implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

microbe clay interactions as a mechanism for the preservation of organic matter and trace metal biosignatures in black shales abstract organic rich fine grained sedimentary rocks such as black shales are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3 billion years while biological productivity and sedimentation rates greatly affect the organic matter content in these rocks mechanisms linking these two processes remain poorly resolved here we examine the interactions of clay minerals with the marine planktonic cyanobacterium synechococcus sp pcc 7002 we suggest that clays settling through the water column could influence carbon and trace metal burial in three ways 1 the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude 2 reactive bacterial cells become completely encased within a clay shroud enhancing the preservation potential of this organic matter and 3 the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments significantly our findings imply that the chemical composition of ancient organic rich fine grained deposits are not only archives of ancient seawater composition and redox state but they also provide a record of the degree of biological activity in the water column through geological time

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

lead isotope and trace element geochemistry of paleoproterozoic metasedimentary rocks in the lead and rochford basins black hills south dakota usa implications for genetic models mineralization ages and sources of leads in the homestake gold deposit abstract mixed silicate carbonate sulfide facies banded iron formations with elevated detrital components were deposited in the northern black hills during middle paleoproterozoic time between 2 012 and 1 974 ga homestake iron formation and equivalents and at 1 887 ga rochford iron formation these iron formations deposited atop shales and basalts in intracontinental rifted basins separated in time by 80 130 myr are the hosts of extensive late paleoproterozoic gold mineralization gold sulfide ore bodies occur within dilated segments of late stage ductile brittle shears that formed after isoclinal folding related to wyoming superior continental collision lead stepwise leaching pbsl data for monazite bearing garnet separated from a sample of homestake iron formation has yielded an isochron age of 1746 10 ma 2 mswd 0 42 which represents a maximum age for both the isoclinal folding and subsequent gold mineralization monazite bearing garnet from a nearby harney peak type granitic pegmatite believed to be coeval with latest stage semibrittle shears and quartz veins that crosscut homestake ore bodies has yielded a pbsl isochron age of 1713 10 ma 2 mswd 4 1 which probably represents a minimum age for the gold event pb isotopes of sulfides from replacement ore and shear ore within the homestake iron formation cannot distinguish between these distinct ore types however both pb isotopes and rey rare earth and yttrium abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation thus revealing an epigenetic sensu stricto component to the gold mineralization the pb isotope characteristics of the ore further reflect an admixed component with low 232th 204pb and variable th u ratios in addition to compositions directly derived from bif typical leads this added component corresponds to pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite the apparent pb isotopic match between ore and granite is further consistent with the pbsl data for shear ore type arsenopyrite which define an isochron age of 1719 45 38 ma 2 mswd 0 2 and by a striking overlap of the pb isotopic data fields defined by the homestake galenas and pegmatitic feldspars the precambrian ore leads are therefore best explained as mixtures of magmatic hydrothermal and host rock mobilized leads the lead area also contains significant gold bearing deposits of tertiary age which are hosted in both paleoproterozoic and phanerozoic rocks tertiary mineralization was temporally and spatially related to coeval plutons emplaced during laramide uplift of the black hills and represents the final mineralization event recognized in the homestake mine area like the precambrian galena leads the leads of tertiary galena hosted by middle paleoproterozoic rocks including the homestake iron formation define a trend best explained by mixtures of pb derived from magmatic fluids exsolved from tertiary intrusives and pb leached from the same country rocks that contaminated the granite derived hydrothermal fluids ascending through them during the much earlier shearing and mineralization event inferred at 1730 1715 ma

sea level changes and the continental freeboard concept general principles and application to the precambrian abstract eustatic sea level changes reflect variation in ocean water or ocean basin volume or changes to the hypsometric curve which is in itself geographically and chronologically variable freeboard the elevation of a continent above mean sea level is closely related to changes in both sea level and this curve relative sea level change occurs due to tectonism sediment supply compaction and eustatic movements hydroisostatic compensation modifies first order sea level variation by about one third short term and localised change in sea level may be ascribed to waves and tides storm winds hurricanes tsunamis and catastrophic sediment slumps wide gently sloping shelves with more uniform circulation systems inferred for the precambrian may have enhanced tide and wave induced sea level changes salinity and water temperature mutually dependent affect eustatic and relative sea levels elevated water temperatures postulated for the early precambrian would have been offset partly by enhanced evaporation and increased salinity longer term changes in sea level regional or eustatic may result from orogenesis volcanism sediment supply compaction and loading isostasy and thermal mechanisms glacioeustatic changes compounded by related salinity variations moderated by glacioisostatic effects and modified by hydroisostasy reach ca 150 m isostatic rebound in interglacial episodes will produce rapid and marked sea level drops up to ca 250 m this is a localised effect and contrasts with the global enhancement of sea level during interglacials however evidence for glaciation in the geological record is limited also for most of the precambrian sea level oscillations in non glacial episodes may reflect flexure of tectonic plates change up to 100 m or changes to earth s geoid 250 m eustasy up to 350 m will result from change to ocean basin volume due to variation in mid ocean ridge volume and spreading rates also related to supercontinent amalgamation and breakup intra plate hot spots and oceanic plateaus will change eustatic sea level more slowly up to 100 m higher heat flow in the archaean and concomitant enhanced volcanism suggest that these were important influences on precambrian sea levels the exponential elevation age relationship observed for phanerozoic oceanic lithosphere may have been more significant in the precambrian as continental crust grew from ca 4 0 ga declining heat flow and concomitant deepening of the ocean basins maintained an approximately constant freeboard during the archaean although still hotly debated there is strong support for episodic growth models inferring rapid formation of continental crust near the archaean proterozoic boundary this would imply constant freeboard conditions after 2 5 ga as well the constant freeboard model however is one based on average global conditions freeboard was still subject to geographic and chronologic variation and these potential changes lie within the bounds of eustatic and relative sea level change continental crustal growth the freeboard concept and sea level changes are thus interdependent variables in precambrian geology

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

a review of the inferred geodynamic evolution of the dharwar craton over the ca 3 5 2 5 ga period and possible implications for global tectonics the geological history and evolution of the dharwar craton from ca 3 5 2 5 ga is reviewed and briefly compared with a second craton kaapvaal to allow some speculation on the nature of global tec

lithospheric thinning during the evolution of continental large igneous provinces a case study from the north atlantic tertiary province lithospheric thickness can act as an important physical control on the composition of asthenosphere derived melts by restricting the minimum depth at which decompression melting can occur a case study of plume related tertiary lavas from the isle of mull scotland has revealed the presence of three successive magma types which become progressively more depleted in the incompatible trace elements geochemical modeling suggests that melting initially took place within the garnet iherzolite stability field but subsequently involved more extensive melting 10 of spinel iherzolite at shallower mantle depths these results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center the observed degree of lithospheric thinning cannot be explained by extension alone and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process and this has important implications for the various starting plume models

multi stage modification of paleoarchean crust beneath the anabar tectonic province siberian craton abstract according to present views the crustal terranes of the anabar province of the siberian craton were initially independent blocks separated from the convecting mantle at 3 1 daldyn terrane 2 9 magan terrane and 2 5 ga markha terrane rosen 2003 2004 rosen et al 1994 2005 2009 previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the markha terrane concluded that the evolution of the crust of the markha terrane is very similar to that of the daldyn terrane to test this conclusion we present results of u pb and hf isotope studies on zircons in crustal xenoliths from the zapolyarnaya kimberlite pipe upper muna kimberlite field located within the daldyn terrane and the botuobinskaya pipe nakyn kimberlite field in the center of the markha terrane the data on xenoliths from the botuobinskaya kimberlite pipe record tectonothermal events at 2 94 2 8 2 7 and 2 ga the event at 2 ga caused pb loss in zircons from a mafic granulite u pb dating of zircons from the zapolyarnaya pipe gives an age of 2 7 ga all zircons from the studied crustal xenoliths have archean hf model ages ranging from 3 65 to 3 11 ga this relatively narrow range suggests that reworking of the ancient crust beneath the nakyn and upper muna kimberlite fields was minor compared with the daldyn and alakit markha fields shatsky et al 2016 this study when combined with dating of detrital zircons implies that tectonic thermal events at 2 9 2 85 2 75 2 7 and 2 0 1 95 ga occurred everywhere on the anabar tectonic province and could reflect the upwelling of superplumes at 2 9 2 7 and 2 ga the presence of the same tectonic thermal events in the daldyn and markha terranes rosen et al 2006a b supports the conclusion that the identification of the markha terrane as a separate unit is not valid

basalt water interactions at high temperatures 1 dissolution kinetic experiments of basalt in water and nacl h2o at temperatures up to 400 c 23 mpa and implications abstract kinetic experiments of basaltic rock water and nacl h2o interactions at temperatures from 20 to 400 c were carried out using flow through a backed bed reactor experimental results indicate that all of the metal release rates vary with temperature and that the relative release rates for the various metals in the basaltic rock vary significantly at constant temperature the rock dissolution rate i e the release rate of si increased as the temperature increased from 20 c to 300 c and then decreased as the temperature continued to increase from 300 c to 400 c the release rate of si reached a maximum at 300 c at 350 c in nacl h2o the maximum release rate of al was reached at 350 c release rates of ca mg and fe decreased with increasing temperature from 20 to 300 c or 350 c and generally ca mg fe and al were dissolved more quickly than si at temperatures in the range from 25 to 300 c but more slowly in the range from 300 to 400 c our experiments demonstrated that si will be fixed on the rock surface at temperatures from 100 to 300 c al will be fixed at 200 300 c or 350 c but mg ca and fe will be fixed on the surface at temperatures from 300 to 400 c the experimental results can be used to illustrate the factors controlling alteration zoning in sub sea floor rocks at mid ocean ridges mor and in continental volcanic rocks in the luzong basin yangtze river valley china the results also demonstrate that alteration zoning in basaltic rocks in a geological profile with a temperature gradient is caused by the variable release rates for different metals with temperature strong variations in those release rates always occurred at 300 400 c which reflect changes in the properties of water as it passes from a sub critical to a supercritical state

paleomagnetic and rock magnetic study of the vredefort impact structure and the johannesburg dome kaapvaal craton south africa implications for the apparent polar wander path of the kaapvaal craton during the mesoproterozoic abstract new paleomagnetic and rock magnetic results are presented for various pre syn and post impact lithologies from the region of the 2023 4 ma originally ca 250 km diameter vredefort impact structure on the kaapvaal craton of south africa after removal of a viscous remanent magnetization vrm component the characteristic remanent magnetization chrm component from 16 vredefort granophyre and pseudotachylitic pt breccia samples d 18 3 i 54 8 95 8 1 was isolated and a paleomagnetic pole at 25 1 n and 43 5 e a 95 10 6 obtained since such a chrm component was isolated also from the archean basement rocks of the vredefort dome we consider its nature as a primary component of the impactites proven the pole falls onto the paleoproterozoic part of the apparent polar wander path of the kaapvaal craton rock magnetic analysis and scanning electron microscopy sem revealed that the pseudotachylitic breccia and granophyre samples contain two distinct magnetite phases one ultra small micrometer size and one altered and larger 50 m phase the ultra small magnetite is interpreted as the carrier of the chrm and the larger grains as the carriers of the vrm component additionally rock magnetic and petrophysical data reveal unusually high koenigsberger ratios q values in all pre impact lithologies in some vredefort impactite samples and in the much younger 1 1 ga anna s rust sheet ars gabbro samples as the high q values which had also been reported by previous studies of vredefort lithologies are now also seen in samples from the johannesburg dome a direct link to the vredefort impact can be ruled out this is also supported by the hysteresis data of this study as the observed magnetization is rather hard and shows multiple components of remanent magnetization we exclude lightning as a cause for all observed high q values except in case of ars gabbros it is instead suggested that the cause of the high q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid crustal original setting and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence moreover paleomagnetic analysis of the rocks around the ars gabbro intrusion in the northern part of the vredefort structure revealed the presence of either a shallow north or a shallow south direction which is tentatively related to emplacement of the umkondo large igneous province analysis of all rocks including the vredefort impactites yields occasionally distinct great circle paths towards these shallow directions a likely explanation for this overprint direction is the heating caused by now eroded ars type gabbro in the area or by regional ca 1 1 1 0 ga orogenic effects no evidence of karoo type 0 18 ga overprint is seen

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

onset of mid cretaceous volcanism by elevation of the 670 km thermal boundary layer a pulse of ocean crustal production during the mid cretaceous begins at the same time that magnetic reversal frequency drops to zero we suggest that slab penetration into the lower mantle caused a thermal boundary layer normally at 670 km to be rapidly advected upward as the slabs descended through the lower mantle the onset of near surface melting due to the rise of this boundary layer coincides with the slabs9 arrival at d which drove up the heat flux across the core mantle boundary and stabilized the magnetic reversal process the slab penetration event was coincident with the breakup of the gondwana supercontinent and triggered the subsequent formation of oceanic plateaus in the central pacific and indian ocean basins

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

why was flood volcanism on submerged continental platforms so common in the precambrian abstract many archean and proterozoic greenstone belts contain volcanic rocks that erupted subaqueously but interacted with continental crust on their way to the surface this points to emplacement in a tectonic setting that was rare to absent throughout the phanerozoic the eruption of flood basalts onto submerged continental platforms to explain the situation requires that sea levels were high during periods of flood volcanism perhaps because the ocean basins were partially filled by extensive active and high standing oceanic ridges or by oceanic plateaus the common preservation of crust contaminated submarine basalts even in the oldest supracrustal sequences may indicate that continental crust was voluminous even at an early stage in earth history

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

a re appraisal of the epoch nickel sulphide deposit filabusi greenstone belt zimbabwe a hydrothermal nickel mineral system abstract the neoarchaean epoch nickel sulphide nis deposit discovered in 1970 and subsequently mined from 1972 to 1998 has been previously interpreted to be a magmatic ore system however on the basis of field petrographic and analytical data obtained during the initial exploration and evaluation stages integrated with limited new analytical data sem and ore microscopy we propose that the epoch nis may be of hydrothermal origin key features that have prompted a re appraisal of the genesis of the epoch mineralisation include a ore lenses are irregularly distributed along foliation fabrics of the host talc carbonate rocks and as such no relationship to basal ultramafic mafic units b the thickness of the ore lenses are highly variable from tens of centimetres to several metres c ore mineralogy is characterised by dominant millerite pyrite chalcopyrite assemblages and d high pd pt ratios although our hydrothermal model is somewhat speculative we suggest that on present evidence there is little doubt that the epoch nis lacks the features of typical magmatic sulphide deposits more geochemical and isotopic data are needed to test our model

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

supercontinent cycles disrupted by strong mantle plumes a theoretical basis for the regularity of supercontinent cycles is lacking here we show that periodic supercontinent cycles are unlikely if thermal instabilities originating at the core mantle boundary are of sufficient strength we couple multiple mobile continents with vigorous mantle convection in a spherical geometry regular supercontinent cycles lasting 400 50 m y occur in idealized models with three continents and a mantle heated purely from within by radioactive decay in a model incorporating six continents and strong mantle plumes this regularity is broken and supercontinents form only sporadically our results suggest that periodic supercontinent cycles are unlikely to occur in realistic earth models

strengths and limitations of zircon lu hf and o isotopes in modelling crustal growth abstract the robust nature of the mineral zircon combined with our analytical ability to readily acquire in situ uranium lead u pb lutetium hafnium lu hf and oxygen o isotopic data has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through earth history in such studies there has been a strong focus on developing methods to determine the timing and or proportion of juvenile magmatic addition to the continental crust one widespread approach to determine the timing of crustal growth has been the construction or fitting of reworking arrays to regional hf isotopic datasets simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on going dilution and refertilisation of ancient crust consistent with hot zone models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism a new compilation of magmatic rock zircon lu hf and o isotope data is used to demonstrate that the use of mantle like o isotope data as a screening tool for meaningful hf model ages is also unlikely to be reliable with independently constrained data indicating that as few as 14 of hf model ages provide a meaningful indicator of the timing of crustal growth the limitations of hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts in an effort to address some of the limitations within existing models we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions

synchrony between early jurassic extinction oceanic anoxic event and the karoo ferrar flood basalt volcanism a well known second order mass extinction took place during the pliensbachian and toarcian stages of the early jurassic first recognized as a minor pliensbachian peak in the global extinction rate it has alternatively been interpreted as a regional response to the early toarcian oceanic anoxic event detailed studies established it as a global long term event spanning five successive ammonoid zones here we present a revised time scale based on highprecision u pb ages resolved to the zone level which suggests that elevated extinction rates were sustained for about 4 m y and peak extinction occurred at 183 ma recent isotopic dating of flood basalts from the southern gondwanan karoo and ferrar provinces documents a culmination in volcanic activity ca 183 ma the onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater 87 sr 86 sr curve the synchrony of voluminous flood basalt eruptions and biotic crises as already noted for three of the major mass extinctions permits a causal relationship which in this case may be mediated by widespread oceanic anoxia

a simulation of mid cretaceous climate a series of general circulation model experiments utilizing genesis have been completed for the mid cretaceous based on geography variable atmospheric carbon dioxide concentrations 2 to 6 times present day concentrations and variable poleward oceanic heat flux 6 to 1 2 1015 w increased from present day by combining all three major variables co2 geography and oceanic heat flux the distribution of mid cretaceous temperatures can be achieved in the simulations increased co2 is required to promote global warmth and increased oceanic heat flux is required to prevent the tropics from overheating with higher levels of co2 four times present day co2 with 1 2 1015 w provided the best match to the distribution of mid cretaceous data the best match to the cretaceous observations was achieved with a globally averaged surface temperature increase of 6 2 c at the lower end of past estimates of mid cretaceous warmth this value may be a better estimate of mid cretaceous global warming finally the model experiments can be used to provide a paleocalibration of the global warming expected for a doubling of atmospheric carbon dioxide the best estimates for the mid cretaceous appear to be a 2 5 to 4 0 c sensitivity in the mid to upper range of the sensitivity of current climate models used to assess future global change

petrogenesis of greenstones from the mino tamba belt sw japan evidence for an accreted permian oceanic plateau abstract permian greenstones in the jurassic mino tamba accretionary complex southwest japan are divided into three distinct series on the basis of their geological occurrence mineralogy and geochemistry a low ti series lts is associated with lower permian chert and limestone and is the most voluminous of the three series the lts shows slightly more enriched geochemical and isotopic characteristics than morb a transition series ts is mainly associated with lower permian chert and has more enriched geochemical signatures than morb its isotopic characteristics are divided into enriched and depleted types a high ti series hts occurs as sills and hyaloclastites within middle permian chert and as dikes intruding the ts some hts rocks have high mgo contents the hts is characterized by enrichment in incompatible trace elements and an isotopic composition comparable to himu type basalt the geochemistry of the voluminous lts is similar to that of the oceanic basalt series of the kerguelen plateau suggesting production by partial melting of a shallow mantle plume head below thick oceanic lithosphere in early permian time we infer that the ts formed simultaneously at the margins of the mantle plume head in contrast the hts may have resulted from partial melting of a deep mantle plume tail in middle permian time permian greenstones in the mino tamba belt may have thus formed by superplume activity in an intra oceanic setting given the presence of two known contemporary continental flood basalt provinces siberia and emeishan and some accreted oceanic plateau basalts the vast magmatism of the mino tamba oceanic plateau suggests a large scale superplume pulse in permian time accretion of oceanic plateaux may have played an important role in the growth of continental margins and island arcs in japan and elsewhere in the circum pacific region

the mid cretaceous super plume carbon dioxide and global warming carbon dioxide releases associated with a mid cretaceous super plume and the emplacement of the ontong java plateau have been suggested as a principal cause of the mid cretaceous global warming the authors developed a carbonate silicate cycle model to quantify the possible climatic effects of these co sub 2 releases utilizing four different formulations for the rate of silicate rock weathering as a function of atmospheric co sub 2 they find that co sub 2 emissions resulting from super plume tectonics could have produced atmospheric co sub 2 levels from 3 7 to 14 7 times the modern pre industrial value of 285 ppm based on the temperature sensitivity to co sub 2 increases used in the weathering rate formulations this would cause a global warming of from 2 8 to 7 7c over today s global mean temperature altered continental positions and higher sea level may have been contributed about 4 8c to mid cretaceous warming thus the combined effects of paleogeographic changes and super plume related co sub 2 emissions could be in the range of 7 6 to 12 5c within the 6 to 14c range previously estimated for mid cretaceous warming co sub 2 releases from oceanic plateaus alone are unlikely to have been directly responsible for more than 20 of the mid cretaceous increase in atmospheric co sub 2

magmatic nickel sulfide mineralization in zimbabwe review of deposits and development of exploration criteria for prospectivity analysis significant magmatic nickel sulfide deposits in zimbabwe occur in archean greenstone belts and archean to proterozoic mafic ultramafic intrusions such as madziwa and the great dyke here we review exploration criteria for magmatic nickel sulfide deposits taking into account nickel sulfide mineral systems in general zimbabwe s cratonic evolution and the geological setting of the deposits we propose that the essential ingredients for nickel sulfide mineral systems in zimbabwe are i proximity to greenstone belts craton margins and orogenic belts ii large mafic ultramafic intrusions and their feeders iii crustal contamination of basic ultrabasic magma with sulfidic crustal lithologies and iv the vicinity to major fault zones that allowed fertile magma to ascend into the upper crust we suggest that there is potential for new nickel sulfide discoveries both within and outside the greenstone belts including the karoo flood basalt province the mashonaland dolerites and the limpopo magondi and zambezi orogenic belts

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

u pb lu hf and ree in zircon from 3 2 to 2 6 ga archean gneisses of the repulse bay block melville peninsula nunavut abstract combined in situ u pb and lu hf isotope and ree analyses of zircons from the tonalite trondhjemite granodiorite ttg type gneisses and granitoids within the repulse bay block rbb of melville peninsula nunavut reveal that the block is composed of ca 2 86 ga and 2 73 2 71 ga granitoids that formed by reworking of a 3 25 3 10 ga basaltic crust the interpretation is supported by initial hf values for ca 2 86 ga and ca 2 73 2 71 ga gneisses of 2 7 to 0 7 and 4 8 to 2 3 respectively and the preservation of high uranium xenocrystic cores at 3 15 0 05 ga neoarchean plutonism also occurred at ca 2 69 2 66 ga and is characterized by two pyroxene bearing charnockite enderbite and hornblende biotite granitoid intrusions that yield initial hf values that range from 7 1 to 1 0 demonstrating that they were derived from juvenile magmas with significant assimilation of the existing crust the pyroxene and non pyroxene bearing granitoids also contain elevated lree in zircon a feature that is interpreted to reflect metasomatism of the source this study also demonstrates that in the rbb the results of lu hf isotopes in zircon combined with in situ u pb geochronology from inherited zircon cores differ from the results of whole rock sm nd isotopes

water contrast between precambrian and phanerozoic continental lower crust in eastern china 1 the presence of water even in small amounts in the continental lower crust may play a critical role in its physical and chemical properties and behavior however the environment and evolution of water in the deep crust remain poorly constrained investigation of water dissolved as h related point defects in minerals of lower crustal granulites may provide clues to clarify this issue the analyzed and compiled water data of nominally anhydrous clinopyroxene cpx orthopyroxene opx and plagioclase plag in lower crustal granulites from hannuoba nushan and daoxian in eastern china reveal significant contrast in water contents ppm h2o by weight between precambrian and phanerozoic samples e g 200 2330 versus 275 720 ppm for cpx 140 1875 versus 60 185 ppm for opx 145 900 versus 65 345 ppm for plag and 155 1120 versus 165 360 ppm for the bulk concentrations our data show consistently higher water contents in the precambrian granulites implying a more hydrous lower crust in the precambrian than in the phanerozoic such a difference may reflect variable water contents in the original melts indicating higher water contents in the precambrian upper mantle or a plume source for that part of the precambrian lower crust

the pangea conundrum geodynamic models for supercontinent assembly whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows fail to account for the amalgamation of pangea such models would predict that the oceans created by continental breakup in the early paleozoic e g iapetus rheic would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo pacific reassembling an extroverted supercontinent as this ocean closed instead pangea assembled as a result of the closure of the younger iapetus and rheic oceans geodynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the ordovician emergence of a super plume that produced a geoid high in the paleo pacific if so the top down geodynamics used to account for the breakup and dispersal of a supercontinent at ca 600 540 ma may have been overpowered by bottom up geodynamics during the amalgamation of pangea

organic carbon exhumation and global warming during the early himalayan collision organic carbon eroded and oxidized from marine sediments on the passive and active neo tethyan margins during the early indian asian collision may have been sufficient to shift the carbon isotopic ratios of late paleocene early eocene marine carbonates toward lighter values and may have contributed to coeval global warming via the greenhouse effect new limits on the timing of collision and the organic carbon content of himalayan sedimentary sources and sinks allow us to estimate the net paleogene flux our calculations suggest that continental collisions play a fundamental role in the global carbon cycle and climate through the exhumation as well as burial of organic carbon

terrestrial record of methane hydrate dissociation in the early cretaceous reconstruction of changing c isotopic composition of early cretaceous atmospheric co2 from fossilized c3 vascular land plant tissue revealed a brief and striking negative excursion d 25 in atmospheric d 13 c followed by a rapid positive compensation d 15 during the aptian ca 117 ma mass balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion this would also result in an increased co2 o2 mixing ratio as o2 is consumed during ch4 oxidation to co2 spurring the exponential phase of angiosperm biogeographic expansion

neoarchean lithospheric strengthening and the coupling of earth s geochemical reservoirs this paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence we show that until the neoarchean continents were unable to sustain topography 2500 m during the neoarchean the continental lithosphere evolved through a rheological threshold allowing for the development of signifi cant topography the consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the earth s geochemical reservoirs the neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values we propose that during the neoarchean the exogenic envelopes which were until then coupled to the mantle through hydrothermal processes and volcanism also became coupled to the continental crust through reliefgenerating tectonics processes and erosion hence changing the balance between mantle versus crustal interaction with the exogenic earth

mantle plume the invisible serial killer application to the permian triassic boundary mass extinction the earth experienced a severe mass extinction at the permian triassic boundary ptb about 252 million years ago this biological catastrophe was accompanied by major changes in geochemical composition of the atmosphere and ocean and the appearance of sedimentary features which had not occurred since the precambrian time the eruption of the largest continental flood basalt the siberian traps overlapped this mass killing many hypotheses have been proposed but no definitive conclusion currently exits here we present characteristics of three sections from iran and china and propose that an active mantle plume initiated a series of processes which led to the mass mortality and produced major sedimentological mineralogical and geochemical changes observed in the transition from the paleozoic to the mesozoic the injection of mantle plume related igneous dike swarms into the continental margin facilitated the release of massive amounts of ch 4 primarily from the dissociation of marine gas hydrates and secondarily from the maturation of organic rich sediments and fracturing of hydrocarbon reservoirs the bulk of the ch 4 was aerobically oxidized in the water column producing dissolved co 2 with low 13 c values this co 2 saturated seawater became acidic to the point of dissolution of shelf carbonates promoting precipitation of siliciclastic rich strata in the transition from the permian to the triassic methane derived co 2 also lowered carbon isotopic composition of seawater leading to the observed decline in 13 c composition of organic and inorganic marine carbon at the ptb gas charged oceans released large volumes of co 2 and ch 4 into the atmosphere which created a severe global warming the end permian inferno causing the release of additional ch 4 from the dissociation of polar gas hydrates these events lowered 13 c compositions of terrestrial carbon simultaneously feeder dikes from the mantle plume formed the siberian traps flood basalt marine mass extinction was the result of a change in seawater composition due to the injection and oxidation of ch 4 in the water column causing low ph high concentrations of co 2 ca 2 and hco 3 and low co 3 2 values combined with a hot seawater temperature these changes made calcification of marine organisms difficult and produced major physiological crisis including reduced metabolic rates high sensitivity to environmental stress and hampered growth and reproduction terrestrial mass extinction can be attributed to severe global warming and soil acidification produced by increased atmospheric co 2 acid rain that was generated by so 2 derived from the siberian trap eruption and loss of habitat cessation of the plume activity during early triassic stopped the release of ch 4 into the ocean and terminated continental flood basalt eruption ending the environment of death on land and in sea the cut off of co 2 production in the ocean instantly increased carbonate saturation of seawater resulting in extensive seafloor cementation it also resulted in the deposition of marine carbonates by microbial activities in the hostile post extinction environment from the trigger to recovery the perturbation which included the end permian mass mortality could have lasted for at least 2 myr several major mass extinctions of the phanerozoic are temporally accompanied by flood basalt eruptions so far these two events have been interpreted in a cause and effect relation flood basalt eruption causes mass extinction this study proposes that flood basalts and their time correlative biological crises are themselves the consequence of a complex perturbation caused by mantle plume activities if so major disturbances in the near surface of the earth are ultimately controlled by changes in the mantle the physical sedimentological observations presented here combined with previous paleontological evidence cast doubt on other interpretations which use geochemical variables numerical modeling biomarkers and ce anomaly to suggest that late permian ocean was anoxic all the way to the photic zone

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

palaeoproterozoic sedimentation on the singhbhum craton global context and comparison with kaapvaal the singhbhum craton has a limited palaeoproterozoic supracrustal record which suggests a three part history comprising a long period of high freeboard and palaeosol formation on granitoids subsequent rift related mafic ultramafic volcanism and subordinate sedimentation c 2 25 2 1 ga dhanjori and jagannathpur basin fills possibly also simlipal malangtoli and ongarbira basin fills which overlapped locally with mafic soil formation and a major regression at around2 0 ga followinga longhiatus theapproximately 1 6 gadhalbhum dalmasuccession was laid down probably under continental conditions this rather truncated record stands in con trast to the chronologically and geographically much more widespread supracrustal basin fills of the kaapvaal craton and there appears to be an overall poor comparison between these two early precambrian crustal blocks however on kaapvaal three analogous events are identified widespread approximately 2 2 ga mafic volcanism followed by a well developed palaeosol and a major transgression prior to 2 05 ga the three shared events between the two cratons are com patible with the postulate of a global approximate 2 45 2 2 ga shutdown of magmatic and tec tonic geodynamics with the origin of the triumvirate directly reflecting its resumption again after about 2 2 ga we recognize here that a large diversity of views on singhbhum s geodynamic history exists predicated on a lack of precise geochronology and commonly poor outcrops and the current hypotheses are presented with these factors in mind

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

fast plumeheads temperature dependent versus non newtonian rheology in a series of two dimensional cartesian simulations with high resolution on the order of 2 km we have compared for the same effective rayleigh number around 106 the dynamics between newtonian and non newtonian rheologies a viscosity contrast due to temperature of 1200 has been employed throughout with surface dissipation numbers varying between 0 05 and 0 2 peak velocities for non newtonian plumes are at least 50 times faster than the peak newtonian velocities and reach magnitudes in excess of m yr upon impinging the surface plume head structures of the non newtonian rheology contain much greater complexity in the thermal mechanical structure than for newtonian plumes

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

the effect of volcanic aerosols on global climate abstract the surface temperature decrease after major volcanic eruptions has been demonstrated by a number of groups to be the order of a few tenths of a degree at first the stratospheric aerosol is usually restricted to a narrow range of latitudes and the volcanically induced temperature decrease is found to be nonuniform over the surface of the earth in fact because of the differential cooling of land and ocean there should always be volcanically induced temperature gradients these temperature gradients will induce a redistribution of air mass and anomalous sea level pressure gradients anomalous winds and anomalous climate on a global scale in this paper a hypothesis is presented which assumes that volcanic aerosols by reducing the solar radiation induces surface temperature gradients and sea level pressure gradients between the land and the oceans since cooling of the land occurs before the oceans there will be at first an increase of sea level pressure over land relative to the oceans conservation of total air mass on a global scale requires the sea level pressure over the ocean to be reduced because the majority of the land area is in the northern hemisphere the volcanically induced cooling will transfer air mass from the oceanic anticyclones in both the northern and southern hemisphere to the continents and especially to southern eurasia the anomalous sea level pressure pattern slp which would be obtained higher slp over southern eurasia and lower slp over the southern oceans is very close to the pattern first named by sir gilbert walker called the negative phase of the southern oscillation the positive phase of the southern oscillation can occur when no volcanic aerosols are present and the stratosphere is clear then the additional radiation and heating of the land increases the transfer of air mass from the northern hemisphere land mass to the world s oceans the positive phase of the global southern oscillation in this paper it will be shown that this hypothesis has the potential to explain the observed association between low latitude volcanic aerosols and the el nino southern oscillation it also has the potential to explain why el chichon s eruption induced a very strong el nino compared to the weaker el nino events induced by the eruptions of agung and krakatau it also explains why almost all monsoon countries experience below normal precipitation just prior to the maximum of the el nino warming the hypothesis also has the potential to explain why low and high latitude aerosols will affect the global climate system differently this hypothesis represents a return to the ideas of sir gilbert walker who first defined the southern oscillation as a global scale anomaly rather than an index between just two stations

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s