sills associated with the bushveld complex south africa an estimate of the parental magma composition abstract four distinct ages of sills intrude the floor rocks under the bushveld complex the first is a metadolerite and pre dates the bushveld complex the middle two are generally related to the complex itself one has quench textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the bushveld complex the second suite is a hypersthene microgabbro representative of the second major injection of magma into the complex the last is doleritic and does not obviously correlate with any portion of the layered sequence the sills genetically related to the complex display several features 1 lack of chilled margins coarse grain size ability to melt country rock xenoliths extreme heterogeneity 2 atypical of normal sills but explicable in their present context

high resolution study of layering within the percolation and soaked facies of the greenland ice sheet within the percolation and soaked facies of the greenland ice sheet the relationship between radar derived internal reflection horizons and the layered structure of the firn column is unclear we conducted two small scale ground penetrating radar gpr surveys in conjunction with 10 m firn cores that we collected within the percolation and soaked facies of the greenland ice sheet the two surveys were separated by a distance of 50 km and 340 m of elevation leading to 40 days of difference in the duration of average annual melt at the higher site 1997 m a s l which receives less melt we found that internal reflection horizons identified in gpr data were largely laterally continuous over the grid however stratigraphic layers identified in cores could not be traced between cores over any distance from 1 5 to 14 0 m thus we found no correlation between firn core stratigraphy observed directly and radar derived internal reflection horizons at the lower site 1660 m a s l which receives more melt we found massive ice layers 0 5 m thick and stratigraphic boundaries that span 15 m horizontally some ice layers and stratigraphic boundaries correlate well with internal reflection horizons that are laterally continuous over the area of the radar grid internal reflection horizons identified at 1997 m a s l are likely annual isochrones but the reflection horizons identified at 1660 m a s l are likely multi annual features we find that mapping accumulation rates over long distances by tying core stratigraphy to radar horizons may lead to ambiguous results because 1 there is no stratigraphic correlation between firn cores at the 1997 m location and 2 the reflection horizons at the 1660 m location are multi annual features

the merensky reef bushveld complex mixing of minerals not mixing of magmas a major geochemical hiatus occurs in the bushveld complex at the level of the platiniferous merensky reef close to the critical main zone boundary the origin of this hiatus and its relationship to mineralization has not been fully resolved initial sr isotope ratios for plagioclase and cr mgo values for orthopyroxene from critical and main zone rocks are significantly different geochemical data through the merensky and bastard cyclic units indicate that orthopyroxene in the merensky and bastard units originated from magma with a composition like that of the critical zone magma it sometimes occurs together with plagioclase that originated from main zone magma in a detailed study of the eastern limb the pyroxenite at the base of the merensky unit contains plagioclase and orthopyroxene that have compositions typical of the critical zone but in the overlying part of the merensky unit plagioclase compositions are more typical of the main zone whereas orthopyroxene retains a critical zone signature similarly in the bastard pyroxenite plagioclase has a range of sr isotope values between that of the main and critical zones but orthopyroxene has values typical of the critical zone these observations of mineral disequilibrium clearly show that the two major minerals in the merensky and bastard units were formed from two different but coexisting magmas a model that accounts for this disequilibrium is proposed here it invokes the influx of main zone magma at the base of the merensky unit during which the critical zone magma was displaced upward but the two magmas did not mix the elevated critical zone liquid continued to crystallize orthopyroxene which sank through the influx of magma of the main zone due to the density contrast these grains collected on the crystal pile to form the merensky pyroxenite the main zone magma into which the orthopyroxene accumulated crystallized interstitial plagioclase that had an sr isotope ratio typical of the main zone whole rock major element geochemical data show that a variable proportion of the plagioclase in both the merensky and bastard pyroxenites is cumulus it is inferred to have accumulated with orthopyroxene and has an initial sr isotope ratio typical of the critical zone thus the two pyroxenites now have a mixed sr isotope signature of cumulus critical zone and intercumulus main zone plagioclase that varies along strike above the pyroxenites the sr signature of the norites and anorthosites of both cyclic units is dominated by cumulus plagioclase from the main zone magma but some plagioclase with a critical zone signature may be present we conclude that the variations in initial sr isotope ratios do not result from mixing of magmas but from variable accumulation of orthopyroxene and plagioclase from a higher isotopically distinct layer of magma into an underlying layer the merensky and bastard cyclic units therefore may have features of either critical or main zone magma depending on which chemical parameter cr mgo and sr isotopes is considered they are therefore classified as a transitional unit between the critical and main zones models that suggest magma mixing was responsible for sulfide saturation and the creation of an immiscible sulfide liquid are not supported by this study and alternative models must be sought to explain the sulfide saturation event in the merensky reef

georadar derived estimates of firn density in the percolation zone western greenland ice sheet 1 greater understanding of variations in firn densification is needed to distinguish between dynamic and melt driven elevation changes on the greenland ice sheet this is especially true in greenland s percolation zone where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt we used georadar to investigate firn density variations with depth along a 70 km transect through a portion of the accumulation area in western greenland that partially melts we estimated electromagnetic wave velocity by inverting reflection traveltimes picked from common midpoint gathers we followed a procedure designed to find the simplest velocity versus depth model that describes the data within estimated uncertainty on the basis of the velocities we estimated 13 depth density profiles of the upper 80 m using a petrophysical model based on the complex refractive index method equation at the highest elevation site our density profile is consistent with nearby core data acquired in the same year our profiles at the six highest elevation sites match an empirically based densification model for dry firn indicating relatively minor amounts of water infiltration and densification by melt and refreeze in this higher region of the percolation zone at the four lowest elevation sites our profiles reach ice densities at substantially shallower depths implying considerable meltwater infiltration and ice layer development in this lower region of the percolation zone the separation between these two regions is 8 km and spans 60 m of elevation which suggests that the balance between dry firn and melt induced densification processes is sensitive to minor changes in melt

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

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

origin of high magnesian andesites in the setouchi volcanic belt southwest japan ii melting phase relations at high pressures melting phase relations of an augite olivine high magnesian andesite and an augite olivine basalt from the miocene setouchi volcanic belt in southwest japan have been studied under water saturated water undersaturated and under anhydrous conditions both the andesite and the basalt are characterized by low feo mgo ratios 0 86 and 0 76 in weight respectively and qualify as primary magmas derived from the upper mantle the andesite melt coexists with olivine orthopyroxene and clinopyroxene at 15 kbar and 1030 c under water saturated conditions and at 10 kbar and 1070 c under water undersaturated conditions 7 wt h2o in the melt the basalt melt also coexists with the above three phases at 11 kbar and 1305 c under anhydrous conditions and at 15 kbar and 1205 c in the presence of 4 wt water present studies indicate that high magnesian andesite magmas may be produced even under water undersaturated conditions by partial melting of mantle peridotite it is suggested that two types of high magnesian andesites in the setouchi volcanic belt augite olivine and bronzite olivine andesites were produced by different degrees of partial melting augite olivine andesite magmas whose mantle residual is lherzolite were formed by lower degrees of partial melting than bronzite olivine andesite magmas which coexist with harzburgite the basalt magmas which were often extruded in close proximity to the high magnesian andesite magmas are not partial melting products of a mantle peridotite which had previously melted to yield high magnesian andesite magmas

southeast greenland high accumulation rates derived from firn cores and ground penetrating radar despite containing only 14 of the greenland ice sheet by area the southeastern sector has the highest accumulation rates and hence receives 30 of the total snow accumulation we present accumulation rates obtained during our 2010 arctic circle traverse derived from three 50 m firn cores dated using geochemical analysis we tracked continuous internal reflection horizons between the firn cores using a 400 mhz ground penetrating radar gpr gpr data combined with depth age scales from the firn cores provide accumulation rates along a 70 km transect we followed an elevation gradient from 2350 to 1830m to understand how progressive surface melt may affect the ability to chemically date the firn cores and trace the internal layers with gpr from the firn cores we find a 52 0 43 m w e a 1 increase in average snow accumulation and greater interannual variability at the lower site than the upper site the gpr profiling reveals that accumulation rates are influenced by topographic undulations on the surface with up to 23 variability over 7 km these measurements confirm the presence of high accumulation rates in the southeast as predicted by the calibrated regional climate model polar mm5

variability in accumulation rates from gpr profiling on the west antarctic plateau isochronal layers in firn detected with ground penetrating radar gpr and dated using results from ice core analyses are used to calculate accumulation rates along a 100 km across flow profile in west antarctica accumulation rates are shown to be highly variable over short distances elevation measurements from global positioning system surveys show that accumulation rates derived from shallow horizons correlate well with surface undulations which implies that wind redistribution of snow is the leading cause of this variability temporal changes in accumulation rate over 25 185 year intervals are smoothed to along track length scales comparable to surface undulations in order to identify trends in accumulation that are likely related to changes in climate results show that accumulation rates along this profile have decreased in recent decades which is consistent with core derived time series of annual accumulation rates measured at the two ends of the radar profile these results suggest that temporal variability observed in accumulation rate records from ice cores and gpr profiles can be obscured by spatial influences although it is possible to resolve temporal signals if the effects of local topography and ice flow are quantified and removed

climatic signal of ice melt features in southern greenland the stratigraphie record of melt features in intermediate depth polar ice cores has provided valuable data on past summer climate1 3 the thermal drilling technique used in previous studies precluded extending this record beyond a few hundred years recently however a 901 m deep ice core was mechanically drilled in southern greenland the excellent core quality and improved techniques for measuring melt features have resulted in an 2 200 yr record with consistently high quality data we report here that the temporal variations in melt feature abundance expressed as annual melt per cent amp provide proxy data on summer climate that can complement other records extending back thousands of years the amp profile indicates that relatively warm summers prevailed from about ad 950 until ad 1500 culminating in the thirteenth and fourteenth centuries the periods from about 300 to 150 bc ad 350 to 550 and ad 1550 to 1800 had relatively little summer melting

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

origin of lead zinc mineralization in the southern benue trough nigeria fluid inclusion and trace element studies the epigenetic pb zn deposits of the southern benue valley nigeria are localized within cretaceous sediments of an intracontinental rift basin fluid inclusion studies of vein minerals from the abakaliki and ishiagu orebodies show that sphalerite and quartz were deposited at relatively low temperatures 102 175 c with ore fluid salinity mostly in the range of 17 25 equiv wt nacl trace element contents of sphalerite and galena are also consistent with the low temperature of formation and epigenetic origin on the basis of the geotectonic setting the mode of occurrence and fluid inclusion characteristics mineralization is attributed to connate brines set into motion by a high geothermal gradient accompanying continental rifting mineral deposition was caused principally by rapid cooling due either to reaction with wall rocks or mixing with meteoric or descending water of low salinity

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

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

characterization and formation of melt layers in polar snow observations and experiments from west antarctica author posting international glaciological society 2005 this article is posted here by permission of international glaciological society for personal use not for redistribution the definitive version was published in journal of glaciology 51 2005 307 312 doi 10 3189 172756505781829395

importance of seasonal and annual layers in controlling backscatter to radar altimeters across the percolation zone of an ice sheet radar altimeters are one of the main tools for measuring elevation changes across the antarctic and greenland ice sheets and larger ice caps a ground based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the greenland ice sheet this radar is a high bandwidth system operating in the ku band the same frequency as several satellite altimeters measurements were made over an elevation range of 1795 to 2350 m along with snow pit and shallow core studies these measurements demonstrate the spatial and temporal variations in the backscatter relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone the extent of percolation will affect elevation estimates made by radar altimeters

wetting front advance and freezing of meltwater within a snow cover 1 observations in the canadian arctic in a naturally stratified snow cover the movement of meltwater into dry snow is complicated by the interaction of the wetting front with stratigraphic horizons field observations showed that when the wetting front reached premelt stratigraphic horizons water ponded at the interface and then flow fingers developed and penetrated the lower stratum the flux in these fingers which was increased to about twice that of the surface flux was used to feed water to the impeding horizons where it froze to form ice layers these ice layers were the major source of latent heat released within the snow cover and they were responsible for the warming of the snow and the underlying soil these continuous ice layers grew only at stratigraphic boundaries because of this ice layer growth the wetting front advance was retarded and the arrival of meltwater at the snow cover base was significantly delayed owing to a cold substrate the strong heat flux from the snow into the soil delays the warming of the snow cover and limits runoff after the snow is isothermal at 0 c by the refreezing of soil infiltration and the development of a basal ice layer

petrology of the tekirova antalya ophiolite southern turkey evidence for diverse magma generations and their tectonic implications during neotethyan subduction the sw antalya complex is an assemblage of mesozoic carbonate platform margin and ophiolitic rocks which record the formation and tectonic emplacement of a small mesozoic ocean basin the late cretaceous ophiolitic rocks are located at two localities namely the relatively intact tekirova ophiolite to the east of kemer zone and the dismembered godene ophiolite to the west of kemer zone the tekirova antalya ophiolite comprises harzburgitic tectonites ultramafic to mafic cumulates isotropic gabbros and sheeted dikes numerous isolated dikes ranging in thickness from 5 cm to 10 m intruded the crustal rocks at different structural levels the isotropic gabbros are represented by gabbro diorite and quartz diorite rocks with granular to ophitic subophitic textures the isolated dikes are characterized by dolerite diabase and microdiorite with ophitic intersertal and microgranular textures these rocks exhibit tholeiitic to alkaline compositions new geochemical data presented in this paper from the isolated dikes and isotropic gabbros suggest that there are three main types of parental basic magmas that form the oceanic crustal rocks of the tekirova antalya ophiolite these are 1 iat series which can be referred to the group i isolated dikes and isotropic gabbros 2 low ti boninitic series characterized by the group ii isolated dike and isotropic gabbros and 3 oib type including the group iii isotropic gabbros the geochemical evidence suggests that the crustal rocks of the tekirova antalya ophiolite were generated from a progressive source depletion from island arc tholeiites iat to boninites therefore a fore arc tectonic setting seems likely for the generation of the crustal rocks from the tekirova antalya ophiolite in the southern branch of neotethys during the late cretaceous the oib type alkaline isotropic gabbros are thought to have resulted from either 1 a late stage magmatic activity fed by melts that originated within an asthenospheric window due to slab break off or 2 subduction of a ridge system which generated oib source across the asthenospheric window that has been no influence of fluids from the subducted slab into the overlying mantle wedge shortly before the emplacement of the tekirova antalya ophiolite onto the tauride platform

gallium germanium indium and other trace and minor elements in sphalerite as a function of deposit type a meta analysis abstract while a significant amount of analytical data on trace and minor element concentrations in sphalerite has been collected over the last six decades no meta analysis of this data has ever been conducted in this study the results of such an analysis are presented while the study focusses on ga ge and in data for six other elements ag cd co cu fe and mn was also included the results show that there are systematic statistically significant differences in the mean concentrations of fe ga ge in and mn in sphalerite from different deposit types while cd and cu concentrations show no such differences and ag and co concentrations are only significantly different for vein type deposits a principal component analysis demonstrates that the differences between deposit types are approximately one dimensional being expressible in terms of a single number this number correlates strongly with the homogenisation temperature of fluid inclusions r 2 0 82 p 16 it may be expressed as follows pc 1 ln c ga 0 22 c ge 0 22 c fe 0 37 c mn 0 20 c in 0 11 with ga ge in and mn concentrations in ppm and fe concentration in wt the relationship is sufficiently strong to be used as a geothermometer ggimfis the empirical relationship between pc 1 and the homogenisation temperature t is t c 54 4 7 3 pc 1 208 10 our results indicate a strong control of sphalerite chemistry by fluid temperature particularly for the concentrations of ga r 2 0 40 ge r 2 0 65 fe r 2 0 30 and mn r 2 0 60 and to a lesser degree in r 2 0 10 the concentrations of ag cd co and cu appear to be independent of temperature as a consequence of the strong temperature control on pc 1 metamorphic overprinting of pb zn deposits even by lower greenschist facies events may lead to significant changes in sphalerite composition namely a relative decrease in ga and ge concentrations and increase in fe mn and to a lesser degree in concentrations the closure temperature of sphalerite in regional metamorphic events appears to be around 310 50 c such that higher grade events will not be reflected in its composition factors other than temperature such as differences in fluid salinity or source rock composition do not appear to be responsible for differences between deposit types but rather appear to cause differences between individual deposits particularly the cu activity in ore forming systems appears to have a strong influence on in concentrations in sphalerite the observed trends in sphalerite compositions provide a useful tool for future studies of different types of pb zn deposits as well as for mineral exploration they should be particularly relevant for the identification of new resources of ga ge and in

a ground based radar backscatter investigation in the percolation zone of the greenland ice sheet abstract satellite radar altimeters and scatterometers deployed over ice sheets experience backscatter from the surface and from within the snowpack termed surface and volume backscatter respectively in order to assess the errors in satellite altimeter measurements it is vital to know where the return is originating from in the snowpack this return can vary spatially and temporally seasonal variations in the volume backscatter can be a major complicating factor in the radar return from the percolation zone ground based step frequency radar was deployed in the percolation zone of the greenland ice sheet at 1945 m elevation 69 51n 47 15w previous measurements in this area made by scientists from the byrd polar research centre and the university of kansas undertaken prior to summer melt events have shown the strongest backscatter from ice features at around 1 m depth buried beneath the previous end of summer surface in autumn 2004 radar measurements in the ku band with bandwidths of 1 and 8 ghz were made alongside detailed stratigraphic observations within a 1 km 2 site the radar results revealed no continuous reflecting horizons in the upper 3 5 m of the firn shallow cores and snowpits also indicated that there were no spatially continuous stratigraphic horizons across the study site an average electromagnetic wave velocity of 2 11 0 05 10 8 m s 1 was determined for the upper metre of the firn surface and volume backscatter at vertical incidence were calculated using a standard model the contribution of the surface backscatter to the total backscatter was on average 6 db higher than that of the volume backscatter however at the higher 8 ghz bandwidth the strongest return frequently originated not from the surface but from within the upper 30 cm of the snowpack most probably from thin ice layers at 1 ghz bandwidth these ice layers were not always resolved their return merged with the surface return causing it to broaden with the peak and leading edge moving down modelling using density and thickness measurements from shallow cores and snowpits showed that the backscatter from these shallow thin ice layers could be stronger than the surface return owing to constructive interference from the top and base of the layers

stratigraphic continuity in 400 mhz short pulse radar profiles of firn in west antarctica we track dated firn horizons within 400mhz short pulse radar profiles to find the continuous extent over which they can be used as historical benchmarks to study past accumulation rates in west antarctica the 30 40cm pulse resolution compares with the accumulation rates of most areas we tracked a particular set that varied from 30 to 90m in depth over a distance of 600 km the main limitations to continuity are fading at depth pinching associated with accumulation rate differences within hills and valleys and artificial fading caused by stacking along dips the latter two may be overcome through multi kilometer distances by matching the relative amplitude and spacing of several close horizons along with their pulse forms and phases modeling of reflections from thin layers suggests that the 37 to 50db range of reflectivity and the pulse waveforms we observed are caused by the numerous thin ice layers observed in core stratigraphy constructive interference between reflections from these close high density layers can explain the maintenance of reflective strength throughout the depth of the firn despite the effects of compaction the continuity suggests that these layers formed throughout west antarctica and possibly into east antarctica as well

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

inverse estimation of snow accumulation along a radar transect on nordenskioldbreen svalbard we present an inverse modeling approach to reconstruct annual accumulation patterns from ground penetrating radar gpr data a coupled surface energy balance snow model simulates surface melt and the evolution of subsurface density temperature and water content the inverse problem consists of iteratively calibrating accumulation serving as input for the model by finding a match between modeled and observed radar travel times the inverse method is applied to a 16 km gpr transect on nordenskioldbreen svalbard yielding annual accumulation patterns for 2007 2012 accumulation patterns with a mean of 0 75 meter water equivalent mwe a 1contain substantial spatial variability with a mean annual standard deviation of 0 17 mwe a 1 and show only partial consistency from year to year in contrast to traditional methods accounting for melt water percolation refreezing and runoff facilitates accurate accumulation reconstruction in areas with substantial melt additionally accounting for horizontal density variability along the transect is shown to reduce spatial variability in reconstructed accumulation whereas incorporating irreducible water storage lowers accumulation estimates correlating accumulation to terrain characteristics in the dominant wind direction indicates a strong preference of snow deposition on leeward slopes whereas weaker correlations are found with terrain curvature sensitivity experiments reveal a nonlinear response of the mass balance to accumulation changes the related negative impact of small scale accumulation variability on the mean net mass balance is quantified yielding a negligible impact in the accumulation zone and a negative impact of 0 09 mwe a 1in the ablation area

thermal tracking of meltwater retention in greenland s accumulation area 1 poorly understood processes controlling retention of meltwater in snow and firn have important implications for greenland ice sheet s mass balance and flow dynamics here we present results from a 3 year 2007 2009 field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west greenland we installed one or two thermistor strings at 14 study sites each string having 32 sensors spaced between 0 and 10 m depth data from our network of over 500 sensors were collected at 15 60 min intervals for 1 2 years thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles we document three types of heating of firn related to different mechanisms of meltwater motion and freezing including heterogeneous breakthrough events wetting front advance and year round heating from freezing of residual deep pore water vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters the offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect the firn is 10 c warmer than the mean annual air temperature at the region where meltwater runoff initiates during 2007 2009 runoff was limited to elevations lower than 1500 m with no sharp runoff limit rather the ratio of retention to runoff transitioned from all retention to all runoff across a 20 km wide zone

radar measurements of melt zones on the greenland ice sheet surface based microwave radar measurements were performed at a location on the western flank of the greenland ice sheet here firn metamorphosis is dominated by seasonal melt which leads to marked contrasts in the vertical structure of winter and summer firn this snow regime is also one of the brightest radar targets on earth with an average backscatter coefficient of 0 db at 5 3 ghz and an incidence angle of 25 by combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter s snow pack this observation has important implications for monitoring and understanding changes in ice sheet volume using space borne microwave sensors

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

distortion of isochronous layers in ice revealed by ground penetrating radar in addition to measuring ice sheet thickness ground penetrating radar can be used to delineate reflections in ice sheets1 2 3 these reflections are generally accepted to result from layers of isochronous deposition of snow and can reveal much about the dynamics of the ice flow here we present ground penetrating radar data from fletcher promontory antarctica which show arches and troughs in isochronous ice layers to a depth of 100 m we demonstrate that the origin of these features can be determined by their growth with depth and many features result from local anomalies in accumulation rate which can be correlated with the ice surface slope one arch appears to be the result of a local anomaly in vertical strain rate its proximity to the ice divide width and growth with depth indicate that this arch is one of a class of features postulated by raymond4 but not previously shown to exist in the field such a feature is an indication of the nonlinear rheology of ice and requires that palaeoclimate records from ice cores extracted from the vicinity of ice divides underlain by similar features should be specifically corrected for such effects on ice flow

mapping of ice layer extent and snow accumulation in the percolation zone of the greenland ice sheet 1 the greenland ice sheet underwent record extensive melt in 2002 and prolonged melt in 2003 the severe melting created a significant and extensive ice layer over the greenland ice sheet an innovative approach is developed to detect the ice layer formation using data acquired by the seawinds scatterometer on the quikscat satellite quikscat backscatter together with in situ data from automatic weather stations of the greenland climate network are used to map the extent of ice layer formation the results reveal areas of extensive ice layer formed by the 2002 melt which is consistent with the maximum melt extent in 2002 moreover during freezing seasons quikscat data show a linear decrease in backscatter in decibels or db that is related to the amount of snow accumulation in the ice layer formation region this snow accumulation signature is caused by the attenuation of radar waves in the snow layer accumulating since the last major melt event whose thickness appears as an exponential function in relation to the backscatter signature we use the greenland climate network data to calibrate the quikscat accumulation rate in order to estimate and map snow accumulation quikscat results capture the extreme snowfall in mid april 2003 which deposited more than 0 5 m of snow in a day as measured by the automated weather station at the nasa south east site large scale quikscat results show an anomalous increase of snow accumulation over the southeast region of greenland during the 2002 2003 freezing season

cooling of the bushveld complex south africa implications for paleomagnetic reversals igneous rocks record the direction of the earth s magnetic field as they cool through their curie temperature the mafic magmas of the 8 km thick bushveld complex of south africa took 65 k y to be emplaced 180 k y to solidify to 900 c and a further 500 k y for the entire intrusion to cool below 580 c the curie temperature of magnetite once solid the cooling of this intrusion occurred mainly from the top downward with slower cooling through its floor as a result the upper rocks cooled through their curie temperature before those at the base the portion 6 km below the upper contact was the last to reach the curie temperature thus the intrusion records a mainly top down sequence of three paleomagnetic reversals starting with n normal direction the last two are also recorded from the base of the mafic sequence upward as it cooled through 580 c later than the top the lateral variations in thickness of the bushveld complex are important in this interpretation because thinner sections cooled more quickly hence reversals do not always correlate with stratigraphy specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering the interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward and has implications for the interpretation of paleomagnetic results from all thick intrusions mafic and felsic

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

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

investigations of meltwater refreezing and density variations in the snowpack and firn within the percolation zone of the greenland ice sheet the mass balance of polythermal ice masses is critically dependent on the proportion of surface generated meltwater that subsequently refreezes in the snowpack and firn in order to quantify this effect and to characterize its spatial variability we measured near surface 26 resulting in a 32 increase in net accumulation this seasonal densification increased at lower elevations rising to 47 10km closer to the ice sheet margin at 1860 m a s l density depth profiles from nine sites within 1 km 2 at similar to 1945ma s l reveal complex stratigraphies that change over short spatial scales and seasonally we conclude that estimates of massbalance change cannot be calculated solely from observed changes in surface elevation but that near surface densification must also be considered however predicting spatial and temporal variations in densification may not be straightforward further the development of complex firn density profiles both masks discernible annual layers in the near surface firn and ice stratigraphy and is likely to introduce error into radar derived estimates of surface elevation

plagioclase content of cyclic units in the bushveld complex south africa the upper critical zone of the bushveld complex south africa has been divided into so called cyclic units ideally they should consist of from the base upward layers of chromitite pyroxenite norite and anorthosite some hypotheses for their origin suggest that they result from the addition and mixing of a magnesian magma having orthopyroxene or olivine as liquidus mineral these models predict that the bulk cumulate composition in a cyclic unit formed from such magma ought to contain excess orthopyroxene to plagioclase relative to their cotectic proportions an hypothesis tested here all rocks investigated here from the upper critical zone at impala platinum mine are mixtures of cumulus plagioclase and orthopyroxene except for the chromitites with very minor intercumulus minerals mainly clinopyroxene hence the proportion of plagioclase can be determined by measurement of the density of a sample the density of every piece of continuous bore core from the upper group 2 ug2 and merensky cyclic units has been determined and the proportion of plagioclase calculated for the bastard cyclic unit estimates of the proportion of plagioclase have been made based on published modal and chemical analyses vertical summation of the proportion of plagioclase indicates that these three cyclic units ug2 merensky and bastard respectively contain 64 74 and over 80 plagioclase by weight for the merensky and bastard cyclic units such proportions exceed that of the plagioclase orthopyroxene cotectic and could not have been produced by processes involving magnesian magma

greenland ice sheet contribution to sea level rise buffered by meltwater storage in firn observations of firn structure and meltwater retention on the greenland ice sheet s percolation zone a region of the ice sheet that is perennially covered by snow and firn quantify the capacity of the firn to store future surface meltwater and to delay expansion of the area contributing to sea level rise the extent and the fate of surface melt on the greenland ice sheet are important factors contributing to sea level change the pore spaces in firn a type of partly compacted snow can be filled by meltwater and thus act as a temporary reservoir here joel harper et al show that the greenland ice sheet parts of which are permanently covered by snow and firn can buffer several hundred gigatonnes of meltwater for several decades although this buffering capacity is only temporary and cannot be easily regained it has the potential to delay the pace of near term sea level rise surface melt on the greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era1 2 3 records for melt were broken in 20054 20075 20106 and 20127 much of the increased surface melt is occurring in the percolation zone a region of the accumulation area that is perennially covered by snow and firn partly compacted snow the fate of melt water in the percolation zone is poorly constrained some may travel away from its point of origin and eventually influence the ice sheet s flow dynamics and mass balance and the global sea level whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects here we quantify the existing water storage capacity of the percolation zone of the greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage we collected in situ observations of firn structure and meltwater retention along a roughly 85 kilometre long transect of the melting accumulation area our data show that repeated infiltration events in which melt water penetrates deeply more than 10 metres eventually fill all pore space with water as future surface melt intensifies under arctic warming a fraction of melt water that would otherwise contribute to sea level rise will fill existing pore space of the percolation zone we estimate the lower and upper bounds of this storage sink to be 322 44 gigatonnes and gigatonnes respectively furthermore we find that decades are required to fill this pore space under a range of plausible future climate conditions hence routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea level rise although once the pore space is filled it cannot quickly be regenerated

contribution to the origin of the serguza lead zinc pyrite deposit northern iraq the origin of the serguza lead zinc deposit of the northern thrust zone has been suggested to be epigenetic hydrothermal structurally controlled by a post tertiary major fault zone reinterpretation of previously obtained data and the evaluation of the present results have provided enough evidence to argue for a different mode of formation based upon trace element content of the ore minerals dating of galena ore and country rock texture and the results of the detailed geochemical and geophysical prospecting in the area it seems that the deposit is strata bound stratigraphically controlled within the triassic dolomites its simple mineralogy chemistry and other features are comparable with other early alpine strata bound leadzinc deposits of the mediterranean belt

determination of timing and location of water movement and ice layer formation by temperature measurements in sub freezing snow measurements of temperature in snow along a vertical profile during the onset of spring melting are used to calculate spatial and temporal temperature gradients and terms of the conduction equation with an internal energy production term are calculated heat transfer information is combined with stratigraphic observations made during melting and allow detailed determination of the timing and location of heterogeneous water movement and of refreezing internal energy production is interpreted as latent heat of refreezing of percolated meltwater times and locations of flow and refreezing of meltwater are calculated and compared to stratigraphic observations of layering and changes in density and liquid water content sequences of melt piping layering and refreezing seen in calculations and in stratigraphy demonstrate retarded flow at fine to coarse transitions flow along such transitions and refreezing at the transitions to form ice layers downslope flow is also observed in the absence of an impermeable horizon to redirect flow from the vertical

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

formation of ice layers by infiltration and refreezing of meltwater melt layer frequency and magnitude in polar and sub polar ice cores have been interpreted as measures of past summer temperature and calibrations have been proposed relating frequency of occurrence of ice layers in ice cores to past summer temperatures but observations in the percolation facies in greenland and an analysis of the combined processes of meltwater infiltration and refreezing of water in snow indicate that in addition to unusually high rates of meltwater input formation of ice layers will also be facilitated by unusually cold initial conditions or early onset of melt uniform warming of both summer and winter conditions has the opposite effect and suppresses ice layer formation in favor of uniform wetting and refreezing of the snowpack numerical modeling of infiltration and refreezing at a stratigraphic fine to coarse transition allows quantification of the effects of significant parameters initial temperature grain size and density contrast across the stratigraphic transition water input rate and minimum impermeable layer thickness calculations are made to distinguish threshold values of parameters at which infiltration progresses faster than refreezing resulting in a break through of water across the stratigraphic transition from values leading to the formation of an ice layer when refreezing progresses faster than infiltration

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

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

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

meltwater percolation impermeable layer formation and runoff buffering on devon ice cap canada the retention of meltwater in the accumulation area of the greenland ice sheet and other arctic ice masses buffers their contribution to sea level change however sustained warming also results in impermeable ice layers or ice slabs that seal the underlying pore space here we use a 1 d physically based high resolution model to simulate the surface mass balance smb percolation refreezing ice layer formation and runoff from across the high elevation area of devon ice cap canada from 2001 to 2016 we vary the thickness of the impermeable ice layer at which underlying firn becomes inaccessible to meltwater thick near surface ice layers are established by an initial deep percolation the formation of decimetre ice layers and the infilling of interleaving pore space the cumulative smb increases by 48 by varying impermeable layer thickness between 0 01 and 5 m within this range we identify narrower range 0 25 1 m that can simulate both the temporal variability in smb and the observed near surface density structure across this range cumulative smb variation is limited to 6 and 45 49 of mass retention takes place within the annually replenished snowpack our results indicate cooler summers after intense mid 2000s warming have led to a partial replenishment of pore space

changes in accumulation area firn stratigraphy and meltwater flow during a period of climate warming devon ice cap nunavut canada 1 to document climate driven changes in firn stratigraphy and their implications for meltwater flow patterns within firn on the devon ice cap nunavut during the 21st century summer warming 500 mhz ground penetrating radar gpr surveys were conducted along a 40 km transect in each spring from 2007 to 2012 these linear gpr surveys were supplemented by four 190 m by 100 m gpr grid surveys and 36 firn cores increased meltwater percolation and infiltration ice formation associated with high surface melt rates since 2005 modified the firn stratigraphy substantially over a horizontal distance of nearly 30 km the most dramatic change involved the growth of a thick ice layer within the firn body this layer grew primarily by upward accretion over an initial widespread ice layer formed during summer 2005 it thickened by between 0 5 and 4 5 m over the study period and filled much of the pore volume in the upper part of the firn reducing vertical percolation of meltwater into deeper sections of the firn and thus the water storage potential of much of the firn reservoir heterogeneous percolation of surface meltwater promoted by rolling topography played an important role in meltwater infiltration and drainage encouraging lateral flow at the tops of small hills and ponding and refreezing of meltwater beneath surface depressions

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

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

high frequency observations of melt effects on snowpack stratigraphy kahiltna glacier central alaska range with an increased volume of melt on many of the world s glaciers study of how meltwater affects the properties of glacial snowpack becomes essential to our understanding of how glaciers will respond to climate change we address this problem by studying how snow properties changed on sub daily timescales on the kahiltna glacier alaska between may 26 and june 10 2010 during this period we dug 1 8 m deep snow pits twice daily to record the stratigraphy of melt layers snow hardness grain size and density and sampled for hydrogen isotopic composition d on four occasions from these data we show that 65 of the melted surface snow infiltrates and refreezes in the snowpack this leads to a densification of the snow a 729 increase in volume of melt layers and a homogenization of isotopic and physical snow properties from visual and stratigraphic observations we show that meltwater flow within the snowpack is conducted primarily along lenses and pipes where melt layers later form but that more homogeneous capillary based flow is also important finally we show using isotope ratios that post depositional alteration is exacerbated with increased melt extent using the d profile below a volcanic ash layer as a case study in the future similar studies would benefit from this high frequency monitoring approach to assessing snowpack evolution as it allows for a greater understanding of short duration processes new directions for study would include longer term monitoring efforts over a wider spatial snow pit network copyright 2012 john wiley sons ltd

crystallization of titaniferous chromite magnesian ilmenite and armalcolite in tholeiitic suites in the karoo igneous province titaniferous chromite up to 8 wt tio2 and magnesian ilmenite up to 10 wt mgo coexist at the base of the differentiated tholeiitic mount ayliff intrusion in the karoo province of southern africa suggesting that the original magma was tio2 rich picritic lavas with 3 tio2 from the lebombo monocline of the karoo province also contain microphenocrysts of magnesian ilmenite up to 6 wt mgo and armalcolite up to 7 wt mgo these oxide mineral associations and compositions are atypical of tholeiitic magmas in which chromite usually has less than 1 wt tio2 ilmenite less than 3 wt mgo and armalcolite is rarely a primary mineral experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of tio2 on the crystallization and composition of chromite ilmenite and armalcolite the results indicate that increasing the tio2 content of picritic magmas increases the tio2 content of the spinel mainly at the expense of al2o3 whereas cr2o3 is not affected spinel compositions in the mount ayliff intrusion with over 45 wt cr2o3 less than 10 wt al2o3 and 8 wt tio2 were duplicated in experiments on a picrite at temperatures of about 1 200 c at the ni nio buffer increasing fo2 from fayalite magnetite quartz to ni nio buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite the total feo content of the liquid has little influence on the crystallization temperature of these phases the tio2 content of the liquid when either ilmenite or armalcolite crystallizes varies inversely with sio2 content the mgo content of the liquid at which ilmenite or armalcolite crystallizes depends upon the tio2 content of the starting composition with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt mgo and 5 5 wt tio2 the partition coefficent for mgo between armalcolite or ilmenite and liquid is about 1 5 observed magnesian armalcolite and ilmenite compositions in picrite lavas both minerals and in the mount ayliff intrusion ilmenite only are consistent with crystallization from a tio2 rich magma with approximately 5 wt mgo the fe23 tio5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1 150 1 110 c and fo2 of the ni nio to ni nio 1 log unit similarities also exist between the compositions of chromite ilmenite and armalcolite and liquid fraction ation trends of some hawaiian high tio2 lavas and the experimental studies presented here

reply to discussion by rn scoon and aa mitchell on the paper the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber by wd maier s j barnes and di groves scoon and mitchell express certain reservations about our paper first the authors suggest that in modeling the bushveld reefs we only appeal to hydrodynamic sorting which they consider unimportant second they feel that we have failed to fully acknowledge and understand the mitchell and scoon 2007 model for the merensky reef in which they proposed formation of the merensky pyroxenite and other platinumgroup elements pge bearing horizons by injection of magma into semi consolidated crystal piles third the authors argue that we misrepresented their model for the origin of the platiniferous pipes our response to these points is the following

rapid expansion of greenland s low permeability ice slabs in recent decades meltwater runoff has accelerated to become the dominant mechanism for mass loss in the greenland ice sheet1 3 in greenland s high elevation interior porous snow and firn accumulate these can absorb surface meltwater and inhibit runoff4 but this buffering effect is limited if enough water refreezes near the surface to restrict percolation5 6 however the influence of refreezing on runoff from greenland remains largely unquantified here we use firn cores radar observations and regional climate models to show that recent increases in meltwater have resulted in the formation of metres thick low permeability ice slabs that have expanded the greenland ice sheet s total runoff area by 26 3 per cent since 2001 although runoff from the top of ice slabs has added less than one millimetre to global sea level rise so far this contribution will grow substantially as ice slabs expand inland in a warming climate runoff over ice slabs is set to contribute 7 to 33 millimetres and 17 to 74 millimetres to global sea level rise by 2100 under moderate and high emissions scenarios respectively approximately double the estimated runoff from greenland s high elevation interior as predicted by surface mass balance models without ice slabs ice slabs will have an important role in enhancing surface meltwater feedback processes fundamentally altering the ice sheet s present and future hydrology observations and regional climate models show that the increasing coverage of ice slabs on the greenland ice sheet could lead to a global sea level rise of up to 74 millimetres by 2100

petrology and geochemistry of cambrian boninites and low ti andesites from heathcote victoria in the heathcote greenstone belt of central victoria a sequence of boninites and low ti andesites is overlain and intruded by tholeiitic basalts with affinities to backarc basin basalts two suites of boninites have been identified one type a with ti zr ratios of 63 4 la yb n of 2 3 and hree 5 times chondritic levels the other suite type b overlies type a boninites and has ti zr ratios of 23 3 and lower tio2 and hree contents 2 3 chondrite but shows significantly greater lree enrichment with la yb n greater than 5 fractionation within both suites was largely controlled by the low ca pyroxenes protoenstatite and enstatite plagioclase phyric low ti high mg andesites occur in fault contact with the boninites and have ti zr and la yb n ratios very close to those of type b boninites but at higher absolute abundances of tio2 and hree they are not related to either boninite suite by any realistic fractionation scheme but originated from the same source as type b boninites by approximately half the degree of partial melting that generated the boninites

na ti zr h2o rich mineral inclusions indicating postcumulus chrome spinel dissolution and recrystallization in the western laouni mafic intrusion algeria disseminated cumulus chrome spinel in the lower most olivine orthocumulates from the western laouni intrusion southern hoggar algeria contains inclusions of silicates enriched in mg ti na and h2o and fe ti zr rich oxides occurring either as numerous micro inclusions or as large solitary cavities regardless of the inclusion type titanian pargasite or kaersutite and the sodium analog of phlogopite predominate over orthopyroxene en86 to en92 and na rich plagioclase an50ab50 to an7ab93 oxide inclusions are magnesian ilmenite up to 37 mol mgtio3 rutile loveringite ca ree ti fe cr mg zr 21o38 and accessory magnesian pseudo brookite kennedyite apatite fe ni cu sulfides and alteration products saussurite bastite are also present apart from kennedyite all minerals included in chrome spinel form larger intercumulus crystals in the host rock as well the micro inclusions were trapped as the consequence of chrome spinel dissolving against the intercumulus liquid 150 to 300 c below its liquidus temperature the solitary cavities are attributed to fluid assisted solid state recrystallization of chains of spinel crystals the role of capillary fluids is demonstrated by both the hydrous nature of solid inclusions and a strong positive correlation between the amphibole content and the abundance of solid inclusions in the rock phlogopite locally showing similar optical orientation within and outside the inclusions indicates that spinel was still recrystallizing down to 950 c in the temperature range 1100 900 c determined for the formation of inclusions chrome spinel probably trapped various combinations of crystals liquids and fluids the respective quantities of which varied greatly over distances of few tens of micrometres in a single spinel the volume ratio of solids to liquids or to fluids must have been low enough to permit magmatic or hydrothermal reactions which are otherwise never possible for the larger intercumulus crystals hence the lack of olivine and clinopyroxene as inclusions in spinel while abundant in the host rock suggests that as trapped they completely reacted with the residual liquid to form phlogopite and pargasite respectively likewise plagioclase and phlogopite have been enriched in na against hydrothermal fluids to form albite and na phlogopite in the mineral inclusions

characteristics and small scale variability of gpr signals and their relation to snow accumulation in greenland s percolation zone we investigate snowpack properties at a site in west central greenland with ground penetrating radar gpr supplemented by stratigraphic records from snow pits and shallow firn cores gpr data were collected at a validation test site for cryosat t05 on the expglaciologiques internationales au groenland egig line over a 100 m 100 m grid and along 1 km sections at fre quencies of 500 and 800 mhz several internal reflection horizons irhs down to a depth of 10 m were tracked irhs are usually related to ice layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season warm conditions in the following melt season can change these characteristics by percolating meltwater in cold conditions smaller melt volumes at the surface can lead to faint irhs the absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data our gpr derived depth of the 2003 summer surface of 1 48 m measured in 2004 is confirmed by snow pit observations the distribution of irh depths on a 1 km scale reveals a gradient of increasing accumu lation to the northeast of about 5 cm w e km 1 we find that point measurements of accumulation in this area are representative only over several hundred metres with uncertainties of about 15 of the spatial mean

the arditurri pb zn f ba deposit cinco villas massif basque pyrenees a deformed and metamorphosed stratiform deposit stratiform banded and massive pb zn f ba mineralizations in metasediments of the westphalian cinco villas formation basque pyrenees show several features related to hercynian deformation and low grade regional metamorphism however some primary depositional textures and structures are preserved minerals like pyrite reveal preregional metamorphic textures framboidal colloform pyrite with automorphic overgrowths dendritic and atoll textures typical of rapid precipitation from supersaturated solutions during hydrothermal sedimentary activity deformation and subsequent micro and mesoscale remobilization have caused morphological and textural changes in the mineralizations producing folding mineral foliation transposition boudinages piercements etc as well as the development of veinlike orebodies pyrite shows a general tendency toward cataclasis whereas sphalerite galena and chalcopyrite exhibit a variety of microstructures subgrains twining recrystallization etc indicative of ductile deformation microscopy suggests the deformation of more plastic sulfide occurred mainly by a dislocation type mechanism at low temperatures 350 c which are compatible with the regional metamorphism recorded in the host rocks

cr and sr keys to parental magmas and processes in the bushveld complex south africa abstract large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization the exact composition s of the replenishing magma s in the case of the bushveld complex south africa has been debated mainly on the basis of major element composition and likely crystallization sequences the intrusion is dominated by orthopyroxene and plagioclase and so their cr and sr contents and likely partition coefficient values can be used to re investigate the appropriateness of the various proposed parental magmas one magma type with about 12 mgo 1000 ppm cr and 180 ppm sr can explain the genesis of the entire lower and critical zones a number of other magma compositions proposed to produce the critical zone fail to match these trace element constraints by being too poor in cr a fundamentally different magma type was added at the base of the main zone but none of the proposed compositions is consistent with the trace element requirements specifically the cr contents are higher than predicted from pyroxene compositions a further geological constraint is demonstrated from a consideration of the cr budget at this level there is an abrupt decrease from about 0 4 to 0 1 cr 2 o 3 in orthopyroxene across this critical zone main zone transition no realistic proportions of mixing between the residual magma at the top of the critical zone and any proposed added magma composition can have produced a composition that could have crystallized these low cr orthopyroxenes instead it is suggested that the resident magma from the upper critical zone was expelled from the chamber possibly as sills into the country rocks during influx of a dense differentiated magma near the level of the pyroxenite marker in the main zone there is further addition of a ferrobasaltic magma with 6 mgo 111 ppm cr and 350 ppm sr that is consistent with the geochemical requirements

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

origin of the pegmatitic pyroxenite in the merensky unit bushveld complex south africa the genesis of the pegmatitic pyroxenite that often forms the base of the merensky unit in the bushveld complex is re examined large 1 cm orthopyroxene grains contain tricuspidate inclusions of plagioclase and chains and rings of chromite grains which are interpreted to have grown by reaction between small primary orthopyroxene grains and superheated liquid this superheated liquid may have been an added magma or be due to a pressure reduction as a result of lateral expansion of the chamber there would then have been a period of non accumulation of grains permitting prolonged interaction with the crystal mush at the crystal liquid interface crystal ageing and grain enlargement of original orthopyroxene grains would ensue only after the pegmatitic pyroxenite had developed did another layer of chromite and pyroxenite with normal grain size accumulate above it immiscible sulphide liquids formed with the second pyroxenite but percolated down as a result of their density contrast even as far as the footwall anorthosite in some cases whole rock abundances of incompatible trace elements in the pegmatitic pyroxenite are comparable with or lower than those of the overlying pyroxenite and so there is no evidence for addition and or trapping of large proportions of interstitial liquid or of an incompatible element enriched liquid or fluid in the production of the pegmatitic rock because of the coarse grained nature of the rock modal analysis especially for minor minerals is unreliable annealing has destroyed primary textures such that petrographic studies should not be used in isolation to distinguish cumulus and intercumulus components geochemical data suggest that the merensky pyroxenite both pegmatitic and non pegmatitic typically consists of about 70 80 cumulus orthopyroxene and 10 20 cumulus plagioclase with a further 10 of intercumulus minerals and could be considered to be a heteradcumulate

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

variations of near surface firn density in the lower accumulation area of the greenland ice sheet pâkitsoq west greenland firn density variations have been studied in the lower accumulation area of the greenland ice sheet 1440 1620 m a s l near p kitsoq west greenland the main control on density in the near surface firn layer of 5 10 m thickness is the formation of ice layers by the refreezing of meltwater that reaches depths of 2 4 m below the surface the density variations are described by the ratio of annual surface melt m to the annual accumulation c the ratio m c is about 0 6 at the run off limit at about 1400 m a s l in the study area where refreezing of meltwater transforms snow into impermeable ice the mean density of near surface firn decreases with elevation reflecting a decrease in melt with elevation there is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface which reflects lower melt rates and or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer the formation of such low density firn may have partially contributed to the 1978 85 thickening of the ice sheet observed by satellite radar altimetry near surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring

experimental determination of the emplacement pressure of proterozoic mafic dikes within portions of the east antarctic shield abstract in order to further constrain previous determinations of the p t pressure time path of two portions of the east antarctic shield the depth pressure under which two proterozoic dike suites were emplaced in the vestfold hills block was determined by experimentally reproducing the equilibrium phenocryst assemblage found in their chilled margins the phenocryst assemblage oliv opx of a high mg tholeiite sample ca 2420 ma could only be experimentally reproduced at 7 5 kbar it was also found that at 7 5 kbar the ca mg fe components of the synthetic orthopyroxene very closely matched the natural phenocryst concentrations based on high pressure experiments the phenocryst assemblage in a sample from the ca 1360 ma fe rich tholeiite suite oliv opx plag opx is only consistent with dike emplacement at pressures 5 5 kbar while the mg values of the natural clinopyroxene phenocrysts match the experimental pyroxene compositions at pressures of 4 5 kbar these results are consistent with pressures of 8 0 and 5 3 kbar obtained for the mg and fe rich tholeiite samples respectively using geobarometry applied to the groundmass assemblages emplacement pressures of compositionally similar dikes from the napier complex were estimated by comparison with previous experimental studies the comparison shows that a napier complex high mg tholeiite suite ca 2350 ma and an fe rich tholeiite suite ca 1190 ma were both emplaced at a pressure 8 kbar

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time

a plot scale study of firn stratigraphy at lomonosovfonna svalbard using ice cores borehole video and gpr surveys in 2012 14 spatial heterogeneity of snow and firn properties on glaciers introduces uncertainty in interpretation of point and profile observations and complicates modelling of meltwater percolation and runof

annual accumulation for greenland updated using ice core data developed during 2000 2006 and analysis of daily coastal meteorological data an updated accumulation map for greenland is presented on the basis of 39 new ice core estimates of accumulation 256 ice sheet estimates from ice cores and snow pits used in previous maps and reanalysis of time series data from 20 coastal weather stations the period 1950 2000 is better represented by the data than are earlier periods ice sheetwide accumulation was estimated based on kriging the average accumulation 95 confidence interval or 2 times standard error over the greenland ice sheet is 30 0 2 4 g cm 2 a 1 with the average accumulation above 2000 m elevation being essentially the same 29 9 2 2 g cm 2 a 1 at higher elevations the new accumulation map maintains the main features shown in previous maps however there are five coastal areas with obvious differences southwest northwest and eastern regions where the accumulation values are 20 50 lower than previously estimated and southeast and northeast regions where the accumulation values are 20 50 higher than previously estimated these differences are almost entirely due to new coastal data the much lower accumulation in the southwest and the much higher accumulation in the southeast indicated by the current map mean that long term mass balance in both catchments is closer to steady state than previously estimated however uncertainty in these areas remains high owing to strong gradients in precipitation from the coast inland a significant and sustained precipitation measurement program will be needed to resolve this uncertainty copyright 2009 by the american geophysical union

crustal influence in the generation of continental flood basalts the suggestion that primordial undifferentiated material may exist in the earth s mantle has recently been revived on the strength of nd isotope data for two types of young continental rocks flood basalts1 and kimberlites2 the limited published data show a clustering of nd isotopic compositions close to those for meteorites with chondritic relative rare earth ree abundance in contrast we now present data for samples from the columbia flood basalt province of the northwestern united states which show large isotopic variability suggestive of mixing processes acting after the separation of the primary magmas from their mantle source