tectonic plate generation and two phase damage void growth versus grain size reduction 1 the generation of plate tectonics from mantle convection requires shear localization in order to yield narrow weak plate boundaries separating broad strong plates a plausible shear localizing mechanism involves damage e g distributed microcracking one theoretical model of which involves two phase mechanics two phase damage theory employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a description of void generation and microcracking and hence weakening failure and shear localization here we examine the application of this theory to the problem of generating plate like behavior from convective type divergent convergent poloidal motion through a source sink formulation we extend the previous damage theory to consider two possible damage effects 1 growth and nucleation of voids associated with dilation of the host matrix and 2 increasing fineness i e reducing coarseness of the mixture by for example grain size reduction void generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate like flow fineness generating damage is found to be very efficient at generating plate like behavior if we assume that the matrix viscosity is a simple function of grain void size as is typical for diffusion creep the implied grain size reduction mechanism is different from that of dynamic recrystallization and appears to be more capable of generating the requisite shear localization for forming tectonic plates from mantle flow

initiation of plate tectonics from post magma ocean thermochemical convection leading theories for the presence of plate tectonics on earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere however it is unknown whether the conditions of the early earth were favorable for plate tectonics or any form of subduction and thus how subduction begins is unclear using physical models based on grain damage a grainsize feedback mechanism capable of producing plate like mantle convection we demonstrate that subduction was possible on the hadean earth hereafter referred to as proto subduction or proto plate tectonics that proto subduction differed from modern day plate tectonics and that it could initiate rapidly scaling laws for convection with grain damage show that though either higher mantle temperatures or higher surface temperatures lead to slower plates proto subduction with plate speeds of 1 75 cm yr can still be maintained in the hadean even with a co2 rich primordial atmosphere furthermore when the mantle potential temperature is high e g above 2000 k the mode of subduction switches to a sluggish subduction style where downwellings are drip like and plate boundaries are diffuse finally numerical models of post magma ocean mantle convection demonstrate that proto plate tectonics likely initiates within 100 myr of magma ocean solidification consistent with evidence from hadean zircons after the initiation of proto subduction non plate tectonic sluggish subduction prevails giving way to modern style plate tectonics as both the mantle interior and climate cool hadean proto subduction may hasten the onset of modern plate tectonics by drawing excess co2 out of the atmosphere and cooling the climate

the geochemistry of deep sea coral skeletons a review of vital effects and applications for palaeoceanography deep sea corals were discovered over a century ago but it is only over recent years that focused efforts have been made to explore the history of the oceans using the geochemistry of their skeletal remains they offer a promising archive of past oceanic environments given their global distribution layered growth patterns longevity and preservation as well as our ability to date them using radiometric techniques this paper provides an overview of the current state of the art in terms of geochemical approaches to using deep sea coral skeletons to explore the history of the ocean deep sea coral skeletons have a wide array of morphologies e g solitary cup corals branching colonial corals and materials calcite aragonite and proteins as such their biomineralization strategies are diverse leading to complex geochemistry within coral skeletons notwithstanding these complications progress has been made on developing methods for reconstructing the oceanographic environment in the past using trace elements and isotopic methods promising approaches within certain coral groups include clumped isotopes and mg li for temperature reconstructions boron isotopes and radiocarbon for carbon cycling end and radiocarbon for circulation studies and 15 n p ca and ba ca for nutrient tracer studies likewise there is now a range of techniques for dating deep sea corals skeletons e g u series radiocarbon and determining their growth rates e g radiocarbon and 210 pb dating studies on historic coral populations in the atlantic southern ocean and pacific point to climate and environmental changes being dominant controls on coral populations over millennial and orbital timescales this paper provides a review of a range of successes and promising approaches it also highlights areas in which further research would likely provide new insights into biomineralization palaeoceanography and distribution of past coral populations

atmospheric noble gases in lake sediment pore water as proxies for environmental change 1 lake sediment pore water has been proposed as a noble gas archive for paleoenvironmental reconstruction but appropriate experimental techniques have not been available until recently here we present noble gas concentrations measured in the sediment pore water of lake issyk kul kyrgyzstan which demonstrate for the first time the value of the sediment pore water archive for paleoclimate reconstruction the noble gas profiles in the sediment indicate that the salinity of the lake water during the mid holocene was more than twice its present value of 6 0 g kg implying a 200 m lower lake level

the copper gold and gold deposits of the neoproterozoic mara rosa magmatic arc central brazil mineral exploration in the neoproterozoic goias magmatic arc central brazil dates back to the beginning of the 1970s the goias magmatic arc extends for more than 1000 km in the western and northern parts of goias into tocantins and disappears under the phanerozoic parnaiba basin two main areas of neoproterozoic juvenile crust the arenopolis and the mara rosa arcs are identified they lie in the southern and northern sectors of the goias arc respectively and are relatively well studied the goias magmatic arc dominantly comprises tonalitic dioritic orthogneisses and narrow nne striking volcano sedimentary belts recent u pb zircon data indicate crystallization of the tonalite protoliths in two main episodes the older between ca 890 and 790 ma and the younger at 670 600 ma nd isotopic data indicate the very primitive nature of the original magmas with tdm model ages mostly within the interval between 0 9 and 1 0 ga and end t values between 3 0 and 4 6 in the chapada mara rosa area the supracrustal rocks form three individual nne belts known as the eastern central and western belts separated from each other by metatonalites metadiorites gold and cu au deposits of the mara rosa area occur in four main associations i au ag ba e g zacarias which are interpreted as stratiform disseminated volcanogenic deposits ii cu au e g chapada which has been interpreted either as volcanogenic or as a porphyry type deposit iii au only deposits e g posse interpreted as an epigenetic disseminated deposit controlled by a mesozonal shear zone and iv au cu bi e g the mundinho occurrence which are considered as vein type deposits controlled by magnetite rich diorites the gold and cu au deposits located within the goias magmatic arc can be spatially and temporally related to the magmatic evolution of a collisional belt or in other words to an orogenic gold deposit model these models are based on the continuous evolution of collisional plates which can be subdivided into four stages with distinct magmatic characteristics i subduction stage ii syntectonic collisional magmatism stage iii post tectonic collisional magmatism stage and iv post orogenic extension stage

the mara rosa arch in the tocantins province further evidence for neoproterozoic crustal accretion in central brazil the mara rosa volcano sedimentary sequence consists of several nne trending belts of metavolcanic metabasalts to metarhyolites and metasedimentary rocks micaschists quartzites cherts exposed over large areas of the tocantins province a neoproterozoic brasiliano orogenic region in the central part of brazil the supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post orogenic granitic and dioritic bodies these volcano sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite greenstone terrain of the archaean basement in this paper we present geochronological and isotopic data for rocks of the metavolcanic metaplutonic rock association samples of a felsic metavolcanic rock from the posse gold mine and a metatonalite yielded u pb zircon ages of 862 8 ma and 856 7 13 ma respectively these are interpreted as crystallization ages of the igneous protoliths titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632 4 ma sm nd isotopes for these rocks indicate primitive compositions with nd t of 4 6 and 3 7 and tdm model ages of 1 0 ga a syn tectonic dioritic intrusion has an u pb crystallization age of 630 6 ma and nd t of 1 9 and tdm of 1 0 ga additional sm nd analyses on metasedimentary rocks and post orogenic granites of the mara rosa region yield tdm ages between 1 2 and 1 0 ga the isotopic data combined with preliminary trace element results suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system off the coast of the sao francisco congo continent at around 860 ma recrystallization and deformational ages of 630 ma may represent the timing of final ocean closure and continental collision the rock associations structural pattern geochronological and isotopic characteristics of the rocks from the mara rosa region are very similar to those of neoproterozoic arc terrains exposed 300 km to the southwest in the arenspolis area proving the regional importance of the neoproterozoic crustal accretion event in the central part of brazil

alpha thermochronology of carbonates abstract step heating experiments on 17 calcites from 11 different samples and 6 dolomites from 5 samples suggest a closure temperature of he in carbonates 70 10 c for a cooling rate of 10 c m y the bulk closure temperature in some samples may tend slightly higher due to the presence of diffusion domains larger and therefore more retentive than the sites in which the majority of he resides the diffusivity of he in calcite is independent of the genesis of the mineral igneous sedimentary metamorphic or the source of the he radiogenic common or laboratory induced and in all samples analyzed the effective diffusion dimension for he is smaller than the size of the crystals investigated although calcite is a low u mineral this shortcoming can be overcome by analyzing large samples 2 mm diameter provided samples have a minimum of 0 3 ppm u samples with smaller concentrations of alpha producers are unlikely to produce enough radiogenic 4he sufficient to overwhelm he present in the crystals at the time they passed through their closure temperature

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

hysteresis in mantle convection plate tectonics systems 1 we use simulations of mantle convection with surface yielding to show that multiple tectonic regimes are possible for equivalent system parameter values models with the same lithospheric strength parameters and the same vigor of convection can display different modes of tectonics within the region of multiple solutions the evolutionary pathway of the system is the dominant factor that determines the tectonic mode e g whether mantle convection operates in a plate tectonic like mode the extent of the multiple regimes window is found to increase with the temperature dependent viscosity contrast across the system the implication for models that seek to predict the tectonic regimes of planets is that the temporal evolution of the planet needs to be taken into account a further implication is that modeling studies can lead to different conclusions regarding the tectonic state of a planet extra solar planets in particular despite the final model parameter values remaining equivalent

diffusive fractionation of noble gases and helium isotopes during mantle melting abstract the large differences in he and ar diffusivities in silicate minerals could result in fractionation of the he ar ratio during melting of the mantle producing he ar ratios in the primary mantle melts that are higher than those of the bulk mantle modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways such as fractures or melt channels suggests that significant order of magnitude he ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks in addition the 15 difference in 3he and 4he diffusivities could also produce isotopic fractionation between the melt and its solid source modeling the behavior of he and ar during melting shows that small increases few in 3he 4he should be correlated with larger variations factor of 5 in 4he 40ar however in order to test this hypothesis the effects of subsequent he ar fractionation that occur during degassing have to be corrected i describe a scheme that can separate he ar variations in the primary melt from overprinted fractionation during magmatic degassing using the degassing corrected data there is a correlation between the primary melt s 4he 40ar and 3he 4he in mid ocean ridge basalts morbs the slope of the correlation is consistent with the models of preferential diffusion of 3he relative to 4he and of 4he relative to 40ar from the solid mantle into the melt diffusive fractionation of noble gases during melting of the mantle can also account for low 4he 40ar ratios commonly found in residual mantle xenoliths preferential diffusion of he relative to ar will produce some regions of the mantle with low 4he 40ar the complement of the high 4he 40ar ratios in basalts diffusive fractionation cannot however account for differences between the he and ne isotopic compositions of morbs compared with ocean island basalts oibs not only are the extremely high 3he 4he ratios of oibs up to 50 ra difficult to produce at reasonable mantle time and lengthscales but also the ne isotopic compositions of morbs and oibs do not lie on a single mass fractionation line therefore cannot result from diffusive fractionation of a single mantle ne source if preferential diffusion of he from the solid mantle into primary melts is a significant process during generation of morbs then it is difficult to constrain the he concentration of the mantle he concentrations in basalts and the he flux to the ocean essentially result from extraction of he from a larger and unknown volume of mantle than that that produced the basalts themselves the he concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available

noble gas anomalies related to high intensity methane gas seeps in the black sea dissolved noble gases and tritium were analyzed at a series of high intensity methane gas seeps in the black sea to study the transport and gas exchange induced by bubble streams in the water column these processes affect marine methane emissions to the atmosphere and are therefore relevant to climate warming the seep areas investigated are located in the dnepr paleo delta west of crimea and in the sorokin trough mud volcano area south east of crimea noble gas concentration profiles at active seep sites revealed prominent anomalies compared to reference profiles that are unaffected by outgassing supersaturations of the light noble gases helium and neon observed relatively close to the sea floor are interpreted as effects of gas exchange between the water and the rising bubbles depletions of the heavy noble gases argon krypton and xenon that were detected above an active bubble releasing mud volcano appear to be related to the injection of fluids depleted in noble gases that undergo vertical transport in the water column due to small density differences in both cases the noble gas anomalies clearly document seep specific processes which are difficult to detect by other methods helium is generally enriched in the deep water of the black sea due to terrigenic input although exceptionally high helium concentrations observed in one seep area indicate a locally elevated helium flux most of the seeps studied seem to be negligible sources of terrigenic helium noble gas analyses of sediment pore waters from the vicinity of a mud volcano showed large vertical gradients in helium concentrations the helium isotope signature of the pore waters points to a crustal origin for helium whereas the deep water of the black sea also contains a small mantle type component

scaling of stagnant lid convection with arrhenius rheology and the effects of mantle melting summary on the basis of numerical modelling and scaling analysis a few modifications are proposed for the scaling of stagnant lid convection in order to make it more applicable to the thermal evolution of terrestrial planets the effect of using arrhenius rheology as opposed to more popular linear exponential rheology is first investigated and the stability analysis of top thermal boundary layer is shown to be able to capture systematic differences caused by the different kinds of temperature dependent viscosity the local stability analysis is then extended to handle the effects of mantle melting such as dehydration stiffening and compositional buoyancy a new heat flow scaling law incorporating these effects suggests that mantle melting may reduce the conventional prediction of surface heat flux by up to a factor of 5 10 and its potential impact on our understanding of planetary evolution is briefly discussed

new views of tropical paleoclimates from corals abstract corals offer a rich archive of past climate variability in tropical ocean regions where instrumental data are limited and where our knowledge of multi decadal climate sensitivity is incomplete in the eastern equatorial pacific coral isotopic records track variations in enso related changes in sea surface temperature further west corals record variability in sea surface temperature and rainfall that accompanies zonal displacement of the indonesian low during enso events these multi century records reveal previously unrecognised enso variability on time scales of decades to centuries outside the enso sensitive equatorial pacific long term trends towards recent warmer wetter conditions suggest the tropics respond to global forcings new coral paleothermometers indicate that surface ocean temperatures in the tropical southwestern pacific were depressed by 4 6 c during the younger dryas climatic event and rose episodically during the next 4000 yr high temporal resolution measurements of sr ca and 18 o in corals provide information about the surface ocean hydrologic balance and can resolve the seasonal balance between precipitation and evaporation radiocarbon measurements in corals coupled with ocean circulation models may be used to reconstruct near surface ocean circulation past mixing rates and the distribution of fossil fuel co2 in the upper ocean most recently seasonal to interannual variations in the radiocarbon of corals from the equatorial pacific have been linked to the redistribution of surface waters associated with the enso

deep sea coral evidence for rapid change in ventilation of the deep north atlantic 15 400 years ago coupled radiocarbon and thorium 230 dates from benthic coral species reveal that the ventilation rate of the north atlantic upper deep water varied greatly during the last deglaciation radiocarbon ages in several corals of the same age 15 41 0 17 thousand years and nearly the same depth 1800 meters in the western north atlantic ocean increased by as much as 670 years during the 30 to 160 year life spans of the samples cadmium calcium ratios in one coral imply that the nutrient content of these deep waters also increased our data show that the deep ocean changed on decadal centennial time scales during rapid changes in the surface ocean and the atmosphere

atomic scale imaging of calcite growth and dissolution in real time we present a new experimental technique for real time observation of aqueous mineral growth and dissolution at the atomic scale using an atomic force microscope afm equipped with a flow through fluid cell we applied this technique to observe changes in surface topography on the 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 4 cleavage plane of calcite during alternating episodes of growth and dissolution growth occurred in a layer by layer fashion by the forward motion of monomolecular steps 0 3 0 1 nm high lying parallel to the edges of the cleavage face under all conditions studied the velocities of positive 48 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document and batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 4 end document 41 steps were the same velocities of negative batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 4 end document 81 and batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 4 end document 41 steps were undetectably small less than 0 1 not s 1 steps were straight passing above perfect crystalline material but roughened into two dimensional dendrites above defective material dissolution nucleated shallow 90 nm deep rhombic etch cores growth into these etch cores was impeded so that steps moved around them afm images of the surface atomic structure revealed rows of atoms along 010 spaced by 0 39 0 05 nm with a periodicity along the rows of 0 43 0 05 run

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

u pb and sm nd constraints on the nature of the campinorte sequence and related palaeoproterozoic juvenile orthogneisses tocantins province central brazil

δd and δ18o variations of the magmatic system beneath deception island volcano antarctica implications for magma ascent and eruption forecasting abstract geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth such as magma evolution and degassing which can be implemented into volcanic hazards assessment deception island is one of the most active volcanoes in antarctica with more than twenty explosive eruptions documented over the past two centuries hydrogen and oxygen isotopic variations in the volatiles trapped in the deception island rocks glass and melt inclusions in phenocrysts provide essential information on the mechanisms controlling the eruptive history in this volcanic suite thus understanding the petrological and related isotopic variations in the island has the potential to foresee the possible occurrence and its main eruptive features of a future eruption the new isotopic results of this study combined with exhaustive petrologic knowledge reveal in deception island i fast ascent and quenching of most magmas preserving pre eruptive magmatic signal of water contents and isotopic ratios with local modification by rehydration due to glass exposition to seawater and by meteoric and fumarolic waters ii a plumbing system s variable with time and currently dominated by closed system degassing leading to explosive eruptions iii control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in deception island and at different magma stagnation depths these results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes and hence forecast volcanic eruptions and related hazards

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

noble gas signals in corals predict submarine volcanic eruptions abstract deep water corals growing in close proximity to the 2011 submarine eruption at tagoro el hierro island canary archipielago spain have revealed their ability to record the magmatic helium 3 he signal during a period of months prior to the eruption with magmatic 3 he 4 he ratios of 3 6 5 0 r a this is similar to the range of he isotope values 3 10 ra obtained from olivine samples from basinites erupted during the 2011 tagoro eruption whip like black corals anthozoa antipatharia antipathidae stichopathes growing on basaltic rocks trapped 3 he within their skeletons we used a theoretical growth rate to estimate the time of trapping of 3 he between release from the magmatic source at depth to trapping by the aquatic organisms our findings suggest that magmatic 3 he emission occurred a few months before the beginning of the seismic activity in the region the latter occurring four months in advance of the beginning of the eruption this discovery implies that corals living in submarine volcanic areas may act as archives of noble gases released before the beginning of an eruption and that the continuous monitoring of 3 he may help to constrain the arrival time of a subaqueous eruption with potential applications in volcano eruption forecasting

release of gas bubbles from lake sediment traced by noble gas isotopes in the sediment pore water abstract the release of gas bubbles from lacustrine or oceanic sediments into the overlying water ebullition is a major mechanism for the discharge of biogenic or geogenic gases into the water body ebullition of methane or carbon dioxide for instance contributes considerably to the release of these potent greenhouse gases through the sediment water interface depending on the rate of ebullition the pore water will show a depletion in dissolved atmospheric noble gases because the poorly soluble noble gases escape from the pore water into the gas bubbles in this study the depletion of dissolved noble gases in sediment pore water was analyzed for the first time to study bubble formation and ebullition in sediments the noble gases in the pore water of the sediments of soppensee switzerland show a distinct depletion due to ebullition of biologically produced methane this depletion is lowest in the deep sediment and increases towards the sediment surface the noble gas isotope ratios in the pore water indicate that vertical diffusion barely affects the observed noble gas profiles the isotope ratios further show that the methane bubbles remain long enough in the sediment to attain noble gas solubility equilibrium before escaping into the overlying water the volume of gas released from the sediment by ebullition can therefore be reconstructed from the extent of the noble gas depletion in the pore water using a simple gas equilibration model the noble gas profiles in the sediment indicate that ebullition increased in soppensee during the holocene and that ebullition contributed strongly to the release of methane from the sediment our case study thus illustrates that noble gases are sensitive proxies for the release of gas from lacustrine and marine sediments or similar aquatic environments

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

diffusive helium emissions as a precursory sign of volcanic unrest significant increases in helium emissions from the soil and 3 he 4 he ratios in groundwater on el hierro island canary islands spain were observed prior to the 2011 2012 submarine eruption off the coast of the island the changes of diffusive helium emissions rate were observed one month prior to the submarine eruption onset 12 october 2011 and the major increase preceded increases in seismic energy release during the volcanic unrest measured 3 he 4 he ratios in groundwaters from a well in el hierro island increased from 2 3 r a to 7 2 r a r a 3 he 4 he ratio in air 1 month prior to the eruption onset and reached a peak of 8 2 r a indicating a dominant magmatic contribution to the dissolved gases in ground waters 3 he 4 he values and diffusive helium emission studies have been extremely important for forecasting the onset of the volcanic unrest and subsequent volcanic eruption an aseismic exsolution of magmatic gases from magma bodies beneath el hierro island through fractures and vertical permeability structures increased the diffusive helium emission rate prior to episodes of seismic energy release associated with the volcanic unrest

nd isotopic study of the neoproterozoic mara rosa arc central brazil implications for the evolution of the brasilia belt abstract the goias magmatic arc is a 0 9 0 64 ga island arc underlying large areas of the western part of the brasilia belt in central brazil it is exposed in two separate sections known as the arenopolis arc in the south and the mara rosa arc in the north it is made of island arc volcanic sedimentary sequences forming narrow nne to nnw belts separated from each other by tonalitic granodioritic granitic orthogneisses all rocks are metamorphosed under upper greenschist to amphibolite facies conditions but have kept major geochemical and isotopic features indicating the juvenile nature of the igneous protoliths the meta igneous and metasedimentary rocks are intruded by a number of late to post orogenic granites e nd 850 ranging from 1 5 to 4 6 with t dm values between 1 0 and 1 1 ga metasedimentary rocks in the mara rosa supracrustal belts display geochemical features suggesting that they represent first cycle immature sediments which preserve chemical characteristics of their igneous sources their nd isotopic characteristics support this interpretation t dm model ages for detrital metasediments are mostly in the interval between 0 9 and 1 2 ga indicating that they are a product of erosion of the arc rocks the absence of older contributions to these sediments seems to indicate an intraoceanic setting this is also suggested by the nd isotopic composition of the 0 8 ga old tholeiitic amphibolites which display positive initial e nd 4 9 indicating derivation from a depleted mantle source syn orogenic deformed granites forming small intrusions within the supracrustal belts have t dm ages ranging from 0 77 to 2 54 ga the older model age values indicate the presence of hidden fragments of ancient crust which were probably tectonically interleaved with younger rocks during accretion of the arc the post orogenic granites are moderately peraluminous and the nd model ages between 1 0 and 1 3 ga indicate that their parental magmas were predominantly re melts of the arc rocks their peraluminous nature reflects the incorporation of substantial amounts of arc sediments in the original magma garnet whole rock sm nd ages for the metasediments may be interpreted to refer to two metamorphic events an older event at 0 76 0 73 ga and a younger event at 604 610 ma mineral chemistry indicate that the older episode was characterized by higher p t and the younger by lower p t the older metamorphic event is interpreted preliminarily as the result of the accretion of island arc terranes to the western edge of the orogen and the younger event as a result of final ocean closure and continental collision between the amazon and sao francisco continents the goias arc formation and accretion is partially coeval with rodinia break up suggesting that the western margin of the sao francisco congo was peripheral in that supercontinent and faced a wide oceanic basin during most of the neoproterozoic

noble gas concentrations in fluid inclusions as tracer for the origin of coarse crystalline cryogenic cave carbonates abstract the chemically inert behaviour of noble gases their well known solubility in water and the long term constancy of atmospheric noble gas mixing ratios make them a unique tool for paleoclimate studies of groundwater this concept has recently been extended to fluid inclusions in speleothems here we use noble gas concentrations of fluid inclusions contained in calcite crystals to constrain the formation history of coarse crystalline cryogenic cave calcites from heilenbecker cave germany cryogenic cave carbonates form under conditions related to the freezing of cave pools and are a new archive for paleo permafrost timing and extent the cryogenic origin of these carbonates is typically deduced from their crystal habit and the carbon and oxygen isotope ratios but this can be ambiguous in some cases a geochemical method that provides clear cut proof of the cryogenic origin of individual crystals has been lacking so far we examined the formation process of cryogenic cave calcites using water and noble gases extracted from inclusions in these crystals based on their 13c and 18o values these calcite crystals could have also formed as normal i e non cryogenic speleothems noble gas concentrations derived from fluid inclusions in these cryogenic calcites however deviate significantly from commonly observed concentrations in atmospheric air surface water groundwater and stalagmites these concentrations cannot be explained simply by a solubility component nor by the addition of various amounts of excess air to a solubility component the data however are consistent with the scenario of a slowly freezing water body lacking exchange with the cave atmosphere the gradual freezing process leads to a partitioning of the noble gases between ice and remaining water and thereby to a pronounced over abundance of heavy noble gases in the liquid phase recorded in the fluid inclusions of the cryogenic calcite crystals

origin and geochemistry of the chapada cu au deposit goias brazil a metamorphosed wall rock porphyry copper deposit chapada is a large 134 x 10 6 metric tons low grade 0 44 cu 0 35 g metric tons au disseminated chalcopyrite pyrite deposit within a geologically complex pan african mobile belt in central goias state brazil mineralization is hosted largely by subhorizontal biotite schist with lesser amphibolite muscovite schist and feldspathic quartzite a foliated diorite is separated from the deposit by a shallow diping 40 m thick mylonite zone discontinuous lenses of mica amphibole schists within the deposit are considered hydrothermally altered equivalents of this diorite which have intruded geochemically related wall rocks such as the locally porphyritic amphibolite seen in the deposit previous assignment of a mid proterozoic age to chapada must be revised in view of our rb sr whole rock isochron which yields a pan african age of 561 or 9 m y for the host schists rare earth element patterns of chapada rocks have steep negative slopes with no eu anomalies petrochemical data and an 87 sr 86 sr initial i value of 0 70414 or 0 00005 indicate chapada host schists and diorite are similar to peraluminous calc alkaline andesites of magmatic arc affiliation mineralization occurred prior to upper amphibolite grade regional metamorphism at 532 or 1 m y as indicated by an rb sr mineral isochron as well as by sulfides paralleling foliation sulfide inclusions in metamorphic minerals and the presence of gahnite and zincian staurolite estimated conditions of peak metamorphism were 650 degrees or 20 degrees c 7 5 or 1 kb log f sub o 2 13 to 14 log f sub s 2 0 5 to 0 5 and a sub h 2 o 0 85 mineral zonations in and around chapada include a central magnetite rich zone a pyritic shell and peripheral pyrrhotite and sphene these mineral zonations and x fe of biotites reflect a gross zonation in oxygen and sulfur fugacities from high within the deposit to relatively low in peripheral rocks pyrite and chalcopyrite have delta 34 s values of 0 4 to 0 6 per mil k 2 o is enriched and cao and na 2 o are depleted in the host schists relative to the unaltered diorite within the deposit several major element abundances describe a symmetrical pattern about the center of the deposit the tonnage grade and ore mineralogy of the chapada deposit are comparable to those reported from porphyry copper deposits of island arc settings much of the gangue mineralogy at chapada is also typical of porphyry deposits and has survived metamorphism whereas the more aluminous original gangue minerals are now represented by such metamorphic minerals as kyanite and staurolite these similarities combined with the textural zoning and geochemical characteristics of the host rocks suggest that chapada is a metamorphosed porphyry copper deposit which hosts mineralization in both wall rocks and intrusive rocks

the chapada cu au deposit mara rosa magmatic arc central brazil constraints on the metallogenesis of a neoproterozoic large porphyry type deposit abstract the large chapada cu au deposit is situated in goias magmatic arc central brazil which is part of an accretionary orogen with tectonic evolution between ca 900 and 600 ma during the brasiliano orogeny with a current reserve proven and probable of 420 94 mt cu 0 29 and 479 84 mt au 0 25 g t or 3 83 moz au the chapada cu au deposit is an important example of a large tonnage low grade porphyry copper gold deposit of precambrian age in which the mineralization is disseminated and hosted mainly by biotite rich schists and gneisses in this work we propose a two stage genetic model for the chapada deposit that includes an early porphyry type cu au event overprinted by a shear zone hosted cu au event the first mineralization event represents a rare case of metamorphosed neoproterozoic porphyry type deposit which preserves distinct hydrothermal alteration mineralization zoning developed at the interface between equigranular 867 8 ma to porphyritic 884 15 ma medium to high kcalc alkaline dioritic rocks and mafic to intermediate metavolcanic and metapyroclastic rocks the e nd values are positive from 2 17 to 5 39 thus indicating the juvenile nature of the original rocks for both metavolcano sedimentary and plutonic rocks in this context kyanite rich rocks are interpreted as products of metamorphosed argillic hydrothermal alteration halo and occur as linear alignments oriented parallel to the mara rosa magmatic arc trend the potassic hydrothermal alteration halo which includes the main host rocks of the chapada deposit is characterized by biotite schist muscovite biotite schist amphibole biotite schist and biotite gneiss the ore is represented predominantly by platy deformed chalcopyrite and pyrite disseminations along foliation plans or axial surfaces of folds and to a lesser extent by small massive concentrations in fold hinges the ore shows a systematic oxide sulfide zonation including the peripheral pyritic shell and the central magnetite rich zone in general the geological context of the deposit lacks preserved stocks dykes and breccias while deformed a type quartz veins veinlets are abundant the second mineralization event represents an epigenetic shear zone hosted cu au mineralization associated with d n 1 and occurred between 630 and 580 ma during the late orogenic to post orogenic stage of the mara rosa magmatic arc which may be interpreted as i a shear zone hosted cu au remobilization ii an orogenic cu au mineralization or iii an intrusion related cu au mineralization in the latter case the mineralization could be related to the ilmenite bearing younger metadiorite 634 8 6 8 ma with disseminations of chalcopyrite pyrite and pyrrhotite the narrow range of pyrite and chalcopyrite 34 s values close to 0 between 2 90 and 0 76 indicates that both the porphyry copper gold mineralization and the epigenetic mineralization were generated by sulfur of magmatic origin

a climate induced transition in the tectonic style of a terrestrial planet abstract we explore the possibility that an increase in the surface temperature of a terrestrial planet due to an enhanced concentration of atmospheric greenhouse gasses and or increased solar luminosity could initiate a transition from an active lid mode of mantle convection e g plate tectonics to an episodic or stagnant lid mode i e single plate planet a scaling theory is developed to estimate the required temperature change as a function of the temperature dependence of mantle viscosity and the yield stress of the lithosphere the theory relies on the assumptions that convective stresses scale with mantle viscosity and that a planet will adjust to surface temperature changes so as to maintain a surface heat flow that balances internal heat production the theory is tested against a suite of numerical simulations of mantle convection the comparisons are favorable the combined theory and numerics suggest that if the yield stress for the earths lithosphere is 30 35 mpa then a surface temperature change of 60 120 could shut down an active lid mode of convection assuming present day conditions lower values are predicted for higher yield stresses and for earlier times in the earth geologic evolution