organic haze in earth s early atmosphere source of low 13c late archean kerogens high concentrations of greenhouse gases would have been required to offset low solar luminosity early in earth9s history enhanced co 2 levels are probably at least part of the solution but ch 4 may have played a significant role as well particularly during the late archean era 2 5 3 0 ga when methanogenic bacteria were almost certainly present indeed biological ch 4 production should have led to co 2 drawdown by way of a negative feedback loop involving the carbonate silicate geochemical cycle we suggest here that the atmospheric ch 4 co 2 ratio approached the value of 1 needed to trigger formation of titan like organic haze this haze was strongly depleted in 13 c relative to 12 c and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments therefore it could provide a novel explanation for the presence of extremely low 13 c kerogens in late archean sediments

origin timing and temperature of secondary calcite silica mineral formation at yucca mountain nevada the origin of secondary calcite silica minerals in primary and secondary porosity of the host miocene tuffs at yucca mountain has been hotly debated during the last decade proponents of a high level nuclear waste repository beneath yucca mountain have interpreted the secondary minerals to have formed from cool descending meteoric fluids in the vadose zone critics citing the presence of two phase fluid inclusions argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids understanding the origin temperature and timing of these minerals is critical in characterizing geologically recent fluid flux at the site and has significant implications to whether waste should be stored at yucca mountain petrographic and paragenetic studies of 155 samples collected from the exploratory studies facility esf and repository block cross drift ecrb tunnels indicate that heterogeneously distributed calcite with lesser chalcedony quartz opal and fluorite comprise the oldest secondary minerals these are typically overgrown by intermediate aged calcite often exhibiting distinctive bladed habits the youngest event recorded across the site is the deposition of mg enriched up to 1 wt and depleted growth zoned calcite intergrown with u enriched opal the cyclical variation in mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years the distribution of secondary minerals is consistent with precipitation in the vadose zone fluid inclusion petrography of sections from the 155 samples determined that 96 of the fluid inclusion assemblages fias contained liquid only inclusions that formed at ambient temperatures 35 c however 50 of the samples n 78 contained relatively rare fia that contain both liquid only and liquid plus vapor inclusions herein termed two phase fias that formed at temperatures above 35 c virtually all of these two phase fias occur in paragenetically old calcite rare two phase inclusion assemblages were also observed in early fluorite and quartz and early intermediate calcite homogenization temperatures trapping temperatures across yucca mountain are generally 45 to 60 c but higher temperatures reaching 83 c were recorded in calcite from the north portal and ramp of the esf cooler temperatures of 35 to 45 c were recorded in the intensely fractured zone multiple populations of two phase fias from lithophysal cavities in the esf and ecrb cross drift indicate early fluid cooling with time from temperatures 45 c in early calcite to 35 to 45 c in paragenetically younger calcite freezing point depressions range from 0 2 to 1 6 c indicating trapping of a low salinity fluid the majority of intermediate calcite and all outermost mg enriched calcite contains rare all liquid inclusions and formed from ambient temperature 35 c fluids carbon and oxygen isotope data reveal a consistent trend of decreasing 13 c from 9 5 to 8 5 and increasing 18 o from 5 2 to 22 1 values from paragenetically early calcite to mg enriched growth zoned calcite depleted d values 131 to 90 of inclusion fluids from intermediate and the youngest mg enriched calcite indicate derivation from surface meteoric fluids recalculation of 18 oh2o values of 12 to 10 is consistent with derivation from paleometeoric fluids results of integrated u pb dating opal and chalcedony and fluid inclusion microthermometry indicate that two phase fias that trapped fluids of 50 c are older than 6 29 0 30 ma two phase fias in parage netically later calcite which formed from fluids of 35 to 45 c are older than 5 32 0 02 ma there is no evidence for trapping of fluids with elevated temperatures during the past 5 32 my the youngest mg enriched calcite intergrown with opal began to precipitate between about 1 9 to 2 9 ma and has continued to precipitate within the past half million years the presence of liquid only inclusions and the consistent occurrence of mg enriched calcite and opal as the youngest event indicate a minor but chemically distinct ambient temperature 35 c fluid flux during the past 2 to 3 my copyright 2003 elsevier science ltd

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

variolites of the paleoproterozoic hekpoort formation transvaal sub basin kaapvaal craton multistage undercooling textures abstract globular structures in volcanic and hypabyssal rocks called varioles or spherulites have been described from a wide variety of places and rock types including komatiites basalts dacites rhyolites as well as alkaline dykes but the processes leading to their formation are still debated here we present a field petrological and geochemical study of variole bearing lavas variolites in the paleoproterozoic volcanic hekpoort formation in the southern part of the transvaal sub basin of south africa variolites were only observed in a restricted area corresponding to a 3 5 km long basin or depression the variolites are locally up to 50 m thick and correspond to a succession of one meter thick lava flows most of the flows consist of two distinct layers with an upper layer rich in spherical varioles 1 32 cm in diameter overlying a layer generally devoid in varioles but often showing vertical jointing both layers show randomly oriented acicular textures of centimetre sized skeletal clinopyroxene crystals al rich augite that precede the formation of varioles the varioles themselves are mainly composed of radiating acicular augite 1 m in diameter the crystallization of which caused diffusion of incompatible elements which became enriched in the groundmass around the varioles the whole rock geochemistry of the hekpoort variolites is indistinguishable from that of non variolitic hekpoort lavas with the same mgo content this excludes liquid immiscibility as the origin of the varioles the acicular texture of al rich augite in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification sub aqueous deposition is thought to be the cause of undercooling

constraints on the origin of archean trondhjemites based on phase relationships of nuk gneiss with h2o at 15 kbar we report the t x h2o phase relations for the trondhjemitic nuk gneiss which comprises the principal component of the second phase of archean 3 0 2 8 by igneous activity in the godthab region of southwestern greenland a pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths under h2o saturated conditions a melting interval of 135 c separates the solidus at 610 c from the liquidus at 745 c h2o saturation at 15 kbar occurs at approximately 15 5 wt h2o the h2o undersaturated liquidus extends along a curved path from 745 c at 15 5 wt h2o to 1100 c at 2 h2o lower h2o contents were not investigated at low h2o contents 7 mafic residues consisting of some combination of hb ga ep and bi are possible and could correspond to amphibolitic source rocks at lower melt h2o contents 5 possible residues consist of na pl qz ga and could correspond to an earlier generation of tonalitic trondhjemitic rocks however such residues would not impart the highly fractionated ree patterns characteristic of archean trondhjemites if a first generation of tonalitic trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth it would crystallize to an assemblage consisting almost entirely of na pl qz with highly fractionated ree patterns these rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated ree patterns because neigher pl nor qz is capable of significantly fractionating hree from lree

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

improved spatial resolution for u series dating of opal at yucca mountain nevada usa using ion microprobe and microdigestion methods two novel methods of in situ isotope analysis ion microprobe and microdigestion were used for 230th u and 234u 238u dating of finely laminated opal hemispheres formed in unsaturated felsic tuff at yucca mountain nevada proposed site for a high level radioactive waste repository both methods allow analysis of layers as many as several orders of magnitude thinner than standard methods using total hemisphere digestion that were reported previously average growth rates calculated from data at this improved spatial resolution verified that opal grew at extremely slow rates over the last million years growth rates of 0 58 and 0 69 mm m y were obtained for the outer 305 and 740 m of two opal hemispheres analyzed by ion microprobe and 0 68 mm m y for the outer 22 m of one of these same hemispheres analyzed by sequential microdigestion these pleistocene growth rates are 2 to 10 times slower than those calculated for older secondary calcite and silica mineral coatings deposited over the last 5 to 10 m y dated by the u pb method and may reflect differences between miocene and pleistocene seepage flux the microdigestion data also imply that opal growth rates may have varied over the last 40 k y these data are the first indication that growth rates and associated seepage in the proposed repository horizon may correlate with changes in late pleistocene climate involving faster growth during wetter cooler climates glacial maximum slower growth during transition climates and no growth during the most arid climate modern data collected at this refined spatial scale may lead to a better understanding of the hydrologic variability expected within the thick unsaturated zone at yucca mountain over the time scale of interest for radioactive waste isolation

the granite problem as exposed in the southern snake range nevada a geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern snake range of eastern nevada the granitoids exposed range in age from jurassic through cretaceous to oligocene and include two calcic intrusions two different types of two mica granites and aplites the younger intrusions appear to have been emplaced at progressively more shallow depths all of these granitoid types are represented elsewhere in the eastern great basin but the southern snake range is distinguished by the grouping of all these types within a relatively small area

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

stable isotopic evidence for a pedogenic origin of carbonates in trench 14 near yucca mountain nevada layered carbonate and silica encrust fault fractures exposed in trench 14 near yucca mountain site of the proposed high level nuclear waste repository in southern nevada comparison of the stable carbon and oxygen isotopic compositions of the fracture carbonates with those of modern soil carbonates in the area shows that the fracture carbonates are pedogenic in origin and that they likely formed in the presence of vegetation and rainfall typical of a glacial climate their isotopic composition differs markedly from that of carbonate associated with nearby springs the regional water table therefore remained below the level of trench 14 during the time that the carbonates and silica precipitated a period probably covering parts of at least the last 300 000 years

geology and petrology of the hormuz dolomite infra cambrian implications for the formation of the salt cored halul and shraouh islands offshore state of qatar abstract geological investigations of the halul and the shraouh islands offshore qatar indicate that most of their calcareous rocks which display abundant stromatolitic bedding belong to the infra cambrian hormuz series mineralogical petrological and geochemical analyses show that these calcareous rocks consist dominantly of dolomite and have formed in a reducing depositional environment faint laminations and small streaks of organic matter furnish evidence for the involvement of algal mats in their genesis and indicate their formation in an intertidal to supratidal setting the halul and shraouh dolomites experienced extensive recrystallization and sulfatization during the emplacement of the halul and shraouh salt domes that form the cores of the islands during mobilization and ascent of the salt the dolomite recrystallized and its sr initial ratios were abnormally enhanced by the incorporation of 87sr from a source which is more radiogenic than the attendant seawater at the time of the dolomite formation near the proterozoic cambrian boundary geochemical analysis show that si al ti zr and of insoluble residue are highly correlative suggesting the presence of detrital minerals such as rutile and zircon a paleosabkha model may well agree with this chemical signature however the infra cambrian age of the hormuz rocks and the presence of stromatolitic layers containing organic materials in the studied rocks suggest that organogenic dolomitization could be an alternative dolomitization model

traces of epigenetic hydrothermal activity at yucca mountain nevada preliminary data on the fluid inclusion and stable isotope evidence samples of opal quartz calcite crusts from the 7 8 km long tunnel excavated under yucca mountain were studied to determine their origin calcite in these crusts commonly forms relatively large up to 1 5 cm euhedral crystal terminations in some crusts layers of chalcedony and patches of opal alternate with calcite layers chalcedony commonly grades into euhedral quartz crystals quartz also forms microdruses of individual crystals up to 8 mm in size fluorite was identified in four samples fluid inclusion studies on calcite revealed formation temperatures of 35 85 c gas filled inclusions are apparently of low density p 1 bar at room temperature and contain aromatic hydrocarbons in four studied calcite samples values of 18o remain virtually constant across crusts 10 6 to 12 1 v pdb in one sample the earliest part of calcite had 18o of 19 in this part of the sample the highest fluid inclusion temperatures 70 85 c were measured the secondary calcite found at yucca mountain is commonly interpreted as being formed through a pedogenic process from rain waters percolating in the vadose zone and carrying dissolved carbonate from overlying soils e g stuckless j s peterman z e muchs d r 1991 u and sr isotopes in ground water and calcite yucca mountain nevada evidence against upwelling water science 254 551 554 nas nrc 1992 ground water at yucca mountain how high can it rise national academy press washington dc roedder e whelan j f vaniman d t 1994 fluid inclusion studies of calcite veins from yucca mountain nevada tuffs environment of formation high level radioactive waste management proc int conf am nucl soc la grande park il pp 1854 1860 our data indicate that the calcite opal quartz with minor fluorite crusts from the presently unsaturated vadose zone of yucca mountain were formed from low temperature hydrothermal aqueous fluids in the saturated phreatic environment the issue has direct bearing on the suitability of yucca mountain as a site for permanent geological disposal of the high level nuclear waste

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

understanding the genesis of nonmarine calcite deposits through quadrupole mass spectrometric analysis of fluid inclusion gases abstract quadrupole mass spectroscopy was used to analyze fluid inclusion volatiles in different types of nonmarine calcites including travertines phreatic calcretes hydrothermal vein calcites and pedogenic calcites the objectives of the study were to determine if there were any diagnostic or characteristic differences between the inclusion volatiles from the different calcites and to see if the quadrupole results could be used to understand the processes and environmental conditions that controlled calcite precipitation the calcites formed under saturated conditions have water dominated inclusions with well constrained n 2 ar ratios and either co 2 or n 2 as the dominant inclusion gas some calcretes and travertines were exceptions to the above characteristics but it is likely that these were subjected to secondary precipitation in an environment different from the primary precipitation phase results for the pedogenic calcites are quite different from calcites formed in water saturated systems pedogenic calcites have gas dominated water poor inclusions unexpectedly large ch 4 contents low o 2 contents and highly variable n 2 ar ratios the dominance of ch 4 and low o 2 contents in the pedogenic calcites suggest that anaerobic conditions occurred during at least part of the precipitation process scanning electron microscope sem examination of the pedogenic calcites shows fungal remains bacteria like forms and gels from fungi or plant roots the quadrupole and sem results suggest pedogenic calcite precipitation occurred as a result of aerobic and ultimately anaerobic microbial decay processes

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

dissolution of primary basaltic minerals in natural waters saturation state and kinetics abstract the state of saturation of olivine orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in iceland including river waters groundwater and geothermal waters with temperatures up to 250 c the stability of olivine and orthopyroxene decreases with increasing mg content similarly the stability of plagioclase decreases with increasing anorthite content the river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene the degree of undersaturation being dominated by the water ph all river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the ph is 200 c are undersaturated with olivine orthopyroxene and plagioclase the first two particularly when fe rich and sufficient to cause olivine to dissolve according to the linear rate law at intermediate temperatures 50 150 c the geothermal waters are close to equilibrium with these minerals except for mg rich olivine it seems likely that dissolution of glass from basalt which is very reactive will favor stability of the igneous minerals

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

modeling of continental weathering under high co2 atmospheres during precambrian times batch experiments were conducted to simulate abiotic weathering of the continental crust under high co2 atmospheric conditions during precambrian times i e corresponding to the general archaean conditions as well as to the immediate aftermath of the neoproterozoic snowball earth ice ages three types of rock basalt granodiorite and tonalite representative of the archaean to proterozoic continental crust were reacted in the form of powders for 1 year at 40 c with pure water under various water rock ratios in oxic and anoxic atmospheres containing 10 co2 that is under conditions assumed to characterize the greenhouse effect which prevailed at that time chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps 1 rapid leaching of the fresh surfaces probably related to the proton donor capacity of the co2 and 2 a steady state reaction under near neutral conditions these observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature using a 1 d weathering model reproducing the two steps we evaluate the atmospheric co2 consumption rate at the snowball earth aftermath focusing on the relative contribution of surface leaching versus steady state reaction the results show that the process is dominated by surface exchange

basaltic volcanism mantle plumes and the mechanics of rifting the parana flood basalt province of south america dynamic modeling of continental extension between south america and africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the parana and etendeka flood basalt provinces on the brazilian and namibian margins the key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension which provided a mechanism for transporting magma generated beneath the incipient sea floor spreading axis into the parana province 100 200 km distant the horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting the model accounts for the large quantity of basalt the asymmetric distribution of basalt on the conjugate margins and the northward migration of the eruptive center with time the rapidity of magma genesis is in agreement with models of decompression melting during rifting the model indicates that although elevated asthenosphere temperatures associated with the tristan plume account for the volume of melt generated the mechanics of rifting control the location and style of emplacement the model suggests that extension in the region began ca 150 155 ma and lasted about 25 m y

magnetic susceptibility of the mont louis andorra ilmenite type granite pyrenees a new tool for the petrographic characterization and regional mapping of zoned granite plutons we have observed remarkably consistent patterns of concentric zonation in the values of low field magnetic susceptibility measurements over the variscan age mont louis andorra granite pluton of the eastern pyrenees of andorra spain and france this zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature as defined by chemical analysis and the magnetic susceptibility magnitude of the rocks it reflects the dominantly paramagnetic nature of the granite carried by fe bearing silicates and this is demonstrated by the fact that low field susceptibilities of representative specimens are almost equal to 1 their high field susceptibilities and 2 their calculated susceptibilities using fe contents assuming a null ferromagnetic contribution we conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons

limited hydrologic response to pleistocene climate change in deep vadose zones yucca mountain nevada abstract understanding the movement of water through thick vadose zones especially on time scales encompassing long term climate change is increasingly important as societies utilize semi arid environments for both water resources and sites viewed as favorable for long term disposal or storage of hazardous waste hydrologic responses to pleistocene climate change within a deep vadose zone in the eastern mojave desert at yucca mountain nevada were evaluated by uranium series dating of finely layered hyalitic opal using secondary ion mass spectrometry opal is present within cm thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface uranium concentrations in opal fluctuate systematically between 5 and 550 g g age calibrated profiles of uranium concentration correlate with regional climate records over the last 300 000 years and produce time series spectral peaks that have distinct periodicities of 100 and 41 ka consistent with planetary orbital parameters these results indicate that the chemical compositions of percolating solutions varied in response to near surface climate driven processes however slow micrometers per thousand years relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

geologic evidence for archean atmospheric and climatic evolution fluctuating levels of co2 ch4 and o2 with an overriding tectonic control the 3 5 3 2 ga sedimentary record shows evidence of surface temperatures of 70 15 c nahcolite nahco 3 as a primary evaporitic mineral and an aggressive weathering regime even in the absence of land vegetation these features are best explained by a mixed ch 4 and co 2 atmospheric greenhouse in which ch 4 co 2 ratios were 1 and pco 2 was at least 100 1000 times the present value and perhaps as high as several bars the formation of large areas of continental crust at 3 2 3 0 ga including the kaapvaal and pilbara cratons resulted in the gradual depletion of atmospheric co 2 through weathering by 2 9 2 7 ga declining pco 2 was associated with climatic cooling and siderite free soils transitory ch 4 co 2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric ch 4 reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 c depleted organic c surface temperatures of 60 c after 2 9 ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes eventual lowering of new continental blocks by erosion reduced loss of atmospheric co 2 due to weathering and continued long term tectonic recycling of co 2 resulted in rising pco 2 and decreasing ch 4 co 2 ratios in the later archean and eventual reestablishment of a mainly co 2 greenhouse similar events may have been repeated in the latest archean and earliest proterozoic but gradually rising production of o 2 effectively kept ch 4 co 2 ratios to 1 at this time

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

how tillite weathering during the snowball earth aftermath induced cap carbonate deposition the development and aftermath of a snowball earth during precambrian time still raise questions particularly the process involved in the deposition of the associated cap carbonates and the time required for the earth to return to more stable and less extreme climatic conditions we present numerical simulations modeling the chemical weathering of regolith exposed to a co 2 rich atmosphere after deglaciation we consider the effects of an early cation proton exchange reaction that promotes fast initial dissolution of the regolith a reaction often omitted in the modeling approach but one that becomes particularly important under acidic conditions and in environments where glacial erosion can substantially refresh the rock surfaces calculations show that this dissolution step may contribute significantly to the riverine ionic input and may promote the deposition of thick cap carbonates over a short period of time the long term weathering of the regolith becomes predominant representing an efficient way of removing atmospheric co 2 over periods ranging from tens to hundreds of thousands of years depending on the grain size of the regolith material

proterozoic links between the borborema province ne brazil and the central african fold belt the congo cc and the sao francisco sfc cratons were joined at about 2 05 ga northern parts of palaeoproterozoic basement subsequently underwent extension at about 1 ga forming intracratonic basins neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 ma the brasiliano and pan african c 620 580 ma assembly of west gondwana extensively altered this system the sergipano domain occurs north of the sfc and the comparable yaoundedomain occurs north of the cc crust north of the sergipano domain com prises the pernambuco alagoas peal domain the ne sw striking tchollire banyo fault in cameroon may extend southwestwards between the peal and sergipano domains defining northern limits of abundant sfc cc basement the adamawa yadedomain in africa does not appear to extend into brazil the transverse domain of brazil is a collage of palaeoproterozoic crustal blocks the 1 0 ga cariris velhos orogen cvo late neoproterozoic basins and brasi liano granites the cvo extends ene for more than 700 km in brazil but eastern continuation into africa has not been identified north of the transverse domain contiguous c 2 15 ga gneisses comprise basement of rio grande do norte and cearadomains which continue eastwards into western nigeria and western sahara

development of magnesian carbonates in quaternary soils on the island of hawaii abstract pedogenic carbonate minerals have progressively developed on basalt in a quaternary weathering chronosequence 5 350 ka on the island of hawaii the sites were isolated from direct contact with seawater or the water table and were minimally influenced by eolian input throughout their history the carbonates consist chiefly of high magnesium calcite hmc and well ordered stoichiometric dolomite pedogenic development of such minerals is rare and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates 87sr 86sr ratios of chronosequence carbonates range from 0 7038 to 0 7048 the data indicate that 75 of the strontium was derived from the weathering of volcanic parent material even in the oldest soils where few primary minerals are available to provide ca and mg this is in contrast to continental calcretes which are commonly dominated by atmospheric inputs and are generally composed of low mg calcite lmc stable isotope data are consistent with carbonate precipitation from soil waters in an arid environment the magnesium content of the carbonate increases with substrate age and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early formed lmc and hmc opal was detected in young 5 10 ka samples and gibbsite is present in older samples indicating a shift from silica dominated to aluminum dominated soil solution high concentrations of dissolved silica and aluminum during early weathering likely precluded formation of mg rich secondary clay minerals such as smectite or palygorskite weathering of basalt with a molar mg ca ratio of 0 8 1 5 and the absence of secondary magnesian silicates can account for elevated soil water mg ca ratios 0 7 1 0 in soils 70 ka that led to magnesian carbonate formation over time the formation of a clay rich soil profile prevented rapid evaporation of soil water possibly facilitating crystallization of stoichiometric dolomite

weathering of basaltic pebbles in a red soil from sardinia a microsite approach for the identification of secondary mineral phases abstract mineral weathering microsites in a basalt rock derived soil were investigated in the mediterranean environment of south western sardinia an in situ approach frequently used for saprolites and rock investigations was employed to examine the weathering microsites of a soil system this study compared clay phases that formed from the weathering of mineral bearing rock pebbles and soil crystal chemical composition with the goal of evaluating the evolutionary trends of the secondary minerals micromorphological mineralogical xrd and ft ir analyses and microscopic sem eds analyses were carried out on i plagioclase enstatite and glassy features weathered volcanic glass microsystems in rock pebbles embedded in the soil ii clay coatings microsystem connecting the soil rock environment and iii the soil itself eds data were plotted on phase diagrams to evaluate developmental and evolutionary trends of the weathered products the weathering of enstatite and volcanic glass resulted in primarily fe bearing montmorillonites while a mixture of fe bearing smectite and kaolinite developed from plagioclases our results indicated formation of secondary phases in open microsystems where transfers of elements among primary minerals and glass occur by slow diffusion along the network of inter mineral micropores cracks that developed inside the soil rock pebbles acted as direct connections between microsystems and the soil environment clay and fe mn coatings deposited in the cracks are an indication of clay migration the mineralogy of these clay coatings was similar to that of the soil matrix and the fine clays since it is likely that the clay migration processes remain active in guspini soils the further mineralogical similarities of clay coatings and soil fine clays suggest that the transformation of fe smectites into k s mlms occurs in the clay phases during transport prior to deposition into the soil environment indeed the similar mineralogy of the in situ analysed soil matrix and fine clays the youngest soil fraction is interpreted as due to same soil processes suggesting stable environmental conditions in the guspini area

reply to the comment on physical and stable isotope evidence for formation of secondary calcite and silica in the unsaturated zone yucca mountain nevada by y v dublyansky s e smirnov and g p palyanova

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

aftermath of a snowball earth 1 using a simple 3 box model of the ocean atmosphere system we simulate the cycling of carbon and strontium in the aftermath of a global glaciation model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pco2 conditions as well as ocean mixing air sea gas exchange and biological productivity the 13c of the first carbonate precipitated after the glaciation depends on the pco2 temperature the saturation state of the surface ocean and kinetic effects associated with mineral precipitation with no biological productivity the model produces 13c values between 1 and 3 consistent with observations this is in direct contradiction with arguments by kennedy et al 2001a who suggest that the 13c value of dissolved carbon in a snowball ocean and directly afterward must be 5 kennedy et al assume the carbon isotope cycle is in steady state which does not apply to a global glaciation and also neglect any effect of high pco2 on the carbonate chemistry of seawater a major difference between our findings and the qualitative predictions of hoffman et al 1998 is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves as a result the 2 drop in the 13c observed in the cap dolostone is unlikely to be the product of rayleigh distillation of atmospheric co2 via silicate weathering instead we interpret the 2 drop in the 13c values as indicative of an increase in sea surface temperature which lowers the fractionation between co2 and carbonate kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important rayleigh distillation of atmospheric co2 via silicate weathering is a viable explanation for the continued drop in the 13c values in the limestone sequence above the cap dolostone with biological productivity and carbonate weathering driving a slow increase in 13c values once pco2 levels decline our study also simulates the cycling of strontium in seawater in contrast to the finding of jacobsen and kaufman 1999 and kennedy et al 2001a model simulations show a drop in 87sr 86sr of less than 0 001 during 5 million years of global glaciation and an increase of less than 0 001 over the entire episode of silicate weathering our calculations emphasize the importance of considering the changes in seawater chemistry due to high pco2 in evaluating the snowball earth hypothesis

evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site yucca mountain nevada usa revealed by the isotope composition of fluid inclusion water abstract secondary calcite residing in open cavities in the unsaturated zone of yucca mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures in order to obtain information on the isotopic composition d and 18 o of the mineral forming water we studied fluid inclusions from this calcite water was extracted from inclusions by heated crushing and the d values were measured using a continuous flow isotope ratio mass spectrometry method the 18 o values were calculated from the 18 o values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid inclusion microthermometry the d values measured in all samples range between 110 and 90 similar to holocene meteoric water coupled 18 o d values plot significantly 2 to 8 to the right of the meteoric water line among the various processes operating at the topographic surface and or in the unsaturated zone only two processes evaporation and water rock exchange could alter the isotope composition of percolating water our analysis indicates however that none of these processes could produce the observed large positive 18 o shifts the latter require isotopic interaction between mineral forming fluid and host rock at elevated temperature 100 c which is only possible in the deep seated hydrothermal environment the stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at yucca mountain precipitated instead they point to the deep seated provenance of the mineral forming waters and their introduction into the unsaturated zone from below i e a hypogene origin

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

evolution in composition of granitic rocks controlled by time dependent changes in petrogenetic processes examples from the archaean of eastern finland abstract the grey gneisses of eastern finland form the basement on which the archaean greenstone belts were developed they are composed of tonalites trondhjemites and granodiorites ttg emplaced during two distinct magmatic episodes at 2 86 ga kivijarvi gneisses and 2 65 ga naavala petrogenetic models based on trace element behaviour indicate that the ttg are ultimately derived from the mantle by a mechanism involving two stages 1 1 melting of the mantle to form a tholeiitic crust and 2 2 re melting of these tholeiites transformed into garnet amphibolites to yield ttg rocks the low yb contents of archaean ttg are explained by the influence of residual garnet and hornblende in the source rocks involved in magma genesis after 2 5 ga the role of these minerals becomes progressively less important and this is interpreted in terms of a cooling earth model at present the subducted oceanic crust is relatively cold and the geothermal gradient at its interface with the overlying mantle wedge is low in addition the descending lithospheric slab undergoes dehydration before partial melting is able to occur and the liberated fluids pass upward into the mantle wedge or base of the continental crust where partial melting may begin these magmas have calc alkaline affinities and have equilibrated with residual olivine and pyroxene in the source consequently the liquids derived from partial melting have high yb contents during the archaean subducted oceanic crust was relatively hot and the geothermal gradients in the interface zone with the mantle were high c 20 25 c km 1 thus partial melting of the subducted oceanic material was possible at temperatures between 650 and 700 c before dehydration would take place residual phases in the source would include garnet and hornblende thus explaining the low yb contents of the ttg association it is concluded that there has been a change in the site of cale alkaline magma genesis since archaean times

protracted felsic magmatic activity associated with the opening of the south atlantic felsic magmatism is often observed in the final stages of the eruption of continental large igneous provinces the occurrence of these felsic extrusive igneous rocks potentially provides key information on the complex interplay of magmatism and tectonics in this paper we investigate the timing of eruption of early cretaceous felsic magmas associated with the parana etendeka flood basalt province using the 40 ar 39 ar technique and interpret the results in terms of the recently proposed petrogenetic histories and dominant stress factors there appears to be a link between lithospheric thinning and a change in the origin of the felsic magmas from open system fractional crystallization to mid lower crustal melting particularly in southern uruguay

ancient oceans and oxygen the ocean chemistry of 1 5 billion years ago inferred from rocks of that age supports the view that marine conditions then were very different from those that pertained at earlier and later times

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

physical and stable isotope evidence for formation of secondary calcite and silica in the unsaturated zone yucca mountain nevada abstract calcite and silica form coatings on fracture footwalls and cavity floors in the welded tuffs at yucca mountain the potential site of a high level radioactive waste repository these secondary mineral deposits are heterogeneously distributed in the unsaturated zone uz with fewer than 10 of possible depositional sites mineralized the paragenetic sequence compiled from deposits throughout the uz consists of an early stage assemblage of calcite fluorite zeolites that is frequently capped by chalcedony quartz intermediate and late stage deposits consist largely of calcite commonly with opal on buried growth layers or outermost crystal faces of the calcite coatings on steep dipping fractures usually are thin 3 mm with low relief outer surfaces whereas shallow dipping fractures and lithophysal cavities typically contain thicker more coarsely crystalline deposits characterized by unusual thin tabular calcite blades up to several cms in length these blades may be capped with knobby or corniced overgrowths of late stage calcite intergrown with opal the observed textures in the fracture and cavity deposits are consistent with deposition from films of water fingering down fracture footwalls or drawn up faces of growing crystals by surface tension and evaporated at the crystal tips fluid inclusion studies have shown that most early stage and some intermediate stage calcite formed at temperatures of 35 to 85 c calcite deposition during the past several million years appears to have been at temperatures 18 o values although others have interpreted the elevated temperatures as evidence of hydrothermal activity and flooding of the tuffs of the potential repository the authors conclude that the temperatures and fluid inclusion assemblages are consistent with deposition in a uz environment that experienced prolonged heat input from gradual cooling of nearby plutons the physical restriction of the deposits and therefore fluid flow to fracture footwalls and cavity floors and the heterogeneous and limited distribution of the deposits provides compelling evidence that they do not reflect flooding of the thick uz at yucca mountain the textures and isotopic and chemical compositions of these mineral deposits are consistent with deposition in a uz setting from meteoric waters percolating downward along fracture flow paths

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

a model for late archean chemical weathering and world average river water abstract interpretations of the geologic record of late archean near surface environments depend very strongly on an understanding of weathering and resultant riverine transport to the oceans the late archean atmosphere is widely recognized to be anoxic p o 2 g 10 5 to 10 13 bars p h 2 g 10 3 to 10 5 bars detrital siderite feco3 pyrite fes2 and uraninite uo2 in late archean sedimentary rocks also suggest anoxic conditions however whether the observed detrital minerals could have been thermodynamically stable during weathering and riverine transport under such an atmosphere remains untested similarly interpretations of fluctuations recorded by trace metals and isotopes are hampered by a lack of knowledge of the chemical linkages between the atmosphere weathering riverine transport and the mineralogical record in this study we used theoretical reaction path models to simulate the chemistry involved in rainwater and weathering processes under present day and hypothetical archean atmospheric boundary conditions we included new estimates of the thermodynamic properties of fe ii smectites as well as smectite and calcite solid solutions simulation of present day weathering of basalt calcite by world average rainwater produced hematite kaolinite na mg saponite and chalcedony after 10 4 moles of reactant minerals kg 1 h2o were destroyed combination of the resultant water chemistry with results for granitic weathering produced a water composition comparable to present day world average river water warw in contrast under late archean atmospheric conditions p co 2 g 10 1 5 and p h 2 g 10 5 0 bars weathering of olivine basalt calcite to the same degree of reaction produced kaolinite chalcedony and na fe ii rich saponite late archean weathering of tonalite trondhjemite granodiorite ttg formed fe ii rich beidellite and chalcedony combining the waters from olivine basalt and ttg weathering resulted in a model for late archean warw with the composition na ca2 fe2 mg2 cl hco 3 sio2 aq the ph of the water was 6 3 and it is much richer in hco 3 and in mg fe relative to ca na compared to present day warw at higher ph2 g e g 10 4 0 bars organic acid anions could be metastable our results are consistent with the thermodynamic stability of fe ii clays pyrite uraninite and under some conditions siderite during weathering and riverine transport overall our results provide a basis for assessing the formation of organic hazes and the mobility of trace elements and nutrients due to fluctuations of the late archean atmosphere

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

reply to the comment on origin timing and temperature of secondary calcite silica mineral formation at yucca mountain nevada by y v dublyansky s z smirnov and g p palyanova

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

geochemistry of precambrian gneisses relevance for the evolution of the east antarctic shield archaean granulite facies orthogneisses of the napier complex in enderby land metamorphosed 3070 maago comprise two chemically distinct suites the more abundant mainly of tonalitic to granodioritic composition shows strong y depletion explicable by hydrous partial melting of a garnet bearing source garnet amphibolite or possibly eclogite it apparently represents new continental crust other gneisses predominantly of trondhjemitic to granitic s s composition do not show y depletion and have higher tio2 zr nb la ce and ga al and lower cao sr and mg mg total fe they probably originated by relatively dry melting of predominantly felsic crystal rocks both suites show evidence for loss of rb relative to k th and u during metamorphism late archaean 2800 ma amphibolite facies gneisses of macrobertson land are of undepleted type and may be representative of a higher crustal level than those of enderby land late proterozoic 1000 ma granulite facies gneisses of enderby land rayner complex are to a large extent remetamorphosed napier complex rocks of igneous derivation in contrast gneisses of similar age in macrobertson land include a much higher proportion derived either directly or by partial melting from sedimentary protoliths

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

magmatic evolution of sulawesi indonesia constraints on the cenozoic geodynamic history of the sundaland active margin abstract tertiary and quaternary magmatic rocks from west sulawesi record the complex history of part of the sundaland margin where subduction and collision have been and are still active the present study based on petrographic data major and trace element chemistry and 40 k 40 ar dating aims to document the age and chemical characteristics of the magmatic formations from west sulawesi and to determine the corresponding constraints on the geodynamic evolution of the sundaland border the west sulawesi magmatic province includes the south arm of sulawesi ujung pandang area the western part of central sulawesi with the toraja and palu areas and finally the north arm extending from palu to manado which includes the tolitoli and manado areas paleocene magmatic activity seems to be restricted to an episode of calc alkaline magmatism in the ujung pandang area 61 59 ma the major eocene 50 40 ma magmatic event is tholeiitic and is documented in all areas except in ujung pandang it led to the emplacement of tholeiitic pillow lavas and basaltic dykes of back arc basin bab affinity these rocks are potential equivalents to the celebes sea basaltic basement from oligocene to miocene magmatic eruptions produced successively island arc tholeiitic iat and calc alkaline ca rock series the youngest iat activity occurred around 18 ma in the central part palu area and around 14 ma in the north arm tolitoli area while ca magmas were emplaced in the north arm at ca 18 ma tolitoli and manado areas typical calc alkaline activity resumed only in the north arm tolitoli and manado areas during the late miocene 9 ma and is still active in the manado region in other areas palu toraja and ujung pandang areas an important and widespread magmatic event occurred between 13 and 10 ma and emplaced k rich magmas either silica undersaturated alkali potassic basalts ak ultrapotassic basanites uk or shoshonites sh k rich activity continued in the south until the pleistocene 0 77 ma with alkali potassic ultrapotassic and shoshonitic magmas in central sulawesi toraja and palu areas the most recent magmatic event occurred between 6 5 and 0 6 ma the corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits all these rocks are acidic in character sio 2 60 with trace element and isotopic signatures sr nd pb typical of a strong continental imprint the most striking tectonic implication of this magmatic evolution is that west sulawesi can no longer be considered as a typical magmatic arc as previously assumed with the exception of the manado area beneath which subduction is still active calc alkaline and island arc tholeiitic lavas and plutonics are volumetrically minor with respect to k rich magmas their occurrence through time is also fairly restricted mostly to the period between 30 and 15 ma another important feature is the occurrence of island arc tholeiitic and calc alkaline magmas crosscutting an older terrane of bab affinity the tinombo formation manado tolitoli and palu areas as this formation is being regarded as an equivalent to the celebes sea floor the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the nw dipping subduction beneath the north arm the late miocene high k magmatic activity in central and south sulawesi reflects the prevalence of a post collisional tectonic regime following the docking of microcontinents of australian origin to central sulawesi during neogene times the incompatible element enriched character of these high k rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event such a model cannot account for the plio pleistocene cak magmatism of central sulawesi the acidic composition of which does not support a derivation from an ultrabasic source the trace element patterns of the cak rocks are very similar to those of the high grade metamorphics of central sulawesi suggesting that the latter might represent their possible source such an anatectic model implies a collisional to post collisional tectonic regime limited to central sulawesi while a post subduction regime prevailed in the south

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

geochemical evolution of archaean granulite facies gneisses in the vestfold block and comparisons with other archaean gneiss complexes in the east antarctic shield the vestfold block like other archaean cratons in east antarctica and elsewhere consists predominantly of felsic orthogneiss mossel and crooked lake gneisses with subordinate mafic granulite tryne metavolcanics and paragneiss chelnok supracrustals two major periods of continental crust formation are represented the mossel gneiss metamorphosed about 3 000 ma ago is mainly of tonalitic composition and is similar to much of the roughly coeval napier complex in enderby land the crooked lake gneiss was emplaced under high grade conditions about 2 450 ma ago and comprises a high proportion of more potassic rocks monzodioritic and monzonitic suites as well as tonalite and minor gabbro and diorite both mossel and crooked lake gneisses are depleted in y and have moderate to high sr ce y and ti y consistent with melting of a mafic source subducted hydrated oceanic crust leaving major residual hornblende garnet most crooked lake gneisses are more enriched in incompatible elements p sr la ce and particularly rb ba and k than mossel gneisses suggesting derivation from a more enriched mafic source the vestfold block contains few orthogneisses derived by melting of older felsic crustal rocks in marked contrast to the archaean napier complex and in particular southern prince charles mountains both mossel and crooked lake tonalites are strongly depleted in rb k th and u and have very low rb sr and high k rb more potassic orthogneisses are depleted in th u and to lesser extents rb tryne metavolcanics are depleted in th and rb but appear to have been enriched in k and probably na possibly during early low grade alteration

natural radionuclide mobility and its influence on u th pb dating of secondary minerals from the unsaturated zone at yucca mountain nevada abstract extreme u and pb isotope variations produced by disequilibrium in decay chains of 238u and 232th are found in calcite opal chalcedony and mn oxides occurring as secondary mineral coatings in the unsaturated zone at yucca mountain nevada these very slowly growing minerals mm my 1 contain excess 206pb and 208pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206pb 238u geochronometers the presence of excess intermediate daughter isotopes does not appreciably affect 207pb 235u ages of u enriched opal chalcedony which are interpreted as mineral formation ages opal and calcite from outer younger portions of coatings have 230th u ages from 94 6 3 7 to 361 3 9 8 ka and initial 234u 238u activity ratios ar from 4 351 0 070 to 7 02 0 12 which indicate 234u enrichment from percolating water present day 234u 238u ar is 1 in opal chalcedony from older portions of the coatings the 207pb 235u ages of opal chalcedony samples range from 0 1329 0 0080 to 9 10 0 21 ma increase with microstratigraphic depth and define slow long term average growth rates of about 1 2 2 0 mm my 1 in good agreement with previous results measured 234u 238u ar in mn oxides which pre date the oldest calcite and opal chalcedony range from 0 939 0 006 to 2 091 0 006 and are 1 in most samples the range of 87sr 86sr ratios 0 71156 0 71280 in mn oxides overlaps that in the late calcite these data indicate that mn oxides exchange u and sr with percolating water and cannot be used as a reliable dating tool in the u poor calcite samples measured 206pb 207pb ratios have a wide range do not correlate with ba concentration as would be expected if excess ra was present and reach a value of about 1400 the highest ever reported for natural pb calcite intergrown with opal contains excesses of both 206pb and 207pb derived from rn diffusion and from direct recoil from u rich opal calcite from coatings devoid of opal chalcedony contains 206pb and 208pb excesses but no appreciable 207pb excesses observed pb isotope anomalies in calcite are explained by rn produced excess pb the rn emanation may strongly affect 206pb 238u ages of slow growing u poor calcite but should be negligible for dating fast growing u enriched speleothem calcite

oxygen isotope studies of illite smectite and clinoptilolite from yucca mountain implications for paleohydrologic conditions abstract illite smectite i s and clinoptilolite mineral separates from drill holes in 11 ma old altered volcanic tuffs of yucca mountain nevada were analyzed for 18 o values we reconstruct the diagenetic and paleohydrologic conditions using the isotopic composition of clay minerals combined with other independent geological observations our isotopic data show that the 18 o values of clay minerals preserve paleohydrologic information from a middle to late miocene episode of hydrothermal alteration at yucca mountain we provide additional evidence for a dual decoupled groundwater circulation system at yucca mountain ca 11 ma ago a near surface system dominated by downward percolation of groundwater is separated from a deeper hydrothermal system by profound thermal and isotopic discontinuities near the r0 r1 i s transition given the set of inferred formation temperatures and the isotopic composition of formation water we show that the current set of isotopic compositions of clinoptilolite is significantly lower than those at the time of formation and are not significantly different from the equilibrium values defined by the current groundwater 18 o and geothermal gradient we conclude that unlike i s clinoptilolite has not preserved its original isotopic composition over the past 11 million years but we do not know how long is necessary to reset the isotopic signature of clinoptilolite under near surface conditions the current groundwater at yucca mountain is 4 more enriched in 18 o than the paleogroundwater 10 11 ma ago as inferred from our reconstruction this shift may be attributed to a change in the pattern of atmospheric circulation such as by surficial uplift of the sierra nevada which caused it to become an orographic barrier to moisture penetrating inland from the west

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

origin and evolution of the late precambrian high grade yaounde gneisses cameroon abstract the yaounde series is composed of low to high grade garnet bearing schists and gneisses belonging to the pan african north equatorial fold belt the high grade rocks consist of kyanite garnet gneisses and garnet plagioclase gneisses containing layers of calcsilicate rocks marbles quartzites and magnetite rich orthopyroxenites the chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite rocks dolomitic marls evaporitic beds quartzites and iron rich sediments some volcanogenic greywackes may have been of alkaline affinity this sequence was probably deposited in shallow water near shore semi barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the congo craton the yaounde gneisses were deformed during two main tectonic phases the first phase d1 is marked by a layering s0 s1 resulting from tectonic transposition the second one d2 corresponds to tangential tectonics with isoclinal folds and flat lying shear zones associated with a s2 schistosity and a l2 lineation a third phase late d2 is marked by the development of low temperature mylonitic shears scarce wrench faults indicate later e w compression the d1 d2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high pressure granulite facies conditions t 750 800 c p 10 12 kbar these peak metamorphic conditions triggered a widespread migmatization dated at 565 22 ma and coeval with the d2 event the gradual decrease of the metamorphic conditions toward the south and the existence of a syn d2 retrograde inverse metamorphic gradient suggest that the overall structure of the yaounde series is a large tectonic nappe thrust onto the congo craton a geotectonic scheme for the north equatorial fold belt is discussed