity unlike dc resistivity sounding the marine csem method is not completely t equivalent and in principle can resolve resistivity and thickness separately smooth inversion provides an estimate of the method s resolving power and highlights the fact that although the radial csem fields contain most of the sensitivity to the thin resistive target inverted alone they produce only increasing resistivity with depth inclusion of the radial mode csem data forces the recovery of the thin resistor but magnetotelluric data can be used more effectively to achieve the same result

laboratory experiments on shear failure of snow using a direct simple shear apparatus snow samples 115 mm in diameter 16 18 mm in height taken from a so called homogeneous layer small rounded particles density 290 kg m 3 were tested in a cold laboratory experiments were performed for strain rates between 7 10 6 s 1 and 5 10 3 s 1 at test temperatures of 5 c 10 c and 15 c the effects of strain rate and temperature on failure stress failure strain stiffness initial tangent modulus and toughness were studied the transition between the ductile and brittle sudden fracture state of failure was found to be at about 1 10 3 s 1 for the snow types tested independent of temperature stiffness proved to be the most temperature dependent property of alpine snow it strongly increases with decreasing temperature failure strain and toughness decrease with decreasing temperature failure stress was found to increase slightly with decreasing temperature the effect is not very distinct but close to statistically significant and might be partly hidden by the scatter in the stress data due to variations inherent in sampling and testing

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

influence of weak layer heterogeneity on snow slab avalanche release application to the evaluation of avalanche release depths slab avalanches are generally caused by the collapse of a weak layer underlying a cohesive slab the two key ingredients for the mechanical description of such slab avalanche releases are the heterogeneity of the weak layer and the redistribution of stresses via the elasticity of the slab the finite element code cast3m is used to build a complete full scale mechanical model of the slab weak layer system including inertial effects the weak layer is modeled as a cohesive mohr coulomb interface with cohesion softening which accounts for shear induced collapse the overlying slab is represented by an elastic layer the system is loaded by increasing the slope angle until rupture we first study the influence of a single weak spot in the weak layer in order to validate the model against analytical solutions the interaction between two weak spots is also analyzed the case of heterogeneous weak layers represented through gaussian stochastic distributions of the cohesion with a spherical spatial covariance is then studied several simulations for different realizations of the heterogeneity of the weak layer are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is examined in particular a heterogeneity smoothing effect caused by slab elasticity and characterized by a typical length scale of the system is evidenced finally the model is coupled with extreme snowfall distributions belonging to the gev class which allows to recover with very good accuracy release depth distributions obtained from field data

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

measurement of crack face friction in collapsed weak snow layers 1 in this letter we analyze the frictional contact forces during and immediately after the collapse of a weak snowpack layer when the sliding plane consists of the freshly collapsed and crushed but not yet eroded granular debris of the weak layer the results from thirty four field experiments show that frictional contact forces per unit area are on the order of 0 6 times the normal stress equivalent to a friction angle close to 30 degrees the measurements show that there is a transient sharp drop in the coefficient of friction during the collapse of the weak layer and relatively constant values afterwards one implication of our findings is that the minimum angle for avalanche release does not depend on shear strength as commonly thought but results from crack face friction which comes into play only as the fracture through the weak layer is already propagating