the mineralogy petrology and origin of the merensky cyclic unit in the western bushveld complex the merensky cyclic unit of the bushveld complex represents the first products to crystallize after the influx and mixing of a large new batch of magma in the chamber excluding the merensky pegmatoid the merensky cyclic unit grades upward from an orthopyroxenite at the base through norite to anorthosite at the top of the sequence it is followed by the very similar bastard cyclic unit the mg mg fe 1 ratio of the pyroxenes changes relatively smoothly from about 0 80 at the base of the pyroxenite to 0 70 at the top of the anorthosite with no abrupt changes between rock types there is a suggestion of an upward increase in the average an content of the plagioclase from midpoint values of an 73 in the norite to an 76 at the top of the anorthosite there also appears to be a decrease in the range of an compositions 10 mole in the norite to 4 5 mole in the anorthosite upward in the sequence as determined by electron microprobe the merensky pegmatoid situated at the base of the cyclic unit commonly but not everywhere contains the nickel copper and platinum group minerals the pegmatoid shows evidence of high fluid activity such as pegmatoidal texture high biotite content redistribution of sulfide phases and the presence of unusual minor phases normally associated with hydrothermal activity variations in the mg fe ratio of the orthopyroxenes from the merensky cyclic unit indicate that this unit formed from a limited thickness of magma between 10 and 30 m whereas mass balance requires that the platinum group element ni and cu mineralization be derived from at least 4 to 5 times this volume of magma to explain these data several processes are inferred to have occurred initially a new influx of slightly more siliceous magma mixed with that already in the chamber by turbulent convection penecontemporaneous warping of the crystal pile and some erosion and solution of the footwall crystal mush occurred resulting in a disconformity the mixing process also led to the formation of dense immiscible sulfide droplets which scavenged platinum group elements ni and cu from the liquid and were then deposited on the floor of the chamber after the mixing event a stable diffusive regime i e a double diffusive convection system became operative as heat was lost from the top of the magma chamber the basal double diffusive convection layer about 10 30 m thick then started bottom crystallization of orthopyroxene resulting in the upward mg fe fractionation observed plagioclase subsequently joined orthopyroxene on the liquidus but this phase nucleated and grew within the convecting liquid which caused convection in the layer to slow as the viscosity sharply increased orthopyroxene continued to crystallize from the bottom up and enclosed small plagioclase crystals in large sievelike orthopyroxene oikocrysts or mottles the plagioclase crystals not enclosed by pyroxene started to impinge mutually and sinter to form large polygonal grains penecontemporaneous infiltration of material from below the merensky unit modified the isotopic and chemical character of the crystal pile by mixing with and displacing the preexisting interstitial liquid the merensky pegmatoid is inferred to postdate both the initial mineralization and formation of the other units in the merensky cyclic unit we conclude that the pegmatoid is the result of trapping of the final late fluids expelled from the footwall rocks this fluid reacted with the basal part of the merensky pyroxenite and also redistributed the platinum group elements ni and cu to form the coarse pegmatoid and its assemblage of platinum group minerals and other sulfide minerals

in situ strontium and sulfur isotope investigation of the ni cu pge sulfide ore bearing kevitsa intrusion northern finland the 2 06 ga kevitsa mafic ultramafic intrusion in northern finland hosts a large disseminated ni cu pge deposit the deposit occurs in the ultramafic olivine pyroxene cumulates and shows a range in ni tenors varying from 4 7 wt regular ore to 10 wt ni pge ore there are also a metal poor sulfide mineralization false ore and contact mineralization that are uneconomic ni tenor 0 7045 in comparison to the estimated depleted mantle sr isotope ratio of 0 702 at 2 06 ga the sulfur 34s values are generally higher than 2 which together with the sr isotope data imply involvement of crustal material in the genesis of the kevitsa intrusion and its ores the 87sr 86sr i values obtained from the ore bearing domain of the intrusion show stratigraphic variation and exceed 0 7050 with the maximum value reaching up to 0 7109 in contrast in rocks around the ore domain the initial sr isotope compositions remain more or less constant 0 7047 0 7060 throughout the intrusive stratigraphy the isotope data suggest that the ore bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma whereas the surrounding part of the intrusion experienced a less vigorous emplacement history no correlation is observed between the strontium and sulfur isotope compositions this is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the kevitsa magma chamber the low level of metals in false ore and the ni depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement cannibalization of early formed sulfides by later magma injections may have been important in the formation of the economic ore deposit

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

the geology and geochemistry of the waterfall gorge section of the insizwa complex with particular reference to the origin of the nickel sulfide deposits the jurassic aged mafic rocks from the waterfall gorge section of the insizwa layered complex transkei range in composition from olivine gabbro and picrite basal zone through a series of gabbros central zone to monzonites roof zone the compositional data for the mineral phases olivine plagioclase bronzite and augite show significant features in the basal gabbro the trend in compositional variation is the reverse of that seen in the central zone of the intrusion and olivines from the picrite unit are the most forsteritic in the intrusion but they are depleted in ni compared to olivines from the basal gabbro and central zone the data for olivines from the picrite unit at tabankulu exhibit a similar but more significant degree of depletion mineral compositional and whole rock geochemical data for the basal olivine gabbro suggest that olivine control is primarily responsible for the compositional variation in this unit at insizwa however to account for the upward increase in the forsterite content of the cumulus olivine at the base the intrusion of successively less fractionated batches of a low mgo magma is suggested we suggest that the depletion of the olivines in ni can be attributed to the equilibration of the parent magma with sulfide and we show that these data are consistent with experimental distribution coefficient data for ni between olivine and sulfide permitting broad constraints to be placed on the grade as well as the tonnage and location of the ore extrapolation of these conclusions to the tabankulu intrusion suggest that the highly ni depleted olivines of the picrite have equilibrated with a magma from which a particularly high proportion of sulfide has segregated geophysical data fail to detect a massive orebody at the base of the intrusion though several small bodies may exist it is suggested that saturation may have occurred at depth and only a small amount of sulfide has been transported to the surface in the picrite and concentrated in depressions at the base of the intrusion massive and disseminated magmatic ni sulfide mineralization is confined to the lower basal zone and shows a consistent fractionation into pyrrhotite pentlandite rich ore at the base and chalcopyrite rich ore at the top in both massive sheets and globular ore the cause of sulfur saturation cannot be attributed to the reduction of the solubility of sulfur in the magma by increasing the sio 2 concentration contamination despite the close association of the ore with granophyric segregations in the basal olivine gabbro indeed the major and trace element data argue strongly against wholesale assimilation of crust at any stage in the production of the basal gabbro

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

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

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

post equinox dynamics and polar cloud structure on uranus abstract post equinox imaging of uranus by hst keck and gemini telescopes has enabled new measurements of winds over previously sampled latitudes as well as measurements at high northern latitudes that have recently come into better view these new observations also used techniques to greatly improve signal to noise ratios making possible the detection and tracking of more subtle cloud features the 250 m s prograde jet peaking near 60 n was confirmed and more accurately characterized several long lived cloud features have also been tracked the winds pole ward of 60 n are consistent with solid body rotation at a westward prograde rate of 4 3 h with respect to uranus interior when combined with 2007 and other recent measurements it is clear that a small but well resolved asymmetry exists in the zonal profile at middle latitudes peaking at 35 where southern winds are 20 m s more westward than corresponding northern winds high s n keck ii imaging of the north polar region of uranus reveals a transition from streaky bands below 60 n to a region from 60 to nearly the north pole where widely distributed small bright spots resembling cumulus cloud fields with several isolated dark spots are the dominant style of cloud features this presents a stark contrast to 2003 detailed views of the south polar region of uranus when no discrete cloud features could be detected in comparable keck ii near ir images the pressure levels of discrete clouds estimated from spatial modulations in h and hcont images indicate that the polar cloud features are generally in the 1 3 to 2 3 bar range as are equatorial and several mid latitude features several of the brighter mid latitude features are found above the 1 2 bar level of methane condensation

simulation of primary phase relations and mineral compositions in the partridge river intrusion duluth complex minnesota implications for the parent magma composition in order to describe the composition and crystallinity of the initial parental magma of the partridge river intrusion of the keweenawan duluth complex and thereby understand the mode of emplacement and solidification of the intrusion we have applied a numerical simulation technique called geochemical thermometry frenkel et al 1988 the parental magma was a low alumina high ti p olivine tholeiite similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex and from the nearby portage lake area of the lake superior region the parental magma was emplaced as a crystal liquid suspension followed by chilling of an evolved leading edge ferrodioritic liquid in the basal zone of the intrusion the conditions of emplacement at the present crustal location were 1 150 c 2 kbar and fo2 slightly above the wustite magnetite wm buffer the main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine calculated crystallization sequence for the parental magma is olivine plagioclase 1 240 c olivine plagioclase magnetite 1 146 c wm 0 5 olivine plagioclase magnetite augite 1 140 c wm 0 5 the calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1 150 c are fo66 7 1 1 and an64 5 2 5 respectively and are similar to the estimated average composition of primary olivine fo69 1 2 8 and the average composition of plagioclase core an66 3 2 8 measured in drill core samples through the intrusion chalokwu and grant 1987

a strontium isotope and trace element profile through the partridge river troctolite duluth complex minnesota initial 87sr 86sr ratios down a 2 315 foot long core ddh 295 through the partridge river troctolite are lower in the depth range 710 1 410 feet 0 70443 22 than in the underlying and overlying rocks 0 70500 18 these initial ratio variations define three zones i ii and iii thought to be three intrusive units within the troctolite the best estimate of the rb sr age of the troctolite is 1 050 105 m y the olivine compositions and the abundances of the incompatible trace elements zr y and rb show identical variations down the core with downward repetitions of olivine iron enrichment and incompatible trace element enrichments discontinuities in the olivine compositions and incompatible trace element abundances match the initial 87sr 86sr ratio discontinuities in the core

experimental data at 3 kbars pressure on parental magma to the bushveld complex experimental studies mainly under 3 kbars pressure have been undertaken on representative samples to determine if any of these compositions could be parental magma to the bushveld complex one such composition with 12 5 mgo mg mg fe of 0 72 and quartz normative crystallizes olivine fo88 as liquidus mineral at about 1 300 c followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure there is a temperature drop of over 100 c before the appearance of plagioclase and finally clinopyroxene this crystallization sequence is in excellent agreement with the observed sequence in the lower part of the bushveld complex

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

an origin of small scale fluctuations in orthopyroxene composition in the lower and critical zones of the bushveld complex south africa abstract numerous small and apparently haphazard reversals in mg fe ratio of orthopyroxene grains have been reported by e n cameron in the lower and lower critical zones of the bushveld complex a hypothesis for their origin is presented by extending the suggestion of cameron that modal variation in layered complexes may be attributed to pressure fluctuation an increase in pressure will cause the magma to be supercooled thus speeding up crystallization rates this results in a greater proportion of interstitial liquid being trapped in the cumulate pile on crystallization this liquid adds relatively fe rich orthopyroxene to the cumulus of mg rich phase homogenization produces minerals whose compositions are displaced from the cumulus composition towards the low temperature end member by an amount proportional to the percent of interstitial liquid decreases in pressure produce the opposite effect and may explain the origin of pure adcumulate rocks quantitative data are presented from the lower zone critical zone contact and from the duluth complex to demonstrate this effect

analytical method for predicting the mobility of slow moving landslides owing to groundwater fluctuations this paper deals with the active landslides that are controlled by pore water pressure changes owing to groundwater fluctuations these landslides are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone above which the unstable soil mass moves as a rigid body taking advantage of some original analytical solutions a method is developed for a preliminary prediction of the landslide mobility this method is based on a simple sliding block model and allows the landslide velocity to be readily evaluated once pore water pressure measurements are available an application to a case study documented in the literature is also shown

a grenville age layered intrusion in the subsurface of west texas petrology petrography and possible tectonic setting the nellie intrusion is a thick more than 4420 m mafic to ultramafic layered intrusion with a radiometric age of 1163 ma rock types change abruptly with stratigraphic height and include norite pyroxenite gabbronorite hornblende gabbro gabbro anorthosite harzburgite and lherzolite norite is most abundant but gabbro and hornblende gabbro are locally abundant rare olivine rich layers are also present the general order of crystallization was olivine orthopyroxene plagioclase clinopyroxene and hornblende mg s expressed as 100 mg mg fe range from 76 3 to 85 8 for olivine 56 7 to 84 9 for orthopyroxene 62 5 to 90 3 for clinopyroxene and 52 4 to 82 8 for amphibole mg s vary with height and display abrupt reversals which indicate open system addition of new mafic magma eleven cyclic units were identified on the basis of evidence for injection of basaltic magma these can be grouped into three megacyclic units the abundance of orthopyroxene and mineral compositional evidence for

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