paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

post depositional tectonic modification of vms deposits in iberia and its economic significance the original stratigraphic relationships and structure of vms deposits are commonly obscured by deformation this can also affect their economic significance as shown by several iberian pyrite belt ipb sw iberia examples the contrasting rheologic properties of the different lithologies present in an orebody massive sulphide feeder stockwork alteration envelope volcanic and sedimentary rocks play a major role in determining its overall behaviour variscan thin skinned tectonics led to stacking of the massive pyrite and stockwork bodies in duplex structures resulting in local thickening and increased tonnage of minable mineralization furthermore differential mechanical behaviour of the different sulphide minerals localised the detachments along relatively ductile sulphide rich bands the result was a geochemical and mineralogical reorganisation of most deposits which now consist of barren massive pyrite horses bounded by base metal rich ductile shear zones metal redistribution was enhanced by mobilisation of the base metal sulphides from the initially impoverished massive pyrite through pressure solution processes to tensional fissures within the already ductile shear zones in nw iberia vms deposits were also strongly overprinted by the variscan deformation during emplacement of the cabo ortegal and ordenes allochthonous nappe complexes but no stacking of the orebodies was produced original contacts were transposed and the orebodies their feeder zones and the country rock acquired pronounced laminar geometry in lower grade rocks greenschist facies cabo ortegal complex solution transfer mechanisms are common in pyrite which remains in the brittle domain while chalcopyrite shows ductile behaviour in higher grade rocks amphibolite facies ordenes complex metamorphic recrystallisation overprints earlier deformation textures the contrasting behaviour of the ipb and nw iberian deposits is explained by key factors that affect their final geometry composition and economics such as pre deformation structure size and mineralogical composition of the orebody and associated lithologies temperature crustal level deviatoric stress and availability of a fluid phase during deformation and the style and rate of deformation

u pb geochronology of vms mineralization in the iberian pyrite belt a geochronology study using u pb isotope dilution tims analyses of zircon has been conducted to determine the ages of volcanic associated massive sulfide vms deposits in the iberian pyrite belt ipb the world s most prolific vms province ages have been determined for host rocks to four vms systems that span the ipb the giant rio tinto and aljustrel districts in the central region lagoa salgada to the west and las cruces to the east a sample of chloritized quartz porphyritic dacite rhyolite in the footwall of the san dionisio massive sulfide deposit of the rio tinto district is 349 76 0 90 ma this is taken as the best age estimate of the mineralization in the rio tinto district probably the world s largest volcanogenic massive sulfide system two xenocrystic zircons from the same sample yielded 207pb 206pb ages of 414 and 416 ma which provide a minimum estimate for the age of the inherited component a biotite tonalite from the campofrio area 3 5 km north of the center of the rio tinto district is chemically similar to the felsic host rock protolith at rio tinto the campofrio sample has an age of 346 26 0 81 ma slightly younger and outside of the 2 error for the rio tinto age therefore this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the rio tinto district the campofrio sample also has three zircon analyses with 207pb 206pb minimum ages of 534 536 and 985 ma indicating inheritance from ordovician and neoproterozoic sources in the aljustrel vms district a u pb zircon age of 352 9 1 9 ma characterizes the altered green tuff host rock of the algares deposit which is slightly older than the rio tinto age two zircons with 207pb 206pb ages of 531 and 571 ma from this sample indicate inheritance from a cambrian or older source the age of mineralization at lagoa salgada is given by essentially identical ages of 356 21 0 73 and 356 4 0 8 ma for footwall and hanging wall samples respectively the hanging wall sample has two zircon analyses with 207pb 206pb ages of 464 and 466 ma indicating inheritance from an ordovician or older source the age for an altered dacite tuff sample from the hanging wall of the las cruces deposit is 353 97 0 69 ma one zircon analysis from the las cruces sample has a 207pb 206pb age of 1048 ma indicating inheritance from a neoproterozoic source these u pb ages refine the ipb geochronology provided by previous studies and they suggest that either volcanism progressed toward the center of the ipb or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation the zircon inheritance provides direct evidence for proterozoic to ordovician sources reflecting either basement rocks beneath the phyllite quartzite group during vms formation in late tournaisian times or a proterozoic to ordovician detrital component in phyllite quartzite group source rocks the presence of an older crustal component is consistent with vms formation during rift development at a continental margin

mixing properties of phengitic micas and revised garnet phengite thermobarometers mixing properties for muscovite celadonite ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end members when a partially ordered solution model is assumed the enthalpy of mixing among the end members muscovite celadonite ferroceladonite is nearly ideal although the gibbs energies of muscovite celadonite and muscovite ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing thermodynamic consistency is achieved for data on phengite compositions inassemblages with a pyrope kyanite quartz coesite b almandine kyanite quartz coesite c talc kyanite quartz coesite and d garnet phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low grade schists the muscovite paragonite solvus has been reanalysed using the asymmetric van laar model and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end members results are applied to a natural pyrope coesite phengite talc rock from the western alps and to investigate the conditions under which biotite bearing mica schists transform to whiteschist like biotite absent assemblages for average pelite bulk compositions

redox and speciation micromapping using dispersive x ray absorption spectroscopy application to iron in chlorite mineral of a metamorphic rock thin section 1 the first dispersive xanes mapping is presented here the experiments have been conducted at the iron k edge on the dispersive exafs beamline of the european synchrotron radiation facility france the mapping has been performed on a polished thin section of 30 m of a natural metamorphic rock including different types of minerals because of the high x ray absorption due to the thickness of the glass sample holder 1 mm the data have been collected in the fluorescence mode using the so called turbo xafs design the effective spot size was approximately 10 10 microns and an area 390 180 microns in size was mapped improvements of the acquisition process allowed collection of each xanes spectrum in 1 5 s and with a step size of 5 microns so that 2808 spectra with full xanes information were collected then automatic procedures for data reduction and mapping reconstruction were developed using matlab the results are maps of iron content oxidation state and speciation with a 5 m spatial resolution after two dimensional deconvolution subsequent analyses of the reconstructed images provide some quantitative calibrations

a new method for quantitative petrography based on image processing of chemical element maps part i mineral mapping applied to compacted bentonites most natural rocks or engineered materials display a multi scale heterogeneity ranging from the nanometer to the centimeter their spatial textural heterogeneity can be approached from chemical element maps acquired using various techniques sem epma sxam synchrotron xrf tem depending on the chosen magnification chemical map processing that yields quantitative petrographic information is improved here according to newly developed mineral thresholding methods that accommodate mixtures and solid solutions the complex case of an mx80 compacted bentonite is used as a test case the 14 major chemical elements of this sample were mapped using an electron probe microanalyzer and chemical map processing yielded a quantitative map of the 18 mineral species of bentonite with a spatial resolution of a few micrometers the textural heterogeneity of the solid part of the sample is thus visualized and quantified on an area ranging between 0 1 1 cm2 the method also provides a complete modal analysis of the sample the methodology is expected to have broad applications in earth and materials sciences

fractionation of bulk rock composition due to porphyroblast growth effects on eclogite facies mineral equilibria pam peninsula new caledonia chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards an effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock to examine this effect on mineral assemblages the bence albee matrix correction was applied to x ray intensity maps collected using eclogite samples from northern new caledonia in order to determine the chemical composition of all parts of the sample the manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix a series of calculated equilibrium volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high p conditions pressure temperature pseudosections are calculated for these compositions in the cao na2o feo mgo al2o3 sio2 h2o system assemblages modal proportions and mineral textures observed in the new caledonian eclogites can be closely modelled by progressively removing elements forming garnet cores from the bulk rock composition the pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage prograde textures and the development of retrograde assemblages

quantification of electron microprobe compositional maps of rock thin sections an optimized method and examples quantification of discrete pressure temperature domains in deformed chlorite white mica bearing metapelites was undertaken on mineral compositions derived by two dimensional microprobe compositional mapping of selected areas of rock thin sections in order to achieve compositional information at sufficient analytical precision spatial resolution and sample coverage within a typical analysis time of 1 day an optimization of measurement methods was necessary the method presented here allows collection of raw counts for eight different element concentrations at an analytical precision of 1 2 wt x ray intensity multiplane maps one map per measured chemical element are translated into concentration multiplane maps utilizing selected conventionally measured spot analyses combined with the castaing approximation for each mineral as this step requires identification of the different minerals present in the mapped area a statistical clustering technique to identify different groups of composition was developed guided by simple petrographic inspection of the thin section to delineate the important minerals in the mapped area finally the compositions of each pixel are translated into a mineral structural formula thus yielding a new kind of image with a high content of petrological information the reliability of the mineral composition images was emphasized by carrying out precision tests on the analytical data the possible use of chemical maps to infer the p t deformation history of metamorphic rocks is illustrated with two samples from the spitzbergen and the sambagawa blueschist facies belts in both samples a strong correlation between structures and chemistry is observed qualitative estimates of p t conditions from the si content of mica and chlorite are in good agreement with their location in microstructures that formed at different times therefore the combination of chemical maps with microstructural observations is a very powerful approach to understand both the evolution of complex metamorphic rocks and the control by deformation of mineral reactivity

petrology and geochemistry of feldspathic impact melt breccia abar al uj 012 the first lunar meteorite from saudi arabia abar al uj aau 012 is a clast rich vesicular impact melt im breccia composed of lithic and mineral clasts set in a very fine grained and well crystallized matrix it is a typical feldspathic lunar meteorite most likely originating from the lunar farside bulk composition 31 0 wt al2o3 3 85wt feo is close to the mean of feldspathic lunar meteorites and apollo fan suite rocks the low concentration of incompatible trace elements 0 39ppmth 0 13ppm u reflects the absence of a significant kreep component plagioclase is highly anorthitic with a mean of an 96 9 ab 3 0 or 0 1 bulk rock mg is 63 and molar feo mno is 76 the terrestrial age of the meteorite is 33 4 5 2kyr aau 012 contains a similar to 1 4x1 5mm 2 exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali suite rocks the im clast is mainly composed of clinopyroxenes contains a significant amount of cristobalite 9 0 vol and has a microcrystalline mesostasis although the clast shows similarities in texture and modal mineral abundances with some apollo pigeonite basalts it has lower feo and higher sio2 than any mare basalt it also has higher feo and lower al2o3 than rocks from the fan or mg suite its lower mg 59 compared to mg suite rocks also excludes a relationship with these types of lunar material

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

silica secreting biota and mass extinctions survival patterns and processes abstract high survival and at least regional blooms of siliceous marine groups with coeval retreat of calcareous organisms is known from the frasnian famennian radiolarians silicisponges and end cretaceous diatoms radiolarians mass extinctions a strictly actualistic approach to the palaeoecology of silica secreting biota is of limited significance especially for silica enriched and oligotrophic palaeozoic epeiric seas and in particular during biotic turnovers adaptation among silica secreting marine plankton has led toward more efficient utilization of shrinking dissolved silica resources within surface waters of cenozoic oceans as shown by both radiolarians and diatoms this biosiliceous signal during the major biotic crises is mostly explained by a large scale increase in volcano hydrothermal activity during major plate boundary re arrangements triggering global ecosystem perturbations the cumulative stimulus favoured siliceous versus calcareous biota growth deposition and preservation because of 1 a higher rate of input of silica and other nutrients promoting eutrophication pulses 2 a punctuated or reversed greenhouse climatic effect episodically coupled with 3 vigorous oceanic circulation due to a variety of volcanogenic upwellings and turnovers siliceous communities adapted to more eutrophic conditions have thrived in the stressed niches on the other hand expanding anoxia and nutrification may lead to a stepwise loss of deep water niches and consequent selective decline of more specialized oligotrophic radiolarians and other pelagic biota by contrast siliceous faunas experienced severe non selective losses during the end permian mass extinction recorded as a chert gap thus the greatest environmental disaster of the phanerozoic was driven by a unique set of killing factors extremely effective for radiolarian productivity e g due to the combination of a drastic volcanic winter with expanding superanoxia

modern style plate subduction preserved in the palaeoproterozoic west african craton the timing of onset of modern style plate tectonics on earth is debated analysis of rocks in the west african metamorphic province which is more than 2 gyr old reveals that some minerals formed under conditions similar to those in modern day subduction zones suggesting that subduction occurred on the palaeoproterozoic earth

origin of fe mn si layers associated with the permian volcanic hosted massive sulphide deposits in the tasik chini district peninsular malaysia abstract the volcanic hosted massive sulphide vhms deposits in the tasik chini district peninsular malaysia are hosted in permian volcano sedimentary rocks and are composed of fe mn si layers which occur stratigraphically above the ore bodies the oxide layers are 1 cm to 2 m thick and occur discontinuously throughout the bukit botol deposit whereas layer of oxides up to 10 m thick occurs continuously over a strike length of over 50 m at the bukit ketaya deposit their mineralogy is dominated by hematite quartz barite magnetite pyrite clastic components and commonly exhibit massive iron silica banding to microbreccia like textures the fe mn si layers from the bukit botol and bukit ketaya deposits generally have high si fe mn and base metals contents various proportions of al zr and ti and high levels of ba as u and v such mineralogical and geochemical concentrations and other geological suggest that the associated fe mn si layers were deposited by the mixing of mainly chemical components and to a smaller degree detrital components in a local submarine environment due to hydrothermal activity on the seafloor

source and evolution of ore forming hydrothermal fluids in the northern iberian pyrite belt massive sulphide deposits sw spain evidence from fluid inclusions and stable isotopes a fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits aguas tenidas este concepcion san miguel san telmo and cueva de la mora located in the northern iberian pyrite belt the isotopic analyses were mainly performed on quartz chlorite carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits and also on some fluid inclusion waters homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 c salinity of most fluid inclusions ranges from 2 to 14 wt nacl equiv a few cases with th 80 110 c and salinity of 16 24 wt nacl equiv have been also recognized in addition fluid inclusions from the soloviejo mn fe jaspers 160 190 c and 6 wt nacl equiv and some late to post hercynian quartz veins 130 270 c and 4 wt nacl equiv were also studied isotopic results indicate that fluids in equilibrium with measured quartz 18ofluid 2 to 4 chlorites 18ofluid 8 14 dfluid 45 to 27 whole rocks 18ofluid 4 7 dfluid 15 to 10 and carbonates 18oankerite 14 5 16 13cfluid 11 to 5 evolved isotopically during the lifetime of the hydrothermal systems following a waxing waning cycle at different temperatures and water rock ratios the results fluid inclusions 18o d and 13c values point to a highly evolved seawater along with a variable but significant contribution of other fluid reservoirs such as magmatic and or deep metamorphic waters as the most probable sources for the ore forming fluids these fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust

distinguishing peperite from other sediment matrix igneous breccias lessons from the iberian pyrite belt abstract breccias composed of coarse monomictic porphyritic igneous clasts and fine grained matrix are common in subaqueous volcanic successions we use the descriptive name sediment matrix igneous breccia for this facies and have recognized at least five different origins among examples in the iberian pyrite belt 1 peperite 2 sediment infill volcanic breccia 3 mud matrix resedimented hyaloclastite 4 mud rich water settled fiamme breccia and 5 apparent sediment matrix igneous breccia because the components and textures are similar discriminating among the different origins can be very difficult both peperite and sediment infill volcanic breccia can occur along top contacts of thick intervals of felsic coherent and monomictic breccia facies the presence of peperite indicates that the contact is intrusive whereas the presence of sediment infill volcanic breccia indicates that the contact is depositional hence correct distinction between peperite and sediment infill volcanic breccia is an important means of discriminating felsic intrusions from felsic lavas and domes the distinction underpins the reconstruction of volcanic centers and facies architecture and the ordering of volcanic intrusive sedimentary and mineralizing events in ancient submarine volcanic successions in addition in volcanic hosted massive sulfide districts such as the iberian pyrite belt paleoseafloor positions are considered highly prospective for massive sulfide ore bodies correct interpretation of sediment matrix igneous breccias is also important in this context because sediment infill volcanic breccia mud matrix resedimented hyaloclastite and mud rich water settled fiamme breccia all form or are deposited at the seafloor and hence delineate paleoseafloor positions in contrast peperite and apparent sediment matrix igneous breccias form subsurface and do not delineate paleoseafloor positions

petrology of pelitic schists in the oligoclase biotite zone of the sanbagawa metamorphic terrain japan phase equilibria in the highest grade zone of a high pressure intermediate type of metamorphic belt the oligoclase biotite zone of the bessi area central shikoku is characterized by sodic plagioclase xca 0 10 0 28 bearing assemblages in pelitic schists and represents the highest grade zone of the sanbagawa metamorphic terrain mineral assemblages in pelitic schists of this zone all with quartz sodic plagioclase muscovite and clinozoisite or zoisite are garnet biotite chlorite paragonite garnet biotite hornblende chlorite and partial assemblages of these two types correlations between mineral compositions mineral assemblages and mineral stability data assuming ph2o psolid suggests that metamorphic conditions of this zone are about 610 25 c and 10 1 kbar based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase biotite zone of the sanbagawa terrain with those in the new caledonia omphacite zone as an example of a typical high pressure type of metamorphic belt and with those in a generalized upper staurolite zone as an example of a medium pressure type of metamorphic belt progressive assemblages within these three zones can be related by reactions such as

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

earliest microbial trace fossils in archaean pillow lavas under scrutiny new micro x ray absorption near edge spectroscopy metamorphic and morphological constraints filament shaped titanite microtextures in early archaean c 3 47 ga metabasalts from the barberton greenstone belt of south africa have been argued to represent earth s oldest microbial trace fossils but petrological data on the environmental conditions that led to the formation of these microtextures are sparse we present here new metamorphic constraints on the equilibrium mineral assemblage containing the titanite microtextures together with fe speciation redox variations in the chlorite matrix surrounding the microtextures and morphospace analysis of the microtextures to test their biogenicity thermodynamic phase diagram modelling using a calculated rock microdomain composition indicated that the titanite mineral assemblage is stable at temperature conditions of t 240 360 c the high resolution quantitative mapping results combined with micro x ray absorption near edge spectroscopy at the fe k edge in chlorite revealed that the titanite microtextures are located in low temperature high xfe3 chlorite bands veins and microdomains supporting an origin as abiotic retrograde mineral cooling textures these new data combined with a late c 2 9 ga u pb date for the titanite are incompatible with the existing biological model which invokes microbial micro tunnel formation followed by titanite infilling and preservation in the early archaean sub seafloor the continuum of titanite morphologies reported herein indicates that titanite morphology cannot be used as reliable evidence in support of a biogenic origin in these metavolcanic rocks it is proposed that the titanite microtextures of purported biogenic origin from other greenstone belts such as the pilbara craton of western australia also deserve to be scrutinized by high resolution petrological investigations

a method for applying matrix corrections to x ray intensity maps using the bence albee algorithm and matlab matlab scripts that apply the bence albee 1968 matrix correction algorithm to x ray intensity data collected as element maps on a cameca sx 50 microprobe are used to produce two dimensional maps of oxide weight percent and cation proportions for sio2 a l 2o3 feo mno mgo cao na2o and k2o once generated large data sets of mapped oxide weight percent values or cation numbers that retain spatial information can be used petrologically the technique is used to evaluate the compositional range of barroisitic amphibole in an eclogite from new caledonia to examine aspects of equilibration during the partial hydration of the eclogite facies mineral assemblage

relation between the intensity of deformation and retrogression in blueschist metapelites of tinos island greece evidenced by chlorite mica local equilibria the late greenschist deformation of the cycladic blueschists unit on tinos island evolves along a ne sw gradient of non coaxial strain across a major extensional shear zone deformation is more intense and the greenschist overprint is also more severe close to the shear zone that separates the cycladic blueschists from the upper ophiolitic unit in the ne of the island the compositional variability of chlorite and k white mica kwm in metapelites sampled along a ne sw cross section is characteristic of their p t conditions of crystallization and is mainly characterized by variations of their si al fe and mg contents due to the tschermak substitution siiv fe mg vi alvialiv multi equilibrium p t estimates based on the assumption of local equilibrium indicate that each sample recorded a part of the p t history which depends on the distance to the contact to the main shear zone i e the intensity and the pattern of deformation the combination of p t estimates obtained from all the samples collected along the transect provides a continuous p t path which is composed of two parts it provides insights on the syn and post orogenic exhumation for some of the cycladic blueschists in particular we could distinguish two stages in the exhumation process a first stage in the blueschist facies occurred along a cold p t path during the construction of the hellenides the second exhumation stage occurred along a warmer p t path during the formation of the aegean sea between the two stages of exhumation there is a thermal reequilibration of the crust and a partial thermal overprint of the blueschist parageneses

bedded jaspers of the ordovician lokken ophiolite norway seafloor deposition and diagenetic maturation of hydrothermal plume derived silica iron gels sedimentary beds of jasper red hematitic chert in the ordovician lokken ophiolite of norway are closely associated with volcanogenic massive sulphide vms deposits the jaspers occur in the immediate hangingwall and laterally peripheral to the large lokken 25 30 mt and small hoydal 0 1 mt vms deposits and are exposed discontinuously for several kilometres along strike massive or laminated types predominate jasper sulphide debris flow deposits are also abundant near vms deposits the jaspers contain hematite rich laminae showing soft sediment deformation structures and microtextural evidence that record the presence of a colloidal precursor and an origin as gels early textures include 1 straight or curved chains of hematitic filaments 3 10 m in diameter and 20 100 m long 2 branching networks of 15 25 m thick tubular structures surrounded by cryptocrystalline hematite and filled with quartz and euhedral hematite 3 small up to 10 m spherules composed of cryptocrystalline hematite and silica and 4 up to 50 m silica spherules with hematitic cores the small filaments seem to have been deposited in varying proportions in the primary laminae possibly together with hematitic and siliceous microspheroids diagenetic changes are represented by polygonal syneresis cracks and the presence of cryptocrystalline originally opaline silica chalcedony quartz carbonate and cryptocrystalline hematite and or goethite forming botryoidal masses and spheroids 10 m to 5 mm in diameter coarser euhedral grains of quartz carbonate and hematite are integral parts of these textures bleached silica rich jaspers preserve only small relics of fine grained hematite rich domains and locally contain sparse pockets composed of coarse euhedral hematite epidote

paleozoic and mesozoic silica rich seawater evidence from hematitic chert jasper deposits laterally extensive beds of highly siliceous hematitic chert jasper are associated with many volcanogenic massive sulfide vms deposits of late cambrian to early cretaceous age yet are unknown in analogous younger including modern settings textural studies suggest that vms related jaspers in the ordovician lokken ophiolite of norway were originally deposited as si and fe rich gels that precipitated from hydrothermal plumes as colloidal silica and iron oxyhydroxide particles rare earth element patterns and ge si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 10 3 to 10 4 similar to modern examples we propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids as suggested by previous workers but instead was deposited from silica rich seawater flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume a model involving amorphous silica opal a precursors to the jaspers suggests that silica contents of cambrian early cretaceous oceans were at least 110 mg l sio 2 compared to values of 40 60 mg l sio 2 estimated in other studies the evolution of ancient silica rich to modern fe rich precipitates in submarine hydrothermal plumes reflects a changeover from silica saturated to silica depleted seawater through phanerozoic time due mainly to ocean wide emergence of diatoms in the cretaceous

development of a mixed seawater hydrothermal fluid geochemical signature during alteration of volcanic rocks in the archean 2 7 ga abitibi greenstone belt canada abstract we investigated a group of silicified volcanic rocks from the 2 72 ga hunter mine group hmg abitibi greenstone belt canada in order to document progressive compositional change associated with alteration in a subaqueous caldera system rocks of the hmg divide into three groups based on mineralogy and texture for petrographic and geochemical analyses volcanic features phenocrysts pseudomorphs after primary glass shards lapilli volcanic clasts are preserved in all groups despite changing mineralogy from primarily quartz feldspar chlorite groups 1 and 2 to quartz hematite and carbonate groups 2 and 3 compositionally group 1 rocks resemble volcanic rocks in the region while group 2 and 3 rocks show a change in mineralogy to iron silica and carbonate minerals which is associated with depletion of many major and trace elements associated with volcanic rocks al2o3 na2o k2o zr in addition rare earth elements display a clear progression from volcanic signatures in group 1 prsn ybsn 1 7 2 96 eusn eusn 0 84 1 72 y ho 25 20 27 41 lasn lasn 0 97 1 29 and zr hf 38 38 42 09 to transitional mixed volcanic hydrothermal and seawater signatures in group 2 prsn ybsn 1 33 2 89 eusn eusn 1 33 2 5 y ho 23 94 30 lasn lasn 0 93 1 34 and zr hf 40 70 to mixed hydrothermal and seawater signatures in group 3 prsn ybsn 0 62 2 88 eusn eusn 1 30 7 15 lasn lasn 1 02 1 86 y ho 25 56 55 and zr hf 35 50 we interpret that silicification of volcanic rocks group 1 produced transitional altered volcanic rocks group 2 and siliceous and jaspilitic rocks group 3 based on preservation of delicate volcanic features building on this explanation we interpret that major trace and rare earth element mobility occurred during the process of silicification during which siliceous and jaspilitic rocks group 3 acquired aspects of the rare earth element geochemical signatures of marine chemical precipitates we conclude that seafloor silicification in hydrothermal depositional settings is capable of producing rocks that resemble marine chemical precipitates such as banded iron formation and could be a process that is widespread in the archean consequently because silicified volcanic rocks from the hmg possess mixed seawater and hydrothermal rare earth element characteristics similar to archean iron formations and cherts we suggest caution must be exercised when interpreting the geochemical information preserved in metamorphosed rocks where original genesis is unknown

a rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh pressure eclogite from the moldanubian zone of the bohemian massif nove dvory czech republic

hydrothermal alteration and mineralization in the neves corvo volcanic hosted massive sulfide deposit portugal i geology mineralogy and geochemistry the neves corvo volcanic hosted massive sulfide deposit is one of the largest 300 mt and richest known deposits of the iberian pyrite belt the deposit is unique because of its extremely high cu and sn grades 45 mt at 6 cu plus 4 5 mt at 12 cu and 2 2 sn more than 300 000 metric tons t of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent sn the deposit consists of five orebodies including corvo the focus of the present study the volcanic volcaniclastic and sedimentary lithofacies that directly host the neves corvo mineralization consist of a rhyolitic dome cryptodome hyaloclastite complex facies interpretation indicates that volcanic activity occurred at moderate water depth below storm wave base in variably subsiding basins of different orders several syn and postore modification events included gravity driven mass transport processes and subsequent low angle thrusting and asymmetric detachment folding which shaped the present deposit the ore geology and geochemistry and the textural mineralogical and geochemical aspects of the ore forming process in the corvo orebody collectively suggest that ore formation resulted from a long lived hydrothermal system including two main mineralizing events an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation two spatially independent stockworks fed these genetically related mineralizing systems the early deposition of stringer and massive cassiterite ores took place almost exclusively in the tin corridor a structural alignment bounded by synvolcanic faults textural analysis and geochemical data both indicate minimal fluid rock interaction during ascent of the tin bearing fluid the massive and semimassive cassiterite ores at corvo most likely formed by direct venting into seawater and or emplacement in water laden unlithified sediments and early hydrothermal products that mantled the footwall volcanic sequence stringer and massive cassiterite precipitation is thought to have been a product of a short lived episode in the overall ore forming system the vein filling mineral assemblage in the cassiterite stockwork quartz cassiterite pyrite donbassite kaolinite chamosite coupled with whole rock and mineral geochemical data indicate that the tin bearing fluid had a low ph and relatively high temperature 380 c the massive sulfide related hydrothermal alteration in corvo is essentially strata bound the permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow the hydrothermal alteration is zoned and consists of an inner chlorite donbassite quartz sulfide sericite core that grades outward into successive enveloping k sericite quartz sulfide and na sericite quartz sulfide halos textural evidence from the mine to the microscopic scale and mass balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and or sedimentary units were major mechanisms of massive sulfide deposition at corvo alteration mineralogy and geochemistry indicate that the neves corvo ore fluids were hotter and more acidic than typical ore forming fluids of the iberian pyrite belt the geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the iberian pyrite belt and imply the involvement of additional metal sources possibly magmatic in the ore forming system copper enrichment due to mechanical and fluid assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit

shallow water bottom topography from radar imagery radar imagery of shallow coastal waters usually 40 m deep contain unexplained features1 3 resembling the bottom topography since microwave radiation does not penetrate far in sea water these features must be the result of hydrodynamic processes coupled to the bottom topography that modulate the amplitude of the short bragg resonant sea surface waves the main contributors to the backscattered radar power4 5 because many different hydrodynamic processes operate in coastal waters and the precise mechanism which generates the radar images is not fully established a programme to explore the mechanism s was initiated at the us naval research laboratory here we give a brief description of the possible mechanisms and results from the pilot experiment conducted in july 1982 40 nautical miles south east of nantucket island

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

petrographic quantification of brecciated rocks by image analysis application to the interpretation of elastic wave velocities abstract this paper presents a new procedure to quantify the petrology of materials with complex and heterogenic rock fabric features the classical methodologies for petrographic quantification by image analysis are not appropriate for this kind of rock due to both the high chromatic complexity of these materials and the scale with which the samples are studied the proposed process is particularly interesting for materials with complex chromatic characteristics such as highly fractured rocks concrete etc two commercial varieties of highly fractured brecciated dolostones were selected for this study in order to test the designed methodology this new procedure is based on the analysis process of colour images obtained from mesoscopic sample scanning six petrographic parameters and diagrams have been designed in order to quantify the most influential aspects in compressional v p and shear v s ultrasonic wave propagation these petrographic parameters and diagrams are brecciation density brecciation density map clast size distribution fine grained matrix content shape preferred orientation and rose diagrams the results demonstrate that the proposed methodology is suitable for quantifying the petrography of these kinds of materials furthermore the variations measured in v p and v s are fully explained by the values measured for the following parameters brecciation density fine grained matrix content clast size distribution and preferred orientation of clasts

electron microprobe petrochronology the term petrochronology has increasingly appeared in publications and presentations over the past decade the term has been defined in a somewhat narrow sense as the interpretation of isotopic dates in the light of complementary elemental or isotopic information from the same mineral s kylander clark et al 2013 although complementary isotopic and elementary information are certainly a central and critical part of most if not all petrochronology studies the range of recent studies that might use the term covers a much broader scope the term petrochronology might alternatively be defined as the detailed incorporation of chronometer phases into the petrologic and tectonic evolution of their host rocks in order to place direct age constraints on petrologic and structural processes as noted by kylander clark et al 2013 the linkage between geochronology and petrology can involve a variety of data including mineral textures and fabrics the distribution of mineral modes or volume proportions compositional zoning mineral inclusion relationships and certainly major element trace element and isotopic composition of the chronometer and all other phases electron probe micro analysis epma has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage the basic instrument is an electron microscope which can be used in either scanning or fixed beam modes with integrated wavelength dispersive spectrometers wds energy dispersive spectrometers eds electron detectors to image secondary and backscattered signals a light optical system and optionally cathodoluminescence cl detection the electron microprobe is used to investigate the distribution composition and compositional zonation of all mineral phases the data that underpin thermobarometric analysis and modeling of p t histories the microprobe with m scale spatial resolution can also characterize compositional zonation in very small accessory phases including monazite xenotime zircon allanite titanite apatite and others this as discussed below can be a

geology fluid inclusions and oxygen isotope geochemistry of the baiyinchang pipe style volcanic hosted massive sulfide cu deposit in gansu province northwestern china the baiyinchang massive sulfide cu deposit zheyaoshan and huoyanshan mines is hosted by an early cambrian submarine felsic volcanic succession within an extrusive cryptodome associated with an overlying basaltic flow in a late proterozoic early paleozoic submarine volcanic belt in the north qilian orogen northwestern china the deposit is comprised of two mineralized zones a 30 cm thick strata bound zn rich sulfide lens associated with hematitic fe mn cherts and an underlying discordant massive ore dominated sulfide zone enveloped by a hydrothermal alteration pipe that is zoned from chlorite in the center to quartz sericite at the margin the discordant sulfide zone accounts for 90 percent of the cu reserves of the zheyaoshan mine it consists of four main ore types 1 pipelike pyrrhotite pyrite chalcopyrite ore 2 massive sulfide ore 3 a disseminated ore halo and 4 footwall stringer ore the pyrrhotite pyrite chalcopyrite pipe has an elliptical shape in plan and is 30 50 m across the pipe partially replaces the overlying massive pyrite lens and extends downward at least 150 m to be gradually replaced by chalcopyrite rich stringer veins and chalcopyritebearing quartz veins surrounded by a discordant hydrothermal alteration envelope massive chalcopyrite pyrite lenses discordant to volcanic bedding containing relict patches of felsic volcanic host rocks are commonly enveloped by a disseminated sulfide halo within a chloritized volcanic unit these features suggest that zheyaoshan is a pipe style deposit that formed mainly by subsea floor replacement of volcanic host rocks studies of fluid inclusions indicate that there are four types 1 type i two phase aqueous fluid inclusions 2 type ii daughter mineral bearing multiphase fluid inclusions 3 type iii co2 rich fluid inclusions and 4 type iv ch4 rich fluid inclusions type ii inclusions have high homogenization temperatures th ranging from 320 to 430 c contain high salinity fluids 31 38 wt nacl equiv and coexist with co2 rich fluids found in vapor rich high th up to 487 c moderate salinity 10 16 wt nacl equiv inclusions in the discordant sulfide zone and associated altered rocks suggesting a possible contribution of a magmatic fluid to the hydrothermal system the coexistence of vapor rich high th 300 c and aqueous low th 220 c type i fluid inclusions in the stringer zone suggests that heated seawater mixed with magmatic fluid gas in the feeder zone most type i fluid inclusions in the massive chalcopyrite pyrite body and in the strongly chloritized pipe have a low th 62 225 c and high salinities 15 0 23 0 wt nacl equiv suggesting that a dense brine zone developed in fractures in the subsea floor where sulfides accumulated by open space filling and replacement of host volcanic rocks eleven quartz samples from the overlying discordant sulfide zone yielded a restricted range of 18o values between 8 8 and 11 1 per mil from which we calculate that the corresponding hydrothermal fluids had 18o values ranging from 5 3 to 3 1 per mil over a temperature range of 160 to 278 c whole rock 18o values

chert in the iberian pyrite belt since lenses of chert are common within the volcano sedimentary succession hosting the massive sulphide deposits of the iberian pyrite belt spain and portugal we examined numerous chert occurrences both petrographically and geochemically to test their possible value for massive sulphide exploration the chert is found at two main lithostratigraphic levels upper and lower that are also interpreted as massive sulphide bearing in both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments we have not been able to observe the relationships between massive sulphides and chert but some of the large orebodies of the province lousal la zarza tharsis planes san antonio body of rio tinto neves are described as being locally capped by chert facies four main types are recognized among the chert and associated facies 1 red hematitic chert magnetite 2 radiolarian and or sedimentary textured conglomeratic chert with hematite and or mn oxides 3 pale sulphidic chert 4 rhodonite and or mn carbonate magnetite facies in the spanish part of the province the radiolarian chert is confined to the upper level the distribution of the other types appears to be haphazard the hydrothermal origin of the south iberian chert is shown by its high fe mn and low co ni cu contents the presence of small positive ce anomalies indicates a shallow marine environment shelf or epicontinental sea which is consistent with the volcanological and sedimentological data the chert was emplaced below the sea floor through chemical precipitation and or through alteration and replacement of the country rock residual traces of which are ghost phenocrysts and high al ti and rare earth contents macro and microscopic relationships indicate that the oxide facies hematite magnetite formed first probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies a phase of mn carbonate and silicate quartz chlorite sulphides appears to be even later it was not possible through discrimination to isolate a chert that could be considered as representing a lateral marker of massive sulphides moreover both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides such is also indicated by the lead isotopic signature of the chert which is appreciably more radiogenic than that of the massive sulphides the lead enrichment in the sulphidic chert facies indicates the participation of a different source sediments sea water from that of the massive sulphides the hypothesis of an independent hydrothermal chert event can thus be envisaged wherein the chert reflects submarine low temperature hydrothermal activity that is most apparent during a break within the volcano sedimentary succession and which may locally have competed with the high temperature hydrothermal activity giving rise to the massive sulphides the interest of the chert thus rests in its palaeodynamic significance as a marker of periods of volcanic quiescence and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides

provenance and geochemistry of sedimentary components in the volcano sedimentary complex iberian pyrite belt discrimination between the sill sediment complex and volcanic pile models two highly contrasting models have been proposed for the palaeovolcanological setting of the massive sulphide deposits in the iberian pyrite belt the long standing view of the host rocks is that they are a pile of effusive and pyroclastic rocks but this position has now been challenged by the proposal that high level peperitic sills predominate discrimination between the volcanic pile and sill sediment complex models is important because they lead to very different conclusions about such key metallogenic features as the timing of mineralization the nature of the ore forming convective system and the source of the metals sedimentary geochemistry particularly ree and ti nb shows that there is no correspondence between the chemistries of mafic igneous sheets and intercalated stratified volcaniclastic rocks that range from andesite to rhyodacite in composition therefore none of the mafic sheets supplied detritus to the sedimentary environment sedimentary rocks of continental provenance persist throughout the host rock sequence especially in mineralized regions implying confinement of the majority of primary volcanic facies in the form of high level intrusions some andesitic and felsic intrusions created a minor stratified volcaniclastic component via hydrovolcanic eruptions the volcanic pile model is invalidated because the expected provenance patterns for this volcanic style are not present and the sill sediment complex setting of the sulphide deposits is confirmed

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

type of volcanoes hosting the massive sulfide deposits of the iberian pyrite belt the volcanic sedimentary complex vsc of the iberian pyrite belt ipb in southern portugal and spain comprises an upper devonian to lower carboniferous submarine succession with a variety of felsic volcanic lithofacies the architecture of the felsic volcanic centres includes felsic lavas domes pyroclastic units intrusions and minor mafic units that define lava cryptodome pumice cone volcanoes the diversity of volcanic lithofacies recognized in different areas of the ipb mainly reflects variations in proximity to source but also differences in the eruption style the ipb volcanoes are intrabasinal range in length from 2 km to 8 km and their thickest sections vary from 400 m to 800 m these volcanoes are dominated by felsic lavas domes that occur at several stratigraphic positions within the volcanic centre however the pyroclastic units are also abundant and are spatially related to the lavas domes the intrusions are minor and define cryptodomes and partly extrusive cryptodomes the hydrothermal systems that formed the neves corvo and lousal massive sulfide ore deposits are associated with effusive units of felsic volcanic centres at neves corvo the massive sulfide orebodies are associated to rhyolitic lavas that overlie relatively thick fiamme rich pyroclastic unit in several other locations within the belt pyroclastic units contain sulfide clasts that may have been derived from yet to be discovered coeval massive sulfide deposits at or below the sea floor which enhances the exploration potential of these pyroclastic units and demonstrates the need for volcanic facies analysis in exploration

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada i comparisons with the iberian pyrite belt abstract the ordovician zn pb cu massive sulphide ore deposits of the bathurst mining camp share many features with those of the devonian carboniferous iberian pyrite belt particularly the tendency to large size tonnage and metal content shape as far as can be determined after allowing for deformation metal content particularly fe cu pb zn and sn mineral assemblages pyrite arsenopyrite pyrrhotite and lack or rarity of sulphates sulphide textures particularly framboidal pyrite lack of chimney structures and rubble mounds irregular metal or mineral zoning and the low degree of zone refining compared to hokuroku ores the major differences between the provinces are the lack of vent complexes and the presence of sn cu ores in the iberian pyrite belt there are also similarities in the geological setting of the two camps both lie within continental terranes undergoing arc continent and continent continent collision and in each case massive sulphide mineralisation followed ophiolite obduction the ore deposits are associated with bimodal volcanic rocks derived from morb and continental crust and marine shales and mineralisation was locally accompanied or followed by deposition of iron formations fluid inclusion data from veins in stockworks from at least six of the iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline ore forming fluids brine pools and it is suggested that most of the major features of the bathurst deposits can be explained by similar processes the proposed model is largely independent of ocean sulphate and o 2 content whereas low values of each are requisites for the current spreading plume model of sulphide deposition in the bathurst camp

the key tuffite matagami camp abitibi greenstone belt canada petrogenesis and implications for vms formation and exploration the key tuffite is a stratigraphic marker unit for most of the zinc rich volcanogenic massive sulfide deposits of the matagami camp in the abitibi greenstone belt this 2 to 6 m thick unit was previously interpreted as a mixture of ash fall andesitic to rhyolitic tuffaceous components and volcanogenic massive sulfide vms related chemical seafloor precipitate exhalative component previous attempts to develop geochemical exploration vectoring tools using metal content within the key tuffite were mostly inconclusive due to the complex nature of the key tuffite unit and a poor understanding of its composition origin and relationship with the vms forming hydrothermal systems detailed mapping and thorough lithogeochemistry of the key tuffite in the vicinity of the perseverance and bracemac mcleod deposits indicate that the key tuffite is a homogeneous calc alkaline andesitic tuff that was deposited before the vms deposits were formed the unit is mostly devoid of exhalative component but it is strongly hydrothermally altered close to orebodies this is characterized by a strong proximal chloritization and a distal sericitization which grades laterally into the unaltered key tuffite neither the key tuffite nor the ore was formed by seafloor exhalative processes for the two studied deposits this probably explains why previously proposed exploration models based on metal scavenging proved unsuccessful and suggests that a re evaluation of the exhalative model should be done at the scale of the mining camp however as shown in this study hydrothermal alteration can be used to vector towards ore along the key tuffite

temperature micro mapping in oscillatory zoned chlorite application to study of a green schist facies fault zone in the pyrenean axial zone spain oscillatory compositional zoning in minerals has been observed in hydrothermal magmatic and metamorphic environments and is commonly attributed to chemical or physical cyclical changes during crystal growth chemical zoning is a common feature of solid solutions which has been rarely reported in phyllosilicates in this study oscillatory zoning in chlorite is described in samples from the pic de port vieux thrust a minor thrust fault associated to the major gavarnie thrust fault zone central pyrenees spain the pic de port vieux thrust sheet comprises a 1 20 m thick layer of triassic red pelite and sandstone thrust over mylonitized cretaceous dolomitic limestone the thrust fault zone deformation comprises secondary faults and cleavage affecting the triassic pelite and sandstone an important feature responsible to this deformation is a set of veins filled by quartz and chlorite chlorite is present in crack seal extension veins and in shear veins both structures opened under the same stress conditions in some shear veins chlorite occurs as pseudo uniaxial plates arranged in rosette shaped aggregates these aggregates appear to have developed as a result of radial growth of the chlorite platelets oscillatory zoning has been imaged by backscattered scanning electron microscopy and by x ray quantitative micro mapping these oscillations correspond to chemical zoning with alternating iron rich and magnesium rich bands the chlorite composition ranges from a fe rich pole to a mg rich pole fe 3 fe values were measured in chlorite using xanes spot analyses and vary from 0 23 to 0 44 the highest values are in the fe rich area temperature maps built from standardized microprobe x ray images and redox state using the program xmaptools indicate oscillatory variations from about 310 to 400 50 c during chlorite crystallization these temperature variations are correlated with a fe 3 fe variation by al 3 fe 3 1 and di trioctahedral substitutions highlighted by mg and fe tot contents fe mg zoning chemical variations could be then explained by alternation of cooling times and cyclical pulses of a fluid hotter than the host rock it is however not excluded that kinetic effects influence the incorporation of mg or fe during chlorite crystallization

the rio tinto basin spain mineralogy sedimentary geobiology and implications for interpretation of outcrop rocks at meridiani planum mars abstract exploration by the nasa rover opportunity has revealed sulfate and hematite rich sedimentary rocks exposed in craters and other surface features of meridiani planum mars modern holocene and plio pleistocene deposits of the rio tinto southwestern spain provide at least a partial environmental analog to meridiani planum rocks facilitating our understanding of meridiani mineral precipitation and diagenesis while informing considerations of martian astrobiology oxidation thought to be biologically mediated of pyritic ore bodies by groundwaters in the source area of the rio tinto generates headwaters enriched in sulfuric acid and ferric iron seasonal evaporation of river water drives precipitation of hydronium jarosite and schwertmannite while mg al fe3 copiapite coquimbite gypsum and other sulfate minerals precipitate nearby as efflorescences where locally variable source waters are brought to the surface by capillary action during the wet season hydrolysis of sulfate salts results in the precipitation of nanophase goethite holocene and plio pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time hematite in meridiani spherules also formed during diagenesis although whether these replaced precursor goethite or precipitated directly from groundwaters is not known the retention of jarosite and other soluble sulfate salts suggests that water limited the diagenesis of meridiani rocks diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry rio tinto environments organic matter does not persist in rio tinto sediments but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers surface blisters formed by biogenic gas and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiological investigation of meridiani outcrops

the relation between iron formation and low temperature hydrothermal alteration in an archean volcanic environment abstract the uppermost section of the hunter mine group hmg 2728 ma a bimodal volcanic complex in the abitibi greenstone belt contains both oxide and carbonate facies banded iron formation bif this paper explores the relationship between volcanic activity and the development of the two types of iron formation the oxide facies represented by chert jasper magnetite iron formation is widespread and recurs at several stratigraphic levels field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard rich tuffs seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes carbonate siderite facies iron formation in contrast formed locally below a silica cap rock by in situ low temperature hydrothermal replacement of chert tuff beds and even of the oxide formation thus the two types of iron formation although spatially and stratigraphically juxtaposed are not the result of the same process and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process the contrasting genesis of oxide and carbonate iron formations indicates that current simplistic models relating banded iron formation to volcanic massive sulphide deposits need to be re evaluated the notion that oxide iron formation represents a distal manifestation of volcano generated hydrothermal activity may be the exception and not the rule because as indicated by this study bif occurs in the central part of a volcanic edifice rather than on the flanks

p t deformation fe3 fe2 mapping at the thin section scale and comparison with xanes mapping application to a garnet bearing metapelite from the sambagawa metamorphic belt japan quantitative x ray maps of composition from a chlorite k white mica albite quartz and garnet bearing thin section from a sambagawa blueschist facies metapelite were combined with a multiequilibrium calculation method to calculate a p t fe3 fe2 deformation map at the millimetre scale the studied sample was chosen because elongated chlorite crystallization tails pressure shadows rimmed by phengite are present which is an appropriate assemblage for the quantification of the p t evolution chlorite temperature and fe3 content maps were calculated by successive iterations for each pixel analysis of fe3 until convergence of the four chlorite quartz h2o equilibria that can be written using the fe and mg amesite clinchlore daphnite and sudoite chlorite end members the calculated map of fe2 fe3 in chlorite is in good qualitative agreement with the in situ mapping of this ratio using xanes x ray absorption near edge structure techniques the temperature map indicates that high temperature chlorite zones with low fe3 contents alternate with lower temperature zones and higher fe3 contents in the crystallization tail late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite 250 c showing fe3 fetotal up to 0 4 groups of chlorite and mica pixels were then identified based on compositional and structural criteria and a p tdeformation map was calculated using representative analyses of these groups the calculated p tdeformation map suggests that in contrast to chlorite the composition of most mica did not change significantly during exhumation mica reequilibrated in late ew shear bands only ew shearing was already active at 0 1 gpa 500 c which corresponds to the peak temperature and probably pressure conditions at reduced redox conditions the intensity of deformation probably decreased with decrease in temperature to 350 400 c at this temperature a second main deformation event corresponding to a further ew stretching occurred and was still active below 250 c and more oxidizing conditions these results indicate that the scale at which p t data can be obtained is now close to the scale of observation of structural geologists a close link between deformation and mineral reaction is therefore possible at the microscopic scale which provides information about the relationship between deformation and mineral reactivity the modalities of deformation with time and the p t conditions at which it occurred

the cu stockwork and massive sulfide ore of the feitais volcanic hosted massive sulfide deposit aljustrel iberian pyrite belt portugal a mineralogical fluid inclusion and isotopic investigation the variscan feitais volcanic hosted massive sulfide deposit in the aljustrel district of the iberian pyrite belt consists of 55 million metric tons of zn pb cu massive sulfide overlying a cu rich stockwork the massive ore is overlain by up to 30 m of feldspar phyric rhyolitic volcaniclastic rock and locally by a jasper and or chert layer up to 15 m thick the massive sulfide orebody consists dominantly of pyrite sphalerite galena chalcopyrite tetrahedrite tennantite arsenopyrite and bournonite together with minor quartz chlorite sericite carbonate and barite the orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated locally silicified felsic volcanic rock the upper 30 to 60 m of which contains chalcopyrite quartzchlorite sericite carbonate bearing stockwork vein let s that prior to deformation were at a shallow angle to the base of the massive orebody chloritized footwall rocks extend up to 20 m below the cu stockwork zone and are underlain by up to 50 m of quartz sericite pyrite altered rhyolitic rock the stockwork veins also contain pyrite tetrahedrite tennantite sphalerite and arsenopyrite pyrite both in stockwork and massive ore locally displays partly recrystallized framboidal reniform and cellular textures two generations of quartz q1 and q2 and carbonate in the stockwork veins contain primary in growth zones and pseudosecondary fluid inclusions with homogenization temperatures of 270 to 315 c and salinities of 2 2 to 8 1 wt percent nacl equiv the 34s cdt values of massive and stockwork ores range from 15 4 to 4 7 mean 2 8 and 11 2 to 11 9 mean 0 4 per mil respectively the lowest values from colloformtextured pyrite with no evidence of oxidation of sulfide sulfur during mineralization the most negative values indicate an origin by biogenic reduction of seawater sulfate the 13c pdb values for carbonates 7 5 to 13 7 and 9 3 to 14 3 per mil in massive and stockwork ore respectively indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon there are no significant lateral or vertical variations in s isotope values in sulfides or c o isotope values in carbonates either in massive or stockwork ore the 18o smow values for quartz in stockwork and massive sulfide are 11 6 to 13 9 and 16 7 to 17 9 per mil respectively coexisting and texturally contemporaneous carbonate and quartz in stockwork veins are not in isotopic equilibrium indicating that the c o isotope values may have been reset the 18o values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4 2 to 5 2 per mil barite from the hanging wall and massive ore yields 34s values 21 9 27 9 equal to or slightly higher than those of coeval seawater 87sr 86sr ratios 0 708438 0 709063 are slightly more radiogenic than those of coeval seawater 0 7080 0 7085 and much more radiogenic than those of coeval volcanic rocks 0 703304 0 706642 probably representing mixtures between seawater sr and radiogenic sr in fluids sourced in the crustal pile deposition of the massive sulfide on the sea floor is suggested by its stratiform nature the stronger alteration of footwall relative to hanging wall rocks the stockwork system terminating sharply at the base of the massive sulfide the presence of sedimentary like textures in the massive sulfide the absence of replacement fronts and the presence of framboidal and other sea floor depositional textures indicative of fluid quenching the sheetlike form lack of rubble mounds and chimneys scarcity of barite reduced mineral assemblage and metal zoning distinguish feitais from kuroko type deposits it shares most of the characteristics of those iberian pyrite belt deposits for which a brine pool origin has been proposed based on fluid inclusion data

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

neotethys closure history of anatolia insights from 40ar 39ar geochronology and p t estimation in high pressure metasedimentary rocks the multiple high pressure hp low temperature lt metamorphic units of western and central anatolia offer a great opportunity to investigate the subduction and continental accretion related evolution of the eastern limb of the long lived aegean subduction system recent reports of the hp lt index mineral fe mg carpholite in three metasedimentary units of the gondwana derived anatolide tauride continental block namely the afyon zone the oren unit and the southern menderes massif suggest a more complicated scenario than the single continental accretion model generally put forward in previous studies this study presents the first isotopic dates white mica 40ar 39ar geochronology and where possible are combined with p t estimates chlorite thermometry phengite barometry multi equilibrium thermobarometry on carpholite bearing rocks from these three hp lt metasedimentary units it is shown that in the afyon zone carpholite bearing assemblages were retrogressed through greenschist facies conditions at c 67 62 ma early retrograde stages in the oren unit are dated to 63 59 ma in the kurudere nebiler unit hp mesozoic cover of the southern menderes massif hp retrograde stages are dated to c 45 ma and post collisional cooling to c 26 ma these new results support that the oren unit represents the westernmost continuation of the afyon zone whereas the kurudere nebiler unit correlates with the cycladic blueschist unit of the aegean domain in western anatolia three successive hp lt metamorphic belts thus formed the northernmost tavsanli zone c 88 82 ma the oren afyon zone between 70 and 65 ma and the kurudere nebiler unit c 52 45 ma the southward younging trend of the hp lt metamorphism from the upper and internal to the deeper and more external structural units as in the aegean domain points to the persistence of subduction in western anatolia between 93 90 and c 35 ma after the accretion of the menderes tauride terrane in eocene times subduction stopped leading to continental collision and associated barrovian type metamorphism because by contrast the aegean subduction did remain active due to slab roll back and trench migration the eastern limb below southwestern anatolia of the hellenic slab was dramatically curved and consequently teared it therefore is suggested that the possibility for subduction to continue after the accretion of buoyant e g continental terranes probably depends much on palaeogeography

deciphering high pressure metamorphism in collisional context using microprobe mapping methods application to the stak eclogitic massif northwest himalaya the stak massif northern pakistan is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the indian continent in the northwest himalaya although this unit was extensively retrogressed during the himalayan collision records of the high pressure hp event as well as a continuous pressure temperature p t path were assessed from a single thin section using a new multiequilibrium method this method uses microprobe x ray compositional maps of garnet and omphacitic pyroxene followed by calculations of 200 000 p t estimates using appropriate thermobarometers the stak eclogite underwent prograde metamorphism increasing from 650 c and 2 4 gpa to the peak conditions of 750 c and 2 5 gpa then retrogressed to 700 650 c and 1 6 0 9 gpa under amphibolite facies conditions the estimated peak metamorphic conditions and p t path are similar to those of the kaghan and tso morari high to ultrahigh pressure hp uhp massifs we propose that these three massifs define a large hp to uhp province in the northwest himalaya comparable to the dabie sulu province in china and the western gneiss region in norway

new high precision p t estimates for oman blueschists implications for obduction nappe stacking and exhumation processes oman blueschists and eclogites lie below the obduction nappe of the semail ophiolite in one of the key areas on earth for the study of plate convergence here new metamorphic and tectonic constraints are provided for the central yet poorly constrained hulw unit sandwiched between the low grade units 10 kbar 300 c and the as sifah eclogites pmax 23 kbar tmax 600 c tweequ multi equilibrium thermobarometry using both compositional mapping and spot analyses and raman spectroscopy of carbonaceous material yield a high precision p t path for the hulw and as sheikh units and reveal that they shared a common p t history in four stages i a pressure decrease from 10 12 kbar 250 300 c to 7 9 kbar 300 350 c ii almost isobaric heating at 8 10 kbar from 300 350 c to 450 500 c iii a pressure decrease at moderate temperatures 450 500 c and iv isobaric cooling at 5 6 kbar from 450 500 to 300 c no significant pressure or temperature gap is observed across the upper plate lower plate discontinuity to the north and west of the hulw unit the combination of tectonic and p t data constrains the stacking chronology of the three main metamorphic units comprising the saih hatat window i e the ruwi quryat the hulw as sheikh and the diqdah as sifah units these results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple n vergent continental subduction of the passive arabian margin below the obduction nappe along a cold p t gradient

thermodynamic properties of white micas on the basis of high pressure experiments in the systems k2omgoal2o3sio2h2o and k2ofeoal3o3sio2h3o abstract experiments in the piston cylinder apparatus were undertaken to synthesize the assemblage of phengite garnet kyanite and quartz or coesite in the system k2o mgo al2o3 sio2 h2o kmash compositional data of phengite were obtained for 13 experimental runs in the range of 30 to 45 kbar and 850 c to 1100 c the same was achieved for 23 runs in the system k2o feo al2o3 sio2 h2o kfash between 600 c and 900 c and 15 to 55 kbar on the basis of the latter runs and 6 additional synthesis experiments the dependence of the lattice dimensions on the composition of 2m1 phengites in kfash was determined to derive thermodynamic data we evaluated the above experiments and those reported by massonne and schreyer 1987 1989 on kmash phengites coexisting with other assemblages considering simple activity model like ams ms we obtained for the mg al celadonite and fe2 al celadonite theoretical endmembers hf0 5832415 3040 and 5492287 6267 j mol s0 288 527 4 008 and 303 148 6 827 j k mol v0 13 870 0 027 and 13 962 0 067 j bar mol the nonideal behavior of the muscovite ms mg al celadomite series was modelled using an asymmetric margules formalism with the following parameters w12h w21h 0 w12s 58 598 8 804 w21s 15 920 3 393 w12v 0 735 0 118 w21v 0 187 0 026 with units as above for the muscovite fe2 al celadonite series we used a symmetric model with wh 0 ws 24 083 3 088 j k mol and wv 0 3394 0 1120 j bar mol the above data were applied to construct a petrogenetic grid the example chosen is related to the kfash system with potassic white mica quartz and h2o in excess it covers the t range from 150 c to 550 c at a p up to 28 kbar the new thermodynamic data are also discussed in regard to their use in geothermobarometry

the higher grade metamorphic zonation of the sambagawa metamorphic belt in central shikoku japan the higher grade metamorphic zonation of the sambagawa sanbagawa belt is established for the first time for the whole area of central shikoku as discontinuous reactions to define the isograd are absent the metamorphic grade is primarily determined by the mg fe partitioning between garnet and chlorite along representative traverses however for regional mapping mineralogical features of the pelitic schists such as using mineral assemblages of more than divariant equilibrium the modal garnet to chlorite ratio and the optical properties of chlorite are employed as auxiliary criteria the presence of the highest grade mineral zone in the middle of the structural level is confirmed but its spatial distribution is far more complex than hitherto accepted thermal axes are now confirmed at three different structural levels a model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re equilibrated local anomalies of metamorphic grade represented by mixing of schists of different metamorphic grade exist but they are due to a later stage event

phase relations and chemical composition of phengite and paragonite in pelitic schists during decompression a case study from the monte rosa nappe and camughera moncucco unit western alps the metamorphic evolution of metapelites from the eastern part of the monte rosa nappe and the camughera moncucco unit both situated in the upper penninic units sw of the simplon line were investigated using microstructural relationships and equilibrium phase diagrams the units under consideration experienced prealpine amphibolite facies conditions and underwent a complex metamorphic evolution during the alpine orogeny peak pressures during an early alpine high pressure stage of 12 5 16 kbar were similar in the monte rosa nappe and camughera moncucco unit a pronounced thermal gradient is indicated during decompression leading to an amphibolite facies overprint as the decompression paths went through the chlorite biotite and plagioclase stability fields in most of the monte rosa nappe through the staurolite field in the easternmost monte rosa nappe and in the camughera moncucco unit and through the sillimanite field in the easternmost camughera moncucco unit in high al metapelites the initial formation of staurolite is related to continuous paragonite breakdown and associated formation of biotite in the course of this reaction phengite becomes successively sodium enriched in low al metapelites in contrast the initial staurolite formation occurs via the continuous breakdown of sodium rich phengite in both low and high al metapelites the largest volume of staurolite is formed during the continuous breakdown of sodium rich phengite below p t conditions of about 9 5 kbar at 600 650 c during this reaction phengite becomes successively potassium enriched as sodium from phengite is used to form the albite component in plagioclase for normal pelitic chemistries phengite becomes na enriched during decompression through the breakdown of paragonite along a nearisothermal decompression path the na content in phengite reaches its maximum when paragonite is entirely consumed during further decompression the paragonite component in phengite decreases again because na is preferentially incorporated into the albite component of plagioclase

p t t estimation of deformation in low grade quartz feldspar bearing rocks using thermodynamic modelling and 40ar 39ar dating techniques example of the plan de phasy shear zone unit brianconnais zone western alps pressure temperature time p t t estimates of the syn kinematic strain at the peak pressure conditions reached during shallow underthrusting of the brianconnais zone in the alpine subduction zone was made by thermodynamic modelling and 40ar 39ar dating in the plan de phasy unit se of the pelvoux massif western alps the dated phengite minerals crystallized syn kinematically in a shear zone indicating top to the n motion by combining x ray mapping with multi equilibrium calculations we estimate the phengite crystallization conditions at 270 50 c and 8 1 2 kbar at an age of 45 9 1 1 ma combining this p t t estimate with data from the literature allows us to constrain the timing and geometry of alpine continental subduction we propose that the brianconnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision

dehydration of dioctahedral aluminous phyllosilicates thermodynamic modelling and implications for thermobarometric estimates we propose a solid solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations including hydration observed in natural smectites interlayered illite smectite illites and phengites from diagenetic to high grade metamorphic conditions the suggested formalism involves dehydrated micas and hydrated pyrophyllite like thermodynamic end members with these end members the equilibrium conditions of quartz water k bearing mica like phyllosilicates of fixed 2 1 composition are represented by a line in p t space along which the interlayer water content varies the relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions which covers a range between 25 c and few mpa to 800 c and 5 gpa the temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data at low temperature and pressure the amount of interlayer water in k deficient phengite and illite is predicted to reach 100 of the apparent vacancies which is consistent with previous values reported in the literature although the amount of interlayer water is predicted to decrease with pressure and temperature it is calculated to be significant in k deficient phengite from lt hp pelites metamorphosed at about 350 c 10 kbar the presence of molecular water in the interlayer site of such phengites has been confirmed by ftir mapping its implications for p t estimates are discussed

mineralogy and geochemistry of the masa valverde blind massive sulphide deposit iberian pyrite belt spain abstract masa valverde is a blind volcanic hosted massive sulphide deposit recently discovered in the spanish sector of the iberian pyrite belt ipb the deposit is more than 1 km long and is located at depths between 400 and 850 m in direct spatial association with a residual gravimetric anomaly the ore body is concealed beneath culm sedimentary rocks which cover the western extension of a hercynian age anticline a few kilometers west of an area where older massive sulphide workings existed the masa valverde deposit consists of two main ore bodies composed dominantly of massive and banded pyrite which are hosted by a volcano sedimentary sequence made up of felsic tuffs interlayered with shale siliceous exhalite and radiolarian chert the abundance of sedimentary host rocks to the deposit is a feature shared by other large massive sulphide deposits in the eastern sector of the ipb and is interpreted as significant with respect to the origin of the deposit a break in volcanic activity the thickness of the upper massive sulphide orebody varies between a few meters and 70 m and it consists of lenses and blankets of massive sulphides with interbedded tuff and shale and occasionally stockwork zones the lower orebody is smaller and thinner than the upper one but this may be due to incomplete drilling two types of stockwork occur beneath the massive sulphide bodies the more common type consists of irregular and anastomosing sulphide veinlets and irregular blebs formed by replacement stockwork with cross cutting generally straight sided sulphide veins is by contrast suggestive of formation by hydraulic fracturing the mineralogy and alteration processes of the ore and host rocks at masa valverde are analogous to those of other ipb massive sulphides the average temperature of formation of the components of the ore zones estimated on the basis of chlorite thermometry are as follows siliceous exhalites 290 c massive sulphides 325 c and stockworks 305 c rare earth element measurements indicate that the lowest metal concentrations occur in chloritites which occasionally host cu rich stockwork and are characterised by a strongly negative eu anomaly by contrast rocks hosting the sulphide lenses where silicification sericitization and carbonatization are more intense are enriched in lree two main stages of hydrothermal activity are suggested for the masa valverde deposit during an early stage ore fluids would have discharged onto the sea floor resulting in the deposition of pyrite with sphalerite and galena later on with continued input of high temperature fluids 290 to 315 c chalcopirite precipitated especially in the stockworks and at the base of the sulphide lenses at masa valverde this late cu rich hydrothermal event would have produced an ascending cu front which overpassed the stockwork zone and reached a higher temperature 325 c at the base of the massive sulphide lenses the combination of geological mineralogical and geochemical features of the masa valverde deposit provide guidelines for exploration for new blind orebodies in this part of the ipb where tuffs and sedimentary rocks are the dominant host lithologies the masa valverde deposit can be considered as a vhms subtype transitional to shms

ree and hf distribution among mineral phases in the cv ck clan a way to explain present day hf isotopic variations in chondrites chondrites are among the most primitive objects in the solar system and constitute the main building blocks of telluric planets among the radiochronometers currently used for dating geological events sm nd and lu hf are both composed of refractory lithophile element they are thought to behave similarly as the parent elements sm and lu are generally less incompatible than the daughter elements nd and hf during geological processes as such their respective average isotopic compositions for the solar system should be well defined by the average of chondrites called chondritic uniform reservoir chur however while the sm nd isotopic system shows an actual spread of less than 4 in the average chondritic record the lu hf system shows a larger variation range of 28 bouvier a vervoort j d and patchett p j 2008 the lu hf and sm nd isotopic composition of chur constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets earth planet sci lett 273 48 57 to better understand the contrast between sm nd and lu hf systems the ree and hf distribution among mineral phases during metamorphism of karoonda ck and vigarano type cv carbonaceous chondrites has been examined mineral modes were determined from elemental mapping on a set of five ck chondrites from types 3 6 and one cv3 chondrite trace element patterns are obtained for the first time in all the chondrite forming minerals of a given class ck chondrites as well as one cv3 sample this study reveals that ree are distributed among both phosphates and silicates only 30 50 of sm and nd are stored in phosphates at least in chondrites types 3 5 as such they are not mobilized during early stages of metamorphism the remaining fraction of sm and nd is distributed among the same mineral phases these elements are therefore not decoupled during metamorphism of the whole rock total of lu the fraction held in phosphate decreases significantly as the degree of metamorphism increases 30 for types 3 and 4 less than 5 in type 6 in contrast to lu hf is mainly hosted by silicates with little contribution from phosphates throughout the ck metamorphic sequence a significant part of sm and nd are stored in phosphates in types 3 5 and these elements behave similarly during ck chondrite metamorphism that explains the robustness of the sm nd ratios in chondrites through metamorphism and the slight discrepancies observed in the present day isotopic nd values in chondrites on the contrary lu and hf are borne by several different minerals and consequently they are redistributed during metamorphism induced recrystallization the lu hf ratios are therefore significantly disturbed during chondrites metamorphism leading to the high discrepancies observed in present day hf isotopic values in chondrites

the iberian pyrite belt a mineralized system dismembered by voluminous high level sills the iberian pyrite belt is a world ranking massive sulphide province in which a reassessment of the palaeovolcanology has dramatically changed understanding of the source of metals and mechanism of ore formation in the northern sector the deposits are hosted by a sill sediment complex in which more than 90 of the sills post date the sulphide sheets because of a very high sill sediment ratio these late intrusions dominate the host succession and have severely disrupted the post mineralization configuration thus obscuring the true genetic relationships for example some oxide deposits have been separated by hectometric sills from sulphide deposits they originally capped creating seemingly totally independent mineralizing systems in addition stratiform sulphide sheets without underlying stockworks are not necessarily allochthonous an early timing for the mineralization with respect to volcanism means that metals had to be predominantly sourced from the sedimentary basin and the continental crust below the volcanogenic sequence

exhumation of eclogite and blueschist cyclades greece pressure temperature evolution determined by thermobarometry and garnet equilibrium modelling high pressure rocks such as eclogite and blueschist are metamorphic markers of paleo subduction zones and their formation at high pressure and low temperature conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years the tectonic mechanisms controlling their exhumation back to the surface are however diverse complex and still actively debated although the cycladic blueschist unit cbu greece is among the best worldwide examples for the preservation of eclogite and blueschist the proposed p t evolution followed by this unit within the hellenic subduction zone is quite different from one study to another hindering our comprehension of exhumation processes in this study we present an extensive petrological dataset that permits refinement of the shape of the p t trajectory for different subunits of the cbu on syros high resolution quantitative compositional mapping has been applied to support the thermobarometric investigations which involve semi empirical thermobarometry garnet equilibrium modelling and p t isochemical phase diagrams the thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions the results were also

prograde pressure temperature paths in the pelitic schists of the sambagawa metamorphic belt sw japan prograde p t paths recorded by the chemistry of minerals of subduction related metamorphic rocks allow inference of tectonic processes at convergent margins this paper elucidates the changing p t conditions during garnet growth in pelitic schists of the sambagawa metamorphic belt which is a subduction related metamorphic belt in the south western part of japan three types of chemical zoning patterns were observed in garnet ca rich normal zoning ca poor normal zoning and intrasectoral zoning petrological studies indicate that normally zoned garnet grains grew keeping surface chemical equilibrium with the matrix in the stable mineral assemblage of garnet muscovite chlorite plagioclase paragonite epidote quartz biotite pressure and temperature histories were inversely calculated from the normally zoned garnet in this assemblage applying the differential thermodynamic method gibbs method with the latest available thermodynamic data set for minerals the deduced p t paths indicate slight increase of temperature with increasing pressure throughout garnet growth having an average dp dt of 0 4 0 5 gpa 100 c garnet started growing at around 470 c and 0 6 gpa to achieve the thermal and baric peak condition near the rim 520 c 0 9 gpa the high temperature condition at relatively low pressure for subduction related metamorphism suggests that heating occurred before or simultaneously with subduction