localized slip controlled by dehydration embrittlement of partly serpentinized dunites leka ophiolite complex norway abstract dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure a process referred to as dehydration embrittlement however the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate in the layered ultramafic sections of the leka ophiolite complex in the central norwegian caledonides prograde metamorphism of serpentinite veins led to local fluid production and to the growth of mg rich and coarse grained olivine with abundant magnetite inclusions and 18o values 1 0 1 5 below the host rock embrittlement associated with the dehydration caused faulting along highly localized

stratiform platinum group element mineralizations in the ultramafic cumulates of the leka ophiolite complex central norway three types of stratiform platinum group element pge enrichment have been documented within olivine cumulates of the layered series in the leka ophiolite complex 1 os ir ru enrichments associated with chromite rich rocks 2 pt dominated enrichments and 3 pd dominated enrichments associated with discrete 10 to 50 cm thick sulfide rich horizons drill cores across the pt and pd enriched horizons have yielded 0 5 and 1 m averages of ca 1 ppm pge au and hand samples from one of the horizons yielded 3 to 4 ppm pge the horizons can be traced laterally for more than 1 5 km beta autoradiography of core samples shows that the enrichments may be only a few cm thick and that they form well defined horizons rich in discrete pge bearing minerals mainly alloys of pt fe ni and cu au pd tellurides and antimonides the pge enrichments are associated with the bases of macrorhythmic units and seem to have formed during or shortly after the influx of primitive magma into the chamber the pt pd enriched horizons are displaced relative to chromite horizons thus explaining why stratiform chromitites in ophiolites in contrast to chromitites in many intracratonic layered intrusions are depleted in pt and pd relative to os ir and ru the study demonstrates that ophiolites should not be disregarded as potential sources for pt and pd

the recycling of chromitites in ophiolites from southwestern north america abstract podiform chromitites occur in mantle peridotites of the late triassic puerto nuevo ophiolite baja california sur state mexico these are high cr chromitites cr cr cr al atomic ratio 0 61 0 69 that contain a range of minor and trace elements and show whole rock enrichment in ipge os ir ru that are similar to those of high cr ophiolitic chromitites crystallised from melts similar to high mg island arc tholeiites iat and boninites in supra subduction zone mantle wedges crystallisation of these chromitites from s undersaturated melts is consistent with the presence of abundant inclusions of platinum group minerals pgm such as laurite rus2 erlichmanite oss2 osmium and irarsite irass in chromite that yield tma trd model ages peaking at 325 ma thirty three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages 2263 44 ma to 278 4 ma and hf o compositions hf t 18 7 to 9 1 and 18o values

anomalous porosity and permeability preservation in deeply buried tertiary and mesozoic sandstones in the cusiana field llanos foothills colombia reply abstract porosity in deeply buried 14000 ft 4200 m sandstones of the lower tertiary mirador formation in the cusiana field llanos foothills colombia is less than 10 porosity units p u however permeability is as much as 1000 md which is believed to be due to the simple quartzose detrital grain composition and large grain size the low porosity has resulted from pervasive quartz cement 10 15 fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated at a temperature of around 100 c during deep burial surprisingly thick 400 ft 120 m sandstones of the underlying upper cretaceous guadalupe group in the cusiana field have porosity as high as 20 p u these sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented quartz cement is not abundant and occurs as rims of microcrystalline quartz on quartz and phosphate grains in contrast interbedded quartz arenites within the guadalupe group are heavily cemented by quartz cement and have porosity similar to the overlying mirador formation the major control on porosity in these sandstones appears to be the presence of quartz cement the quartz cement is controlled by grain composition the reasons for the lack of quartz cement in the lithic sandstones of the guadalupe group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility the presence of high porosity sweet spots in deeply buried sandstones may be linked to the occurrence of phosphate beds a primary depositional control

transfer of olivine crystallographic orientation through a cycle of serpentinisation and dehydration our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from one stage to another within the leka ophiolite complex in the central norwegian caledonides prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures the outer part of the veins comprises coarse grained olivine that forms an unusual brick like microstructure the inner part of the veins surrounding a central fault is composed of fine grained olivine where the fault movement included a dilational component optically clear equant olivine occurs in the centre electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation cpo from the olivine in the porphyroclastic host rock however misorientation is weaker and associated to different rotation axes we propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its cpo growth towards pre existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick like microstructure in the veins when dehydration embrittlement induced slip a strong strain localisation on discrete fault planes prevented distortion of the cpo due to cataclastic deformation grain size reduction did not significantly modify the olivine cpo this illustrates how a cpo can be preserved though an entire metamorphic cycle including hydration dehydration and deformation processes and that the cpo and the microstructures e g grain shape of one phase do not necessarily record the same event

geomorphological processes and their palaeoenvironmental significance at the shum laka cave bamenda western cameroon abstract the climatic environmental history of the shum laka cave western cameroon and its surroundings has been established by means of geomorphological sedimentological and botanical data tied in a well dated stratigraphical sequence since 30 000 yr bp this mountainous area 1 650 2 000 m asl has never been a desert nor a tropical forest temperatures seldom significantly dropped below freezing mountain forest with galleries along the water courses and grassland in between prevailed most of the time a certain evolution can be observed from 31 000 to 20 000 yr bp annual precipitation was initially higher than today and subsequently somewhat lower from 13 000 yr bp onwards humidity and temperatures increased again around 9 000 yr bp they probably reached higher values than today from 9 000 yr bp human activity became very apparent this may have influenced the evolution of the natural landscape and obliterated climatological effects this evolution has been interrupted three times by dryer climatic conditions which each time lasted for only 1 or 2 millennia respectively at 33 000 at 11 000 and at 3 000 yr bp another important less humid episode occurred between 6 070 and 3 180 yr bp finally there is some evidence for a drier period between 8 480 and 7 150 yr bp the climatic evolution established for the shum laka cave confirms and refines former data from the area and is in good agreement with data from barombi mbo and chad

modelling the erosional susceptibility of landslide catchments in thick loess chinese variations on a theme by jan de ploey abstract in his 1990 paper catena 17 175 183 jan de ploey proposed a system whereby catchments are viewed as functional units in which geomorphic work results from the combined effects of water erosion and mass movements his measure of erosional susceptibility of catchments es introduced the gravitational potential in addition to the kinetic energy of the eroding agents this modelling of the erosional susceptibility of catchments in terms of energy was being used to explore time dependent variations of es when jan de ploey died also at this time he extended an invitation to the present authors to test and extend his model using our data base on the landslide dominated catchments in the thick loess country of north central china this paper presents a series of new equations expressing variations in es using a data base for over 200 landslides in the loess covered mountainous terrain relative relief up to 1500 m of the lanzhou region of eastern gansu province these equations express the loss of potential energy of the landslide mass in relation to the relative relief and the pressure energy input in the form of rainfall these indices of erosional susceptibility can be used to analyse the differences in response from one catchment to another and through time between the characteristics of vegetation cover including land use hydrology geomechanical behaviour and mechanisms of movement on slopes using certain assumptions on percentage area of catchments affected by mass movement and long term mean annual precipitation in the lanzhou region the es equation suggests an estimate for non clayey materials of about 10 4 m 2 s 2 which is a significantly higher susceptibility value than the mean worldwide value for non clayey materials obtained by de ploey

a model to explain the occurrence of platinum and palladium rich ophiolite complexes during the last ten years significant concentrations of pt and pd more than 1000 ppb pt pd have been reported from several ophiolite complexes including shetland leka al ays zambales ceruja and thetford similar studies on others including those in cyprus and oman have failed to reveal equivalent concentrations here we compare the characteristics of these ophiolite sequences and propose that pt and pd rich ophiolites contain thick sulphur bearing ultramafic crustal units that crystallized from pge rich boninitic magmas produced by excess partial melting in a water rich subduction zone

reservoir performance of late eocene incised valley fills cusiana field llanos foothills eastern colombia the cusiana field is located in the llanos foothills of eastern colombia the principal reservoir is the late eocene mirador formation which comprises 50 of reserves currently the mirador reservoir is providing nearly all of the 150 00bopd of production from the cusiana field the mirador reservoir comprises a stack of incised valley deposits the fills of the valleys are dominated by quartz arenite sandstones the average porosity of the valley sandstones is 8 which reflects abundant quartz cement approximately 14 and significant compaction during deep burial approximately 20 000feet single valleys are up to 70 feet thick and exhibit a distinctive bipartite fill that reflects changing energy conditions during filling bases of valleys have the coarsest grain size and have sedimentological and trace fossil evidence for deposition in highly stressed brackish water environments the upper parts of the valleys are typically finer grained and were deposited in more saline settings despite the low porosity of the mirador valleys drill stem tests and production log data show that they have phenomenal performance characteristics rates of ge 10 000bopd are achieved from single valleys bases of the valley fills are the key contributors to flow integration of detailed core and pore system analysis with the reservoir performancemore data shows that the permeability fabric of the mirador can be explained by original depositional architecture and simple loss of primary porosity comparison of cusiana with other quartz rich sandstones from around the world suggests that it s low porosity high performance is predictable less

the giant chromite deposits at kempirsai urals constraints from trace element pge ree and isotope data the investigation of stable and radiogenic isotopes and of platinum group pge and rare earth elements ree in chromitites and associated ultramafic rocks of the kempirsai massif southern urals gives strong evidence for a multistage formation of giant ophiolitic podiform chromite deposits present in the southeastern part of the massif the kempirsai ophiolite massif is divided by a shear zone into two parts in the northwestern area small bodies of al rich chromite formed from basaltic melts between 420 to 400 ma according to sm nd mineral isochrons of harzburgite pyroxenite websterite and gabbro harzburgites and pyroxenites in this area are enriched in light ree and have nd 400 6 and sr 400 5 chromitites have scattered pge distributions pd ir 0 4 7 0 being partly enriched in pd and pt os 400 of one chromitite is 4 4 the southeastern part of the kempirsai massif well known for its world class deposits of podiform low al magnesiochromite is characterized by harzburgite and dunite enriched in light ree with very low nd 400 4 3 to 17 1 and positive sr 400 10 values chromitites are strongly enriched in ir os and ru and depleted in pd and pt os 400 of three chromitites is uniform and approaches c1 and dmm compositions in veins and pods postdating crystallization of massive chromite pargasitic amphibole formed in equilibrium with fluid inclusion bearing chromite at temperatures close to 1000 c these amphiboles give 40ar 39ar stepwise heating ages of 365 to 385 ma and are characterized by low nd 400 0 6 to 4 6 and general enrichment in ree the cooling ages correspond to a 379 3 1 6 ma rb sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif from these data it is concluded that parts of the kempirsai massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments subduction occurred at least 15 35 ma after a melting event that produced a typical ophiolitic sequence in the paleozoic sakmara zone we conclude that large chromite orebodies formed from second stage high mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime and in a fore arc position to the magnitogorsk island arc

assessment of the interrill and rill contributions to total soil loss from an upland field plot abstract govers g and poesen j 1988 assessment of the interrill and rill contributions to total soil loss from an upland field plot geomorphology 1 343 354 erosion measurements were carried out on an upland field plot in the loam region of belgium the sediment being detached by splash on interrill areas is transported to the channel system mainly by interrill wash over the entire 7500 m field plot rill and gully erosion is more important than interrill erosion however the relative importance of interrill erosion varies in time and in space due to changes of the interrill surface characteristics and the activation of sidewall and gullying processes in the channel network these factors should be taken into account in a realistic rill interrill erosion model introduction at present few available field data permit the separation of rill and or gully and interrill contributions to sediment removal from arable land the scarcity of this type of field data is a major factor inhibiting the validation and fur ther development of recently developed physi cally based models describing upland erosion by water e g foster 1982 rose et al 1983 knowledge of the contribution of rill and inter rill processes to total soil loss has also some practical interest if the ratio between rill and interrill erosion were known even approxi mately it would be possible to obtain a quick estimate of total erosion by making a volumet ric recording of rill patterns plot measure

aerodynamic and geometric diameter of airborne particles discussion the paper by chen and fryrear 2001 addresses a much neglected topic in the general field of grain dynamic behavior but it is unfortunately marred by a number of omissions and inaccuracies first some minor errors have to be pointed out which may mislead some readers if not corrected 1 equation 1 should read r f 6 rv without the minus sign 2 equation 7 is valid only for reynolds numbers less than 1 not more than 1 as stated 3 in their table 1 the legend states that t m is the calculated mean settling velocity which should read the calculated mean settling time 4 the legend in fig 2 should read measured average settling times and not settling velocities 5 the values for v t and the geometric particle diameters given in table 1 do not correspond to the graph in fig 5 6 on page 366 it is mentioned that sieve intervals of 75 m were used for particles between 125 and 600 m and 10 m intervals between 125 and 10 m however in table 1 the intervals apparently vary from 105 m to 25 m for the first range and between 19 and 8 m for the second 7 it is implied on the same page that perfect spheres were selected by examining them under a microscope this cannot be done because all the grains would lie on their shortest axes they may therefore just as well

erosion processes in a dune landscape along the dutch coast abstract the landscape of the dunes along the dutch coast is the result of the interaction of biological and geomorphological processes where the latter prevail new landforms are created blowouts by wind erosion and rills and accumulation footslopes by overland flow spatial extent and rate of the erosion processes have been assessed by mapping and by measuring it appears that dune sand is either sensitive to erosion by wind or erosion by water depending on the presence of organic material clean yellow sand is cohesionless and easily blown away but has no surface runoff grey sand containing organic matter on the other hand will resist wind erosion but it is waterrepellent and will be washed from slopes when rain follows a period of dry weather once the grey surface sand is removed from the surfaces by water erosion wind erosion begins in the underlying yellow sand present day geomorphological processes reshape the slopes of the original parabolic dunes wind is particularly active in summit areas where new blowouts are formed whereasoverland flow creates concave south exposed slopes the stepped surface of the north exposed slopes is due to mass movements following rabbit activity

formation of h2 ch4 and n species during low temperature experimental alteration of ultramafic rocks abstract hydrogen methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities it is well known that production of h 2 through serpentinization of the primary fe ii containing minerals olivine and pyroxene in ultramafic rocks and reduction of co 2 to ch 4 are processes that occur at higher temperatures knowledge is however limited about these reactions under low temperature conditions and how they are affected by fe ii containing secondary minerals such as brucite and serpentine which are commonly found in these environments in this experimental study we explored the formation of h 2 ch 4 and nh 4 during low temperature 25 c reactions between deionised water and 1 unaltered 2 medium altered and 3 highly altered dunites over a period of 99 days hydrogen was detected in all three experiments and the concentration increased over time the o 2 concentration decreased over time but o 2 was still present at significant levels when h 2 started to form this suggests that the h 2 was formed locally at the surface of mineral grains rather than in the hypoxic solution the highly altered dunite gave the highest and the unaltered dunite the lowest h 2 concentrations and while the o 2 concentration decreased fast with the medium altered dunite significant amounts of o 2 still remained with the unaltered dunite at the end of the experiments both the medium and highly altered dunite gave o 2 concentrations close to or below the detection limit during the last part of the experiment factors that likely affected the h 2 production are fe content mineralogy dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite methane was also detected and increased with time in all experiments the results indicate that ch 4 formation also occurred locally on the surface of minerals detection of both nh 4 and no 3 in the aqueous solutions indicates that previously absorbed n species were leached out during the experiment increasing nh 4 and decreasing no 3 concentrations over time suggest no 3 reduction likely catalysed by mineral surfaces the results imply that low temperature water rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy yielding metabolic processes by subsurface microorganisms such reactions could play an important role in sustaining subsurface microbial communities particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow spreading mid ocean ridges and possibly also in other rocks and sediments the results suggest that brucite and possibly also serpentine may be more important sources of fe ii and thus h 2 formation than olivine in low temperature ultramafic systems formation of h 2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of h 2 formation and the subsurface habitat for aerobic h 2 oxidizing microorganisms

geomorphology and natural hazards abstract natural hazards research was initiated in the 1960 s by gilbert white and his students who promulgated a research paradigm that involved assessing risk from a natural event identifying adjustments to cope with the hazard determining people s perception of the event defining the process by which people choose adjustments and estimating the effects of public policy on the choice process studies of the physical system played an important role in early research but criticismsof the paradigm resulted in a shift to a prominence of social science geomorphologists are working to fill gaps in knowledge of the physical aspects of individual hazards but use of the information by social scientists will only occur if information is presented in a format that is useful to them one format involves identifying the hazard according to seven physical parameters established by white and his colleagues magnitude frequency duration areal extent speed of onset spatial dispersion and temporal spacing geomorphic hazards are regarded as related to landscape changes that affect human systems the processes that produce the changes are rarely geomorphic in nature but are better regarded as atmospheric or hydrologic an examination of geomorphic hazards in four fields soil erosion mass movement coastal erosion and fluvial erosion demonstrates that advances in those fields may be evaluated in terms of the seven parameters geomorphologists have contributed to hazard research by focusing on the dynamics of the landforms the prediction of occurence the determination of spatial and temporal characteristics the impact of physical characteristics on people s perception and the impact of physical characteristics on adjustment formulation opportunities for geomorphologists to improve our understanding of geomorphic hazards include research into the characteristics of the events particularly with respect to predicting the occurence and increased evaluation of the impact of human activities on natural systems

a regional scale geochemical survey of soil o and c horizon samples in nord trondelag central norway geology and mineral potential abstract a highly efficient low density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in central norway soil o and c horizon samples n 752 were collected in nord trondelag and parts of sor trondelag and analysed for 53 chemical elements ag al as au b ba be bi ca cd ce co cr cs cu fe ga ge hf hg in k la li mg mn mo na nb ni p pb pd pt rb re s sb sc se sn sr ta te th ti tl u v w y zn and zr and pb isotopes in an aqua regia extraction at the sample density of one site 36 km2 the four metal deposits which have been mined in the area within the last 50 years were all detected as geochemical anomalies in addition a number of new anomalies that may warrant follow up surveys were found in terms of geology the grong olden culmination is marked by a distinct 206pb 207pb isotope anomaly geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area though the fosdalen iron ore deposit is only marked in the soil o horizon the c horizon outlines the more prominent anomalies of possibly economic interest climatic factors like the input of marine aerosols along the coast are clearly visible in the soil o horizon low density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in nord trondelag county

diversity of platinum group mineral assemblages in banded and podiform chromitite from the kraubath ultramafic massif austria evidence for an ophiolitic transition zone we report highly unusual platinum group mineral pgm assemblages from geologically distinct chromitites banded and podiform of the kraubath massif the largest dismembered mantle relict in the eastern alps the banded chromitite has a pronounced enrichment of pt and pd relative to the more refractory platinum group elements pges of the ipge group os ir ru similar to crustal sections of ophiolites on the contrary the podiform chromitite displays a negatively sloping chondrite normalised pge pattern typical of ophiolitic podiform chromitite the chemical composition of chromite varies from cr 73 77 in the banded type to 81 86 in the podiform chromitite thirteen different pgms and one gold rich mineral are first observed in the banded chromitite the dominant pgm is sperrylite 53 of all pgms which occurs in polyphase assemblages with an unnamed pt base metal bm alloy and pd rich minerals such as stibiopalladinite mayakite mertieite ii unnamed pd rh as and pd pt as sb minerals this banded type also contains pge sulphides about 7 represented by a wide compositional range of the laurite erlichmanite series and irarsite 8 os ir alloy geversite an unnamed pt pd bi cu phase and tetrauricupride are present in minor amounts by contrast the podiform chromitite which yielded 21 different pgms is dominated by laurite 43 of all pgms which occurs in complex polyphase assemblages with pge alloys ir os os ir pt fe pge sulphides kashinite bowieite cuproiridsite cuprorhodsite unnamed fe cu ir rh 2s4 braggite unnamed bm ir and bm rh sulphides and pd telluride keithconnite a variety of pge sulpharsenides 33 including irarsite hollingworthite platarsite ruarsite and a number of intermediate species have been identified whereas sperrylite and stibiopalladinite are subordinate 2 the occurrence of such a wide variety of pgms from only two 2 5 kg chromitite samples is highly unusual for an ophiolitic environment our novel sample treatment allowed to identify primary pgm assemblages containing all six pges in both laurite dominated podiform chromitite as well as in uncommon sperrylite dominated banded chromitite we suggest that the geologically geochemically and mineralogically distinct banded chromitite from kraubath characterises the transition zone of an ophiolite closely above the mantle section hosting podiform chromitite rather than being representative of the crustal cumulate pile

modelling the erosional susceptibility of catchments in terms of energy summary basically the paper proposes to consider catchments as functional units in which geomorphic work generally results from combined effects of erosion by water and by mass movements or from a combination of erosion by wind by runoff and by mass movements therefore a measure of the erosional susceptibility e s of catchments introduces the gravitational potential beside the kinetic energy of the eroding agents furthermore expressions of e s stress the impact of the spatial distribution of erosion as a much differentiating factor for future research it is recommended to look after timedependent variations of e s for representative drainage basins

ground water silicification and leaching in sands example of the fontainebleau sand oligocene in the paris basin in the fontainebleau sand oligocene superposed lenses of quartzite are restricted to outcrops and structurally low zones of the formation the quartzite lenses result from a silicification process controlled by water table levels during the recent geomorphologic evolution of the landscape the quartzite lenses pinch out rapidly under the limestone capped plateaus and are related to the development of a bleached sand profile above the water table the sand is bleached and the former quartzite lenses are corroded along their margins moreover flint pebbles are dissolved and primary smectite and illite are weathered to kaolinite dissolved silica can thus reach supersaturated levels allowing quartz precipitation quartz grains are overgrown at the base of the profile in zones of ground water outflow related to spring lines the adjustment of the water table during the landscape incision led to formation of quartzite lenses at deeper levels this silicification process has no climatic significance but is dependent on the stability of the landscape and on its progressive dissection the rate of quartz cementation indicated by geomorphologic considerations is consistent with hydraulic and geochemical calculations it appears to be very rapid approximately 30 000 yr for a quartzite lens

platinum group element pge mineralisation and chromite geochemistry in the berit ophiolite elbistan kahramanmaras se turkey abstract the high al chromitite bearing stratigraphic levels cr ranges from 0 38 to 0 47 are located at the mantle crust transition zone mtz and in the cumulate dunites in the berit ophiolite in se turkey iridium group elements ipge ir os and ru and related mineralisation were previously reported from ophiolitic high cr chromitites in the berit elbistan area palladium group elements ppge pt pd and rh and associated mineralisation are predominantly found in the high al chromitites and they are presented in this study for the first time our results indicate that ipge enriched phases of the pgms such as laurite are present in both the ppge enriched and the ppge poor chromitites in berit the high al chromitite chondrite normalised patterns in the berit ophiolite show an enrichment of ppge with values of up to 1700 ppb pt and up to 4469 ppb pd during their occurrence sulphur saturation is consistent with the production of minor quantities of base metal sulphides bms which took place in the form of droplets enclosed in chromite commonly with ppge enriched patterns these ppgm assemblages include pd pt tellurides sperrylite platarsite pd antimonide pd sb as and pt pd cu au alloys the ppge enrichment in the berit ophiolite is unusual and the presence of such occurrences is known only in a few other ophiolites worldwide the geochemical evolution of the high cr ratios of the high cr chromitites indicates an origin due to high degrees of partial melting whereas the high al chromitites indicate lower degrees of partial melting both high degree and low degree melts may have been fluxed through the upper mantle either synchronously or possibly with a hiatus between the generation of different melts

an overview of the pge concentrations in the shetland ophiolite complex extremely anomalous platinum group element pge concentrations are known at one site cliff in the shetland ophiolite complex systematic pge analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous pge occurrences throughout the cumulate ultramafic sequence and their distribution is described here more sites with pge enrichments as anomalous as those at cliff have not been found at cliff the anomalous values of over 60 ppm pt pd are restricted to a chromite rich sulphide bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite lower values of up to 6 4 ppm pt pd occur in chromite poor sulphide rich dunites also within this dunite lens the only other chromite rich dunite lens in the cliff area also has detected pt and pd but with low pt pd values of 200 ppb in contrast harzburgites metasediments underlying the basal thrust serpentinites from the basal thrust and internal faults are all barren of pt and pd detection limit 20 ppb in the cliff area therefore detected pge values are restricted to magmatic chromite rich dunite lenses pge analyses of drill core from beneath the disused chromite quarries at cliff are low 227 ppb pt pd indicating that the extremely anomalous pge values are very restricted this supports the idea that pge concentrated magmatically within the dunite lens at cliff may have been remobilized locally to produce extremely anomalous pge values in an alteration zone only a few metres in diameter elsewhere in the ophiolite anomalous pt pd values of up to 4 ppm occur in chromite rich sulphide bearing dunite within the dunite unit which stratigraphically overlies the harzburgite pt pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences high level wehrlites and pyroxenites within the gabbro unit representing late ultramafic differentiates contain lower levels of pge of up to 310 ppb pt pd the gabbros all have less than 20 ppb pt and pd this distribution indicates that the pge concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with ni cu au and cr pge have not been found above detection limits in fault zones away from primary pge mineralization sites and it is concluded that pge are not significantly hydrothermally reconcentrated at cliff remobilization may have occurred over short distances of only a few metres at most

review of platinum group element distribution and mineralogy in chromitite ores from southern iran abstract the platinum group element pge potential of four chromite mining districts in southern and south eastern iran was investigated using geochemical and mineralogical methods a total of 20 mines and prospects were sampled in the neyriz ophiolite the abdasht and sikhuran complexes in the esfandagheh district and the faryab district chromitite and dunite were analysed for major trace element and pge geochemistry and the platinum group element mineralogy pgm in all mining districts chromites analysed from mantle tectonite probable transition zone and cumulate zone chromitites are refractory low in tio2 pge concentrations range from 5000 ppb with median values around 200 ppb in all complexes investigated the 187os 188os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial os mantle normalised pt ir ratios are generally low in the sikhuran complex pods of sulphide bearing chromitite 0 2 1 2 sulphides occur in a dunite zone interpreted as an ultramafic cumulate zone chromites in chromitite are characterised by high cr 75 81 and intermediate mg numbers 45 65 in associated dunite chromites cr 74 80 are more fe rich mg 29 50 chromitite and dunite carry up to 5 2 ppm total pge and are characterised by high normalised pt ir up to 5 pt pd large degrees of partial melting in a supra subduction zone geotectonic setting probably in a two stage melting process produced boninitic melts that crystallised refractory cr mg rich and ti poor chromite in some cases sulphide saturation was achieved during and after chromite formation saturation during chromite crystallisation is evidenced by ni cu rich sulphide droplets preserved in some chromitites sulphur saturation after chromite crystallisation governed the formation of ni rich sulphide liquids that exsolved monosulphide solid solution which later crystallised to form pge bearing pentlandite platinum and pd did not partition into pentlandite but crystallised as pt and pd rich pgm associated with the pentlandite

evidence for the emplacement of ca 3 0 ga mantle derived mafic ultramafic bodies in the ukrainian shield abstract the archean bedrock of the ukrainian shield is divided into western and central blocks by a n w trending tectonic zone known as the golovanevsk suture zone within this zone there are numerous mafic ultramafic bodies including the chromitite bearing lipovenki and kapitanov massifs the chromitites of these massifs possess mineralogical and textural features and platinum group element systematics similar to those typically observed in ophiolitic podiform chromitites although there is no field evidence for an ophiolitic structure the geochemical evidence strongly suggests an origin in the oceanic lithosphere the re os ratios of the chromites and whole rock chromitites are extremely low and consequently their 187 os 188 os ratios have increased little since crystallization osmium isotope model ages indicate formation at approximately 3 0 ga suggesting that the massifs represent portions of the archean oceanic lithosphere

the de ploey erosional susceptibility model for catchments es abstract this article is based on an unpublished manuscript by the late prof dr jan de ploey it focusses on the erosional susceptibility of catchments in terms of total energy input by the meteorological factors water and wind e s model the form and the universal character of the basic expression of the erosional susceptibility are explained applications of the model are illustrated for different processes such as landsliding and debris flows gullying creep and rill interrill combinations characteristic e s values exist for these processes also e s values vary in regions with differing pedo botanical characteristics and land use the difference between e s values for single storm events and longer periods are related to the magnitude frequency distribution of erosive activities and allow in many cases an assessment of the age of erosion phenomena and an estimation of the recurrence period of extreme single events

paragenesis of multiple platinum group mineral populations in shetland ophiolite chromitite 3d x ray tomography and in situ os isotopes abstract chromitite from the harold s grave locality in the mantle section of the shetland ophiolite complex is extremely enriched in ru os and ir at g g concentrations high resolution x ray computed tomography on micro cores from these chromitites was used to determine the location size distribution and morphology of the platinum group minerals pgm there are five generations of pgm in these chromitites small average 5 m in equivalent sphere diameter esd euhedral laurites often with os ir alloys are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised also within the chromitite there are clusters of larger 50 m esd aligned elongate crystals of pt rh ir os and ru bearing pgm that have different orientations in different chromite crystals these may have formed either by exsolution or by preferential nucleation of pgms in boundary layers around particular growing chromite grains thirdly there is a generation of large 100 m esd composite os ir ru rich pgm that are all interstitial to the chromite grains and sometimes form in clusters it is proposed that os ir and ru in this generation were concentrated in base metal sulfide droplets that were then re dissolved into a later sulfide undersaturated magma leaving pgm interstitial to the chromite grains fourthly there is a group of almost spherical large 80 m esd laurites hosting minor os ir ru rich pgm that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer these may be hosted in an annealed late syn or post magmatic fracture finally a few of the pgm have been deformed in localised shear zones through the chromitites the vast majority of the pgm including small pgm enclosed within chromite larger interstitial pgm and elongate aligned pgm have os isotope compositions that give re depletion model ages approximately equal to the age of the ophiolite at 492 ma a number of other pgm not confined to a single textural group fall to more or less radiogenic values with four pgm giving anomalously unradiogenic os corresponding to an older age of 1050 ma the 187 os 188 os isotopic ratios for pgm from cliff and quoys from the same ophiolite section are somewhat more radiogenic than those at harold s grave this may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal os

pge rich podiform chromitites in the al ays ophiolite complex saudi arabia an example of critical mantle melting to extract and concentrate pge the al ays ophiolite complex in saudi arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements pge in podiform chromitite with maximum values of 2 570 ppb pt 6 870 ppb pd 840 ppb rh 5 800 ppb ru 6 200 ppb ir and 3 300 ppb os smooth chondrite normalized pge profiles indicate igneous pge ratios these suggest that in situ alteration of the pgm caused only minor mobility of pge during secondary modification of the mineralogy thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration three main types of pge mineralization are observed that are defined by their relative abundances of individual pge type 1 has ru both pt and pd with negative slope chondrite normalized profiles type 2 has ru either pt or pd pt pd ir ratios of 1 to 5 and convex upward chondrite normalized profiles type 3 has ru either pt or pd pt pd ir ratios of 5 to 60 positive slope chondrite normalized profiles and is associated with elevated cu and ni concentrations the unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition for example cr2o3 varies from 39 to 69 wt percent pge mineralization types 1 2 and 3 are related to the composition of the chromite type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent cr2o3 type 2 occurs in chromitite having a range of 53 to 61 wt percent cr2o3 and type 3 occurs in chromitite having a range of 39 to 51 wt percent cr2o3 the pge form a great variety of platinum group minerals pgm and they differ among the three types of pge mineralization type 1 is characterized by euhedral os ir and ru ipge alloys and laurite both commonly enclosed in chromite as well as members of the irarsite hollingworthite solid solution series and pt ipge bearing pgm both commonly interstitial to the chromite grains type 2 pge enrichment is characterized by ipge pt and rh bearing pgm type 3 pge enrichment hosts predominantly pd and pt bearing pgm associated with ni and cu bearing minerals where exposed to the serpentinization process the pgm are altered to alloys arsenides antimonides and oxides that form irregular shapes or may form pseudomorphs of former pgm they are commonly associated with ni and cu bearing minerals including ruthanian pentlandite millerite arsenides and pge bearing awaruite mantle melting and subsequent crystallization were at an optimum to concentrate pge in the al ays ophiolite complex crystallization of ipge commonly prior to chromite crystallization and latterly with some pt and rh occurred across the range of chromitite composition crystallization of pd with some remaining pt occurred during sulfur saturation in chromitite formed from a more evolved magma we propose that this crystallization was from a magma that was enriched in pge because the degree of mantle melting was just sufficient to extract the pge but not dilute them in a melt that includes further mantle melting this feature is likely to be common to other pge rich ophiolite complexes such as in shetland in the united kingdom leka in norway thetford in canada pindos in greece tropoja in albania and in new caledonia if equilibrium partial melting had continued in al ays then the magma would have been diluted by subsequent pge poor melt this would have prevented sulfur saturation until much higher in the sequence producing pt and pd bearing base metal sulfides in the crustal wehrlite and gabbro as has occurred in the cyprus and oman ophiolites