regionally differentiated contribution of mountain glaciers and ice caps to future sea level rise the contribution to sea level rise from mountain glaciers and ice caps has grown over the past decades a projection of their melting during the twenty first century based on temperature and precipitation projections from ten climate models suggests that by 2100 these glaciers will lose about 21 of their total global volume

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

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

potholes of the merensky reef at brakspruit shaft rustenburg platinum mines primary disturbances in the magmatic stratigraphy potholes are locations where parts of the magmatic stratigraphy are missing in the foot wall sequence of the merensky reef in general potholes are circular to elliptical in plan and funnel shaped in cross section within narrow limits most potholes reveal the same three dimensional orientation to the strike and dip of the magmatic layering the updip pothole flank is steep to vertical the downdip flank is horizontal the pothole bottom generally coincides with one of the marker horizons of the footwall stratigraphy and is conformable with layering where the merensky reef fills potholes it is more variable in thickness than it is at its normal stratigraphic level in the potholes it changes over short distances from an unusually thin chromite rich contact phase of the reef to thick unlayered lenses of harzburgite pegmatite in deep seated parts of the potholes when the bastard reef is observed in potholes it sometimes grades into massive graphite rich transgressive gabbroic pegmatite the marker horizons in the anorthositic footwall units around potholes are often intensely mineralized with stratiform and crosscutting sulfides such marginal footwall mineralization is absent in the undisturbed footwall sequence away from potholes potholes are traditionally considered to be resorption pits due to the influx of a new hot primitive magma at the merensky reef level or to represent scars of a late magmatic fumarole activity the present study suggests that cumulate resorption played a minor role only potholes are sites of nondeposition where the missing rock units did not crystallize they formed where locally high concentrations of dissolved c h o s volatiles lowered the liquidus temperature of the magma and suppressed the crystallization of cumulus plagioclase potholes seem to be concentrated above anticlines of the metasedimentary basement rocks in the footwall of the bushveld complex volatiles derived by pyrometamorphism were focused upward into the overlying melt above such topographic highs and were accumulated at the level of the merensky reef

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

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

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

defining modern day area altitude balance ratios aabrs and their use in glacier climate reconstructions abstract with the increasing use of digital elevation models in palaeo glacier reconstructions and the availability of freeware spreadsheets the area altitude balance ratio aabr and balance ratio br methods are becoming increasingly used in palaeo glacier reconstruction for estimating equilibrium line altitudes ela and subsequently deriving quantitative estimates of palaeo climate while there are many data detailing contemporary accumulation area ratios there are still only a few studies that have established from contemporary environments aabr br ratios publicly available glacier mass balance world glacier monitoring service us geological survey and norwegian water directorate and spatial extent datasets provided the basis for this research from a time series of mass balance regressing specific net balance against ela allows the zero net balance ela to be identified once the zero balance ela is established the glacier hypsometry above and below the ela is defined the aabr br is calculated by using the right hand side the following aabr b n a b b n a c z a c a a c z a b a a b where b nab and b nac are the net mass balance gradients in the ablation and accumulation zones respectively z a c and z a b are the area weighted mean altitudes of the accumulation and ablation areas respectively and a ac and a ab are the areas of the accumulation and ablation areas respectively aabrs are calculated for a suite of glaciers located across a range of climatic zones and glacier types with antarctica being excluded the following representative aabrs are found a global aabr 1 75 0 71 mid latitude maritime 1 9 0 81 high latitude 2 24 0 85 north america west coast 2 09 0 93 north america eastern rockies 1 11 0 1 canadian arctic 2 91 0 35 svalbard 2 13 0 52 western norway 1 5 0 4 european alps 1 59 0 6 central asia 1 75 0 56 kamchatka 3 18 0 16 this study provides an empirically derived dataset characterising aabr ratios which may be used for ela estimation in palaeo glacier reconstructions and for palaeo climate quantification

application of inventory data for estimating characteristics of and regional climate change effects on mountain glaciers a pilot study with the european alps a parameterization scheme using simple algorithms for unmeasured glaciers is being applied to glacier inventory data to estimate the basic glaciological characteristics of the inventoried ice bodies and simulate potential climate change effects on mountain glaciers for past and potential climate scenarios glacier changes for assumed mass balance changes are calculated as step functions between steady state conditions for time intervals that approximately correspond to the characteristic dynamic response time a few decades of the glaciers in order to test the procedure a pilot study was carried out in the european alps where detailed glacier inventories had been compiled around the mid 1970s total glacier volume in the alps is estimated at about 130 km3 for the mid 1970s strongly negative mass balances are likely to have caused a loss of about 10 20 of this total volume during the decade 1980 90 backward calculation of glacier length changes using a mean annual mass balance of 0 25m w e a 1 since the end of the little ice age around 1850 ad gives considerable scatter but satisfactory overall results as compared with long term observations the total loss of alpine surface ice mass since 1850 can be estimated at about half the original value an acceleration of this development with annual mass losses of around 1 m a 1 or more as anticipated from ipcc scenario a for the coming century could eliminate major parts of the presently existing alpine ice volume within decades

mass balance and a glacier surge of guliya ice cap in the western kunlun shan between 2005 and 2015 abstract contemporary estimates of glacier changes are necessary to assess the impact of climate change associated hazards and water resources management glaciers in high mountain asia hma are mostly retreating except the karakoram and western kunlun shan confirmed by remote sensing measurements however ground validation with precise measurements of these mass balance estimates are scarce this study selected guliya ice cap in the western kunlun shan to observe its recent changes regarding surface dynamics and mass balance using aster dem of 2005 and 2015 our findings indicate that one of the north facing glaciers surged with no previous surging history during july and early november 2015 advancing at about 8 m per day on average the mass balance shows a balance condition 0 01 0 02 m w e a 1 the icesat data 2004 2008 at selected locations compared to dgps data collected in the field during 2015 indicating a minute difference of 0 03 0 05 m a 1 with aster data in the same area our findings suggest that guliya ice cap in the western kunlun shan is in equilibrium state following a similar pattern mass gain or stable conditions as observed previously

estimating equilibrium line altitude ela from glacier inventory data a glacier s most fundamental altitude is the equilibrium line altitude ela because it divides the glacier into ablation and accumulation areas the best parameterization of the ela for glacier inventory is the balanced budget ela we discuss direct estimation of balanced budget ela from mass balance data for individual glaciers and indirect estimation of balanced budget ela from simple topographic parameters available from the world glacier inventory wgi i e the area median and maximum and minimum altitudes mass balance and ela for individual glaciers are usually strongly correlated and we calculate balanced budget ela from the regression equation linking the two we then compare balanced budget ela with area median and mid range altitudes for the 94 glaciers for which we have all the necessary data the different ela estimates agree well enough 82 to 125 m to describe geographical variations in ela and for application of glacier climate models to glacier inventory data mid range and area median altitudes are already available for tens of thousands of glaciers in the current wgi and should be evaluated in future inventories

a macroscopic approach to glacier dynamics a simple approach to glacier dynamics is explored in which there is postulated to be a relationship between area and volume with three parameters the time for area to respond to changes in volume a thickness scale and an area characterizing the condition of the initial stale this approach gives a good fit to the measurements of cumulative balance and area on south cascade glacier from 1970 97 the area time scale is roughly 8 years the thickness scale about 123 m and the 1970 area roughly 4 larger than required for adjustment with volume combining this relationship with a version of mass continuity expressed in terms of area and volume produces a theory of glacier area and volume response to climate in which another time constant the volume time scale appears area and volume both respond like a damped spring and mass system the damping of the south cascade response is approximately critical and the volume time scale is roughly 48 vexes six times the area time scale the critically damped spring and mass analogy reproduces the time dependence predicted by the more complicated traditional theory of nyc

the physical basis of glacier volume area scaling ice volumes are known for only a few of the roughly 160 000 glaciers worldwide but are important components of many climate and sea level studies which require water flux estimates a scaling analysis of the mass and momentum conservation equations shows that glacier volumes can be related by a power law to more easily observed glacier surface areas the relationship requires four closure choices for the scaling behavior of glacier widths slopes side drag and mass balance reasonable closures predict a volume area scaling exponent which is consistent with observations giving a physical and practical basis for estimating ice volumes glacier volume is insensitive to perturbations in the mass balance scaling but changes in average accumulation area ratios reflect significant changes in the scaling of both mass balance and ice volume

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

response time of glaciers as a function of size and mass balance 1 theory a simple interpretation of the traditional definitions of glacier and ice sheet response time e g thickness divided by mass balance rate h b suggests that larger glaciers respond more slowly than small glaciers to a perturbation in climate however with reasonable choices for mass balance behavior a scaling analysis shows that the response time of valley glaciers decreases as a function of increasing size when other variables are held constant in essence this is because larger valley glaciers push further into the ablation zone and ablation increases more rapidly than the thickness so that h b gets smaller ice sheets have different mass balance regimes than valley glaciers and as they grow larger the ablation does not increase faster than the thickness therefore as ice sheets grow in surface area the response time increases for both ice sheets and valley glaciers the response time also depends on a mass balance index which is defined as the slope of the balance curve as a function of horizontal distance along the glacier surface rather than elevation the response time decreases as the balance index increases so for valley glaciers an increase in the balance index and an increase in glacier size have opposite effects on the response time the balance index is typically larger for smaller valley glaciers therefore a small glacier will typically respond faster than a large glacier but this quicker response is because of mass balance considerations and not because of the dynamic characteristics of the glacier arising from its small size as often assumed

improved structural interpretation of legacy 3d seismic data from karee platinum mine south africa through the application of novel seismic attributes abstract seismic detection of faults dykes potholes and iron rich ultramafic pegmatitic bodies is of great importance to the platinum mining industry as these structures affect safety and efficiency the application of conventional seismic attributes such as instantaneous amplitude phase and frequency in the hard rock environment is more challenging than in soft rock settings because the geology is often complex reflections disrupted and the seismic energy strongly scattered we have developed new seismic attributes that sharpen seismic reflections enabling additional structural information to be extracted from hard rock seismic data the symmetry attribute is based on the invariance of an object with respect to transformations such as rotation and reflection it is independent of the trace reflection amplitude and hence a better indicator of the lateral continuity of thin and weak reflections the reflection continuity detector attribute is based on the hilbert transform it enhances the visibility of the peaks and troughs of the seismic traces and hence the continuity of weak reflections we demonstrate the effectiveness of these new seismic attributes by applying them to a legacy 3d seismic data set from the bushveld complex in south africa these seismic attributes show good detection of deep seated thin 1 5 m thick platinum ore bodies and their associated complex geological structures faults dykes potholes and iron rich ultramafic pegmatites they provide a fast cost effective and efficient interpretation tool that when coupled with horizon based seismic attributes can reveal structures not seen in conventional interpretations

full wave effects on shear wave splitting in this chapter we develop an approach to the inversion of 3d anisotropy structure using the sensitivity frechet kernels calculated by an efficient and flexible full wave algorithm based on the normal mode theory

what do glaciers tell us about climate variability and climate change glaciers respond to long term climate changes and also to the year to year fluctuations inherent in a constant climate differentiating between these factors is critical for the correct interpretation of past glacier fluctuations and for the correct attribution of current changes previous work has established that century scale kilometre scale fluctuations can occur in a constant climate this study asks two further questions of practical significance how likely is an excursion of a given magnitude in a given amount of time and how large a trend in length is statistically significant a linear model permits analytical answers wherein the dependencies on glacier geometry and climate setting can be clearly understood the expressions are validated with a flowline glacier model the likelihood of glacier excursions is well characterized by extreme value statistics although probabilities are acutely sensitive to some poorly known glacier properties conventional statistical tests can be used for establishing the significance of an observed glacier trend however it is important to determine the independent information in the observations which can be effectively estimated from the glacier geometry finally the retreat of glaciers around mount baker washington state usa is consistent with but not independent proof of the regional climate warming that is established from the instrumental record

noble metals in the marginal rocks of the bushveld complex two major subdivisions of marginal rocks occur in the eastern bushveld complex a pyroxenitic group borders the lower and lower critical zones and a gabbroic group borders the upper critical and main zones three main facies of the gabbroic group can be recognized on the bases of field relations petrography and chemical composition five sampling sites were chosen that would best represent the varieties of marginal rocks by study of the field relations and bulk rock compositions of 184 previously analyzed samples these sites were resampled and the rocks analyzed for major and trace elements and the noble metals ir rh pt pd and au the field relations petrography and mineral and bulk rock composition of the samples are presented the range in noble metal abundances is ir 0 2 to 0 6 rh 3 to 12 pt 30 to 50 pd 10 to 20 and au 10 to 40 ppb the values are considerably higher than corresponding abundances in most basalts and chilled margins of other layered complexes especially for au all marginal rocks have broadly similar chondrite normalized noble metal distribution patterns with enrichment in rh and au relative to pt and pd abundances of noble metals in a liquid from which the ug 2 chromitite layer and the merensky reef may have crystallized is calculated from marginal rock compositions assuming a conical magma chamber the liquid is composed of a approximately 52 48 percent pyroxenitic gabbroic mixture and has a composition of ir 0 41 rh 7 7 pt 40 pd 16 and au 21 ppb maximum distribution ratio values d super sulfide liquid super silicate liquid computed for the mixture assuming no fractional crystallization are between 1 8 and 26 times higher in the ug 2 chromite environment than in the merensky reef silicate environment distribution ratio values of ir 6 600 rh 1 100 pt 3 900 pt 4 000 au 480 and cu 480 were obtained for the merensky reef with 50 percent fractional crystallization and incompatible behavior of the metals the distribution ratios attain more realistic values of ir 3 300 rh 550 pt 1 950 pd 2 000 au 240 and cu 240

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

the petrogenesis of merensky reef potholes at the western platinum mine bushveld complex sr isotopic evidence for synmagmatic deformation potholes represent areas where the normally planar pge rich merensky reef of the upper critical zone of the bushveld complex transgresses its footwall such geometric relationships being unusual in layered intrusions the recognition of vertical dykes of merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull apart sites resulting from tensional deformation due to the loading effects of major new magma additions in contrast crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement the merensky reef and its footwall have distinct initial sr isotope ratios r 0 0 7066 and 0 7066 respectively which may be used to constrain these processes related to pothole formation merensky reef in potholes r 0 0 7069 0 7078 shows no isotopic evidence of assimilation of or reaction with footwall material discrete discordant replacement bodies of anorthosite extend from the footwall lithologies to cross cut the merensky reef and its hanging wall the initial sr isotope ratio in these replaced rocks is totally reset to footwall values r 0 0 7066 and immediately adjacent stratiform lithologies are slightly modified towards footwall values in contrast neptunian pyroxenitic merensky dykes cross cutting the footwall lithologies with a large surface area to volume ratio and low sr content do not display footwall like sr isotope initial ratios r 0 0 7077 and thus show no evidence for assimilation of or reaction with footwall material furthermore pegmatoidal replacement pyroxenite replacement pegmatoid at the base of the merensky reef within potholes has a high initial ratio r 0 0 7071 and so models of pervasive metasomatism by footwall material are not applicable this isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during or after the formation of merensky reef potholes a basic tenet of existing pothole formation hypotheses involving footwall mass transfer in contrast the isotopic data are entirely consistent with an extensional model for pothole formation with the more radiogenic merensky magma migrating laterally to fill extensional zones in the footwall layers

lowermost mantle anisotropy beneath the northwestern pacific evidence from pcs scs sks and skks phases 1 in this study we utilize data from broadband seismic stations of the japanese f net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern pacific the comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle such as pcs scs or sks skks can yield constraints on anisotropy in the d region we measured splitting for sks skks pcs and scs phases at f net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region we observed many examples of discrepant sks skks splitting and splitting of scs pcs sks and skks phases that does not agree well with the known upper mantle anisotropy beneath individual stations the most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle in particular for sks skks phases that sample the lowermost mantle just to the south and east of the kamchatka peninsula we observed generally n s fast directions with delay times between 0 5 and 1 5 s these data suggest the presence of a fairly large coherent region of deformation in the lowermost mantle beneath the northwestern pacific our preferred model for these observations is that solid state flow at the base of the mantle induces a lattice preferred orientation of lowermost mantle minerals generating a seismically anisotropic fabric

structures at the base of the upper group 2 chromitite layer bushveld complex south africa on karee mine lonmin platinum abstract exposures in a now infilled pit mined for platiniferous ug2 chromitite in the bushveld complex south africa are described the layer of chromitite is underlain by anorthosite providing a dramatic colour contrast the interface between these two rock types shows evidence of various scales of irregularities in plan view small circular depressions less than 3 cm across and 5 mm deep occupy about 20 of the surface between them the contact is planar the anorthosite immediately underlying the chromitite has a planar fabric visible in thin sections that is not disturbed beneath these small depressions another set of depressions occurs about 40 cm in diameter and with variable depth several possible processes are reviewed for the origin of these irregularities especially the small scale structures but none explains all the features noted these processes include remelting diapirism impact generated dimpling gas escape and interference rippling we present a photographic record of these structures but present no definitive model for their interpretation

distributed ice thickness and volume of all glaciers around the globe 1 a new physically based approach for calculating glacier ice thickness distribution and volume is presented and applied to all glaciers and ice caps worldwide combining glacier outlines of the globally complete randolph glacier inventory with terrain elevation models shuttle radar topography mission advanced spaceborne thermal emission and reflection radiometer we use a simple dynamic model to obtain spatially distributed thickness of individual glaciers by inverting their surface topography results are validated against a comprehensive set of thickness observations for 300 glaciers from most glacierized regions of the world for all mountain glaciers and ice caps outside of the antarctic and greenland ice sheets we find a total ice volume of 170 103 21 103 km3 or 0 43 0 06 m of potential sea level rise

the merensky cyclic unit and its impact on footwall cumulates below normal and regional pothole reef types in the western bushveld complex the merensky reef of the bushveld complex occurs in its highest stratigraphic position as a heterogeneous pegmatitic feldspathic melanorite bounded by two narrow chromitite stringers at the base of the merensky cyclic unit mcu in the swartklip facies of the rustenburg layered suite the occurrence of widespread thermal and mechanical erosion termed potholing has led to the subdivision of the merensky reef into normal reef and regional pothole reef sub facies the transition between the two sub facies occurs where the mcu transgresses the lower chromitite stringer of the normal merensky reef and cuts down into the underlying cumulate lithologies in the regional pothole reef at the northam platinum mine several economic reef types are identified where the merensky reef becomes conformable to cumulate layering in particular to the footwall marker np2 reef type and the upper pseudoreef p2 reef type the normal merensky reef as well as the p2 and np2 reefs contains economic platinum group element pge grades and includes the lower portion of the mcu melanorite and the merensky chromitite whole rock geochemistry indicates that this package is compositionally identical in normal p2 and np2 reefs suggesting that the base of the mcu is a relatively homogeneous drape over both normal and regional pothole reef regions however the lower sections of the three reefs are variables depending on the depth of transgression of the mcu in the normal and p2 reef types transgression by the mcu was arrested within harzburgites melanorites and norites resulting in coarse pegmatitic textures in the immediate footwall units for the np2 reef transgression by the mcu was arrested within leucocratic rocks and resulted in the formation of troctolites below the merensky chromitite these troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and pge contents relative to equivalent units in the footwall of the normal reef along with micro textural relationships these features suggest that troctolization of leucocratic cumulates in the np2 reef beneath the merensky chromitite was a result of a reactive infiltration of a chromite saturated melt and an immiscible sulphide liquid from the overlying mcu rather than a significant fluid flux from below in all reef types the concentration of s defines symmetrical peaks centred on the merensky chromitite and chromitites from pre existing cyclic units in normal and p2 reefs whereas pge concentrations define asymmetrical peaks with higher pge contents in reconstituted footwall rocks relative to the mcu melanorite this signature is attributable to a magmatic model of pge collection followed by deposition towards the base of the mcu and within reconstituted footwall rocks the continuity of the asymmetrical magmatic pge signature between the normal reef and regional pothole reef sub facies indicates that pge mineralization inherent to the merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall

the randolph glacier inventory a globally complete inventory of glaciers the randolph glacier inventory rgi is a globally complete collection of digital outlines of glaciers excluding the ice sheets developed to meet the needs of the fifth assessment of the intergovernmental panel on climate change for estimates of past and future mass balance the rgi was created with limited resources in a short period priority was given to completeness of coverage but a limited uniform set of attributes is attached to each of the 198 000 glaciers in its latest version 3 2 satellite imagery from 1999 2010 provided most of the outlines their total extent is estimated as 726 800 34 000 km 2 the uncertainty about 5 is derived from careful single glacier and basin scale uncertainty estimates and comparisons with inventories that were not sources for the rgi the main contributors to uncertainty are probably misinterpretation of seasonal snow cover and debris cover these errors appear not to be normally distributed and quantifying them reliably is an unsolved problem combined with digital elevation models the rgi glacier outlines yield hypsometries that can be com bined with atmospheric data or model outputs for analysis of the impacts of climatic change on glaciers the rgi has already proved its value in the generation of significantly improved aggregate estimates of glacier mass changes and total volume and thus actual and potential contributions to sea level rise

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

a regional test of global models for flow rheology and seismic anisotropy at the base of the mantle abstract the study of flow patterns and seismic anisotropy in the lowermost mantle is fraught with uncertainties given the limitations in our understanding of the physical properties of the lowermost mantle and the relationships between deformation and anisotropy here we use a set of sks skks and scs splitting measurements that sample the eastern edge of the african large low shear velocity province to test predictions of seismic anisotropy derived from previously published 3d global mantle flow models and anisotropy modeling walker et al 2011 the observations can be fit by a model that invokes flow directed to the southwest with a component of downwelling in our study region and slip that occurs along the 0 1 0 plane of post perovskite most importantly we demonstrate the ability of a regional shear wave splitting data set to test the robustness of models for flow and deformation in the lowermost mantle

an intrusive origin of some ug 1 chromitite layers in the bushveld igneous complex south africa insights from field relationships abstract the ug 1 chromitite in the bushveld complex is a 1 m thick massive layer overlain by orthopyroxenite 4 9 m thick and underlain by anorthosite 1 5 2 0 m thick which is strongly interleaved with numerous thin layers of ug 1 chromitite mm to 10 s of cm thick the chromitite bearing anorthosite is further underlain by mottled anorthosite which is devoid of chromitites sometimes the massive ug 1 chromitite may be hosted entirely within orthopyroxenite in this study however the orthopyroxenite occurs intermediate to the massive ug 1 layer and the footwall interleaved anorthosite unit which has a 10 cm thick chromitite layer at the contact though the ug 1 chromitites display complex features in the field outcrops e g presence of numerous anorthosite autoliths bifurcations and dimpled contacts with anorthosite the ones discussed in this study are distinctive and diagnostic of the emplacement mechanism for this chromitite layer in this study of the ug 1 chromitite at the impala platinum mine the chromitite is seen to form a small pothole 1 5 m deep 3 6 m wide that cuts down into the underlying orthopyroxenite exposed only on the right side of the pothole and anorthosite layers a 15 cm thick ug 1 chromitite layer 85 modal chromites is observed to protrude or branch out of the pothole margin and laterally extend into the anorthosite with which it shows several crosscutting features examples include truncation of layering defined by interstitial pyroxene mottles within the footwall anorthosite against the protruding ug 1 layer and truncation of several thinner ug 1 layers in the hanging wall anorthosite against erosive outlines of this chromitite layer these observations strongly attest to the emplacement of the protruding ug 1 chromitite layer as a sill into the anorthosite the presence of thin anorthosite lenses with high aspect ratios within this ug 1 layer along with lateral variations in thickness of the layer its irregular contact with the host anorthosite and disturbances observed for the ug 1 layers in the footwall anorthosite adjacent to the erosive contact implies that the superheated melt parental to the chromitite chemically dissolved the anorthosite and made space for it to flow as a sill within the latter the probable mode of emplacement of the laterally protruding ug 1 layer would be through the opening of fractures related to the event of pothole formation where there is ponding of hot thick and dense melt above the anorthosite the melt subsequently filled the fractures and flowed laterally into the anorthosite thereby eroding it an open magmatic system ensured continuous addition of melts in the pothole and their supply into the sill this study thus emphasizes that some of the ug 1 chromitite layers within anorthosite can be emplaced as sills and not all need to form from discrete cyclical pulses of chromite saturated and plagioclase saturated melts being brought into the chamber

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma

a pilot study of geochemical and redox relationships between potholes and adjacent normal merensky reef of the bushveld complex lateral and vertical variations of f sub o 2 and carbon in whole rock and f sub o 2 fe cr mg and al variations in separated chromites are employed to elucidate the chemical relationships between the merensky reef and associated structures termed potholes these chemical distributions are utilized to develop a model relating the origin of the potholes to the platiniferous merensky reef potholes are large roughly circular and or rarely irregular depressions up to hundreds of meters wide and tens of meters deep in the merensky reef footwall filled with merensky equivalent material pothole and adjacent merensky reef samples studied herein were the footwall anorthosite the overlying basal chromitite and the platinum bearing gabbroic pegmatoidal pyroxenite three potholes were analyzed in this study intrinsic oxygen fugacity analyses demonstrate that pothole bottoms are more reduced than adjacent normal merensky reef 10 super 13 9 10 super 12 1 at 1 150 degrees c and analyses show that noncarbonate inorganic carbon concentration is up to 14 times greater in the pothole bottom cation distributions in the basal chromitite also reflect the variation in f sub o 2 for example the ratio feo super feo super mgo where feo super total iron as feo increases from pothole bottom to adjacent normal merensky reef 0 78 0 84 the carbon and intrinsic oxygen fugacity gradients in addition to the variations in spinel chemistry are all consistant with more reduced conditions within the pothole as compared to the adjacent normal merensky reef for the studied parameters geochemical gradients not only exist along stratigraphically equivalent units lateral gradients but also exist between units vertical gradients various hypotheses which address the genesis of pothole formation are reviewed in light of these discovered gradients a model of potholes acting as plutonic fumaroles releasing reducing fluid rich in carbon and or sulfur is discussed as a possible mechanism for merensky reef petrogenesis the model integrates our geochemical data with published research from the literature to present an integration of ideas for platinum group element enrichment in the merensky reef

synmagmatic deformation in the underplated igneous complex of the ivrea verbano zone the ivrea verbano zone northern italy contains an igneous complex up to 10 km thick that is thought to have been intruded near the interface between the continental crust and mantle during the late paleozoic new data indicate that this complex is pervasively deformed and concentrically foliated widespread deformation under hypersolidus conditions is indicated by growth of undeformed poikilitic phases across the foliation and segregation of late stage melts into high temperature faults and pressure shadows of boudins the presence of analogous features in ophiolitic gabbros suggests that emplacement of the ivrea verbano zone plutonic rocks involved large scale flow of crystal mush in a dynamic and possibly extensional tectonic environment

an innovative seismic and statistical approach to understand 3d magmatic structures and ore deposits in the western bushveld complex south africa abstract in this paper we demonstrate the effectiveness of a novel approach to identify and quantify structural features that disrupt ore bearing horizons using 3d seismic attributes and a horizon surface processing technique difference of two surfaces on high resolution 3d reflection seismic data from the lonmin platinum mine in the bushveld complex south africa we demonstrate that seismic attribute analysis is an effective tool to enhance the intelligibility of the ug 2 a pge enriched chromitite layer as well as potholes slump structures faults iron rich ultramafic pegmatite bodies irups and other subtle geological features furthermore the horizon surface processing technique is able to quantify the dimensions and geometry of potholes in total we identify 66 potholes and show that they are probably randomly distributed in 2d on the ug 2 horizon with some extent of clustering roughly two classes of potholes are identifiable by size the larger potholes are clustered around a major normal fault the marikana fault and its branches in the area using a first order pothole structure geometry statistical analysis we demonstrate that 1 the diameters and surface areas of the potholes are strongly related to their depths and 2 the volume of the potholes is substantially log normally distributed we interpret these findings to implicate a coherent and pervasive pothole formation mechanism furthermore we show that statistically derived pothole structure geometry knowledge can be used to constrain the validity of several hypotheses our findings indicate the most probable pothole formation hypothesis is a widespread extensional regime in the bushveld complex magma chamber therefore our study provides important large scaled and highly data driven insights with an approach that can be applied to a wide range of magmatic structures furthermore relationships identified between various pothole characteristics in addition to their academic value also allows for predictive modelling that can guide mine planning and resource extraction