mobility of rhenium platinum group elements and organic carbon during black shale weathering this study investigates the effects of black shale weathering on the re os isotope system platinum group element concentrations and the degradation of organic matter samples from a weathering profile in late devonian 365 myr ohio shale show a pronounced decrease 77 in organic carbon corg near the present soil surface relative to the interior portion of the outcrop a similar trend is observed for total n 67 loss conversely organic phosphorus porg concentrations increase by 59 near the soil surface the decrease in corg is accompanied by a pronounced decrease in re 99 and to a lesser extent os 39 palladium and pt do not appear to be significantly mobile the effects of re and os mobility on the re os isotope system are significant none of the samples plots on a 365 myr isochron rather the samples define a trend with a slope corresponding to an age of 18 myr with an initial 187os 188os of 6 1 this indicates recent disturbance of the re os system isotope mass balance calculations imply that the labile fraction of os is significantly more radiogenic 187os 188os of 7 8 than the average of the unweathered samples 187os 188os of 6 4 based on data from this study the molar ratio of labile re to corg in ohio shale is estimated at 7 10 8 we estimate the present day riverine black shale derived re flux to seawater using literature data on re burial in anoxic marine sediments and assuming steady state between re release during black shale weathering and re burial in anoxic marine sediments then the labile re corg observed in this study implies that 0 5 tmol of corg is released annually from weathering of black shales a trace lithology in the continental crust this flux corresponds to 12 of the estimated annual co2 flux from oxidative weathering of sedimentary rocks the labile molar re os of 270 indicates that black shale weathering releases 130 mol os per year which accounts for 7 of the riverine os input to seawater the data from this study support the notion that the crustal cycles of labile corg re and os are tightly coupled gray shales which are less corg re and os rich but much more abundant in the continental crust than black shales are likely to be even more important continental sources of re and os to seawater

contradictory correlations of paleoproterozoic glacial deposits local regional or global controls abstract there is little agreement among recently proposed correlation schemes for paleoproterozoic glaciogenic rocks scattered thinly around the globe correlations are hindered by the dearth of tight geochronological control on the ages of glacial deposits which are notoriously difficult to date most attempts at global correlation are based on comparison to the huronian supergroup in canada which contains three glaciogenic formations although the two lower glacial units were deposited in a rift setting and have a restricted distribution even in north america they have been used in attempts to effect international correlations the gowganda formation together with correlatives comprises the thickest and by far the most widespread paleoproterozoic glacial deposits in north america glacial deposits are also reported from south africa western australia and elsewhere where they appear to fall within the same time period as the huronian supergroup 2 45 2 2 ga but attempts to correlate individual diamictite bearing units to the three huronian glaciogenic formations have proved difficult none of the huronian glacial formations has been precisely dated until such geochronological data are available it is practical and prudent to recognize that during the early paleoproterozoic as in the cryogenian the earth was susceptible to glaciations that could be triggered by a variety of local events including uplift related to rifting and compressional orogeny within the 250 million years of the huronian glacial event 2 45 2 2 ga it is commonly assumed that ice sheet fluctuations were globally synchronous but this has not been demonstrated

global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

shallow water anoxia in the mesoproterozoic ocean evidence from the bashkir meganticlinorium southern urals abstract the apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the proterozoic eon 2 5 0 542 ga it is generally accepted that from 1 85 ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous fe ii rich with episodic euxinia confined to productive continental margins and intracontinental basins the geochemical record that informs this picture however is currently sparse and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time here we report fe s c systematics alongside major al mn and trace metal mo u data for 1 6 0 8 ga marine sediments from the bashkir meganticlinorium bma southern urals russia our fe speciation data reveal a water column dominated by ferruginous conditions no evidence for euxinia and oxygenated waters confined to only the shallowest near coastal settings trace metal data support these findings with mo and u enrichment factors in anoxic sediments implying the operation of a particulate fe oxyhydr oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic mo enrichments sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the bma succession which in combination with a low organic carbon loading promoted the development of ferruginous rather than euxinic water column conditions the restriction of oxic conditions to the shallowest near coastal depositional settings within the bashkir basin contrasts to other redox studies from this period such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the mesoproterozoic era and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period

pliocene cu au bearing igneous intrusions of the gunung bijih ertsberg district irian jaya indonesia k ar geochronology nine distinct potassic intermediate igneous bodies have been identified during detailed surface and underground mapping of 100 km 2 in the gunung bijih erstberg cu au district in the highlands of irian jaya indonesia west new guinea fourteen biotite k ar ages range from 2 6 to 3 8 ma while another from a small isolated pluton is distinctly older at 4 4 ma these ages represent times of intrusion and crystallization which followed regional kilometer scale folding that resulted from collision at a subduction zone of the northern edge of the australian continental margin with the melanesian oceanic island arc complex the grasberg and ertsberg intrusions are cut by numerous faults with only minor hundreds of meters offset throughout the island of new guinea dated neogene igneous rocks form two clusters separated in age and location a distinct belt of miocene 20 10 ma plutons maramuni arc stretches the entire length of the northern highlands of papua new guinea and recurs in western iri

biomarker evidence for photosynthesis during neoproterozoic glaciation laterally extensive black shales were deposited on the sao francisco craton in southeastern brazil during low latitude neoproterozoic glaciation 740 to 700 million years ago these rocks contain up to 3 0 weight organic carbon which we interpret as representing the preserved record of abundant marine primary productivity from glacial times extractable biomarkers reflect a complex and productive microbial ecosystem including both phototrophic bacteria and eukaryotes living in a stratified ocean with thin or absent sea ice oxic surface waters and euxinic conditions within the photic zone such an environment provides important constraints for parts of the snowball earth hypothesis

allochthonous oceanic basalts within the mesozoic accretionary complex of alexander island antarctica remnants of proto pacific oceanic crust the mesozoic lemay group accretionary complex of alexander island antarctica contains thrust bound slices of accreted ocean floor ocean islands and seamounts they represent fragments of proto pacific oceanic crust of which only a tiny remnant the phoenix plate remains off northern antarctic peninsula they therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern pacific ocean all the basalts experienced sea floor and subduction accretion metamorphism ranging from zeolite to transitional blueschist facies on the basis of rare arth and other immobile trace element characteristics the basalts are divided into depleted morb n morb e morb and tholeiitic and alkaline oib oceanic basalts occur within two rock associations on alexander island basalt volcaniclastite chert and basalt volcaniclastite tuff the basalt volcaniclastite chert rock association is dominated by pillow lavas which have light ree depleted n morb geochemical characteristics and is interpreted as representing ocean floor formed at spreading centres locally sills of tholeiitic oib intrude the sequence the basalt volcaniclastite tuff rock association exposed in the lully foothills was formed in shallow water during the early jurassic it is geochemically varied consisting of basalts with n morb e morb and tholeiitic oib characteristics the association is interpreted to have been formed on a large seamount or ocean island

evidence for paleoproterozoic cap carbonates in north america the early paleoproterozoic snowy pass supergroup of the medicine bow mountains and sierra madre wyoming usa and huronian supergroup ontario canada were deposited along the present day southern flank of the wyoming and superior cratons whereas three discrete levels of glacial diamictite are developed in both successions carbonate strata are known only directly above the middle diamictite vagner and bottle creek formations in medicine bow mountains and sierra madre respectively and espanola formation in southern ontario in these thick correlative siliciclastics dominated strata the carbonates from each succession record negative 13 c values 4 0 to 0 8 v pdb and attenuated carbon isotopic difference between organic and inorganic phases oxygen in carbonates is strongly depleted in 18 o suggesting exchange with hot fluids which is consistent with pervasive recrystallization of carbonates and remobilization of elements however the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between 13 c and either 18 o values or trace element concentrations supports a primary origin for 13 c depleted carbonates which are interpreted here to reflect anomalous oceanic compositions the intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post glacial transgression although these early paleoproterozoic carbonates are similar to neoproterozoic cap dolomites in stratigraphic position and carbon isotopic compositions the older post glacial accumulations begin with limestone and lack many of the sedimentary structures typical of neoproterozoic deposits furthermore it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above neoproterozoic diamictites the differences might reflect lower overall carbonate saturation in early paleoproterozoic oceans which contrasts sharply with archean and later paleoproterozoic intervals and higher siliciclastic inputs in rift environments which shut down carbonate deposition geological and geochemical indicators suggest a stepwise increase in atmospheric oxygen across the paleoproterozoic glacial epoch the tempo and mode of atmospheric oxygen rise has significant consequences for the abundance of the important greenhouse gases ch4 and co2 and hence for oceanic acidity if we accept that atmospheric oxidation of methane to carbon

continental basalts of the boise river group near smith prairie idaho the boise river group of late cenozoic age consists of basaltic lavas and intercalated fluvial and lacustrine sediments deposited in drainages of the boise river and boise river south fork from 1 8 to 0 2 m y basalts of the boise river group can be divided into two groups based on major and trace element chemistry boise river group 1 basalts brg 1 which comprise the three oldest flows in the smith prairie area are silica saturated olivine tholeiites characterized by low alkalis mg fe and ni high concentrations of high field strength elements hfse and enrichment in the heavy isotopes of sr nd and pb they are chemically similar to basalts of the snake river plain and both units probably derive from a similar enriched mantle lithosphere source some brg 1 flows long gulch rock creek record the affects of crustal assimilation and fractional crystallization boise river group 2 basalts brg 2 are all younger than brg 1 0 7 m y and are transitional between olivine tholeiites and alkali olivine basalts brg 2 basalts are characterized by high alkalis mg fe and ni lower hfse concentrations and isotopic compositions of sr and nd near bulk earth these lavas have distinct geochemical characteristics and are not cogenetic with the older brg 1 flows or with coeval lavas of the snake river plain chemical variations within individual flows are consistent with low pressure crystal fractionation of the observed phenocryst phases olivine plagioclase chemical variations between different flows within the same group cannot result from low pressure fractionation but can be modeled by combined high pressure pyroxene fractionation and low pressure olivine plagioclase fractionation the high mg numbers and ni of the younger brg 2 basalts are not consistent with enrichment of the light field strength elements by crustal assimilation this enrichment must reflect a mantle source region characteristic the trace element and isotopic systematics of this source region are similar but not identical to the asthenospheric source inferred for ocean island basalts in contrast chemical and isotopic systematics of the brg 1 basalts imply derivation from ancient subcontinental lithosphere which has been isolated from the asthenosphere for at least 1 5 eons the subcontinental lithosphere was affected by an early enrichment in rb sr which supported growth of high 87sr 86sr a second enrichment event resulting in fe ti metasomatism of the lithosphere occurred later in response to intrusion of partial melts from a mantle plume rooted in the underlying asthenosphere this is second enrichment event is required by the high nb rb ratios of the brg 1 lavas and of similar lavas of the snake river plain the transition from saturated olivine tholeiites of boise river group 1 to transitional alkalic lavas of brg 2 in the smith prairie region implies a time dependent change in mantle source region with early magmas derived from a shallow lithospheric source giving way to younger magmas derived from a deeper asthenospheric source the eruption of these younger lavas along the flanks of the snake river plain coeval with eruption of saturated tholeiites of the snake river group to the south further implies an axial zonation to magmatism in the snake river province the nature of this axial zonation suggests that the deeper asthenospheric melts were preferentially tapped along the margins of the snake river plain but were blocked from reaching the surface in the axial zone these time space relationships are consistent with a model in which thinned lithosphere near the rift axis undergoes extensive partial melting which overwhelms any asthenospheric melts that may attempt passage the lithosphere is too cool to melt along the rift margins where pressure release by extensional thinning is minor subsequent small volumes of asthenospheric melt are able to traverse this region more easily and erupt with little or no lithospheric contamination

geochemical characterization evolution and source of a mesozoic accretionary wedge the torlesse terrane new zealand whole rock major and trace element compositions of quartzofeldspathic permian cretaceous sandstones of the torlesse terrane new zealand display progressive changes which compare well with published petrographic data chemical indices in three permian early late triassic rakaia subterrane petrofacies show small contrasts as do the modes reflecting original source compositions a late triassic rakaia petrofacies displays sharp increases in sio 2 al 2 o 3 k 2 o na 2 o th sc la sc ce sc la n y n and decreases in ti zr and v la consistent with partial cannibalistic recycling and a small influx of lithic volcanic detritus in the post collisional late jurassic early cretaceous pahau subterrane sio 2 al 2 o 3 remains high but other indices decrease to levels seen in the permian due to recycling of the earlier rakaia petrofacies and influx of 10 30 mafic intermediate volcaniclastic detritus derived from inboard volcanogenic terranes the results are confirmed by additional suites including mudstones which cover a larger area but also show that some spatial variation occurs within the terrane as a whole whole rock data and chemical index of alteration indices indicate that the torlesse was derived from a relatively unweathered source with granodioritic bulk composition comparisons with data from a proposed rakaia source in the new england and hodgkinson orogens of eastern australia suggest that the observed chemical and modal compositions cannot be simply derived from that area as presently exposed

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

carbon isotope records in a mesoproterozoic epicratonic sea carbon cycling in a low oxygen world abstract carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in earth s biogeochemical cycles chemostratigraphic interpretations however often rely on the assumption that the isotopic composition of dissolved inorganic carbon dic in the surface oceans is broadly homogeneous at times of globally high sea level when epicratonic epeiric environments dominate our sedimentary record carbon isotope records reveal substantial lateral variability here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the mesoproterozoic 1 1 ga atar el mreiti groups of the taoudeni basin west africa restricted marine epicratonic environments are consistently 2 4 lighter than open marine pericratonic environments suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments in contrast to epicratonic environments in the paleozoic where input of isotopically light carbon is generally attributed to input of terrestrial organic matter we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability within epicratonic el mreiti group strata the extent of organic matter remineralization and thus degree of 13 c depletion is correlated with water depth and associated with distinct differences in both total organic carbon toc and pyrite concentration suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments in such settings the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition or the intermittent oxidation of an enhanced dissolved organic carbon doc pool

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

primitive os and 2316 ma age for marine shale implications for paleoproterozoic glacial events and the rise of atmospheric oxygen abstract re os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the rooihoogte and timeball hill formations transvaal supergroup south africa provides a precise isochron age of 2316 7 ma 4 statistical uncertainty if error on decay constant is excluded and a chondritic initial 187os 188os ratio of 0 1121 0 0012 these units were deposited between what are interpreted as the second and third of three paleoproterozoic global glacial events and thus provide minimum and maximum ages respectively for these events the rooihoogte formation is correlative with the duitschland formation which records previously undated carbon isotope excursions because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative 34s values the rise of atmospheric oxygen most likely began prior to 2 32 ga the chondritic initial 187os 188os ratio requires that primitive hydrothermal sources of os dominated the os budget of seawater at 2 32 ga os is mobile only under oxidizing conditions therefore the chondritic initial 187os 188os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic os to offset the primitive hydrothermal magmatic flux even if atmospheric oxygen levels had increased significantly by 2 32 ga anoxic conditions throughout the archean most likely limited os enrichment in archean marine shales so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic os to paleoproterozoic seawater

the generation of sodic granite magmas western palmer land antarctic peninsula subduction related mesozoic to cainozoic granites s l in western palmer land antarctic peninsula have similar chemical compositions to archean tonalite trondhjemite granodiorite ttg suites phanerozoic slab melts adakites and to experimental partial melts of basaltic material in equilibrium with amphibole pyroxene garnet they are predominantly sodic metaluminous and most have al2o3 15 wt and y 36 km between triassic and tertiary times the initial nd t of the magmatism increased and sr t decreased suggesting that new continental crust was produced during this period underplating by mafic magma was an important crustal growth mechanism in the arc the generation of abnormally thick crust and its subse quent fusion is considered to be a consequence of ca 180 ma of subduction and associated magmatism in the region an implication of the model is that dense garnet amphibolite and eclogite residues from partial melting of the lower crust will accumulate in theory the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle such a process would leave the rest of the crust enriched in large ion lithophile elements lree but depleted in hree y

carbonate chemostratigraphy of the vazante group brazil a probable tonian age abstract the thick carbonate siliciclastic succession of the vazante group was deposited on the western border of the sao francisco craton central brazil which was deformed during the brasiliano orogeny that ended at 540 ma in this research we studied five drill cores and outcrops to obtain integrated isotope data 13ccarb 18ocarb and 87sr 86sr as well as u pb ages of detrital zircons to understand the chronostratigraphic relationships between the tonian and cryogenian periods the vazante group forms a succession of more than 3 km thick of limestones dolostones shales and rare sandstones these rocks were deposited on a marine shelf that shows distinct 13c variations throughout the stratigraphic succession the lower limestone of this group lagamar formation has low positive 13c values 0 3 1 0 and 87sr 86sr ratios of 0 70646 0 70692 with 423 1 719 ppm sr and possible vase shaped microfossils vsms that may be equivalent to those found in huaibei group in northern china lower tonian or has stratigraphic correlations to the vsms bearing units of the upper tonian the serra do poco verde formation has a plateau of moderate 13c positive values 0 0 4 followed by dolostones of the morro do calcario formation which is a 50 m thick interval with negative values as low as 4 the upper unit of the vazante group serra da lapa formation begins with a sharp negative 13c excursion locally overlying a few layers of diamictite which may be the equivalent of the sturtian glacial age restrictions on establishing the depositional age of the vazante group continue however the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a tonian age for the vazante group

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

margarite corundum phyllites from the appalachian orogen of south carolina mineralogy and metamorphic history phyllites from the charlotte belt central piedmont of south carolina contain porphyroblasts of black corundum in a matrix of margarite and minor muscovite the margarite corundum phyllites formed during ordovician amphibolite facies metamorphism of an aluminous protolith with a probable mineralogy of pyrophyllite diaspore and calcite through reactions such as 6dia prl 2cal 2mrg 2co 2 2h 2 o and 2dia crn h 2 o the protolith was probably a hydrothermally altered felsic tuff of late proterozoic to cambrian age based on a whole rock chemical analysis of margarite corundum phyllite the protolith is believed to have consisted of about 7 mol calcite 23 mol pyrophyllite and 70 mol diaspore a t x co 2 plot of pertinent reactions in the system cao al 2 o 3 sio 2 h 2 o co 2 shows that in the presence of an h 2 o rich fluid x co 2 0 08 margarite began forming by the reaction 6dia prl 2cal 2mrg 2co 2 2h 2 o at about 350 c at 5 kbar this reaction continued along a univariant path until pyrophyllite and calcite were consumed the remaining diaspore was converted to corundum by the reaction 2dia crn h 2 o at about 430 c for this h 2 o rich fluid at 5 kbar the upper temperature limit of this assemblage is constrained by the reaction mrg crn an h 2 o which should occur at about 575 c under this pressure the absence of plagioclase in these rocks suggests that the conditions of this reaction were never achieved fluids associated with subsequent alleghanian greenschist facies metamorphism reacted with co rundum to form fine grained margarite at corundum margins

mesozoic radiolarian faunas from the antarctic peninsula age tectonic and palaeoceanographic significance new assemblages of radiolaria including some of the few occurrences of high southern latitude jurassic and cretaceous radiolarian faunas show that several localities in the lemay group of alexander island range in age from latest jurassic earliest cretaceous to at least albian by demonstrating that sedimentation and deformation in the lemay group was diachronous younging oceanwards to the northwest these new age assessments support the model of the lemay group as an accretionary complex the polarity of subduction beneath alexander island was not affected by arc collisions from at least the lower jurassic to the oligocene and such a long period of continuous accretion appears to be unusual deposition of the lemay group spans the kimmeridgian to albian sedimentation in the fossil bluff group fore arc basin thus making the earlier concept of the lemay group as pre jurassic basement untenable allochthonous latest jurassic earliest cretaceous radiolarian assemblages with some supposed tethyan affinities are present in the lemay group in contrast an in situ latest jurassic assemblage from the nordenskjold formation of the back arc basin and a further jurassic assemblage from a probable trench slope basin have characteristics believed diagnostic of high latitudes the biogeographic affinities of radiolarians from cherts in the lemay group accretionary complex suggest that both these cherts and associated basalts are far travelled slices of the phoenix plate rocks from the probable trench slope basin formerly assigned to the younger fossil bluff group fore arc basin sequence now appear to be part of a new previously unrecognized formation

ductile thrusting within subduction complex rocks on signy island south orkney islands abstract a model involving repeated nnw directed ductile thrusting in subduction complex rocks on signy island south orkney islands accounts for the consistent nnw trending alignment of fold axes stretching and shearing directions in the absence of major discrete thrust planes stratigraphic control or younging criteria the evidence for consistent ductile thrusting is more cryptic and is found in folding stretching relations porphyroblast rotations and mylonite fabrics early fold axes parallel to stretching syn tectonic garnet rotations augen lineations and sheath folds all support nnw directed thrusting as do a common late shear band foliation and minor shear zones development of late open folds with axes parallel to stretching may be due to constrictional deformation a lateral ramp or differential movement during thrusting the structures described from signy island may characterize the type of deformation in deep level ductile thrust zones at the base of an accretionary wedge or in a zone of subcretion the thrusting direction on signy island is inferred to be parallel to the direction of subduction in the south orkney islands in the early jurassic plate reconstructions suggest that this direction was at a high angle to the arcuate trend of the antarctic peninsula continental margin

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

sediment hosted lead zinc deposits of the neoproterozoic bambui group and correlative sequences sao francisco craton brazil a review and a possible metallogenic evolution model abstract the proterozoic sediment hosted zn pb sulfide and non sulfide deposits of the sao francisco craton brazil are partially syn diagenetic and epigenetic and were probably formed during extensional events the majority of the deposits occur within shallow water dolomites the pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the plumbotectonic model some of the deposits are characterized by highly radiogenic lead 206 pb 204 pb 21 originating from the highly radioactive crust of the sao francisco craton pb and s isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments respectively the relatively high temperatures of formation 100 to 250 c and moderate salinity 3 to 20 nacl equiv of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation the steep paleo geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids

fluid evolution in the southern part of the proterozoic vazante group brazil implications for exploration of sedimentary hosted base metal deposits abstract the proterozoic vazante group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical fluid inclusion and isotopic studies of the upper morro do pinheiro and lower pamplona members from the serra do poco verde formation in the southern part of the paleo basin these units are the main hosts for the hypogene structurally controlled zinc silicate deposits in the vazante zinc district including the vazante mine which is considered to be the largest willemitic zn 2 sio 4 deposit in the world with estimated total resources of 40 60 mt at 20 zn five hydrothermal alteration types were identified in the southern extension of the vazante group i early stage alteration comprises dolomite substitution bands and nodules associated with moderate salinity h 2 o nacl cacl 2 fluids with temperatures around 90 c interpreted as late diagenetic the calculated c and o isotopic compositions of the fluids suggest meteoric and or connate origin and interaction with organic carbon ii pre ore stage alteration is evidenced by dog tooth dolomite and quartz with minor fe oxi hydroxides which fills dissolution voids formed by h 2 o nacl cacl 2 mgcl 2 fluids at temperatures around 100 to 150 c the c isotopic data also indicate interaction with organic carbon the two first stages are poor in ore related elements iii ore stage encompasses four phases of mineral precipitation the first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite the second is the main phase of the ore stage composed of massive red dolomite massive hematite and willemite with enrichment mainly in fe 2 o 3 sio 2 ag as cd cu ni pb se and zn the third phase comprises white dolomite hematite and traces of willemite with enrichment in mno cd ni and pb the calculated c and o isotopic compositions of the fluids at 180 c associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock the fourth phase corresponds to zn chlorite and quartz which are associated with h 2 o nacl fluids of variable salinities and distinct temperatures 90 140 c and 170 190 c iv pyrite bearing vein characterized by pyrite sphalerite white dolomite fine hematite and late quartz with c and o isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage these data suggest a co genetic relationship of these two alteration styles v the late sulfide stage characterized by rare galena bearing stringers with sphalerite chalcocite greenockite covellite and white dolomite that cross cut the main ore stage phases the ore related fluid compositions associated with the silicate zinc mineralization in the southern part of the vazante group are also similar to the fluids reported in previous work for the sulfide zinc lead deposits in the northern part indicating favorable conditions for metal transport during the brasiliano orogeny sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water this finding implies that carbonate sequences in other districts with sulfide lead zinc deposits may also host hypogene silicate zinc deposits

confirmation of the carolina slate belt as an exotic terrane an assemblage of middle cambrian atlantic faunal province trilobites has been found in the rocks of the carolina slate belt near batesburg south carolina geologic and paleomagnetic data suggest that the carolina slate belt and the adjacent charlotte belt constitute an exotic terrane that was accreted to north america in early to middle paleozoic time

rhenium osmium isotope systematics of an early mesoproterozoic sedex polymetallic pyrite deposit in the north china craton implications for geological significance and the marine osmium isotopic record abstract the rhenium osmium re os isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the os composition of ancient seawater in particular the age of sedimentary exhalative sedex deposits can be determined by measuring re os isotopes in syn depositional pyrite a common constituent of sedex deposits here we report on the gaobanhe sediment hosted polymetallic pyrite deposit which is hosted within the gaoyuzhuang formation in the central part of the northern margin of the north china craton ncc re os isotopic data from syn depositional pyrite yield an isochron age of 1505 55 ma constraining the timing of primary mineralization as well as the minimum age of the host sediments to the mesoproterozoic analyzed pyrite grains indicate an initial 187os 188os osi value of 0 181 0 066 suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids with the os being primarily derived from seawater the osi value is slightly higher than that of paleoproterozoic seawater 0 13 suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased continental radiogenic os input to the oceans we propose that the re os isotopic system as recorded by these sedex deposits may provide key insights when examining changes in the os isotopic composition of seawater throughout the precambrian

continental rift to back arc basin jurassic cretaceous stratigraphical and structural evolution of the larsen basin antarctic peninsula the larsen basin developed in jurassic times as a result of continental rifting during the early stages of gondwana break up lower upper jurassic non marine sedimentary and volcanic rocks constitute a syn rift megasequence recording initial largely amagmatic extension and subsequent widespread extension related silicic volcanism a succeeding kimmeridgian early berriasian transgressive megasequence consisting largely of anoxic dysoxic hemipelagic mudstones is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak jurassic eustatic sea levels served to maximize sediment starvation the fragmentary record for late berriasian barremian times suggests that a regressive megasequence may have developed in the earlier part of this period recording increased sediment yield to the larsen basin from the increasingly emergent antarctic peninsula arc subsequently strata in the southern but not the northern part of the basin underwent relatively intense eastward verging deformation possibly during the formation of a retro arc fold thrust belt where exposed the lower part of the succeeding aptian eocene megasequence consists of a deep marine clastic wedge deposited along the fault bounded western basin margin during a phase of arc uplift and related differential subsidence following partial basin inversion in late cretaceous times regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward

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

chemostratigraphic correlation of neoproterozoic successions in south america abstract the neoproterozoic successions of south america largely accumulated during extensional events associated with the breakup of rodinia these units are preserved as thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments on passive cratonal margins which have been intensely deformed in mobile belts surrounding the craton volcaniclastic and siliciclastic materials are associated with some of the marginal fold belts three mega sequences are represented in the cratonal area glaciogenic carbonate and molasse at least two transgressive regressive sea level cycles occurred during the evolution of the carbonate mega sequence which lies above iron cemented glacio marine diamictite of probable sturtian age we report high resolution carbon oxygen strontium and sulfur isotope trends from analyses of well preserved sample sets and use principally strontium and carbon isotopes from the best preserved samples in concert with lithostratigraphic and biostratigraphic observations to provide detailed correlations of the south american neoproterozoic successions lead zinc deposits some of which with important metal reserves seem to be restricted to the sturtian carbonate succession and associated with a sequence boundary at the end of the first transgressive regressive cycle phosphate deposits are found in both sturtian and varangerian marinoan carbonate successions above glaciogenic sequences

role of strike slip faulting in the tectonic evolution of the antarctic peninsula the antarctic peninsula mesozoic magmatic arc has had a long history of dextral strike slip deformation the deformation was initially associated with the development of a wide accretionary complex by the migration of fore arc slivers and the formation and inversion of a thick fore arc basin succession it also formed an important component within major shear zones in the arc and may have controlled the formation of sedimentary basins in the back arc region although some transcurrent motion within the fore arc region was related to a component of oblique subduction the main movement occurred during the breakup of gondwanaland and the formation of a major transtensional rift system a new reconstruction for this part of gondwanaland is presented taking this transcurrent motion into consideration

the osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

a global transition to ferruginous conditions in the early neoproterozoic oceans eukaryotic life expanded during the proterozoic eon1 2 5 to 0 542 billion years ago against a background of fluctuating ocean chemistry2 3 4 after about 1 8 billion years ago the global ocean is thought to have been characterized by oxygenated surface waters with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron containing ferruginous deeper waters5 6 7 the spatial extent of sulphidic waters probably varied through time5 6 but this surface to deep redox structure is suggested to have persisted until the first neoproterozoic glaciation about 717 million years ago8 9 10 11 here we report an analysis of ocean redox conditions throughout the proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops we find a global transition from sulphidic to ferruginous mid depth waters in the earliest neoproterozoic coincident with the amalgamation of the supercontinent rodinia at low latitudes we suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on rodinia we propose that this transition essentially detoxified ocean margin settings allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

stable isotopic compositions of carbonates from the mesoproterozoic bangemall group northwestern australia marine carbonate rocks from the mesoproterozoic bangemall group of northwestern australia show little deviation 1 3 in whole rock delta 13c carb values about a mean of 0 5 this narrow range persists despite close sampling every 10 20 m through long sections up to 2500 m that are geographically widespread up to 250 km apart over many depositional environments supralittoral to outer shelf sediment sources stromatolitic bioherms to detrital calcilutites and rock types pure limestones to dolomitic shales the only major excursions from the norm seem related to unusual environmental or post depositional processes as they are correlated with large enrichments to 3 or depletions to 16 in 18o relatively heavy delta 13c values up to 2 5 occur in a single bed of brecciated ferruginous dolostone at a single locality these abnormal values may result from local evaporitic conditions limey and shaley nodular dolostones have delta 13c values as low as 4 3 probably caused by remineralization of organic matter during late and patchy dolomitization most notably sharp negative excursions in delta 13c up to 8 4 occur in bleached kerogen free rocks with mineral assemblages of dolomite quartz calcite tremolite talc reflecting isotopic re equilibration in thick metamorphic aureoles around dolerite intrusions general environmental variations are minor with delta 13c values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative there are no strong secular trends but subtle fluctuations within the range 2 to l can be correlated along the northwestern margin of the basin this resembles the pattern seen in other mesoproterozoic successions but is markedly unlike the heavy background 5 and extreme variations up to l0 in delta 13c evident in neoproterozoic successions of similar thickness and environmental setting hence in contrast to the neoproterozoic the global rate of organic carbon burial was probably fairly constant during deposition of the bangemall group and perhaps generally during the mesoproterozoic as was the redox state of the atmosphere and hydrosphere

further evidence of fracture zone induced tectonic segmentation of the antarctic peninsula from detailed aeromagnetic anomalies aeromagnetic anomaly data collected between 67 s and 70 s crossing the antarctic peninsula and adjacent offshore areas show a prominent nw se trend in the magnetic fabric apparent lateral offsets previously recognized in the pacific margin anomaly have been mapped in detail and are shown to be much smaller than previously suggested a 35 km wide zone of subdued magnetic anomalies at the western edge of the pacific margin anomaly bounded by these apparent offsets is interpreted as a downfaulted block of the mafic intermediate batholith thought to be responsible for the pacific margin anomaly the trends of both fracture zones and magnetic lineaments strongly support the link between faulting in the antarctic peninsula magmatic arc and offshore tectonics

a complete arc trench system recognized in gondwana sequences of the antarctic peninsula region prior to late triassic early jurassic times the geological history of the antarctic peninsula region was dominated almost entirely by gondwana sequences that together comprised a major arc trench system subduction complex trench slope break and fore arc basin sedimentation can all be recognized and deposition was at least partly on early palaeozoic or older continental crust the only evidence for a contemporaneous magmatic arc situated in the antarctic peninsula at this time consists of patchy occurrences of metavolcanic rocks possibly representing the frontal edge of the arc and the major outcrop area of these rocks is believed to lie under the broad shallow continental shelf e of the antarctic peninsula this is contrary to most current hypotheses in which a marginal basin presumably floored by oceanic crust is thought to crop out close to the e coast of the antarctic peninsula however the complete absence of substantial outcrops of pre jurassic volcanic rocks anywhere in eastern antarctica and south africa which are the closest and most likely places in which these should exist supports the new proposal moreover the identification of back arc elements of the arc trench system foreland fold thrust belt and retro arc foreland basin in eastern antarctica and south africa greatly strengthens the likelihood of the arc cropping out in the area suggested towards the end of the triassic and during the early jurassic periods an intense diastrophic event or culmination of events of orogenic magnitude occurred gondwanian orogeny causing substantial redistribution of the pre existing elements of the arc trench system in particular all the fore arc sequences were strongly deformed some possibly for the first time e g in the fore arc basin and became firmly accreted to the continental margin moreover the magmatic foci migrated trenchwards to intrude the deformed rocks in the antarctic peninsula because many of the plutons were emplaced synkinematically they often closely resemble basement gneisses the distinction between these rocks remains a serious problem in the antarctic peninsula and it can be solved only partially by radiometric dating

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

global correlation of the vazante group sao francisco basin brazil re os and u pb radiometric age constraints abstract the vazante group consists of precambrian carbonate dominated platform deposits that extend along more than 300 km in the external zone of the brasilia fold belt of the sao francisco basin in east central brazil the sequence is about 4 8 km thick and contains a preserved glaciomarine diamictite unit containing dropstone at the top and a lower diamictite unit at the bottom previous c and sr isotope profiles suggested the correlation of the upper diamictite unit with the sturtian glacial event ca 750 643 ma however new re os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 46 and 1100 77 ma u pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 15 and 1000 25 ma respectively based on the re os and u pb ages the best age estimate of the vazante group is constrained to be 1000 1100 ma and thus the two diamictite units are not correlative with the sturtian glaciation s but most likely are records of glacial events that occurred during the late mesoproterozoic

the carbon isotopic composition of proterozoic carbonates riphean successions from northwestern siberia anabar massif turukhansk uplift thick carbonate dominated successions in northwestern siberia document secular variations in the c isotopic composition of seawater through mesoproterozoic and early neoproterozoic early to early late riphean time mesoproterozoic dolomites of the billyakh group anabar massif have delta 13c values that fall between 0 and 1 9 permil versus pdb with values in the upper part of the succession yusmastakh formation consistently higher than those of the lower ust il ya and kotuikan formations consistent with available biostratigraphic and radiometric data delta 13c values for billyakh carbonates compare closely with those characterizing early mesoproterozoic carbonates about 1600 1200 ma worldwide in contrast late mesoproterozoic to early neoproterozoic limestones and dolomites in the turukhansk uplift exhibit moderate levels of secular variation only the lowermost carbonates in the turukhansk succession linok formation have delta 13c values that approximate billyakh values higher in the turukhansk succession delta 13c values vary from 2 7 to 4 6 permil with outliers as low as 5 0 permil interpreted as diagentically altered again consistent with paleontological and radiometric data these values compare well with isotopic values from 1200 to 850 ma successions elsewhere five sections measured in different parts of the turukhansk basin show nearly identical patterns of variation confirming that carbonate delta 13c correlates primarily with time and not facies the siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra and interbasinal correlation of mesoproterozoic and early neoproterozoic rocks

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

isotope stratigraphy of the lapa formation sao francisco basin brazil implications for late neoproterozoic glacial events in south america the lapa formation is a thick carbonate sequence 900 m that constitutes the upper part of the vazante group on the s francisco craton brazil it conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age the sequence above the glacial unconformity consists predominantly of organic rich shale subtidal rhythmic dolomicrites and microbialaminites and intertidal stromatolites four boreholes spanning different depositional settings were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation the 13 c and 18 o values of well preserved lapa carbonate microsamples range from 8 2 to 3 3 vpdb and from 13 6 to 0 9 vpdb respectively each of the 13 c profiles of the investigated cores reveals two strong negative excursions of up to 8 an event in post glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic rich shale and a second near the top of the sequence associated with a shale interval based on the observation of dropstones and sedimentary iron formation in the underlying diamictite as well as the distinguishable carbon isotope trends the lapa formation is considered as a cap carbonate lithofacies the age of the lapa formation is presently unknown but the least radiogenic 87 sr 86 sr value 0 7068 associated with a negative carbon isotope excursion matches that from the rasthof formation in namiba on the congo craton which is radiometrically constrained to be younger than ca 750 ma

the low temperature geochemical cycle of iron from continental fluxes to marine sediment deposition suspended sediments from 34 major rivers geographically wide spread and 36 glacial meltwater streams have been examined for their variations in different operationally defined iron fractions fehr iron oxides soluble in dithionite fepr iron soluble in boiling hcl but not in dithionite and feu total iron less that soluble in boiling hcl river particulates show a close association between fehr and total iron fet reflecting the effects of chemical weathering which derive oxide iron from and retain it in close association with total iron consistent with this continental scale average fehr fet ratios vary with runoff ratios average river runoff per unit area average precipitation per unit area by contrast the diminished effects of chemical weathering produce no recognizable association of fehr with fet in glacial particulates and instead both fepr and feu are closely correlated with fet reflecting essentially pristine mineralogy a comparison of the globally averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in fehr fepr and fet but enriched in feu the river and glacial particulate data are combined with estimates of authigenic hydrothermal atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for fehr f e pr f e u and fet this budget suggests that the differences between riverine particulates and marine sediments can be explained by i preferentially removing fehr from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains salt marshes and estuaries and ii mixing the resulting riverine particulates with fehr depleted glacial particulates preliminary measurements of inner shore sediments are consistent with i above phanerozoic and modern normal marine sediments have similar iron speciation characteristics which implies the existence of a long term steady state for the iron cycle this steady state could be maintained by a glacioeustatic feedback where fehr enriched riverine particulates are either more effectively trapped when sealevel is high small ice masses diminished glacial erosion or are mixed with greater masses of fehr depleted glacial particulates when sealevel is low large ice masses enhanced glacial erosion further important controls on the steady state for fehr operate through the formation of euxinic sediments and ironstones which also provide sealevel dependent sinks for fehr enriched sediment

provenance of the vazante group new u pb sm nd lu hf isotopic data and implications for the tectonic evolution of the neoproterozoic brasilia belt abstract the vazante group show varied u pb provenance patterns along the basin zircon ages range from 936 to 3409 ma but paleo and mesoproterozoic terrains constitute the main sources of the original sediments the youngest population 930 ma establishes the maximum depositional age of the group sm nd t dm data show the predominance of paleoproterozoic ages 1 90 2 08 ga and also indicate some input from younger sources in rocks of the lapa formation 1 67 to 2 0 ga in the upper part of the group whereas rocks of the serra do garrote formation present the oldest model ages 2 03 to 2 76 ga hf isotopic compositions of the detrital zircons indicate that they were derived mainly from recycled paleoproterozoic crust with a minor mesoproteroic juvenile component terranes within the sao francisco craton represent the main sources of detrital sediments of this group and reinforce the interpretation that it may be a passive margin sequence developed along the western margin of the original continent however the origin of mesoproterozoic grains remains uncertain slightly younger sm nd model ages in the lapa formation however are not entirely consistent with derivation solely from the craton and may indicate contribution from younger sources such as the neoproterozoic goias magmatic arc

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

mesoproterozoic paleo redox changes during 1500 1400 ma in the yanshan basin north china abstract the mesoproterozoic 1600 1000 million years ago ma is thought to have experienced important changes in ocean chemistry and eukaryote evolution our understanding of global redox conditions remains in its infancy and redox data derived from the mesoproterozoic thus far indicate heterogeneity and complexity in the realm of ocean redox which hampers our understanding of the potential significance of variations in ocean redox on eukaryote evolution in this study we report analyses of fe species and redox sensitive trace metals of u and mo from the jixian group including the wumishan hongshuizhuang and tieling formations as well as the xiamaling formation in the yanshan basin in north china the fe speciation data from the wumishan hongshuizhuang and tieling formations first reported in this study help bridge the global data gap of fe species data during 1500 1400 ma our data suggest likely oxic bottom water conditions for the wumishan formation and ferruginous bottom water conditions for the tieling formation the fe species data can distinguish ferruginous conditions for the upper hongshuizhuang from sulfidic depositional conditions for the lower middle hongshuizhuang formation based on fe species and trace metal data redox conditions likely varied from anoxic conditions with transient euxinia in the lower middle xiamaling formation to oxic conditions in the upper xiamaling formation the new fe speciation data from the yanshan basin when combined with previously published data show a surprisingly high proportion of middle proterozoic samples with very low highly reactive to total iron ratios this suggests either a relatively large extent of oxic seafloor or non uniformitarian iron cycling resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes

radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late mesoproterozoic borden basin arctic canada

reverse fault step at engel peaks antarctic peninsula abstract a steep reverse fault at engel peaks antarctic peninsula cuts granophyre and overlying volcanic rocks and incorporates a lower angle zone or fault step the reverse fault step differs from the more familiar thrust ramp in that it cuts down section in the footwall in the direction of tectonic transport accommodation of the fault zone to this step and its associated sidewall are discussed with reference to kinematic models for pull apart zones and for the movement of thrust sheets over non planar surfaces during movement on the main fault the step acted as a fault jog and pull apart zone preserving cataclasite breccia and other structures demonstrating a brittle to ductile deformation history ultra fine grained rock in the step zone is the result of dynamic recrystallization of ultracataclasite during episodes of brittle slip the fault step acted as a dilational fault jog between the main steep reverse fault segments rotation of mylonite into the reverse fault step and its sidewall involved flexural flow parallel to foliation movement of material through the step zone resulted in shear parallel to the base of the step enhancement of breccia layering mixing of lithologies and cleavage formation it also involved extension at a high angle to the main movement direction to accommodate the extra length of the fault zone late steep reverse fault zones propagating from the main fault folded the breccias in the step and crenulated the earlier cleavage the fault step was a zone of brittle dilation hydration diffusive mass transfer and mineralization

on the origin of fore arc basins new evidence of formation by rifting from the jurassic of alexander island antarctica the middle jurassic lower cretaceous fossil bluff group of alexander island antarctica represents the fill of a fore arc basin unconformably overlying an accretionary complex like most fore arc basins this example had been considered to have a passive origin as a topographic hollow between the arc and the trench slope break recent discoveries of igneous rock coeval with sedimentation have altered this view oxfordian kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin within a single formation chemically most basalts are high nb types which cannot have originated in a supra subduction zone setting since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the antarctic peninsula volcanic arc it is concluded that a late jurassic pause in subduction led to active rifting to form the fore arc basin

evolution of the late paleozoic accretionary complex and overlying forearc magmatic arc south central chile 38 41 s constraints for the tectonic setting along the southwestern margin of gondwana stratigraphic structural metamorphic and geochronologic studies of basement rocks in the andean foothills and coast ranges of south central chile 39 41 s suggest a protracted late paleozoic to middle mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern gondwana margin in the study area the late paleozoic paired metamorphic belt coeval magmatic arc and overlying triassic sedimentary units preserve a record of late carboniferous to early permian subduction and arc magmatism subsequent deep exhumation of the western series subduction complex and diminished uplift and erosion of the eastern series arc forearc region by the late triassic late paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism collectively referred to as dl are largely erased in the western series by the dominant d2 schistosity and lower greenschist grade metamorphism d1 structural features as well as original sedimentary textures are relatively well preserved in the less penetratively deformed eastern series the regional distribution of late paleozoic arc magmatism suggests that the late paleozoic convergent margin deviated from a n s trend north of this area to a nw se trend near this latitude and faced an open marine environment to the southwest a transition from f2 isoclinal folding to more open larger scale f3 folds interpreted as change in ductility during differential uplift of the western series is not apparent in the eastern series despite a lesser degree of uplift during the main exhumational d2 event delineation of unconformities and u pb dating of detrital zircons and intrusions into the eastern series allow tighter constraints to be placed on timing of uplift and denudation of the eastern series than on that in the western series a regional unconformity exposed in the lake district that separates more highly deformed eastern series lithologies from late triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the d2 event we propose that dextral oblique convergence initiated during the middle permian along this segment of the gondwana margin resulted in the transpressional uplift and juxtaposition of high pressure temperature p t western series against low p t eastern series lithologies and culminated with deposition of late triassic continental to shallow marine coarse clastic sedimentary rocks in fault bounded strike slip basins adjacent to the exhumed western series large scale dextral transpression and northward displacement of the accretionary complex during late permian to late triassic time along the chilean margin of gondwana are synchronous and kinematically compatible with widespread regional transpression extension and silicic magmatism inboard of the southern gondwana margin at this time we thank c mpodozis m gardeweg and j munoz of the servicio de geologia y mineria de chile sernageomin for their support of this work fruitful discussions with n blanco f herve h moreno c mpodozis and f munizaga have aided in our understanding of the geology of the region the hard work by the staff of sernageomin s puerto varas office is graciously appreciated we thank j d walker and w r van schmus at the university of kansas for allowing mwm use of their u pb and mass spectrometer facilities and j vargas and the staff of sernageomin s geochemistry laboratory for their assistance in this project f munizaga allowed us to cite an unpublished 40ar 39ar date we thank g ya nez for access to aeromagnetic data t kato wishes to thank w g ernst comments by i dalziel s kay and v ramos helped clarify ideas presented in this paper and are greatly appreciated this work is dedicated to our friend and colleague alberto campos c who died in a climbing accident on calbuco volcano 1996

very low and low grade metamorphism in the trinity peninsula group permo triassic of northern graham land antarctic peninsula the permo triassic trinity peninsula group is a widespread regionally metamorphosed metasedimentary sequence in northern graham land antarctica which forms the local basement to the mainly jurassic cretaceous antarctic peninsula magmatic arc the metamorphic grade thermal evolution and pressure series of this major tectono stratigraphical unit are largely unknown determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases mainly in rare volcanic beds however diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas using white mica illite crystallinity of interbedded mudrocks the trinity peninsula group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest outwith thermal aureoles near plutonic intrusions the alteration temperatures ranged mainly from 250 to 325 c exceeding 300 c in the highest grade epizone greenschist facies parts of the sequence the facies series k white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under moderate geothermal gradients 35 c km corresponding to burial depths of c 7 10 km unroofing and substantial erosion of the trinity peninsula group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern graham land the geological setting of the trinity peninsula group is ambiguous and could have been a foreland or back arc basin or the mid to upper levels of an accretionary prism

sedimentology and structure of the trench slope to forearc basin transition in the mesozoic of alexander island antarctica the mesozoic forearc of alexander island antarctica is one of the few places in the world where the original stratigraphic relationship between a forearc basin and an accretionary complex is exposed newlydiscovered sedimentary rocks exposed at the western edge of the forearc basin fill the kimmeridgian albian fossil bluff group record the events associated with the basin formation these strata are assigned to the newly defined selene nunatak formation bathonian and atoll nunataks formation bathonian tithonian within the fossil bluff group the selene nunatak formation contains variable thicknesses of conglomeratesand sandstones predominantly derived from the lemay group accretionary complex upon which it is unconformable the formation marks emergence and subsequent erosion of the inner forearc area it is conformably overlain by the1 km thick atoll nunataks formation characterized by thinly bedded mudstones and silty mudstones representing a marine transgression followed by trench slope deposition the atoll nunataks formation marks a phase of subsidence possibly in response to tectonic events in the accretionary prism that are known to have occurred at about the same time the atoll nunataks formation is conformably overlain by the himalia ridge formation a thick sequence of basin wide arc derived conglomerates this transition from fine to coarse grained deposition suggests that a well developed depositional trough and hence trench slope break had formed by that time the atoll nunataks formation therefore spans the formation of the forearc basin and marks the transition from trench slope to forearc basin deposition

limestone and chert in tectonic blocks from the esk head subterrane south island new zealand the esk head subterrane is a continuous belt generally 10 20 km wide of tectonic melange and broken formation on the south island of new zealand this subterrane separates older and younger parts of the torlesse terrane which is an extensive accretionary prism composed mostly of quartzo feldspathic submarine fan deposits ranging from permian to early cretaceous in age the torlesse is the most pacific ward of several permian and mesozoic accreted terranes in new zealand that record tectonic amalgamation and ultimate accretion against the pacific facing gondwana margin the esk head subterrane of the torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt associated seamount and sea floor limestones and cherty rocks thought to be representative of the subducted plate limestones in tectonic blocks are of late triassic and probably jurassic ages and include 1 submarine cemented pelagic bivalve geopetal packstone grainstone 2 brachiopod bryozoan encrinite and 3 radiolarian pelagic lime mudstone most of the triassic blocks have been dated using conodonts which have remarkably low color alteration index cai values paleogeographic inferences drawn from megafossils bioclasts and radiolarians as well as from carbonate cements indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the new zealand part of the gondwana margin but higher than paleoequatorial latitudes these oceanic sediments and their basaltic substrates were evidently emplaced in the torlesse accretionary prism following off scraping from an extensive subducting oceanic plate probably the phoenix plate which was obliquely convergent with the northwest trending gondwana margin during late jurassic and or early cretaceous time

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

extensional collapse of the northern taiwan mountain belt as an active collision zone between the luzon arc and the china continental margin the taiwan mountain belt particularly its south central part is undergoing crustal shortening and rapid uplift the northern part of the orogen is however subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest facing luzon arc system to the south facing ryukyu arc system taiwan and its neighboring southern ryukyu used to be part of the rifted china continental margin before the collision started in late miocene time engineered by the northwest movement of the philippine sea plate in the past 5 m y the north south trending luzon arc obliquely overrode the northeast southwest trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest following the southwest propagating collision the north dipping philippine sea plate which was subducting beneath the ryukyu arc also extended westward and caused flipping of subduction in the northern part of the collision orogen in consequence the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction the collision orogen collapsed as a result of crustal thinning and its central part foundered into a deep submerged basin a whole spectrum of active tectonic scenarios ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence can be readily observed in the taiwan ryukyu area and provides a vivid example for the process of orogenic collapse

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

re os and mo isotope systematics of black shales from the middle proterozoic velkerri and wollogorang formations mcarthur basin northern australia joint application of the mo isotope paleoredox proxy and re os deposition age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time here we report new re os and mo isotope data for the mesoproterozoic velkerri formation roper group and paleoproterozoic wollogorang formation tawallah group mcarthur basin northern australia new re os ages of 1361 21 ma 2 n 14 mean square of weighted deviates mswd 1 3 model 1 and 1417 29 ma 2 n 12 mswd 1 3 model 1 constrain the depositional age of the velkerri formation and its contained biomarkers as well as acritarchs and microfossils from the roper group black shales from the upper velkerri formation have high mo abundances 105 119 ppm and degree of pyritization dop values 0 90 0 92 implying quantitative conversion of molybdate moo42 to thiomolybdate mos42 in overlying bottom waters the average 97 95mo 0 72 0 10 2 n 6 of these shales is consistent with previous data but represents a significantly more precise determination for global seawater 97 95mo at 1 4 ga this value is lighter than present day seawater by 0 85 and reflects expanded strongly euxinic deep ocean conditions h2s aq 11 m relative to oxic suboxic and weakly intermittently euxinic h2s aq 11 m marine deposition in the 1 4 ga oceans mass balance modelling suggests mo removal into strongly euxinic and oxic sediments may have comprised 30 70 and less than 15 respectively of the oceanic mo sink at 1 4 ga as opposed to 5 and 35 today respectively the re os radioisotope system in organic rich shales serves as a test for post depositional alteration that could affect the usefulness of paleoredox tracers such as mo stable isotopes re os isotope data for the wollogorang formation black shales are scattered and yield a highly imprecise date of 1359 150 ma 2 n 21 mswd 85 model 3 this age is younger than u pb zircon ages from interbedded tuffs that constrain the age of deposition at ca 1730 ma in conjunction with previous petrological geochemical and paleomagnetic data the re os isotope data suggest hydrothermal fluid flow through the wollogorang formation possibly associated with formation of the ca 1640 ma mcarthur river pb zn ag sedimentary exhalative deposit resulted in post depositional mobilization of re and os based on the degree of deviation of the re os data from a 1730 ma reference line open system behavior of re and os was greatest near the base of the black shale unit wollogorang formation black shales are enriched in mo 41 58 ppm but are characterized by variable 97 95mo 0 3 0 8 and dop 0 57 0 92 the lightest 97 95mo values occur near the base of the black shale unit based on the re os systematics hydrothermal fluids have probably overprinted the authigenic 97 95mo signature in those shales however the heaviest 97 95mo values in the wollogorang formation come from stratigraphically higher shales and are similar to those observed for the velkerri formation and thus may reflect seawater 97 95mo at 1 73 ga

tectonic implications of fore arc magmatism and generation of high magnesian andesites alexander island antarctica alexander island situated off the west coast of the antarctic peninsula contains a suite of late cretaceous to early tertiary subduction related magmatic rocks the rocks occupy a fore arc position 100 200 km trenchward of the main arc antarctic peninsula and they become younger northward along the length of the island major and trace element geochemistry for 222 samples shows the suite to be a medium to high k calc alkaline series ranging in composition from picro basalt to rhyolite andesite samples show a large range in mgo and mg with nine samples representing high magnesian andesites sr and nd isotopic data indicate that the andesites range isotopically to more depleted mantle compositions than the associated basalts the dacite rhyolites can be related compositionally to the andesites by assimilation of typical pacific rim accretionary material to produce high magnesian andesite lavas it is necessary to introduce a suitable source of heat into the fore arc thus enabling partial melting of depleted sub fore arc hydrous peridotite a causative link with ridge subduction prior to magmatism is proposed with successive ridge trench collisions producing a temporal migration of the magmatism and high geothermal gradients in an anomalously hot fore arc region

chemostratigraphy of the late mesoproterozoic atar group taoudeni basin mauritania muted isotopic variability facies correlation and global isotopic trends a b s t r a c t sedimentary strata of the atar el mreiti groups taoudeni basin mauritania are characterized by car bon isotope values that fall largely between 2 5 and 4 0 and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from 1 2 ga to 1 1 ga chemostratigraphic results are consistent with recent re os dates of 1 1 ga for atar el mreiti group strata rooney et al 2010 and support an emerging dataset that identifies the late mesoproterozoic as an isotopically distinctive interval in earth history the relatively low amplitude carbon isotope variabil ity limits the use of chemostratigraphy in detailed correlation during this time interval in this study we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of deposi tional facies to construct a rigorous and testable model for the intrabasinal correlation of atar and el mreiti group strata data support a depositional model that results in broadly uniform stratigraphic thicknesses across the west african craton with predominantly stromatolitic facies of the atar group mauritania and the hank and dar cheikh groups algeria representing deposition on shallow water craton margins and clayey carbonate and shale facies of the el mreiti group mauritania representing deposition on a broad shallow water epicratonic platform within this stratigraphic framework regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely asso ciated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean similarly distinct differences in trace element composition recorded in multilayered marine cements within deeper water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox gradients in the water column we use this multifaceted approach to test a recent hypothesis that suggests correlation of atar el mreiti group strata with glaciogenic strata of the vazante group sao francisco craton critically preservation of shallow water carbonate and evaporite strata across more than 1200 km of the predominantly epicra tonic taoudeni basin with minimal evidence for high amplitude sea level change is consistent with inferences of late mesoproterozoic highstand conditions in a global greenhouse climate such charac teristics are inconsistent with suggestions of a time equivalency with glaciogenic strata of the vazante group sao francisco craton despite broad similarities in re os geochronology and carbon isotope com position together these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution 2012 elsevier b v all rights reserved

the eastern palmer land shear zone a new terrane accretion model for the mesozoic development of the antarctic peninsula a major ductile fault zone the eastern palmer land shear zone has been identified east of the spine of the southern antarctic peninsula this shear zone separates newly identified geological domains and indicates that during late jurassic terrane accretion and collision two and possibly three separate terranes collided resulting in the palmer land orogeny the orogeny is best developed in eastern palmer land and eastern ellsworth land there shallow marine sedimentary rocks of the latady formation and a metamorphic and igneous basement complex of possible lower palaeozoic to pre early jurassic age are thrust and folded this forms an arcuate east directed foreland fold and thrust belt up to 100 km wide and 750 km long parallel to the axis of the antarctic peninsula the newly identified antarctic peninsula domains include 1 a parautochthonous eastern domain that represents part of the margin of the gondwana continent comparable to the western province of new zealand the ross province superterrane of marie byrd land the eastern series of south central chile the pampa de agnia and tepuel rocks of north patagonia and the cordillera darwin rocks of tierra del fuego 2 a suspect central domain that represents an allochthonous microcontinental magmatic arc terrane comparable to the median tectonic zone of new zealand the amundsen province superterrane of marie byrd land and coastal cordillera of north chile and 3 a suspect western domain with strong similarities to the eastern province of new zealand western series of south central chile and chonos metamorphic complex of north patagonia that represents either a subduction accretion complex to the central domain or another separate crustal fragment although an allochthonous terrane hypothesis for the antarctic peninsula remains to be fully tested this has much in common with models for the new zealand and south american parts of the pacific margin of gondwana the identification of a potential allochthonous terrane continent collision zone allows us to define the edge of the gondwana continent in the antarctic peninsula sector of the supercontinent margin which has implications for mesozoic reconstructions of gondwana

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

raman spectroscopy and biomarker analysis reveal multiple carbon inputs to a precambrian glacial sediment extractable biomarkers can help elucidate the environment and biota of ancient glaciations although the method must be applied with care as glacial sediments have a potential for incorporation of older detrital carbon in phanerozoic glacial sediments the distinct elemental molecular and isotopic compositions of the terrestrial and marine biomass allow discrimination between primary marine and redeposited terrestrial organic matter however as the proterozoic biosphere was largely microbial and marine biomarker and isotopic analyses are insufficient for distinguishing primary organic matter from secondary reworked organic matter here we report the combined application of raman spectroscopy and biomarker analysis to precambrian glacial sediments which together allows discrimination between mixed pools of organic carbon and provides a promising new approach for rapidly screening precambrian sediments for immature organic matter amenable to biomarker analysis

oxygen climate and the chemical evolution of a 1400 million year old tropical marine setting the xiamaling formation is an exceptionally well preserved sedimentary succession deposited on a marine passive margin about 1400 million years ago we used a multi proxy approach including iron speciation trace metal dynamics and organic geochemistry to explore the evolution of ocean chemistry through most of the xiamaling formation this evolution is put in the context of the paleogeography and the sedimentological evolution of the xiamaling depositional system overall the xiamaling formation is informally divided into six units based on both sedimentological and geochemical criteria of the six units we fully explored four of them unit 4 the lowest unit we studied is comprised of deep water red muds periodically interrupted by green colored silt and sandy turbidites iron extraction results show that the red muds are enriched in highly reactive iron indicating a water column source for the iron however the low organic carbon contents low hydrogen index hi values and the oxidized nature of the reactive iron pool indicate deposition in oxygenated bottom waters we interpret unit 4 to represent a low productivity ferruginous oxygen minimum zone omz environment underlain by oxygenated bottom waters the transition to unit 3 reflects an increase in primary productivity and the development of a more biologically active omz that supported anoxygenic phototrophic bacteria still in this unit the bottom waters remained oxygenated the overlying unit 2 represents the transition to deep water deoxygen ation and anoxic waters at the sediment surface these waters were ferruginous in the bottom part of the unit and sulfidic euxinic towards the top in the uppermost unit 1 euxinic conditions continued punctuated by more frequent water column oxygenation towards the upper part of the unit we place the evolution of these chemical dynamics in the context of climate and climate change and in particular the placement of the xiamaling formation in relation to the intertropical convergence zone itcz and the resulting hadley cell dynamics also while our results demonstrate the persistence of anoxic water column conditions high in the water column during the deposition of the xiamaling formation they also demonstrate bottom water oxygenation near the seafloor during the deposition of three of the four units and over a time interval extending to 10 s of millions of years less

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

re os age constraints and new observations of proterozoic glacial deposits in the vazante group brazil abstract a new re os radiometric age date for an organic rich shale horizon from the vazante group in brazil coupled with geological observations provide evidence for late mesoproterozoic glacial episodes conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time field observations of a reverse fault juxtaposing older mesoproterozoic sedimentary rocks above younger neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of u pb detrital zircon ages through the succession combined the geochronological geochemical and stratigraphic evidence suggest that the vazante group sediments accumulated along a passive margin of the sao francisco craton and are correlative with the neighboring paranoa group biomarker sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post glacial transgression and deposition of one of the bituminous shale horizons the relationship of the glaciogenic vazante group to other late mesoproterozoic successions such as the non glacial atar group in west africa and bylot supergroup in arctic canada however remains enigmatic

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

a coupled carbon silicon cycle model over earth history reverse weathering as a possible explanation of a warm mid proterozoic climate abstract the balance between carbon outgassing and carbon burial controls earth s climate on geological timescales carbon removal in carbonates consumes both atmospheric carbon and ocean carbonate alkalinity sourced from silicate weathering on the land or seafloor reverse weathering rw refers to clay forming reactions that consume alkalinity but not carbon if the cations of alkalinity end up in clay minerals rather than in the carbonates carbon remains as atmospheric co 2 warming the climate higher silicate weathering fluxes and warmer temperatures are then required to balance the carbon cycle it has been proposed that high dissolved silica concentrations resulting from the absence of ecologically significant biogenic silica precipitation in the precambrian drove larger rw fluxes than today affecting the climate here we present the first fully coupled carbon silica cycle model for post hadean earth history that models climate evolution self consistently available as open source code rw fluxes and biogenic silica deposition fluxes are represented using a sediment diagenesis model that can reproduce modern conditions we show that a broad range of climate evolutions are possible but most plausible scenarios produce proterozoic warming 5 k relative to without rw which could help explain the sustained warmth of the proterozoic despite lower insolation rw in the archean is potentially more muted due to a lower land fraction and sedimentation rate key model uncertainties are the modern reverse weathering flux the rate coefficient for rw reactions and the solubility of authigenic clays consequently within the large uncertainties other self consistent scenarios where proterozoic rw was unimportant cannot be excluded progress requires better constraints on parameters governing rw reaction rates including explicit consideration of cation limitations to precambrian rw and perhaps new inferences from si or li isotopes systems

direct dating of the sete lagoas cap carbonate bambui group brazil and implications for the neoproterozoic glacial events the end of the neoproterozoic is punctuated by glacial deposition but the chronology of these deposits is hindered presently by the paucity of geochronological data here we present new radiometric dating for the basal sete lagoas cap carbonate deposits that overlie glacial units in the sao francisco craton six samples from aragonite pseudomorph crystal rich facies showing pristine textures and constant 87sr 86sr ratios around 0 7075 yielded a pb pb isochron age of 740 22 ma which is interpreted as the depositional age for these remarkably preserved rocks this age can be used to infer a low to moderate palaeolatitude of 20 30 for carbonate and glacial deposition in addition as it overlaps the ages obtained for the oldest neoproterozoic glacial successions our result reinforces the idea of a long standing sturtian interval suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 ma

re os geochronology of organic rich sediments an evaluation of organic matter analysis methods rhenium and osmium in organic rich sedimentary rocks are dominantly hydrogenous but any nonhydrogenous component will influence the accuracy and precision of the re os date obtained to minimize the influence of any nonhydrogenous re and os we evaluate analysis of isolated organic matter from the whole rock together with whole rock analysis using a cro3 h2so4 digestion medium instead of inverse aqua regia for a black shale unit of the exshaw formation canada this unit previously returned a whole rock re os date of 358 10 ma model 3 geochim cosmochim acta 2002 using inverse aqua regia dissolution organic matter isolated from the whole rock matrix using the hf bf3 technique org geochem 20 1993 249 yields scattered data and a re os date of 449 220 ma model 3 mswd 616 the organic matter analyses show similar 187os 188os values but significantly lower 187re 188os values in comparison to the whole rock analyses we show that the re os systematics of organic matter are altered during chemical isolation and as such we suggest that the hf bf3 method should not be used for re os analysis of organic matter whole rock re os analysis using a cro3 h2so4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method and the re os data identify two distinct initial 187os 188os values for the sample set separate regressions of these data yield precise dates 366 1 9 6 mswd 2 2 and 363 4 5 6 ma mswd 1 6 model 3 which are indistinguishable from the age constraints for this formation 363 4 0 4 ma u pb monazite comparison of the re os dates obtained from aqua regia and cro3 h2so4 methods suggests that the former may contain nonhydrogenous re and os whereas the cro3 h2so4 method dominantly liberates hydrogenous re os from organic matter allowing for better stratigraphic age determinations and evaluation of the os isotope composition of seawater