precious metals in volcanic peridotite associated nickel sulfide deposits in western australia ii distribution within the ores and host rocks at kambalda a detailed study of the distribution of pd ir and au in the sulfide component of contact ore layers at lunnon shoot has shown consistent differences between the matrix and massive layers and significant lateral variation within them palladium values in sulfides increase upward in ore sections and when average values are compared with those predicted by the relationship between pd and ni in mined ore ross and keays 1979 loss of pd is indicated the pd au cu enriched footwall sulfide stringers are believed to be the principal repository of remobilized pd iridium has a more uniform distribution and there is no evidence of important loss or gain from contact ore even though massive layer sulfides contain more ir at nearly all sample locations average values of au are higher in matrix layer sulfides and some loss of au from contact ore into footwall stringers and perhaps more distant sites is likely if it is assumed that the distribution of these metals and of ni and cu was uniform during emplacement of sulfide melt their present distribution can be attributed to the following processes redistribution of ir could have occurred during igneous cooling in the presence of a high thermal gradient at the base of the komatiitic volcanic pile and could have been accompanied by some movement of pd in areas of active stress most redistribution of pd is attributed to stress during the existence of metamorphic monosulfide solid solution and significant redistribution of cu may also have been induced by stress the circulation of latestage sulfur bearing solutions through contact ore layers seccombe et al 1981 could have transferred pd au and cu from contact sulfides particularly from the more permeable massive layer to footwall stringers and resulted in the close association of anomalous values of au and cu transfer of about 5 percent of the s in the matrix layer to the massive layer by these solutions would account for the consistently higher ni content of the matrix layer and development of secondary pyrite in the massive layer analyses of sulfide mineral separates from several shoots have consistently shown that pentlandite is the dominant host for pd ir is more uniformly distributed with higher values in pyrite and chalcopyrite and perhaps pyrite is the most important host for au concentrates of secondary pyrite appear to have normal values which suggests some movement of ir during its development the precious metals appear to be uniformly distributed through sulfide mineral hosts but observed variation in ore sections and layers requires that their abundance in the preferred sulfide host must vary considerably further work has strengthened the conclusion of ross and keays 1979 that olivine in the original komatiitic magmas was the principal host for ir and the liquid component was the principal host for pd olivine control lines for pd and ir in primary melts predict values at lower mgo in good agreement with those in hanging wall basalts at kambalda at higher mgo they agree closely with the most recent estimate for pyrolite and a chondritic pd ir ratio of about unity these predicted values result in a substantial increase in pd ir with decreasing mgo results from kambalda and other ni sulfide deposits indicate that the pd ir ratio of the sulfide melt may approximate that of the coexisting silicate melt even though absolute values may vary widely this variation exceeds that expected from the relative volumes of silicate and sulfide melt and may result from differences in factors such as f sub s 2 and f sub o 2 when immiscible sulfides formed measurements of pd and ir offer scope for defining the mgo contents of primary silicate melts associated with immiscible sulfides and may allow a more precise definition of f sub s 2 and perhaps f sub o 2 during formation of immiscible sulfide melts

integrating palaeocaves into palaeolandscapes an analysis of cave levels and karstification history across the gauteng malmani dolomite south africa abstract the drimolen palaeocave system in the fossil hominid sites of south africa unesco world heritage site is well known for numerous remains of early hominins such as paranthropus robustus and early homo these hominin fossils along with bone tools and notably diverse accumulation of non hominin primates and fauna have all been excavated from the main quarry area of the site where extensive lime mining took place here we report the first radiometric age of 1 712 0 269 ma for hominin bearing deposits associated with the dnh7 paranthropus robustus cranium in the main quarry area of the site which is consistent with recent biochronological estimates this age is similar to recent estimates for swartkrans member 1 hanging remnant somewhere between 2 3 and 1 8 ma which also contains paranthropus and early homo simultaneously we integrate the newly radiometrically dated main quarry deposits with a new fossil deposit the drimolen makondo discovered in 2013 that is situated some 50 m up the hill to the west from the main quarry it has experienced only limited disturbance from mining but much more extensive erosion preliminary excavations and analysis have revealed that the makondo infill is older than the main quarry dating to 2 706 0 428 ma its greater age is confirmed by biochronology the makondo thus overlap with the suggested end of deposition of australopithecus bearing sterkfontein deposits although it is yet to yield any hominin remains these new dates for the two drimolen palaeocave system deposits indicates that contrary to prior age estimates the drimolen site as a whole records the critical hominin and faunal turnover in south african palaeocommunities that occurred around 2 3 1 7 ma finally as the drimolen makondo represents a rare example of a pre 2 ma fossil bearing deposit in the gauteng exposures of the malmani dolomite we also integrate our results into the greater south african record of palaeodeposit formation most of which occur between 2 0 and 1 0 ma an analysis of the age of palaeocave infillings across the malmani dolomite suggests that as is classically the case with karst the height within the dolomite is broadly correlated to their age although with some notable exceptions that are likely related to localised geological features our analysis also indicates that most caves have undergone some form of secondary karstification related to a younger phase of cave formation contrasting with models that suggest the cavities all formed at the same time and that infill is related to erosion and the opening up of cave passages as such the reason that few pre 2 ma deposits have been identified in the gauteng exposures of the malmani dolomite is probably because these older caves have been eroded away identifying such early caves is critical in understanding whether earlier hominins may have once existed in south africa or if erosion of older deposits or an absence of speleogenesis at this time has made such early periods absent from the geological record

genesis and geodynamic significance of chromitites from the orhaneli and harmancik ophiolites bursa nw turkey as evidenced by mineralogical and compositional data abstract chromitites from orhaneli and harmancik ophiolites bursa nw turkey are mainly associated with dunites they occur as banded and lenticular bodies showing massive and disseminated textures and are composed of magnesiochromite grains characterized by the following composition cr 2 o 3 58 30 61 19 wt al 2 o 3 8 32 10 03 wt and tio 2 0 14 0 21 wt the cr cr cr al values of magnesiochromite from both ophiolites range between 0 80 and 0 83 and their mg mg mg fe 2 values vary from 0 52 to 0 69 however one chromitite sample bo13b from the orhaneli ophiolite contains magnesiochromite with relatively low cr 2 o 3 48 38 wt high al 2 o 3 15 16 wt and tio 2 0 30 wt contents and average cr and mg values of 0 68 and 0 56 respectively although the total content of platinum group elements pge in most chromitite samples varies between 83 and 169 ppb two samples from the orhaneli ophiolite show enrichments in total pge up to 500 ppb several platinum group mineral pgm grains occur as minute inclusions in magnesiochromite grains 1 15 m across laurite is dominant pgm phase followed by erlichmanite unidentified ru ni fe sulfide irarsite hollingworthite native osmium and iridium millerite and pentlandite represent the most abundant primary base metal mineral inclusions in magnesiochromite whereas bornite awaruite heazlewoodite violarite and orcelite are mostly observed in the secondary silicate groundmass olivine clinopyroxene and amphibole are also identified as silicate inclusions in the magnesiochromite the composition of the magnesiochromite and the parental melts in equilibrium indicate that these magnesiochromite grains were most likely crystallized from boninitic melts in an arc setting however bo13b chromitite sample from the orhaneli chromitites may have crystallized from arc related melt of probably an island arc tholeiite affinity

numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

secular variation from mexican stalagmites their potential and problems abstract as part of a feasibility study to see whether stalagmites could provide useful records of secular variation nine oriented stalagmites were collected from the states of chiapas and san luis potosi mexico of these six have yielded measurable natural remanent magnetizations nrms throughout their length the cleaned magnetizations of one of these samples were shown unequivocally to have recorded the ambient field and there is no reason to believe that this is not true of the primary magnetizations of other samples the sequences of palaeofield directions up the samples length have varying degrees of resolution and serial correlation depending on the rate of field change as averaged by the stalagmite growth rate and the thickness of the measured specimens ages and growth rates were estimated by the u th method the main problem in stalagmite palaeomagnetic analysis is a weak nrm although this may be avoided by judicious choice of the sample samples which possess significant viscous components may be cleaned by alternating field or thermally if there is sufficient magnetic material unlike many sediments stalagmites do not appear to suffer from depositional error problems dating problems may include low initial u content yielding ages with large errors and the presence of allogenic 230 th in detritus which causes older apparent ages the dating limit is 350 ka it is usually not possible to obtain long records comparable with those of most lake sediments and there are aesthetic reasons for not spoiling caves adorned with stalagmites the method is seen to be complementary to the use of sedimentary sequences to study palaeosecular variation studies of the rock magnetism of stalagmites are presented to suggest the mineral carriers of the magnetization and the origins of the natural remanence

the role of viscous magma mush spreading in volcanic flank motion at kīlauea volcano hawai i 1 multiple mechanisms have been suggested to explain seaward motion of the south flank of k lauea volcano hawai i the consistency of flank motion during both waxing and waning magmatic activity at k lauea suggests that a continuously acting force like gravity body force plays a substantial role using finite element models we test whether gravity is the principal driver of long term motion of k lauea s flank we compare our model results to geodetic data from global positioning system and interferometric synthetic aperture radar during a time period with few magmatic and tectonic events 2000 2003 when deformation of k lauea was dominated by summit subsidence and seaward motion of the south flank we find that gravity only models can reproduce the horizontal surface velocities if we incorporate a regional decollement fault and a deep low viscosity magma mush zone to obtain quasi steady state horizontal surface velocities that explain the long term seaward motion of the flank we find that an additional weak zone is needed which is an extensional rift zone above the magma mush the spreading rate in our model is mainly controlled by the magma mush viscosity while its density plays a less significant role we find that a viscosity of 2 5 1017 2 5 1019 pa s for the magma mush provides an acceptable fit to the observed horizontal surface deformation using high magma mush viscosities such as 2 5 1019 pa s the deformation rates remain more steady state over longer time scales these models explain a significant amount of the observed subsidence at k lauea s summit some of the remaining subsidence is probably a result of magma withdrawal from subsurface reservoirs

modeling and mechanistic interpretation of creep of rock salt below 200 c abstract upper and lower bound estimates of the steady state strain rates of rock salt from five locations are analyzed the measurements were obtained in triaxial creep tests including systematic changes in stress and temperature the various data sets are correlated in activation analyses and fitted by models and combinations of models for power law creep cross slip and dislocation glide to show that a cross slip model yields the best mechanistically most credible fit good fits for cross slip and high power law stress exponents rule out the possibility that pressure solution affected the data in a major way to the lowest recorded strain rates of order 10 10s 1

reservoir monitoring and characterization using satellite geodetic data interferometric synthetic aperture radar observations from the krechba field algeria deformation in the material overlying an active reservoir is used to monitor pressure change at depth a sequence of 11 field estimates allows us to construct a measure of diffusive traveltime throughout a reservoir the dense distribution of traveltime values means that we can construct an exactly linear inverse problem for reservoir flow properties application to interferometric synthetic aperture radar insar data gathered over a carbon dioxide c o2 injection site in algeria reveals pressure propagation along two northwest trending corridors an inversion of the traveltimes indicates the existence of two northwest trending high permeability zones the high permeability features trend in the same direction as the regional fault and fracture zones model parameter resolution estimates indicate the features are well resolved

relating viscosities from postseismic relaxation to a realistic viscosity structure for the lithosphere summary models of geodetic observations of postseismic relaxation most often represent the lithosphere with only a few layers with constant viscosity this is surpr ising because rock mechanical experiments consistently demonstrate that we should expect a profound vertical gradient in lithospheric viscosities due to the geothermal gradient i n this thermal boundary layer we isolate the effect of lower crustal flow where the effect of visc osity gradients has the largest impact on surface deformation we therefore explore postseismic deformation in models with realistic vertical viscosity gradients and seek to illustrate th e differences between these models and those with idealized uniform viscosity layers by means of synthetic experiments with a semianalytical viscoelastic relaxation model we show how for a given earthquake the averaged viscosity value obtained for a thick viscoelastic layer is d ependent on both the layout of the geodetic network and on the observation time window

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

some remarks on the relation between vertical motions of the lithosphere during extension and the necking depth parameter inferred from kinematic modeling studies a well established fact of continental stretching is that during the extension phase the lithosphere retains a finite flexural strength vertical loads caused by the redistribution of material with crustal and mantle densities during lithospheric thinning can therefore be supported by the lithosphere whether these loads cause flexural uplift or subsidence depends on the strength distribution within the continental lithosphere in kinematic model studies stratigraphic and other data acquired in the vicinity of sedimentary basins and passive margins are inverted for these loads in these models vertical rebound loads are parameterized in terms of a necking depth parameter we use thermal mechanical finite element simulations to investigate whether rheological information about the continental lithosphere can be extracted from the necking depth parameter derived from kinematic models we conclude that spatial and temporal variations in necking depths near basin edges and passive margins as derived from our dynamic simulations are not consistent with the uniform and stationary necking depth that is usually assumed in kinematic models this is very important since typically the stratigraphic data used in kinematic model studies are acquired in these regions we therefore conclude that there is no one to one relation between the necking depth parameter and the strength distribution in the lithosphere

rheology of rocksalt abstract we review progress in experimental determinations of transient and steady state flow properties and processes of natural rocksalt aggregates focusing primarily on results from avery island louisiana domal salt the steady state flow field established from constant stress and constant strain rate tests at temperatures from 50 to 200 c strain rates from 10 5 to 10 9 s 1 and differential stresses from 20 7 to 2 5 mpa has been separated into two flow regimes each fit by a power law relation at the higher stresses and strain rates this relation is e 1 6 10 4 exp 68 1 rt 10 3 5 3 for which the pre exponential constant is expressed in mpa 5 3 s 1 and the apparent activation energy is in j mol 1 creep rates predicted by equation a do not differ appreciably from those predicted previously the relatively low stress low strain rate data are very well fit by e 8 1 10 5 exp 51 6 rt 10 3 2 4 which is comparable conditions predicts creep rates higher and equivalent viscosities lower than does equation a by two orders of magnitude the change in behavior from a to b is ascribed to a change in rate limiting mechanism from cross slip of screw dislocations to the climb of edge dislocations in this dry material subgrain formation dominates microstructural development during steady state flow of salt and from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies flow stress levels between 0 6 and 1 4 mpa have been estimated these values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because apparently evidence relating to the early deformational history has been overprinted at that stage fluid assisted grain boundary diffusional processes might dominate dislocation creep leading to a linear stress strain rate mechanical response considering buoyancy forces alone behavior described by equation b rather than a reduces the relief necessary to initiate pillow growth but a 1000 fold amplification is required to produce stress differences near 1 mpa that forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures

sulfide paragenesis and sulfur mobility in fe ni cu sulfide ores at lunnon and juan main shoots kambalda textural and sulfur isotopic evidence sulfur isotope data from a range of ore types at lunnon and juan main shoots kambalda have been determined in an attempt to constrain models for the crystallization of the sulfide ores the place of pyrite in the sequence of ore formation and modification is evaluated from data representing the widest possible range of pyrite types showing variable textural relations to coexisting pyrrhotite the sulfur isotopic distribution 68 mineral analyses yields a mean value of 2 4 per mil range 0 5 to 3 8 standard deviation or 0 8 the distribution is characterized by a small spread in values and isotopic disequilibria between coexisting minerals there is no statistical difference between pyrrhotites in massive ores and matrix ores at lunnon shoot and no systematic variation in isotopic composition of pyrrhotite and sparse pyrite in a single ore section from the same deposit these data are consistent with previous suggestions that massive and matrix ores crystallized independently at the magmatic stage variable but overall less positive delta 34 s values for sulfides from disseminated ores from lunnon shoot are consistent with control of isotopic compositions by fe related redox mechanisms during alteration of the ultramafic hosts the delta 34 s values of pyrite and pyrrhotite from massive ores at lunnon and juan main shoots are indistinguishable and the origin of coarse grained pyrite remains equivocal despite the isotopic data the data are however consistent with derivation of fine grained pyrite by late stage relatively low temperature sulfurization processes as suggested by textural studies the greater permeability of the massive ore relative to more disseminated ores and barren ultramafics due to thermal contraction following magmatic and or metamorphic cooling is considered the controlling factor the secondary nature of a significant proportion of the pyrite in massive ores severely limits attempts to use present bulk compositions to predict magmatic crystallization sequences within ore and the use of sulfur metal ratios to estimate parameters such as f sub o 2 at the magmatic stage

concepts of the evolution of the austroalpine basement complex eastern alps during the caledonian variscan cycle the austroalpine basement complex has a complicated pre alpidic history which begins with the caledonian era in the late precambrian and early paleozoic a magmatic sedimentary rock sequence is formed presumably in an island arc or active continental margin environment subduction with eclogite formation is followed by collision high grade metamorphism and anatexis in the ordovician this caledonian basement is preserved in parts of the austroalpine crystalline mass the post caledonian deposits are mainly shelf type sediments with intercalated volcanics although there is evidence for an oceanic basin to the south the variscan facies zones are arranged in sw ne direction oblique to the alpidic trend in a first stage of variscan orogeny in the carboniferous south east vergent decollement nappes syntectonic flysch deposits and granitoids are formed along with regional metamorphism this is followed by a second stage in the permian with north west vergent thrusting renewed granite formation and metamorphism the variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast

the paleomagnetism and u th dating of three canadian speleothems evidence for the westward drift 5 4 2 1 ka bp three speleothems from western canada have been analyzed for their paleomagnetism and u th activity ratios these cave deposits do not appear to show the kind of depositional effects on the recorded magnetic signal as is often the case with sediments in addition the signal may be referred directly to geographic coordinates so that virtual geomagnetic pole plots may be constructed this in turn has allowed assessment of any possible bias of the paleofield the u th activity ratios of two flowstones from the crowsnest pass area of british columbia indicated ages greater than the 350 ka dating limit of the dating method one of the samples was reversely magnetized besides their use in the study of the ancient field these samples have useful geomorphic applications a stalagmite from vancouver island was shown to have recorded clockwise loops of the field vector in the period 5 4 2 1 ka bp this implies a mainly westward drift of the paleofield for this period the corresponding vgp s were mostly far sided

geoarchaeological and 3d visualisation approaches for contextualising in situ fossil bearing palaeokarst in south africa a case study from the 2 61 ma drimolen makondo abstract south africa contains a wealth of palaeokarst deposits that have yielded hominin fossils and early stone age archaeology despite the complex nature of deposition within many of these caves there has been a dearth of detailed geoarchaeological studies undertaken on these sites many sites in south africa have been interpreted using an overly simplistic member system based on simplified sedimentological attributes rather than chronostratigrahic units many of the defined members thus identify different but contemporary geological processes occurring in the caves this has caused serious confusion in reconstructing the life histories of palaeocaves and the ages of the fossil remains interned within them it is critical to uncover new sites that have not been extensively altered by decades of data collection and destructive mining techniques employed early in their discovery although unmined sites present their own problems with regards to extensive colluvium cover and access to fossil bearing units analysing strata that is found in situ enhances overall confidence of interpretations drawn a wealth of geoarchaeological and 3d visualisation techniques can now be employed to aid in the understanding of cave life histories as well as their excavation in this paper we present the first attempt to integrate and publish data from a range of such methods on south african fossil bearing palaeokarst using the newly discovered drimolen makondo deposit as a case study this includes the use of ground penetrating radar 3d visualisation through photogrammetry and multi scale 3d scanning micromophology and petrography palaeomagnetism mineral magnetism synchrotron radiation electron spin resonance uranium lead dating and biochronology our analysis has allowed us to successfully uncover the full extent of this new 2 61 ma fossil bearing palaeokarst deposit and to visualise and interpret its chronostratigraphy

distribution of aluminum rich and chromium rich chromite pods in ophiolite peridotites detailed study of the acoje and the coto districts zambales philippines and comparison with new caledonia the troodos massif and other ophiolites cr rich pods are located near the lherzolite harzburgite transition zone whereas the al rich pods are distributed along the harzburgite dunite transition zone the chromite pods have probably been generated from different sources during the upwelling of mantle peridotites modified journal abstract

mineralogy and magnetism of fe cr spinel series minerals from podiform chromitites and dunites from tąpadla sudetic ophiolite sw poland and their relationship to palaeomagnetic results of the dunites summary this paper presents the mineralogy and magnetic properties of two varieties of chromitite sampled in the same exposure of a serpentinite massif regarded to be a fragment of the sudetic ophiolite the varieties differ mineralogically and magnetically one of them labelled taa and altered to a low degree according to scanning electron microscope sem and microprobe results comprises an unaltered al cr spinel core some secondary chromite and less abundant cr magnetite it has a high magnetic susceptibility and natural remanence nrm and its dominating magnetic phase is ferrichromite with t b t c of about 530 c occurring in grains in the single domain sd pseudo single domains psds and superparamagnetic sp states the second variety labelled tab is highly altered and comprises apart from al spinel two kinds of secondary chromites that do not differ much from the primary al spinel and no cr magnetites this chromitite variety does not reveal one well defined t b t c but a series ranging between 200 and 450 c their grain sizes correspond to the sd sp domain states the same pattern was observed in chromitite taa after annealing in air at a temperature of 700 c dunites associated with the chromitites contain apart from magnetite chromite grains the majority of specimens reveal magnetic characteristics similar to the taa like variety but in some specimens tab like characteristics were also observed a standard palaeomagnetic study performed earlier on the dunites showed that their nrm has three components lower devonian carried by magnetite permian and tertiary recent the results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the taa and tab chromitites

ground motions induced by a producing hydrocarbon reservoir that is overlain by a viscoelastic rocksalt layer a numerical model hydrocarbon reservoir pressure depletion leads to stress changes inside the reservoir and ground deformation which is registered at the surface as subsidence as reservoirs are often overlain by layers of rocksalt or other evaporites which are materials that flow so as to relax stresses inside them there is the potential for time varying surface subsidence this work focuses on understanding the macroscopic mechanisms that lead to rocksalt flow induced ground displacements a finite element model is used for this purpose in which the rocksalt layer is represented by a viscoelastic maxwell material two distinct mechanisms that lead to displacement are observed these are active during different stages of the deformation and have different timescales associated with them an important observation is that the timescale for deformation that is measured at the ground surface is not equal to the timescale for deformation of a viscoelastic material element but can be many times larger than that the sensitivity of the response to the thickness and location of the rocksalt layer is also presented conclusions are drawn which allow for the relative importance of the presence of the rocksalt layer to be assessed and for a framework for understanding time dependent subsidence above producing hydrocarbon reservoirs to be developed finally the changes in stress distribution around a producing reservoir are also briefly discussed

mineralogy and mineral chemistry of detrital heavy minerals from the rhine river in germany as evidence to their provenance sedimentary and depositional history focus on platinum group minerals and remarks on cassiterite columbite group minerals and uraninite in the course of studying the gold bearing heavy mineral spectrum of sediments from the upper rhine river a distinct suite of detrital grains comprising platinum group minerals pgm cassiterite columbite group minerals and uraninite was identified and investigated using conventional and modern analytical methods this study aimed to characterize the selected mineral groups mineralogically and geochemically in order to identify possible source areas and to reconstruct different aspects of the complex sedimentary history of the rhine river sediments the pgm assemblage is dominated by grains of ru os ir alloys 70 followed by pt fe alloys sperrylite and rare other pgm accordingly this pgm assemblage represents highly mature physically and chemically extremely resistant compounds which may have experienced and survived repeated reworking during their sedimentary history pt fe alloys and sperrylite may originate from various sources however the predominant ru os ir alloy grains point to an origin from ophiolite sequences of unknown age but likely pre alpine variscan or older the exact locations of the primary sources and the complex prolonged sedimentary history of the detrital pgm with possibly multiple intermittent storages remain unknown detrital cassiterite grains were dated by the u pb method using la icp ms the age dates of cassiterite largely overlap with zircon age distributions by peaking distinctly at ca 325 ma majority of ages thereby implying a predominantly variscan age of the cassiterite grains and possible derivation from mineralization in the black forest area columbite group minerals are dominantly tapiolite originating from pegmatites rare uraninite grains attest that this mineral experienced rapid erosion transport and deposition in a reducing environment

composition and textures of chromite and platinum group minerals in chromitites of the western ophiolitic belt from pampean ranges of cordoba argentina abstract chromitite bodies hosted in the neoproterozoic western ophiolitic belt of pampean ranges of cordoba argentina were studied at los congos and los guanacos ultramafic bodies with regard to the composition and textures of the chromite and platinum group minerals primary chromite composition is only preserved in some massive chromitites from the los guanacos ultramafic body and is similar to al rich ophiolitic chromitites suggesting that they crystallized from melts with back arc basin basalts babb affinity in the suprasubduction mantle subsequently these chromitites underwent a prograde metamorphism chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends a first trend defined by chromite from massive chromitite in which there is an enrichment in fe3 and fe2 cr remain relatively constant and slightly depleted in al mg a second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite in which a fe rich phase is replacing the al rich chromite this alteration trend is characterized by enrichment in the total iron content fe3 fe2 and a strong depletion in al and mg the chemical composition of all analyzed spinels from los guanacos and los congos as plotted on the ternary fe3 cr al diagram correlates well with the cr spinels from the upper amphibolite to granulite facies metamorphism platinum group minerals pgm identified include native osmium laurite erlichmanite irarsite platinum and a number of inadequately identified phases such as an oxide or hydroxide of ru pt and ir ru pt telluride ir ru as se and ir ru ti compounds native osmium was the only pgm which remained unaltered other pgm underwent mineralogical reworking during metamorphism although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation our results suggest that the ru os ir pgm in the los guanacos and los congos chromitites were modified in situ producing re distribution of these pge on a small scale the presence of rare pt and pge as se minerals was possibly related to remobilization of pt as and se by fluids during the alteration processes

chromitites in ophiolites how where when why part i a review and new ideas on the origin and significance of platinum group minerals abstract platinum group minerals are the dominant control on the budget of platinum group elements in ophiolitic chromitites they vary widely in distribution morphology chemistry and os isotope compositions their distribution in different types of chromitites and different microstructural sites within them may reflect the role of chromite as a physical collector of the platinum group elements during crystallisation of both phases from melts the platinum group minerals show significant os isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains this micro scale isotopic heterogeneity and their random spatial distribution within the chromites argue strongly against an origin of the platinum group minerals via exsolution from the chromite lattice during post magmatic cooling a re interpretation of their microstructures and os isotopes analysed in situ had led us to review the traditional models and to propose several origins for the platinum group minerals in chromitites 1 the suites of platinum group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite 2 some platinum group minerals in chromitites may have been scavenged from wall rock peridotite during migration of the parental melts of the chromitite 3 other platinum group minerals may have precipitated from metasomatic fluid melts that infiltrated existing chromitites the recrystallization of pre existing platinum group minerals during polyphase metamorphism or recycling of the chromitite into deeper mantle levels can explain the presence of micrometric platinum group minerals with distinct os isotope compositions

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

the paleomagnetism and u th dating of mexican stalagmite das2 a 72 cm high stalagmite from mexico das2 has been analyzed for its paleomagnetism and has provided a record of secular variation beginning about 1200 years ago to present the stalagmite did not grow uniformly and a low content of magnetic material has rendered the record less precise in some parts a topographic test involving the magnetic measurement of coeval specimens from the sides of the stalagmite has shown that the record is detectably free from depositional errors age data were provided by the u th method and concordancy checked by14c and226ra methods shallow inclinations at the base of the record are followed by a partial anticlockwise loop in the declination inclination plot there is good agreement between the das2 data and the southwest u s a archeomagnetic record and with a lacustrine record from oregon from 750 to 1400 ad comparison of the record with the present field at the site latitude shows that westward drift of the field has persisted for at least 1200 years at a drift rate of about 0 30 yr 1 a close match in declinations with the present field at the site latitude suggests that the stalagmite may have recorded the passing of the northern edge of the gulf of guinea negative anomaly about 1000 years ago though the corresponding inclination feature is not present in the record an archeomagnetic synthesis by merrill and mcelhinny of worldwide data to estimate positions of the main dipole shows that it was tilted away from mexico from 500 to 1250 ad and this may account for the shallow inclinations seen at the start of the record

buckling of a pervasively faulted lithosphere we investigated the buckling response of a faulted elastic plate under horizontal compression using the finite element technique to better understand the effect of faults on the elastic behavior of a plate we studied the effect of changes in fault spacing depth and dip on the effective young s modulus buckling stress and wavelength our model consists of a thick elastic plate whose entire upper surface is cut by evenly spaced faults we impose either an initial sinusoidal deformation with a fixed wavelength or a random deformation to the grid a fault is represented as a free surface with no resolved shear stress and is allowed to slip in a specified direction using the method of slippery nodes with the assumption of free slip on the faults our model results represent an end member case in which the buckling wavelength and buckling stress are minimized by the presence of the faults

the internal geometry of salt structures a first look using 3d seismic data from the zechstein of the netherlands abstract we present a first look at the large scale complexly folded and faulted internal structure of zechstein salt bodies in nw europe using 3d reflection seismic reflection data from two surveys on the groningen high and the cleaver bank high we focus on a relatively brittle folded and boudinaged claystone carbonate anhydrite layer the z3 stringer enclosed in ductile salt a first classification of the structures is presented and compared with observations from salt mines and analogue and numerical models z3 stringers not only are reservoirs for hydrocarbons but can also present a serious drilling problem in some areas results of this study could provide the basis for better prediction of zones of drilling problems more generally the techniques presented here can be used to predict the internal structure of salt bodies to estimate the geometry of economic deposits of all kinds and locate zones suitable for storage caverns structures observed include an extensive network of zones with increased thickness of the stringer these we infer to have formed by early diagenesis karstification gravitational sliding and associated local sedimentation later this template was deformed into large scale folds and boudins during salt tectonics salt flow was rarely plane strain producing complex fold and boudin geometries deformation was further complicated by the stronger zones of increased thickness which led to strongly non cylindrical structures we present some indications that the thicker zones also influence the locations of later suprasalt structures suggesting a feedback between the early internal evolution of this salt giant and later salt tectonics this study opens the possibility to study the internal structure of the zechstein and other salt giants in 3d using this technique exposing a previously poorly known structure which is comparable in size and complexity to the internal parts of some orogens

compositional zoning in unusual zn rich chromite from the sykesville district of maryland and its bearing on the origin of ferritchromit

impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks accurate forward modeling of surface subsidence above producing hydrocarbons reservoirs requires an understanding of the mechanisms determining how ground deformation and subsidence evolve here we focus entirely on rock salt which overlies a large number of reservoirs worldwide and specifically on the role of creep of rock salt caprocks in response to production induced differential stresses we start by discussing available rock salt creep flow laws we then present the subsidence evolution above an axisymmetric finite element representation of a generic reservoir that extends over a few kilometers and explore the effects of rock salt flow law choice on the subsidence response we find that if rock salt creep is linear as appropriate for steady state flow by pressure solution the subsidence response to any pressure reduction history contains two distinct components one that leads to the subsidence bowl becoming narrower and deeper and one that leads to subsidence rebound and becomes dominant at later stages this subsidence rebound becomes inhibited if rock salt deforms purely through steady state power law creep at low stresses we also show that an approximate representation of transient creep leads to relatively small differences in subsidence predictions most importantly the results confirm that rock salt flow must be modeled accurately if good subsidence predictions are required however in practice large uncertainties exist in the creep behavior of rock salt especially at low stresses these are a consequence of the spatial variability of rock salt physical properties which is practically impossible to constrain a conclusion therefore is that modelers can only resort to calculating bounds for the subsidence evolution above producing rock salt capped reservoirs

sulfides associated with podiform bodies of chromite at tsangli eretria greece small bodies of pyrrhotite chalcopyrite minor pentlandite and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at tsangli eretria central greece banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding a magmatic origin has been proposed for similar sulfide masses in the troodos ophiolite panayiotou 1980 the compositions of the host rocks chromite and of the sulfides have been investigated on average the sulfide mineralization recalculated to metal content in 100 sulfide contains 0 55 ni 5 15 cu 0 29 co 9 ppb pd 179 ppb pt 16 ppb rh 112 ppb ru 31 ppb ir 58 ppb os and 212 ppb au these metal contents particularly the high cu cu ni ratio of 0 78 and the pt pd pt ratio of 0 95 are inconsistent with the sulfides having reached equilibrium with their ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin the average 34s value of the sulfide bodies is 2 while that of a sample of pyrite from country rock schist is 15 6 these values are inconclusive as to the origin of the sulfur it is suggested that the sulfides have been precipitated by hydrothermal fluids possibly those responsible for the serpentinization of the host rocks the source of the metals may have been the host rocks themselves

a multi disciplinary seriation of early homo and paranthropus bearing palaeocaves in southern africa abstract fossils of early homo and paranthropus have been recovered from several sites in southern africa unfortunately their precise age has historically been difficult to assess hampering the reconstruction of their relationships to each other and to fossils from eastern africa multi dating strategies combining biochronological archaeological palaeomagnetic electron spin resonance esr and uranium series techniques are now clarifying their age the following sequencing of sites is suggested swartkrans member 1 2 0 ma gondolin 1 8 ma kromdraai 1 8 1 7 ma sterkfontein m5a 1 8 1 4 ma swartkrans m2 1 7 1 1 ma sterkfontein m5b 1 4 1 1 ma sterkfontein m5c 1 3 0 8 ma swartkrans m3 1 0 0 6 ma the position of coopers d and drimolen is difficult to access because they only have faunal age ranges 1 9 1 6 ma esr suggests mixing is a potential major problem in multi generational sites the oldest southern african representatives of early homo and paranthropus occur around 2 1 1 9 ma in swartkrans member 1 and are recorded almost continuously in the palaeocave deposits until around 1 0 0 6 ma in swartkrans member 3 currently these data suggest that paranthropus and homo first occur significantly later in the southern african record than the eastern african record moreover paranthropus persists much later in southern africa than in eastern africa

salt structures and hydrocarbon accumulations in the tarim basin northwest china the tarim basin is one of the most important hydrocabon bearing evaporite basins in china four salt bearing sequences the middle and lower cambrian the mississippian the paleogene and the neogene have various thickness and areal distribution they are important detachment layers and intensely affect the structural deformation in the basin the kuqa depression is a subordinate structural unit with abundant salt structures in the tarim basin salt overthrusts salt pillows salt anticlines salt diapirs and salt withdrawal basins are predominant in the depression contraction that resulted from orogeny played a key function on the formation of salt structures growth strata reveal that intense salt structural deformation in the kuqa depression occurred during the himalayan movement from oligocene to holocene with early structural deformation in the north and late deformation in the south growth sequences also record at least two phases of salt tectonism in the yingmaili tahe and tazhong areas low amplitude salt pillows are the most common salt structures and these structures are commonly accompanied by thrust faults the faulting and uplifting of basement blocks controlled the location of salt structures the differences in the geometries of salt structures in different regions show that the thickness of the salt sequences has an important influence on the development of salt cored detachment folds and related thrust faults in the tarim basin salt sequences and salt structures in the tarim basin are closely linked to hydrocarbon accumulations oil and gas fields have been discovered in the subsalt intrasalt and suprasalt strata salt deformation has created numerous potential traps and salt sequences have provided a good seal for the preservation of hydrocarbon accumulations large and small scale faults related with salt structures have also given favorable migration pathways for oil and gas when interpreting seismic profiles special attention needs to be paid to the clastic and carbonate interbeds within the salt sequences because they may lead to incorrect structural interpretation in the tarim basin the subsalt anticlinal traps are good targets for hydrocarbon exploration

geomechanical modeling of stresses adjacent to salt bodies part 1 uncoupled models we compare four approaches to geomechanical modeling of stresses adjacent to salt bodies these approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt as well as their treatment of fluid pressures in modeling we simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete simulate effective stress in an elastic medium and use assumed pore pressure during calculations simulate total stress in an elastoplastic medium either ignoring pore pressure or approximating its effects by decreasing the internal friction angle and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations to evaluate these approaches we compare stresses generated by viscoelastic stress relaxation of a salt sphere in all cases relaxation causes the salt sphere to shorten vertically and expand laterally producing extensional strains above and below the sphere and shortening against the sphere flanks deviatoric stresses are highest when sediments are assumed to be elastic whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support so stress perturbations are smaller these comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions

cobalt arsenide orebodies related to an upper proterozoic ophiolite bou azzer morocco the bou azzer cobalt deposits which have produced to date 50 000 tons of cobalt metal are apparently unique they are associated with serpentinites of an upper proterozoic ophiolite and they consist of cobalt ni fe arsenides with accessory sulfoarsenides copper sulfides molybdenite and gold in a quartz carbonate gangue the ore has undergone several phases of brecciation and recrystallization related to late pan african and hercynian deformations which produced the varying shapes of the orebodies lodes veins stocks complex shells and flat lenses the cobalt ore is mainly controlled by tectonic structures along the borders and top of the precambrian serpentinite massifs the orebodies which are buried under the infracambrian volcanic cover may be localized by geochemical zoning the serpentinites are the most obvious source rock for cobalt the quartz carbonate gangue grades laterally into a weathering crust of late proterozoic age however the origin of the arsenic is not yet resolved it may be consanguineous with the serpentinites or derived from an unknown hydrothermal source

silicate and base metal sulfide inclusions in chromites from the maqsad area oman ophiolite gulf of oman a model for entrapment abstract al rich chromites from the maqsad area oman ophiolite have been studied to propose a genetic model for silicate and base metal sulfide bms inclusion entrapment samples were collected in both concordant and discordant nodular pods and in a peculiar stratiform ore chromite ore contains olivine clinopyroxene plagioclase as interstitial silicates regardless of the ore type the silicate inclusions are in decreasing order of abundance composed of ti and na rich phlogopite ti pargasite enstatite mg diopside olivine albite and secondary hydrothermal products chromite hosted bms are dominated by pentlandite heazlewoodite millerite and are nearly devoid of fe and cu sulfides the samples which come from concordant ores are very poor in mineral inclusions a consequence of the strain induced grain boundary migration and recrystallization which they suffered during plastic deformation in contrast the nodular and stratiform samples which escaped post magmatic deformation and densification are inclusion rich the inclusions display negative crystal shape following the chromite symmetry if present in nodular samples they may be arranged as coronas which mimic the shape of the host chromite grains all these textural features are considered to be evidence for mineral inclusion entrapment during magmatic precipitation of chromite by reference to a previous dynamic model of chromite ore genesis we consider that the nodules of chromite crystallized from basaltic magma were flowing inside a narrow cavity in which a non turbulent convective system was established we deduce from the presence of several inclusion coronas or patches in the largest nodules and their aabsence in the smallest that the inclusions were trapped preferentially at the point in which the primitive magma flow meets the convective current the peculiar mineralogy of mineral inclusions dominated by hydroxyl sodic phases indicates a complex interaction at the magmatic stage between the chromite precipitating magma and an unrelated aqueous fluid phase this fluid which was sodium and sulfur bearing travelled through the top of the mantle sequence and the basis of cumulate rocks of the ophiolite it could have been injected at certain stages of its evolution in the same conduits as the chromite precipitating magma locally giving rise to volatile enriched differentiated melts the latter have been sealed together with the liquidus phases of magma olivine clinopyroxene orthopyroxene and plagioclase as inclusions during chromite growth and or dissolution olivine and pyroxenes have partly reacted with the trapped volatile rich melt producing phlogopite and pargasite several hundred degrees below trapping temperatures bms have formed through sulfidization reactions with enclosed silicates and or host chromite

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

ephemeral crustal thickening at a triple junction the mendocino crustal conveyor as the north american crust interacts with the migrating mendocino triple junction the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers within a time frame of 5 m y or less this process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward migrating gorda slab and the base of north america south of the triple junction a time dependent thermal mechanical finite element model is developed to test this hypothesis of plate boundary tectonics results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area the observed crustal structure and triple junction regional seismicity and localized regions of crustal extension coincident with the position of a hypothesized lower crustal melt zone

initiation of asymmetric extension in continental lithosphere govers r and wortel m r 1993 initiation of asymmetric extension in continental lithosphere in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 x 96 the physical conditions are investigated under which the lithospheric scale style of extension is pure shear or simple shear we focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions continental collision magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension we use a finite element method to model the thermo mechanical evolution of continental lithosphere experimental flow laws are used to model the elastic hutle power law creep or diffusion creep rheology of lithospheric rocks our results indicate that if in plane forces change from compressive to tensile immediately after a rapid mountain building phase initiation of a lithosphere scale detachment fault is possible we find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase this result is consistent with observations of gravitational collapse in regions like the aegean and the basin and range province where detachment faults have exhumed lower crustal rocks the predicted dip direction of the fault also agrees with observations in these areas intrusion of magma into continental lithosphere which is subject io in plane tensile forces will cause localization of pure shear deformation the style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase when in plane forces change sign from compressive to tenstle this result also strongly depends on the assumed residual stress distribution if delamination occurs during the mountam huilding phase the style of deformation will be pure shear another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep reduces the tendency to strain localization a large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of mckenzie 1978a in some basins it was found that the kinematics of deformation could be more accurately described by depth dependent stretching factors royden and keen 19801 which introduced asymmetry near basin edges in the simple shear model of wemicke

deformation and recrystallization mechanisms in actively extruding salt fountain microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size qum kuh central iran microstructural study of rocksalt samples from an active salt fountain qum kuh central iran enabled to identify the relative contribution of different deformation mechanisms on extrusive salt flow the microstructural study combined reflected and transmitted light microscopy of gamma irradiated thin sections textural analysis of digitized microstructures and electron back scattered diffraction ebsd deformation microstructures record the strongly variable deformation conditions of salt flow in the diapiric system from the diapiric stem towards the distal part of the mature viscous fountain high stress deformation conditions typical for diapiric stems are recorded in the small subgrains within the porphyroclasts of all documented samples recovery and recrystallization due to divergent and decelerating flow associated with differential stress drop in the salt extrusion above the diapiric orifice is reflected by abundant growth band microstructures this study reveals also evidence for penetration of rainwater into the salt mass and documents the switch from the dominant dislocation creep into dominant solution precipitation creep from the upper part to the distal part of the fountain this deformation mechanism switch is provided by influx of meteoric water and grain size decrease likely controlled by subgrain rotation and grain boundary migration recrystallization

effect of confining pressure on dilatation recrystallization and flow of rock salt at 150 c microstructural evidence for fluid assisted dynamic recrystallization fadrx is widespread in naturally deformed rock salt however the principal experimental evidence for fadrx in salt has been obtained from stress relaxation experiments and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be here we report deformation experiments performed on natural rock salt at a constant strain rate of 3 5 10 7 s 1 150 c and confining pressures pc of 3 30 mpa samples deformed at pc 3 mpa showed continuous work hardening and minor dilatation microstructurally they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide creep electron backscatter diffraction analysis revealed a high frequency of boundaries with low angle misorientations in the range 5 10 in contrast samples deformed at pc 6 5 mpa showed work hardening followed by steady state flow at strains 6 7 these samples compacted slightly and crystal plastic deformation was accompanied by extensive fadrx with a predominance of high angle boundaries 30 50 over low angle boundaries we infer that fadrx is suppressed by dilatation at low pressures as a result of grain boundary disruption at pressures high enough to prevent dilatation however fadrx acts as a recovery mechanism counteracting work hardening the results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 mpa a simple rate model for diffusion and interface controlled fadrx indicates that fadrx should become increasingly important toward natural halokinetic conditions although the effect on flow stresses is likely to be small

rheological behaviour of synthetic rocksalt the interplay between water dynamic recrystallization and deformation mechanisms abstract the ductile deformation of rocks in nature can be greatly enhanced by the presence of water part of the water induced weakening of rocks at depth may come from fluid assisted deformation or recrystallization mechanisms that are absent in dry rocks in this study we investigate the effect of water on the rheological behaviour of rocksalt we focus on quantification of the contribution of individual deformation and recrystallization mechanisms to deformation we also aim to calibrate a flow law that incorporates the effect of all the relevant microphysical processes and hence more accurately describes the flow of rocksalt in nature for this purpose the mechanical behaviour and microstructural evolution of synthetic rocksalt samples that are similar except for differences in water content determined using ftir analysis are investigated the samples are deformed to natural strains of 0 07 0 46 at 50 mpa confining pressure strain rates of 5 10 7 1 10 4 s 1 and temperatures of 75 240 c resulting in flow stresses of 7 22 mpa the flow stress of samples with a water content below 5 ppm dry is higher than that of samples with a water content of 9 46 ppm wet at all strains under the investigated conditions the difference in flow stress can be explained as due to the operation of only work hardening dislocation creep without dynamic recrystallization in the dry material versus combined dislocation and solution precipitation creep plus fluid assisted grain boundary migration in the wet material the results allow us to calibrate a flow law for wet rocksalt that incorporates the effects of solution precipitation creep and fluid assisted grain boundary migration the results also suggest that strain localization in natural rocksalt is more likely to be localized due to fluid infiltration and associated rheological weakening than due to progressive removal of strain hardening substructure by grain boundary migration

quaternary geochronology using the uthpb method abstract we describe a method of uranium thorium lead u th pb isotopic age dating for quaternary rocks the approach uses an instrumental mass discrimination correction for lead isotope ratios which allows small enrichments of radiogenic 206ph and 208ph to be detected at the level of 0 001 igneous rocks hosting minerals with a range in 238 u 204 pb values of 100 can be dated with uncertainties of approximately 15 20 kyr a quaternary rhyolite dated at 1 19 ma by k ar yields a 238u 206ph age of 1 03 0 10 ma a holocene dacite 9 5 ka has uniform 206 pb 207 pb to within 0 0015 in groundmass phases but 1 mm plagioclase phenocrysts have lower 206 pb 207 pb by 0 105 0 002 indicating contamination of the magma after plagioclase crystallization high precision 206 pb 207 ph ratios may be a useful new tool for petrogenetic studies

modeling stress evolution around a rising salt diapir abstract we model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses close to the diapir we model the salt as a solid visco plastic material and the sediments as a poro elastoplastic material using a generalized modified cam clay model the salt flows because ongoing sedimentation increases the average density within the overburden sediments pressurizing the salt stresses rotate near a salt diapir such that the maximum principal stress is perpendicular to the contact with the salt the minimum principal stress is in the circumferential direction and drops near the salt the mean stress increases near the upper parts of the diapir leading to a porosity that is lower than predicted for uniaxial burial at the same depth we built this axisymmetric model within the large strain finite element program elfen our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of mudrocks close to salt diapirs such an understanding is critical for predicting stress porosity and pore pressure in salt systems

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

deformation and recrystallization mechanisms in mylonitic shear zones in naturally deformed extrusive eocene oligocene rocksalt from eyvanekey plateau and garmsar hills central iran abstract microstructural processes in mylonitic shear zones from extrusions of eocene oligocene rocksalts from garmsar hills and eyvanekey plateau central iran are described and analyzed the halite samples were studied by transmitted light microscopy of gamma irradiated thin sections subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by electron backscatter diffraction ebsd the studied shear zones are thin using published flow laws the strain rate in the fine grained mylonites is about 10 10 l s which is in a good agreement with earlier in situ measurements on glacier salt flow rate

strain accumulation across the carrizo segment of the san andreas fault california impact of laterally varying crustal properties 1 major strike slip faults juxtapose geologically dissimilar terrain which may vary in mechanical properties leading to an asymmetric pattern of strain accumulation we present new gps data on the carrizo segment of the san andreas fault separating the salinian block southwest of the fault from franciscan terrane northeast of the fault to better quantify asymmetric strain accumulation we also present a series of finite element models to investigate the possible role of variable elastic layer thickness and material properties of the upper crust the geodetic data are well fit with a simple model comprising a weak upper crustal zone 10 25 km wide northeast of the fault this model is also consistent with geologic data on the distribution of major rock types and corresponding laboratory data on their material properties as well as paleoseismic seismic and magnetotelluric data using this model we estimate a long term average over several seismic cycles slip rate for the san andreas fault of 36 1 5 2 mm yr in agreement with the known holocene rate within uncertainties and a viscosity for the combined lower crust upper mantle of 2 5 1019 pa s

chromite deposits of the tiebaghi ultramafic massif new caledonia the tiebaghi ultramafic massif northern new caledonia has been the main chromite producer of the island it is a small part of a huge ultramafic nappe emplaced during the upper eocene and is mainly composed of peridotites with tectonite fabrics it has suffered a complex tectonic history involving several phases of folding and fracturing the different ultramafic facies ranging from dunite to plagioclase lherzolite appear to form a coherent lithostratigraphic succession the tiebaghi series which reflects the successive appearance upward in the series of olivine and spinel then orthopyroxene and then clinopyroxene plagioclase results from metamorphic reequilibration of the upper part of the series in the field the tiebaghi series is a succession on a centimetric to metric scale of dunite peridotite and pyroxenite with a sequential organization evidenced by cyclic units of decametric thickness geochemically there is a continuous enrichment in ca al si fe mg from harzburgite and dunite to plagioclase lherzolite the mineral compositions also show evolution by a decrease of the mg fe and ni content of silicates and the cr al of chrome spinel and by an increase of the al ti and na contents of pyroxenes with conspicuous geochemical cyclic units the chromite deposits appear at three levels of the host series as schlieren in thick dunitic layers near the base of the series as small lenses in the lherzolite unit and for most of the deposits in or near the transition zone between harzburgite and lherzolite where they may have formed a small number of chromite rich layers of large lateral extension the three most important deposits tiebaghi chagrin and fantoche display typical podiform morphologies and ores with the elongation directions of the orebodies parallel to the tectonic structures of the country rocks in contrast the alpha and vieille montagne 1 deposits situated higher in the series show chromite rich layers of regular thickness with large open folds and with well preserved cumulate textures geochemically ore forming chromite has a rather uniform mg and cr rich composition except for al rich ores rarely found within the lherzolitic unit within a given deposit the mg fe super 2 ratios of chromite and olivine increase with the chromite olivine modal amount the mineral compositions otherwise compare well with those of dunites of the host series the tiebaghi massif shows typical features of both alpine type ultramafic massifs and ultramafic zones of stratiform tholeiitic complexes the differentiation of the tiebaghi series is explained by fractional crystallization of a mixture of basaltic liquid and mg rich minerals mainly olivine accumulation of chromite rich layers during differentiation of the series is due to cr supersaturation of the liquid which may result from drops in total pressure

switching between alternative responses of the lithosphere to continental collision summary we study possible responses to arc continent or continent continent collision using numerical models our short term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones 1 mantle wedge 2 plate interface and 3 lower continental crust depending on which of these is the weakest zone in the system three different responses can be recognized 1 subduction polarity reversal 2 continuation of subduction and 3 delamination and back stepping subduction polarity reversal occurs if the mantle wedge is the weakest zone in the system this happens only if the viscosity of the mantle wedge is at least one order of magnitudelowerthantheaverageviscosityofthelithosphere incontinent continentcollision one additional condition needs to be satisfied for subduction polarity reversal to occur for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0 006 the time required for polarity reversal depends on several parameters the convergence rate the sinking velocity of the detached slab and the relative strength of the mantle wedge arc and backarc the response to collision is continued subduction if the plate interface is the weakest zone and is delamination and back stepping if the lower continental crust is the weakest area in the system our finding that a low viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed

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

metamorphic evolution of sulphide rich chromitites from the chernichevo ultramafic massif se bulgaria abstract the upper mantle rocks of the metamorphosed ophiolite of chernichevo rhodope metamorphic complex in southern bulgaria host small chromite ores with unusual mineralization of base metal sulphides rich in platinum group elements mineralogical and chemical data indicate that after their formation in the mantle the chernichevo s chromite ores were modified by the intrusion of an alkaline mafic melt which resulted in the precipitation of a suite of metasomatic minerals sulphides calcite apatite and ilmenite accompanied by an increase in feo tio2 ga zn vn mn and especially ti and fe2o3 contents in the chromite the degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt sulphide segregation promoted the concentration of high amounts of pges up to 3661 ppb particularly pt and pd yielding chromite ores with a typical flat to positive sloped chondrite normalized pattern subsequently the chromite ores were deformed and metamorphosed together with their host rocks at ultra high pressure uhp 2 5 gpa 1200 c to be later retrograded under eclogite and finally hydrous amphibolite facies conditions giving rise to three microstructural types metamorphism of the most metasomatized i e sulphide rich chromitites at temperatures 700 c within the conditions of uhp and eclogite facies resulted in the formation of 1 non porous recrystallized chromite consisting of a granoblastic microstructure made up of coarse grained blasts and finer grained chromite neoblasts in contrast hydrous metamorphism on the less metasomatized i e sulphide poor chromitite under the conditions of amphibolite facies ca 482 483 c resulted in the formation of 2 partly altered chromite characterized by unaltered cores surrounded by fe2 rich and al depleted porous chromite containing abundant clinochlore and 3 porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to fe2 rich and al depleted porous chromite during metamorphism magmatic ni fe cu sulphides originally formed during the metasomatic event in the mantle were altered resulting in a major leaching of cu rich sulphides leading to significant remobilization of pt and pd

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

solid inclusions in chrome spinels and platinum group element concentrations from the hochgrossen and kraubath ultramafic massifs austria a great variety of platinum group mineral sulfide and silicate inclusions in chrome spinel from hochgrossen and kraubath ultramafic massifs and platinum group element contents of three different rock types have been investigated both ultramafic massifs are tectonically isolated bodies variably serpentinized and metamorphosed greenschist to lower amphibolite facies and show ophiolitic geochemical affinities the chromite from massive chromitites and disseminated in serpentinized dunites and serpentinites exhibits compositional zonation as the result of alteration during serpentinization and metamorphism three distinctive alteration stages are indicated in the chrome spinels from the hochgrossen whereas alteration is less significant in chromites from kraubath the core of chrome spinel represents the least altered part surrounded by an inner rim characterized by slight compositional differences in cr mn fe2 and al with respect to the core the outer rim is formed by ferritchromite with a sharp boundary to the inner rim and shows a significant decrease of al mg cr and increase of fe2 fe3 and ni compared to the core two different groups of inclusions in chrome spinel are present the first group occurs within the chromite core and comprises olivine orthopyroxene amphibole sulfides and platinum group minerals i e dominated by ru os ir sulfides the second group is formed by chlorite serpentine galena pyrite arsenopyrite pt pd rh dominated sulfarsenides and sperrylite in particular the abundance of pt pd rh sulfarsenides and arsenides is typical of both ultramafic massifs and is very unusual for chromitites from ophiolites morphology paragenesis and chemical composition indicate a different origin for these two groups of inclusions the first group is intimately related to the crystallisation of the chromite host the second group of inclusions clearly displays a secondary formation during serpentinization and metamorphism closely related to the alteration of chrome spinel and the development of ferritchromite the distribution patterns of the platinum group elements from massive chromitites disseminated chrome spinel bearing serpentinites and serpentinites exhibit variable enrichment of rh pt and pd rh pt for the hochgrossen and kraubath massifs respectively these results are in accordance with the occurrence and distribution of platinum group mineral phases a remobilisation of pt pd and rh together with ni cu and possibly fe as bisulfide and or hydroxide complexes and deposition of metals by the reaction of the metal bearing hydrothermal fluid with chromite is proposed

dynamic recrystallization of wet synthetic polycrystalline halite dependence of grain size distribution on flow stress temperature and strain abstract it is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress however it is still open to discussion if theoretical models that underpin recrystallized grain size stress relations offer a satisfactorily microphysical basis the temperature dependence of recrystallized grain size predicted by most of these models is rarely observed possibly because it is usually not systematically investigated in this study samples of wet halite containing 10 ppm water by weight were deformed in axial compression at 50 mpa confining pressure the evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0 07 0 12 and 0 25 at a strain rate of 5 10 7 s 1 and a temperature of 125 c the stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0 29 0 46 and one to a strain of 0 68 at constant strain rates of 5 10 7 1 10 4 s 1 and temperatures of 75 240 c yielding stresses of 7 22 mpa the microstructures and full grain size distributions of all samples were analyzed the results showed that deformation occurred by a combination of dislocation creep and solution precipitation creep dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration during deformation grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection i e moving boundaries that crosscut or consume parts of neighbouring grains at steady state grain growth and grain size reduction processes balance yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the grain size sensitive solution precipitation and grain size insensitive dislocation creep fields application of a piezometer calibrated using the recrystallized grain size data to natural halite rock revealed that paleostresses can vary significantly with temperature up to a factor of 2 5 for t 50 200 c and that the existing temperature independent recrystallized grain size stress piezometer may significantly underestimate flow stresses in natural halite rock

secular variation of the earth s magnetic field from 18 5 to 15 0 ka bp as recorded in a vancouver island stalagmite a record of secular variation has been obtained from a stalagmite from vancouver island british columbia canada covering the period 18 5 15 0 ka bp the ages obtained by the u th method were not fully constrained because of the low u content of the sample the record contributes to a regional type curve obtained from the sediments of bessette creek 31 19 ka bp and to which it shows similar features in particular a 1600 year long signal of easterly bias and shallow inclinations is similar to three earlier such features attributed by m e evans to oscillating geomagnetic sources to the west of north america the later part of the composite record correlates fairly well with other western north american lake records there is no evidence for a geomagnetic excursion during this period for the western north american continent

construction of type curves of geomagnetic secular variation for dating lake sediments from east central north america type curves of palaeomagnetic secular variations of declination and inclination through the last 12 000 radiocarbon years have been constructed by combining data from the minnesota lakes with paral

noble metals in thetford mines ophiolites quebec canada part i distribution of gold iridium platinum and palladium in the ultramafic and gabbroic rocks the gold iridium palladium and platinum contents of ophiolites from the thetford mines area quebec were determined by radiochemical neutron activation analysis the main lithologies studied are the tectonite peridotites harzburgites of the lower structural unit and the ultramafic mafic plutonic members of the overlying cumulates average noble metal contents of the main rock types are au ppb ir ppb pt ppb pd ppb tectonite peridotitesharzburgite 1 5 3 2 10 0 3 8dunite lenses in harzburgite 0 27 3 6 2 1 0 29cumulate rocksolivine chromitite 0 28 30 0 1 5 0 33dunite 1 3 2 4 28 0 21 0pyroxenite 1 4 0 24 17 0 29 0gabbro 1 7 0 017 4 5 3 0the harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite the cumulates are highly variable in noble metal content iridium is enriched in early olivine chromitite and progressively depleted in later rocks palladium increases from early to late ultramafic cumulates and decreases in gabbros whereas pt shows no systematic trend in the cumulates gold increases very slightly from early to late cumulates the noble metal trends in cumulate rocks result mainly from fractional crystallization but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals sulfur rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks there is little preferential partition of noble metals into any ferromagnesian silicate but the platinum group elements particularly ir are strongly associated with chromite acid leaching experiments suggest that in harzburgites ir is concentrated along chromite silicate grain boundaries whereas in cumulus chromitites ir occurs partly within chromite grains it is suggested that ir was originally incorporated into the chromite structure during a high temperature magmatic stage and in the case of the harzburgites has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress serpentinization probably occurred in both oceanic and continental environments cumulates suffered intense serpentinization but little noble metal mobility resulted harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of au in conjunction with formation of asbestos veins

platinum group minerals in alpine chromitites from southwestern oregon electron microprobe and scanning electron microscopy the platinum group minerals found in the chromitites are laurite porous ru rich alloys os ir and pt rich alloys with highly variable compositions an ir cu rich sulfide a pt arsenide and three relatively rare platinum group sulfides the platinum group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains some of these inclusions also contain silicates and millerite base metal alloys largely ni fe occur in cracks in the chromitites along with magnetite ferritchromite chromian chlorite and serpentine the primary platinum group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite modified journal abstract

high cr and high al chromitites from the sagua de tanamo district mayari cristal ophiolitic massif eastern cuba constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum group elements article i nfo several chromitite bodies of variable sizes are hosted in dunite harzburgite of the small b3 km mining district of sagua de tanamo in the mayari cristal ophiolitic massif eastern of cuba the chromian spinel in these bodies displays a large range in cr cr cr al atomic ratio from 0 74 to 0 45 in high cr chromitites cr and ti contents increase from harzburgite to dunite to chromitite a roughly opposite trend of variation in cr is observed in high al chromitites although in this case ti distributes randomly the differences in the type of chromian spinel i e high cr or high al in the chromitite coincide with a different behavior of the platinum group elements pge whereas high cr chromitites are rich in pge and contain abundant grains of platinum group minerals pgm high al chromitites are systematically poor in pge and in pgm the calculated melts in equilibrium with chromian spinel of high cr chromitite are island arc thoeliites iat with boninitic affinity whereas those in equilibrium with chromian spinel of high al chromitites are back arc basin basalts babb the formation of high cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity melt rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing mingling of melts promotes crystallization of mononomineralic high cr chromian spinel in contrast high al chromitites formed by the mixing mingling of babb melts within conduits not in equilibrium with dunite percolation of primitive babb melts through pre existing dunite dissolved olivine producing melt conduits in which babb melts mixed and formed high al chromian spinel the higher pge and pgm in high cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite the coexistence of both types of chromitites in one small mining district reflects the temporal and or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back arc basin in a supra subduction zone environment

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins

salt geology and tectonics of selected brazilian basins in their global context this work addresses the geological and geophysical interpretation of salt structures in selected brazilian sedimentary basins from intracratonic palaeozoic evaporites in the amazon and solimoes basins to divergent margin evaporite basins formed during the mesozoic break up of gondwana there is an intriguing correlation between evaporite basins and hydrocarbon accumu lations in all the selected basins discussed the solimoes and amazonas basins developed eva porite depositing environments as the middle carboniferous sea was closing during a plate convergence phase the salt basin along the eastern brazilian and western african margins devel oped along the mesozoic rifts of the south atlantic regional seismic interpretation and potential field gravity and magnetic data over the eastern brazilian and west african margins suggest a very thick autochthonous salt layer deposited over rifted continental crust and particularly above the thick sag basin sediments over the hyperextended crust that marks the transition from continen tal to oceanic crust most of the hydrocarbon discoveries in the eastern brazilian and western african margins are in post salt turbidite and carbonate reservoirs but recent discoveries in the deepwater salt basins along the southeastern brazilian margin indicate that pre salt plays will rep resent an important contribution to hydrocarbon production in the near future