iron and oxygen isotope fractionation during iron uv photo oxidation implications for early earth and mars abstract banded iron formations bifs contain appreciable amounts of ferric iron fe 3 the mechanism by which ferrous iron fe 2 was oxidized into fe 3 in an atmosphere that was globally anoxic is highly debated of the three scenarios that have been proposed to explain bif formation photo oxidation by uv photons is the only one that does not involve life the other two are oxidation by o 2 produced by photosynthesis and anoxygenic photosynthesis whereby fe 2 is directly used as electron donor in place of water we experimentally investigated iron and oxygen isotope fractionation imparted by iron photo oxidation at a ph of 7 3 the iron isotope fractionation between precipitated fe 3 bearing lepidocrocite and dissolved fe 2 follows a rayleigh distillation with an instantaneous 56 fe 54 fe fractionation factor of 1 2 such enrichment in the heavy isotopes of iron is consistent with the values measured in bifs we also investigated the nature of the mass fractionation law that governs iron isotope fractionation in the photo oxidation experiments i e the slope of the 56 fe 57 fe relationship the experimental run products follow a mass dependent law corresponding to the high t equilibrium limit the fact that a 3 8 gyr old bif sample if g from isua greenland falls on the same fractionation line confirms that iron photo oxidation in the surface layers of the oceans was a viable pathway to bif formation in the archean when the atmosphere was largely transparent to uv photons our experiments allow us to estimate the quantum yield of the photo oxidation process 0 07 iron atom oxidized per photon absorbed this yield is used to model iron oxidation on early mars as the photo oxidation proceeds the aqueous medium becomes more acidic which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing uv photons iron photo oxidation in centimeter to meter deep water ponds would take months to years to complete oxidation by o 2 in acidic conditions would be slower iron photo oxidation is thus likely responsible for the formation of jarosite hematite deposits on mars provided that shallow standing water bodies could persist for extended periods of time the oxygen isotopic composition of lepidocrocite precipitated from the photo oxidation experiment was measured and it is related to the composition of water by mass dependent fractionation the precipitate fluid 18 o 16 o isotope fractionation of 6 is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water

element geochemistry and neodymium isotope systematics of the neoarchean banded iron formations in the qingyuan greenstone belt north china craton abstract pure marine chemical sediments such as banded iron formations bif are excellent archives of ancient seawater and may also provide constraints on the coeval hydrosphere atmosphere system however on the bases of interpreted depositional settings it has long been argued that algoma type bif merely recorded the intrabasinal volcanic or hydrothermal conditions here we couple systematic major and trace elements data with samarium sm neodymium nd isotopic analyses of 2 56 ga old algoma type bifs and associated schists from different stratigraphic horizons of the qingyuan greenstone belt qgb in the north china craton ncc to characterize the source and depositional environment over a short period 30 ma meanwhile we also compare our results with data for other archean to paleoproterozoic superior and algoma type bifs especially those near contemporaneous counterparts in the ncc to further evaluate the reliability of qgb bifs as proxies for the neoarchean seawater low al2o3

distribution of high field strength elements y zr ree hf ta th u in adjacent magnetite and chert bands and in reference standards fer 3 and fer 4 from the temagami iron formation canada and the redox level of the neoarchean ocean abstract adjacent magnetite and chert bands and reference standards fer 3 and fer 4 from the neoarchean temagami iron formation if show shale normalized rare earth and yttrium rey patterns with low nd yb ratios and positive eu gd and y anomalies indicating that they formed as marine chemical sediments in contrast to previous claims none of the samples shows any ce anomaly indicating the absence of oxidative ce rey decoupling and arguing against oxic conditions in the wider vicinity of the neoarchean temagami seabasin the distribution of zr hf and ta yields zr hf and hf ta ratios that differ from those of chondrites average upper continental crust and local shales suggesting that the temagami if is the only case observed so far in which a significant fraction of these elements is non detrital but sourced from seawater if neoarchean seawater was characterized by zr hf and hf ta ratios similar to those of modern seawater these ratios point towards preferential scavenging of hf over zr and ta as is typical of the modern ocean similar to the 2 9 ga old mozaan if in the pongola supergroup south africa the temagami if shows low th u ratios that differ from those of the respective local shales and from that of average upper continental crust decoupling of u and th results from u4 oxidation in the earth s surface system and fractionated th u ratios in these marine chemical sediments are therefore at odds with the lack of ce anomalies this suggests a different redox sensitivity of the two paleo redox proxies th u and ce rey and demonstrates that the temagami if and the mozaan if warrant further study of other paleo redox proxies

the hafnium neodymium isotopic composition of atlantic seawater the first full water column hafnium isotopic compositions of atlantic seawater have been obtained at seven locations from the labrador sea to the drake passage despite subpicomolar concentrations in seawater a precision of the hf isotopic measurements of 03 epsilon hf units was achieved an overall epsilon hf range between 3 1 in the labrador sea and 4 4 in antarctic bottom water was determined the distribution of which broadly reflects continental weathering inputs within particular water column profiles significant differences of up to 4 epsilon hf units occur combined with nd isotope data of the same samples it is evident that the hf isotopic composition of seawater is too radiogenic for a given nd isotopic composition and that the largest difference between expected and measured hf isotopic compositions in seawater occurs near the oldest continental crust in the labrador sea this corroborates the previous proposition which was mainly based on ferromanganese crust data that the hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly to some extent by hydrothermal contributions hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years which is significantly shorter than previously assumed the hf isotopic composition of past seawater can therefore serve as a proxy for short distance basin scale mixing processes and the regime and intensity of nearby continental weathering processes c 2009 elsevier b v all rights reserved

geological archive of the onset of plate tectonics plate tectonics involving a globally linked system of lateral motion of rigid surface plates is a characteristic feature of our planet but estimates of how long it has been the modus operandi of

the control of weathering processes on riverine and seawater hafnium isotope ratios hafnium 176 hf 177 hf isotope ratio variations in marine records are thought to reflect changes in continental weathering through time but the behavior of hf in rivers and during weathering is not well understood here we present 176 hf 177 hf data for rivers bedrock soils and leaching experiments for the moselle basin vosges france these data strongly suggest that the 176 hf 177 hf composition of river waters is controlled by prefer ential dissolution of accessory phases i e apatite sphene versus more resistant minerals e g k feldspar and linked to the intensity of silicate weathering estimates for the global isotopic composition of riverine hf suggest that the ocean hf budget may be dominated by river input and variations seen in marine records can be directly related to changes in silicate weathering intensity

new constraints on the sources and behavior of neodymium and hafnium in seawater from pacific ocean ferromanganese crusts the behavior of dissolved hf in the marine environment is not well understood due to the lack of direct seawater measurements of hf isotopes and the limited number of hf isotope time series obtained from ferromanganese crusts in order to place better constraints on input sources and develop further applications a combined nd hf isotope time series study of five pacific ferromanganese crusts was carried out the samples cover the past 38 myr and their locations range from sites at the margin of the ocean to remote areas sites from previously unstudied north and south pacific areas and water depths corresponding to deep and bottom waters for most of the samples a broad coupling of nd and hf isotopes is observed in the equatorial pacific end and ehf both decrease with water depth similarly end and ehf both increase from the south to the north pacific these data indicate that the hf isotopic composition is in general terms a suitable tracer for ocean circulation since inflow and progressive admixture of bottom water is clearly identifiable the time series data indicate that inputs and outputs have been balanced throughout much of the late cenozoic a simple box model can constrain the relative importance of potential input sources to the north pacific assuming steady state the model implies significant contributions of radiogenic nd and hf from young circum pacific arcs and a subordinate role of dust inputs from the asian continent for the dissolved nd and hf budget of the north pacific some changes in ocean circulation that are clearly recognizable in nd isotopes do not appear to be reflected by hf isotopic compositions at two locations within the pacific ocean a decoupling of nd and hf isotopes is found indicating limited potential for hf isotopes as a stand alone oceanographic tracer and providing evidence of additional local processes that govern the hf isotopic composition of deep water masses in the case of the southwest pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of antarctic glaciation

fault assisted vertical pluton growth coastal cordillera north chilean andes abstract extensive tracts of magmatic arc plutonic suites in the mesozoic coastal cordillera of northern chile imply that large volumes of granitic to dioritic magmas were transferred from a lower crustal source to the upper crust space for single intrusions was created by interaction of vertical pluton growth and dip slip reactivation of arc parallel faults during episodic emplacement of subhorizontal compositionally distinct magma pulses cross sectional pluton shapes are broadly tabular in geometry and were controlled by differential subsidence of the pluton floors during fault reactivation and incremental assembly of subhorizontal sheets equal amounts of floor subsidence of the fault footwall and hanging wall during magma accommodation led to symmetrical intrusions whereas differential subsidence caused plutons to grow asymmetrically strongly asymmetrical plutons resulted when only the hanging wall was reactivated to accommodate floor subsidence late post intrusion contractional deformation resulted in open folding of some tabular plutonic units and local inversion of bounding faults which has modified but not obscured the original syn emplacement geometry and fault kinematics

hafnium and neodymium isotopes in seawater and in ferromanganese crusts the element perspective abstract hafnium and nd isotopes are increasingly used as paleoceanographic proxies comparing the mantle crust array and the seawater array in plots of e hf vs e nd it has been observed that for a given e nd value the corresponding e hf value is higher for seawater than it is for terrestrial rocks while this difference had initially been explained by significant hydrothermal input of mantle hf into seawater the currently favoured explanation is incongruent weathering of continental rocks producing radiogenic riverine hf input we here address this topic from the perspective of the behaviour of these two elements in seawater and in ferromanganese fe mn crusts we distinguish between a truly dissolved and a dissolved hf and nd pool the latter being comprised of truly dissolved and colloid bound colloidal hf and nd while there exists a hydrothermal pathway for colloid bound dissolved mantle hf into the oceans there is in marked contrast to nd no important riverine pathway for colloidal or truly dissolved continental hf owing to their respective chemical speciation in seawater there exists truly dissolved nd in the ocean while the amount of truly dissolved hf is insignificant neodymium is in exchange equilibrium between local seawater and both the hydrous fe and mn oxides hydrogenetic fe mn crusts are composed of due to continuous ad and desorption there is continuous isotopic re equilibration and the isotopic composition of nd in a fe mn crust reflects that of truly dissolved nd in local ambient seawater in contrast hf is only associated with the hydrous fe oxides on which it forms surface precipitates that do not exchange with seawater due to this lack of isotopic re equilibration the isotopic composition of hf in a fe mn crust is the average of that of all the hf scavenged during the lifetime of the hydrous fe oxide particles since the hf bearing hydrous fe oxides in a fe mn crust do not form from local ambient seawater at the crust s growth site but are advected as colloids or fine particles their hf isotopic composition depends on the origin and migration pathway of these colloids hence while nd isotopes in fe mn crusts provide reliable information on truly dissolved nd in local ambient seawater hf isotopes rather indicate the origin and pathway of hydrous fe oxide colloids and might differ from truly dissolved hf in local ambient seawater this may explain the occasional decoupling of nd and hf isotopes in fe mn crusts and supports the notion of a significant hydrothermal mantle signal of hf in seawater

maximum link distance between strike slip faults observations and constraints the maximum fault link distance lateral separation between two interacting strike slip faults is studied in relation to fault length combined length of the two interacting faults data collected for laboratory generated strike slip faults indicate that fault linkage takes place when the lateral separation between two strike slip faults is less than one tenth of the combined length of the two faults strike slip faults developed in clay models require a smaller link distance than those developed in gouge materials about 98 of the data collected for natural strike slip faults and 93 of earthquake rupture data derived from knuepfer 1989 follow the rule these observations support a simple scaling relationship for strike slip faults i e w 0 1 l where wis lateral link distance and l the combined fault length an and sammis 1996a two possible explanations are discussed assuming a fault continues to be dominantly strike slip after various fault coalescences the fault link distance for both compressional coalescence leading to restraining bend and extensional coalescence leading to releasing bend can be constrained within 0 134 l assuming fault coalescence is caused by interaction between breakdown zones at fault tips then the scaling relationship b 0 1 l between the size of a breakdown zone b and the individual fault length l scholz et al 1993 leads to the scaling relationship w max 0 1 l where w max is the maximum link distance the above observed relationship between w max and l may not hold if faults coalesce through preexisting faults if a fault has a mixed mode other than strike slip or if a fault bend is caused by cross faulting rather than fault coalescence limited exposure of natural fault traces may also lead to errors in link distance measurements

the hf isotopic composition of ferromanganese nodules and crusts and hydrothermal manganese deposits implications for seawater hf we present hf and pb isotopic data and chemical compositions of the outermost layers of marine ferromanganese deposits of different types hydrogenous and hydrothermal with a worldwide distribution the hf isotopic compositions display a broad range and refine previously reported regional differences as follows atlantic ocean hf 4to 2 indian ocean hf 2to 4 pacific ocean hf 3to 10 the most radiogenic hf isotopic compositions in the pacific samples are for hydrothermal manganese deposits that also have low 207pb204pb demonstrating that this signature reflects a contribution from hydrothermal venting of hf leached from oceanic volcanic rocks rather than from riverine inputs volcanic ash or eolian dust hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing hf the hf and pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and morb the range in hf for ferromanganese crusts is narrower than it is for 206pb204pb compared to the differences in isotopic composition of the sources of hf and pb this is consistent with hf having a longer residence time than pb the concentration of hf in ferromanganese crusts has been found to co vary with growth rate and inversely correlates with hf isotopic compositions hf isotope ratios may be used to determine not only the source of hf but possibly the source of fe and mn measurements of hf and hf concentrations in nodule tops bottoms and associated sediments show that the hf of nodules is sensitive to sedimentary oxic and sub oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of hf in seawater

granite magma migration and emplacement along thrusts this paper investigates the influence exerted by brittle tectonic structures in the emplacement of granite plutons in contractional settings we address both cases where contractional tectonics and magma intrusion are 1 coeval to study how active contractional tectonics controls the transport of magma and 2 diachronous to study the role of pre existing structures on the transport of magma in light of new experimental models we show that magma can rise along thrusts ramps and flats this phenomenon occurs for both low viscosity magma basalts to andesite and high viscosity magma dry granite the experimental results also allow the evaluation of the role played by magma viscosity in determining pluton geometries in addition a review of literature demonstrates a spatial and causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the pluton final shape the abundance of subhorizontal and tabular granitic intrusions indicates that the location of inflating granitic sills along thrust flats can be common we argue that active and pre existing flats and ramps thrusts provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

the ratio of tellurium and selenium in geological material as a possible paleo redox proxy abstract only few data for both tellurium and selenium in different geological materials including international reference materials have been available so far in this study the te se ratio has been investigated in more detail to provide a better understanding of the correlation of these two elements in the geological environment nine international reference materials irms and 79 different geological samples spanning a wide range of formation times and conditions from the paleoproterozoic to present were analyzed with quadrupole dynamic reaction cell drc icp ms these samples include basalts shales river and beach sediments loess dolomites a recent cold water coral precambrian manganese and iron formations and cenozoic marine ferromanganese nodules and crusts a wide range of concentrations between 0 01 and 1 5 g g for se and 0 01 and 90 g g for te was measured and the most interesting feature was found to be the te se ratio in the different geological matrices while most rock and sediment samples fall within the range of te se ratios typical of the average earth s crust a significant enrichment of te over se is evident for modern marine mn fe oxide precipitates with the most pronounced te enrichment found for hydrogenetic ferromanganese crusts which form under fully oxidizing conditions the apparent positive trend of the degree of te se fractionation with the redox potential of the formation environment suggests that the te se ratio may represent a potential paleo proxy for redox environments

age and timing of igneous activity in the temagami greenstone belt ontario a preliminary report the southernmost remnants of archean supracrustal and intrusive rocks in eastern ontario are exposed through a window in the early proterozoic huronian supergroup near the town of temagami u pb zircon ages from this area indicate the presence of some of the oldest felsic magmatism so far discovered in this portion of the superior province the iceland lake pluton 2736 2 ma and a nearby rhyolite flow are contemporaneous which establishes that at least some of the intrusive rocks in the region are synvolcanic and coeval with the oldest volcanic cycle the youngest plutonic activity is the emplacement of a late rhyolite porphyry dike at 2687 2 ma an age that is bracketed by the 2675 2700 ma emplacement ages of late internal plutons found throughout the abitibi subprovince the 2736 ma dates however are older than the nearest portion of the exposed abitibi some 120 km to the north near kirkland lake

a warm or a cold early earth new insights from a 3 d climate carbon model abstract oxygen isotopes in marine cherts have been used to infer hot oceans during the archean with temperatures between 60 c 333 k and 80 c 353 k such climates are challenging for the early earth warmed by the faint young sun the interpretation of the data has therefore been controversial 1d climate modeling inferred that such hot climates would require very high levels of co2 2 6 bars previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the hadean and the archean here we revisit the climate and carbon cycle of the early earth at 3 8 ga using a 3d climate carbon model we find that co2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects however such high co2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts producing an efficient stabilizing feedback moreover the weathering of impact ejecta during the late heavy bombardment lhb would have strongly reduced the co2 partial pressure leading to cold climates and potentially snowball earth events after large impacts our results therefore favor cold or temperate climates with global mean temperatures between around 8 c 281 k and 30 c 303 k and with 0 1 0 36 bar of co2 for the late hadean and early archean finally our model suggests that the carbon cycle was efficient for preserving clement conditions on the early earth without necessarily requiring any other greenhouse gas or warming process

the neoarchaean storo supracrustal belt nuuk region southern west greenland an arc related basin with continent derived sedimentation we present new major and trace element data as well as sm nd isotope compositions for archaean supracrustal rocks and a gabbro anorthosite complex from the island of storo in godthabsfjord southern west greenland we also provide new u pb isotope data for zircon extracted from these rocks the storo rocks have experienced amphibolite facies metamorphism and the entire sequence forms an east dipping frontal thrust ramp in tectonic contact with mesoarchaean nuk gneisses to the west and eoarchaean itsaq gneisses to the east these orthogneisses record a complex regional accretion history as established by previous work the present study aims at explaining the geochemical features of the storo supracrustal belt ssb which comprises metavolcanic and metasedimentary rocks that are situated within this collisional zone the ssb has a maximum age of ca 2800 ma constrained by the youngest detrital zircon population found in a metasedimentary unit the minimum age of the ssb is 2707 8 ma defined by previously published re os isotope data the metavolcanic rocks have a tholeiitic basaltic composition with relatively flat primitive mantle normalised trace element patterns and generally negative nb anomalies nb nb 0 30 0 90 they plot above the mantle array in th yb nb yb space consistent with a subduction zone affinity as also proposed by previous studies a thin fault contact separates the ssb from a gabbro anorthosite complex formally defined here as the storo anorthosite complex which has an age of ca 3050 ma the sm nd isotope data of the ssb suggest that these metavolcanic rocks experienced contamination by a crustal source that was isotopically similar to the storo anorthosite complex sac this in turn suggests that the sac could have formed the basement for the younger volcanic sequence of the ssb or alternatively that the mantle source of the ssb was contaminated by melts derived from sac aged crust or sediments the metasedimentary rocks of ssb show a mixed mafic felsic component with variable degrees of maturity highly mature metasediments of the ssb contain several age populations of regionally well known magmatic events and thus support a significant local crustal provenance furthermore the youngest documented detrital zircon is found in a thin metasedimentary unit within the metavolcanic rocks and thereby shows that the ssb formed in close proximity to subaerially exposed continental crust i e a back arc environment and not a distal island arc setting as previously proposed finally this suggests that relatively cool lower continental crust which was capable of supporting subaerial mountains existed at least locally during the meso neoarchaean

paleobasinal to band scale ree y distribution in iron formations from carajas amazon craton brazil abstract banded iron formations bifs from the carajas mineral province cmp southeastern amazonian craton are the protolith of the world class iron deposits of the cmp besides being one of the largest iron ore deposits in the world they recorded important paleoenvironmental and regional geodynamic markers across the neoarchean paleoproterozoic transition we investigate the secular variation of geochemical signature from several deposit of carajas province the low contents of al2o3 and of 19 other immobile trace elements e g zr th hf ti in 26 high resolution detritus free bif samples indicate their detritus free nature their positive shale normalized subscript sn la and gd anomalies are similar to those of modern seawater whereas their y ho ratios are higher these bif samples also display large positive chondrite normalized subscript cn eu anomalies reflecting the influence of high temperature hydrothermal vents and bottom solutions in contrast to modern seawater and to most other neoarchean paleoproterozoic detritus free bifs the carajas bifs do not show the typical heavy rare earth element ree enrichment relative to light ree instead they do show variable light to heavy ree fractionation trends ybsn prsn ratios 0 05 4 91 the th u ratios of both bulk bifs and mesobands are generally lower than the upper crustal ratio although the bulk bifs show mostly chondritic values the individual fe and si bands exhibit distinctly higher ratios the general absence of negative cesn anomalies attests for rather reducing paleoenvironmental conditions unlike the great majority of the data which lack negative anomalies of ce a few samples do display such anomalies which may be compatible to the oxidizing conditions shown by the low th u ratios of most the carajas bifs such low th u ratios may register terrestrial weathering conditions which cause preferential u input into the oceans in response to the oxidation of immobile u4 to more mobile u6

chemical evolution of seawater in the transvaal ocean between 2426 ma ongeluk large igneous province and 2413 ma ago kalahari manganese field abstract drill core and outcrop samples of pure marine chemical sediments banded iron formation bif manganese formation mnf jaspilites lutites and cherts from the transition of the 2426 ma old ongeluk formation into the 2413 ma old hotazel formation transvaal supergroup south africa reveal remarkable changes of seawater chemistry in the transvaal ocean similar to pre ongeluk chemical sediments the shale normalized rare earths and yttrium reysn patterns of jaspilites intercalated with the volcanic rocks of the ongeluk large igneous province and directly overlying cherts do not show positive eusn anomalies indicating that high temperature 250 c hydrothermal fluids did not contribute significantly to the rey budget of ambient waters however a 10 cm drill core section in the lower hotazel formation is characterized by conspicuous positive eusn anomalies revealing temporary inflow of water masses strongly affected by high temperature hydrothermal fluids after this short episode the reysn pattern of transvaal seawater returned to that of pre ongeluk times showing heavy reysn enrichment positive lasn gdsn and ysn anomalies but no cesn or eusn anomalies higher up in the stratigraphy the hotazel formation shows negative cesn anomalies in some of the lutites bifs and mnfs reflecting ce depletion in ambient seawater all hotazel lutite bif and mnf samples studied show unradiogenic end t values 0 5 0 2 to 2 4 0 2 indicating a mostly continental rey source the rey distribution and nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved rey to local transvaal seawater precipitation of the hotazel lutites bifs and mnfs with negative cesn anomalies therefore suggests that oxic conditions prevailed on the kaapvaal craton and in hotazel seawater already at 2 413 ga i e 80 m y before the disappearance of mass independent sulfur isotope fractionation mif s that defines the great oxidation event at 2 33 ga

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

mechanics of active salt diapirism abstract an active diapir forcefully intrudes its overburden driven by diapir pressure that overcomes the resistance of the overburden strength possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion for a typical density contrast between salt and a sedimentary overburden a simple force balance demonstrates that the diapir height must be more than two thirds to three quarters the thickness of the surrounding overburden to initiate substantial active diapirism narrow diapirs must be even taller physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward curved reverse faults can separate the flaps from the regional overburden normal faults in the crestal graben propagate downward with new faults created farther outward as the roof arches over the rising diapir thin wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps in cases leading to emergence the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward the numerical models show that overburden flexure contributes to early formation of active diapir structures but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle a round cornered rectangle a semicircle to a triangle with a pointed crest

criteria helpful in recognizing shear zone and diapiric melanges examples from the hoh accretionary complex olympic peninsula washington a study of the middle miocene hoh melanges exposed on the western olympic peninsula washington provides criteria for differentiating between shear zone and diapiric melanges the shear zone melange is characterized by a consistent foliation orientation throughout the outcrop intense pervasive scaly foliation uniform fold vergence directions exotic clasts predominantly phacoidal clasts and consistent clast long axis lineations the diapiric melange is characterized by radial foliation in map view well developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange opposite fold vergence directions across the melange and rare exotic clasts additionally clasts vary in shape from phacoidal at the contacts to angular in the center in the diapiric melange large clasts are found only in the center of the melange and clast long axes vary from tightly clustered at the contacts to near random orientations in the center the systematic variation in clast size in the diapir may be due to increased shear at the margins and or clast migration to the center of the diapir caused by a velocity viscosity contrast interpretation of field data indicates that some clasts rotate continuously in the matrix yet clast long axes have a definite preferred orientation this contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane thus although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane parasitic folds in the matrix indicate that clast long axes align parallel to transport direction small scale folds in the matrix clast long axis lineations and the mean foliation indicate that the hogsback section of the hoh melange was emplaced by a high angle oblique thrust directed to n10 e

experimental studies of the controls of the geometry and evolution of salt diapirs abstract experimental models were conducted to study the controls of the evolution and geometry of salt diapirs and the transition from passive to active diapirism both constant and variable sedimentation rate experiments were conducted constant sedimentation rate experiments show that the evolution of diapirs is strongly dependent on the rates of sand deposition and the thickness of the source layer low rates of sedimentation result in high ratios of salt flow to aggradation leading to cylindrical diapirs which subsequently develop flared shapes with overhangs whereas high sediment rates result in low ratios of salt flow to aggradation resulting in tapered shapes and eventual eclipse and occlusion of the diapirs thick source layers result in higher salt flow rates than thin layers variable sedimentation rates result in changing shapes of diapirs over time a small increase in sedimentation rates for flared or cylindrical diapirs result in an initial eclipse followed by tapering and a transition to active diapirism enabling the diapir to pierce the overlying sediments the narrower diapir continues to grow by passive diapirism a large increase in rates results in a permanent eclipse because the tapered diapir is unable to penetrate the greater thickness of the overburden finally thicker source layers generally result in wider diapirs during both phases of deformation tapering and flaring are mechanisms of maintaining equilibrium between the rate of sedimentation and net salt flow rate the resulting models are directly applicable to the analysis of poorly or partially imaged salt diapirs in natural subsurface examples

influence of crystallised igneous intrusions on fault nucleation and reactivation during continental extension abstract continental rifting is commonly accommodated by the nucleation of normal faults slip on pre existing fault surfaces and or magmatic intrusion because crystallised igneous intrusions are pervasive in many rift basins and are commonly more competent i e higher shear strengths and young s moduli than the host rock it is theoretically plausible that they locally intersect and modify the mechanical properties of pre existing normal faults we illustrate the influence that crystallised igneous intrusions may have on fault reactivation using a conceptual model and observations from field and subsurface datasets our results show that igneous rocks may initially resist failure and promote the preferential reactivation of favourably oriented pre existing faults that are not spatially associated with solidified intrusions fault segments situated along strike from laterally restricted fault intrusion intersections may similarly be reactivated this spatial and temporal control on strain distribution may generate 1 supra intrusion folds in the hanging wall 2 new dip slip faults adjacent to the igneous body or 3 sub vertical oblique slip faults oriented parallel to the extension direction importantly stress accumulation within igneous intrusions may eventually initiate failure and further localise strain the results of our study have important implications for the structural of sedimentary basins and the subsurface migration of hydrocarbons and mineral bearing fluids

anoxic to suboxic mesoproterozoic ocean evidence from iron isotope and geochemistry of siderite in the banded iron formations from north qilian nw china abstract banded iron formations bifs provide important constraints on geochemical cycling of fe and ancient marine environment globally the formation of bifs peaked during late archean and early paleoproterozoic with limited reappearance during neoproterozoic here we investigate a rare example of mesoproterozoic ca 1 3 ga bifs from jingtieshan in the north qilian region of nw china we present rare earth element and yttrium ree y compositions together with iron isotope features of siderite from the bifs and integrate these with our published data on carbon isotope compositions from the siderite rich bifs including carbonate facies bifs and mixed carbonate oxide facies bifs siderite from the bifs shows ree y characteristics that are consistent with submarine hydrothermal fluids mixed with seawater including positive eu eu anomalies and high y ho ratios the siderite also displays homogeneous iron isotope compositions 56fe range from 0 71 to 0 41 one sample down to 1 62 and moderate negative carbon isotope compositions 13c values 8 40 to 3 0 these findings argue against the traditional view that siderite associated with bifs was formed through microbial dissimilatory iron reduction dir instead siderite from jingtieshan bifs is in equilibrium with submarine hydrothermal fluids and seawater suggesting an inorganic origin and was directly precipitated from the co2 and fe oversaturated water column with only insignificant role for dir these mesoproterozoic bifs are also characterized by ce anomalies and the siderite falls within the pe log s t and po2 pco2 stability fields suggesting that the bifs were deposited in an anoxic to suboxic ocean the results from our study suggest the presence of iron rich mesoproterozoic deep ocean in the north qilian area

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract the recognized worldwide gap in bif deposition between 2 4 and 2 0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time however the recently dated 2 3 2 2 ga yuanjiacun banded iron formation bif in the north china craton ncc offers a possibility to redress these uncertainties shale normalized rare earth element yttrium ree y patterns of the bif and interlayered meta chert samples show features characteristic of other archean and paleoproterozoic bifs with hree enrichment relative to lree positive la and eu anomalies and superchondritic y ho ratios comparable to modern seawater very low al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd t 2 4 which obtained its signature through weathering of the nearby landmasses and associated with high si fluxes our findings suggest that the ree budget of the oceans prior to 2 3 ga was generally dominated by hydrothermal circulation of seawater through depleted mantle derived source rocks however where evolved local continental crust and or an enriched mantle source were present this positive mantle nd signal became discernible in the bifs by comparing the nd isotopic features of similar aged bifs and marine carbonates it is concluded that similar to modern oceans the early precambrian ocean was not well mixed with respect to its nd isotopic composition

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

decoupled hf and nd isotopes in suspended particles and in the dissolved load of late archean seawater abstract it is generally agreed that decoupling of the hf and nd isotope systems in modern aqueous systems is a result of incongruent release of hf during terrestrial weathering of the continental crust although the mechanism s behind this process are not yet fully understood we here present hf nd isotope data for the neoarchean krivoy rog banded iron formation bifs ukraine and combine observations on modern aqueous environments with those of the early earth to further evaluate the mechanism s behind hf nd isotope decoupling in aqueous systems the pure late archean krivoy rog chemical sediment endmember representing the dissolved pool of ancient seawater shows decoupled end2 60ga ehf2 60ga values of 2 3 and 9 48 respectively and suggests that decoupled hf nd isotopes had been a global rather than a local phenomenon in neoarchean seawater this further reveals that incongruent hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by late archean time impure krivoy rog bif samples composed of a mixture of seawater derived and detrital nd and hf show systematically more positive end2 60ga values but still reveal decoupled end ehf values relative to an associated schist that plots slightly below the terrestrial array this suggests that mineral sorting between a zircon bearing sand sized fraction and fine grained sediment particles occurred on in late archean continents rivers and oceans and had significant impact on the chemical compositions of the suspended and dissolved element loads of late archean seawater less radiogenic hf isotope compositions in the krivoy rog seawater relative to detritus contaminated bifs further suggest a pathway for high temperature hydrothermal hf into anoxic archean seawater that diluted the even more radiogenic hf isotopic composition of continental run off created by the mineralogical composition of the continental hinterland and the zircon effect alternatively the less pronounced decoupling of ehf end in late archean seawater may be related to a shorter residence time of hf relative to nd furthermore systematically more positive initial end values in detritus contaminated archean bifs relative to respective dissolved seawater loads suggest that weathered and eroded material of ultra mafic rock suites had significant impact on the suspended and dissolved fractions in archean seawater

variscan granitoids related to shear zones and faults examples from the central sudetes bohemian massif and the middle odra fault zone the granitoid intrusions of the central sudetes cs and of the middle odra fault zone mofz ne part of the bohemian massif are both spatially and temporally related to large scale shear zones and faults including possible terrane boundaries that provided effective channels for melt migration summarizing common features of the cs and mofz granitoids we have delineated a set of characteristics of the fault related and shear zone related granitoids 1 they are mainly generated by partial melting of crustal sources with variable contribution or no contribution of mantle materials 2 the sheet like steeply inclined narrow and rather small granitoid intrusions are emplaced within shear zones at mid crustal level c 20 km depth whereas the larger flat lying plutons intrude into the upper crust outside or above these shear zones 3 the magmatic foliation and lineation in granitoids of the deeper sheet like intrusions are concordant with those in the surrounding metamorphic rocks suggesting that the solidification of granitoids was coeval with the deformation in the shear zones instead the magmatic foliation in the shallower and larger dome like plutons reflects magma flow 4 ductile transcurrent movements along the shear zones postdate medium pressure regional metamorphism and are accompanied by an increase in the local thermal gradient as documented by the crystallization of cordierite andalusite and sillimanite 5 the increase in the thermal gradient precedes the emplacement of granitoids and their concomitant thermal influence on the country rocks the granitoids related to the final stages of tectonothermal activity of the shear zones are good time markers of their evolutionary path

evidence from the oman ophiolite for sudden stress changes during melt injection at oceanic spreading centres the system of dikes in the uppermost mantle below oceanic spreading centres bears witness to the processes involved in the supply of melt to the ridge such systems cannot be studied on currently active ridges but can be seen in ophiolites in particular the oman ophiolite allows us to examine segments of a fast spreading ridge1 here we present a statistical analysis of the orientation of intrusions in the uppermost mantle section of the oman ophiolite spreading centres which points to the existence of two systems dikes whose azimuth is parallel to the sheeted dikes which we thus consider as related to the extensional lithospheric stress field and sills roughly parallel to the moho2 branching contemporaneous dikes and sills are frequently observed we suggest that these relations can be explained by repeated sudden changes of the dominant stress field from lithospheric to asthenospheric such changes could result from the episodic relaxation of lithospheric stress in response to tension fracturing of the lithospheric lid related to sudden melt surges this episodic behaviour is recorded in the sheeted dike complex with each dike each about one metre wide representing one of these fractures

neodymium isotopes in archean seawater and implications for the marine nd cycle in earth s early oceans abstract published neodymium nd isotopic data for archean iron formations if suggest that overall seawater throughout the archean typically displayed 143nd 144nd close to bulk earth values with nd t between 1 5 and 2 5 neodymium isotopic ratios in seawater during deposition of the 3 8 isua greenland if likely displayed positive nd 3 8 ga of 2 5 as suggested by if g an isua reference if that is considered the best archive for early archean seawater seawater 143nd 144nd ratios dominated by radiogenic nd positive nd t seem to have persisted for much of the archean as if from the pietersburg greenstone belt south africa suggest seawater nd 2 95 ga 1 however similarly aged 2 9 ga ifs from south africa indicate that significant variations in seawater 143nd 144nd occurred and clearly show influences from isotopically distinct crustal sources these variations are apparently related to depositional environment with cratonic margin shallow water ifs possessing a continental nd t of 3 while ifs associated with sub aqueous mafic volcanics display more radiogenic positive nd t values such variation in seawater 143nd 144nd is not possible in an isotopically well mixed ocean and similar to today it appears that marine nd cycling in the archean produced water masses with distinct nd isotopic ratios since the presence of banded iron formations requires a reducing archean ocean capable of transporting fe metal oxide precipitation and scavenging processes near deep sea hydrothermal vent systems would not have scavenged mantle nd i e nd sourced from alteration of oceanic crust we propose that bulk anoxic seawater prior to 2 7 ga possessed relatively constant positive nd t of 1 to 2 whereas local shallow water masses associated with exposed evolved crust could possess distinctly different lower nd t

hafnium and neodymium isotopes and rey distribution in the truly dissolved nanoparticulate colloidal and suspended loads of rivers in the amazon basin brazil abstract radiogenic isotopes in river sediments and river waters have been widely used in provenance studies as these samples naturally integrate the geology chemistry of the entire catchment while the hf and nd isotope systems are coupled during igneous processes they are decoupled during supergene processes at the earth s surface which is reflected by the isotope composition of riverine sediments we present the first data for both hf and nd isotope compositions of the dissolved 0 2 m filtrates rich in nanoparticles and colloids npcs and the truly dissolved 1 kda ultrafiltrates load of rivers hafnium and nd isotope compositions and concentrations of the rare earths and yttrium rey and hf were determined for suspended particles 0 2 m as well as for the dissolved and the truly dissolved load of the rio solimoes the amazon s largest tributary draining the andes and of the rio negro an organic npc and particle rich river draining the rainforest of northern amazonia we also analyzed the nd isotope compositions of suspended sediments and 0 2 m filtered water samples from the amazon river and its tributaries rio tapajos rio xingu and rio jari our novel results clearly show that the decoupling of the hf and nd isotope systems is related to incongruent weathering processes on the continent as this decoupling can already be observed in the different hf and nd pools i e in the particulate the npc dominated dissolved and the truly dissolved load of rivers in the rio negro and rio solimoes a strong particle size dependent difference in hf isotope composition is observed values of e hf become more radiogenic as filter poresize decreases which can be related to the density and size dependent distribution of hf rich minerals e g zircons and their absence from the truly dissolved pool in contrast the nd isotope composition of amazonian river waters reflects that of their catchment geology tributaries draining the precambrian brazilian and guyana shields show very unradiogenic e nd values of 19 to 25 in their dissolved load whereas the rio solimoes draining the andes yields a more radiogenic e nd signal of only 7 the dissolved nd isotope composition of the amazon is dominated by its andean tributaries and averages at 8 although nd isotope compositions are thought to not being fractionated by earth surface processes significant differences of 1 3 1 9 e units can be observed between the dissolved and suspended loads in the amazon river and its main tributary the rio solimoes in these rivers the dissolved load is more radiogenic than the suspended sediment which is likely due to incongruent weathering and related mineral sorting in the andean headwaters in contrast the organic rich and mineral poor shield rivers do not show a difference between the truly dissolved dissolved and suspended load as the nd in all these pools is controlled by surface and solution complexation and hence isotopically homogenized by continuous exchange and re equilibration

a case for late archaean continental emergence from thermal evolution models and hypsometry abstract the secular cooling of the earth s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans in the oceanic bathymetry and in the area of emerged continental crust the evolution of these variables is of fundamental importance to the geochemical coupling of mantle continental crust atmosphere and ocean to explore this further we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature continental area and hypsometry in this paper we investigate the continental freeboard predicted using different models for the cooling of the earth we show that constancy of the continental freeboard 200 m is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110 210 c hotter than present such numbers imply either a very limited cooling of the planet or most likely a change in continental freeboard since the archaean during the archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere thus limiting crustal thickening due to mountain building processes and the maximum elevation in the earth s topography rey p f coltice n neoarchean strengthening of the lithosphere and the coupling of the earth s geochemical reservoirs geology 36 635 638 2008 taking this into account we show that the continents were mostly flooded until the end of the archaean and that only 2 3 of the earth s area consisted of emerged continental crust by around 2 5 ga these results are consistent with widespread archaean submarine continental flood basalts and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from 2 5 ga the progressive emergence of the continents as shown by our models from the late archaean onward had major implications for the earth s environment particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the earth s deep and surface reservoirs

trace element and nd isotope composition of shallow seawater prior to the great oxidation event evidence from stromatolitic bioherms in the paleoproterozoic rooinekke and nelani formations south africa abstract pure stromatolitic limestones from the 2 44 to 2 43 ga old rooinekke formation transvaal supergroup south africa provide unique insight into the geochemistry of paleoproterozoic shallow seawater and therefore into the evolution of ambient redox levels of the earth s atmosphere hydrosphere system in contrast to the stromatolitic klipput dolostones from the overlying nelani iron formation if the stromatolitic rooinekke limestones do not show post depositional alteration and are virtually free of detrital contamination this makes them excellent geochemical archives of proxies of ambient shallow marine transvaal seawater shale normalized sn rare earth element and yttrium rey patterns of pure rooinekke limestones show seawater like rey distributions with positive lasn and gdsn anomalies super chondritic y ho ratios and a depletion of the light lrey relative to the heavy reysn hrey however the lack of negative cesn anomalies implies that redox conditions at the earth s surface were not oxidizing enough to stabilize significant amounts of ce iv in contrast to the majority of archean marine chemical sediments the pure early paleoproterozoic rooinekke limestones lack positive eusn anomalies suggesting that the fraction of rey derived from high temperature 250 c hydrothermal fluids in shallow rooinekke seawater was negligible the predominance of continental sources in the geochemical rey budget of rooinekke seawater is corroborated by unradiogenic end 2 44 ga values close to those of contemporaneous upper crustal material the rooinekke limestones therefore reveal the absence of a high temperature hydrothermal component from shallow seawater already at least 100 million years prior to the great oxidation event in contrast older chemical sediments from the transvaal supergroup such as the campbellrand carbonates and the penge and kuruman ifs show positive eusn anomalies and more radiogenic end t values indicating a significant contribution of mantle derived rey via high temperature hydrothermal fluids to the late neoarchean shallow marine rey inventory on the kaapvaal craton the declining impact of high temperature hydrothermal fluids on surface seawater that is recorded by the late neoarchean to paleoproterozoic shallow water transvaal carbonates is similar to the general trend of decreasing positive eusn anomalies with decreasing depositional age that is observed in eoarchean to proterozoic ifs which recorded the composition of deeper marine waters

decoupled hf nd isotopes in neoarchean seawater reveal weathering of emerged continents marine chemical sediments from the temagami banded iron formation bif in canada exhibit nonchondritic zr hf and y ho ratios and seawater like rare earth element patterns indicating that their hf and nd are not detrital but derived from seawater this is confirmed by sm nd and lu hf isochron ages of 2605 140 ma initial end 0 03 4 1 and 2760 120 ma initial ehf 7 2 5 3 respectively that overlap within error the 2 7 ga u pb age of associated igneous rocks the temagami bif is therefore an excellent archive of the nd hf isotopic composition of neoarchean seawater whereas graphic 1 values cluster around 1 graphic 2 values range from 6 7 to 24 1 substantially more radiogenic than those of ambient neoarchean mantle and continental crust such an ehf end distribution is typical of modern seawater plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks the only mechanism known to produce natural waters with decoupled nd and hf isotope compositions is the incongruent mobilization of hf from continental crustal material therefore input of such highly radiogenic hf into seawater requires substantial amounts of evolved neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

in defense of magmatic diapirs abstract diapirism as a crustal magma ascent mechanism has been recently criticized we contend that this reflects an overly simplistic view that diapirs must resemble modeled hot stokes diapirs and the perception that magma ascent in dikes is a more problem free mechanism for the construction of plutons we describe four cordilleran plutons that have characteristics much more compatible with diapirs than dike fed chambers these plutons were emplaced at depths ranging from 10 to 30 km and record different parts of diapiric ascent paths most ascended during complex visco elastic flow of host rock during regional deformation have narrow structural aureoles indicating power law behavior of host rock and were constructed of multiple batches of magma attributes enabling them to ascend greater distances than single hot stokes diapirs some features of these plutons are not typically attributed to diapirs and thus we introduce the term visco elastic diapir for bodies consisting of one or more batches of magma rising together with length to width ratios

magma emplacement during arc perpendicular shortening an example from the cascades crystalline core washington it is increasingly inferred by others that active arcs at all crustal levels are undergoing arc perpendicular extension and that magma utilizes this extension or extension along arc parallel strike slip faults during emplacement however a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc perpendicular shortening without the aid of strike slip faulting this conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens evaluation of host rock material transfer processes that operated during emplacement of the 93 ma 10 to 14 km deep mount stuart batholith washington provides one example and includes the following 1 ductile downward flow in a narrow 0 2 body radii structural aureole 2 stoping and of lesser importance 3 regional folding thrusting and subhorizontal arc parallel extension we see no evidence that arc perpendicular extension or local extension along strike slip faults made space for this pluton nor for most other plutons in contractional arcs instead we argue that both localized and rapid around plutons and regional and slower downward flow during growth of crustal roots is a common process in such arcs

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

linking mineral deposit models to quantitative risk analysis and decision making in exploration this paper describes a methodology of translating mineral deposit models into flexible probabilistic structures based on 1 extraction of ore components fluids metals and ligands from crustal or mantle sources or both 2 fluid or melt assisted transport of ore components from source to trap zones 3 formation of trap zones i e effective melt or fluid channels that can focus melt or fluid migration and accommodate large amounts of metal and 4 operation of the physicochemical processes that promote and sustain the deposition of metal from fluids or melts passing through a particular trap site our approach integrates these critical mineralization processes and conditions with concepts of probability theory decision analysis and financial modeling the principal objective is to make mineral deposit models amenable to financial risk and value analysis and suitable for communication of value creating geologic concepts to financial stakeholders in economic terms a case study based on an actual porphyry copper project illustrates how the resulting probabilistic mineral systems model can generate a measure of the probability of ore occurrence as an input for exploration decision trees and simulations to calculate the expected value of an exploration project and the probability distribution of all possible surrounding net present values npvs within a minimum and maximum range formulation of the probabilistic model closely follows and combines principles of the well established petroleum and mineral systems approaches and makes use of excel based model templates with decision tree and simulation add in software packages

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

decoupling of neoarchean sulfur sources recorded in algoma type banded iron formation abstract neoarchean algoma type banded iron formations bifs are widely viewed as direct chemical precipitates from proximal volcanic hydrothermal vents however a systematic multiple sulfur isotope study of oxide facies bif from a type locality in the ca 2 74 ga temagami greenstone belt reveals mainly bacterial turnover of atmospheric elemental sulfur in the host basin rather than deposition of hydrothermally cycled seawater sulfate or sulfur from direct volcanic input trace amounts of chromium reducible sulfur that were extracted for quadruple sulfur isotope 32s 33s 34s 36s analysis record the previously known mass independent fractionation of volcanic so2 in the archean atmosphere s mif and biological sulfur cycling but only minor contributions from juvenile sulfur despite the proximity of volcanic sources we show that the dominant bacterial metabolisms were iron reduction and sulfur disproportionation and not sulfate reduction consistent with limited availability of organic matter and the abundant ferric iron deposited as fe oh 3 that sulfur contained in the bif was not a direct volcanic hydrothermal input as expected changes the view of an important archive of the neoarchean sulfur cycle in which the available sulfur pools were strongly decoupled and only species produced photochemically under anoxic atmospheric conditions were deposited in the bif forming environment

the shaping of salt diapirs abstract six parameters shape the geometry of passive diapirs associated with stiff overburden rates of salt supply s dissolution d sediment accumulation a erosion er extension e and shortening sh these parameters change in space and time and hence influence the geometry of the structure as it forms a complex six parameter plot representing the evolution history of a salt diapir can be simplified into three separate graphs this study recommends adding to plots of s against a a second of rate of salt supply s this time with extension rate e and a third of rate of sediment accumulation a plotted against extension rate e integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three dimensional data however if applied to a profile the plot shows only the two dimensional evolution history of the diapir lateral forces extension or compression have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies by incorporating extension in the moulding plots of salt diapirs this study introduces the rate of sediment accumulation multiplied by extension e a as a significant factor in moulding salt diapirs by using this rate against the rate of salt supply this study redefines the conventionally accepted interpretation of upward narrowing upward widening and columnar diapirs upward narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension upward widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension this new relationship explains the absence of columnar and upward widening diapirs in passive margins where thin skinned extension dominates and emphasizes the significance of lateral movement extension and shortening in moulding the geometry of salt diapirs

eocene shoshonitic mafic dykes intruding the monashee complex british columbia a petrogenetic relationship with the kamloops group volcanic sequence the newly named three valley suite tvs kersantite lamprophyre to shoshonitic mafic dykes of the monashee complex are inferred to be hypabyssal feeder dykes to an alkaline to calc alkaline volcanic suite related to the kamloops group these dykes were emplaced in a subvertical north trending orientation coincident with inferred eocene crustal extension 50 0 ma based on the flat 40ar 39ar step heating plateau of contact metamorphic muscovite on the margin of a tvs dyke these weakly altered mafic dykes are fine grained with phenocrysts 0 5 2 0 mm of phlogopite augite amphibole and olivine pseudomorphed by clays rare labradorite and both primary and secondary carbonates set in a fine grained groundmass of similar mineralogy consistent with their classification as plagioclase bearing potassic diorite to kersantite lamprophyre the dykes are weakly silica undersaturated and alkalic 2 8 wt k2o 7 7 wt mgo with high large ion lithophile element contents 300 times primitive mantle and ele

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

submarine hydrothermal venting related to volcanism in the lesser antilles evidence from ferromanganese precipitates 1 radiogenic isotope compositions sr nd pb hf and os of sediment hosted seafloor ferromanganese crusts and sediments incrusted with ferromanganese oxyhydroxides from the lesser antilles island arc were measured to distinguish between hydrogenous seawater derived and hydrothermal metal sources the ages of the precipitates range between recent last few thousand years and a few 100 kyr as deduced from 10be and co concentrations evidence from the presence of bladed todorokite and nontronite together with the major element and ree composition suggests that a significant proportion of these sediment hosted precipitates formed at relatively low temperatures from a mixture of seawater and hydrothermal fluids associated with island arc volcanism the radiogenic isotope compositions of all metals mentioned above except pb show large differences in hydrothermal versus hydrogenous contributions over space and time in contrast to precipitates of high temperature fluids which mainly scavenge their ree contents from seawater the crusts of this study show 143nd 144nd of up to 0 512817 nd 3 5 this is close to the signature of the nearby island arc rocks and far above the expected local seawater ratio of 0 51209 nd 10 7 these crusts also show high 176hf 177hf up to 0 283102 low 87sr 86sr up to 0 7069 and low 187os 188os up to 0 16 compared with local seawater as expected from hydrothermal island arc derived metal contributions in contrast the pb isotope signatures of the crusts cannot be explained by mixing between seawater and hydrothermal sources it is suggested that pb was either removed from the ascending fluids within the sediment column before they reached seawater or the temperatures were too low to leach significant amounts of pb from the rocks or sediments external sources such as saharan dust particulate inputs from the orinoco river or even incongruent release of pb isotopes from the island arc rock derived particles must have contributed to the observed pb isotope variability our results suggest that submarine hydrothermalism originating from intraoceanic island arc volcanism creates distinct geochemical environments for the dispersion of hydrothermal fluids and may be an important mechanism to supply metals of hydrothermal origin to seawater

erosion of archean continents the sm nd and lu hf isotopic record of barberton sedimentary rocks abstract knowing the composition nature and amount of crust at the surface of the early earth is crucial to understanding the early geodynamics of our planet yet our knowledge of the hadean archean crust is far from complete limited by the poor preservation of archean terranes and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants to address this problem and get a more comprehensive view of what an archean continent may have looked like we investigated the trace element and sm nd lu hf isotopic records of archean metasedimentary rocks from south africa we focused our study on sandstone and mudstone from drill core in the fig tree group 3 23 3 26 ga of the barberton granite greenstone belt but also analyzed the 3 4 ga buck reef cherts and still older 3 5 3 6 ga meta igneous rocks from the ancient gneiss complex swaziland based on principal component analysis of major and trace element data the fig tree metasedimentary rocks can be classified into three groups crustal detritus rich sediments si rich sediments and ca fe rich sediments the detritus rich sediments have preserved the sm nd and lu hf isotopic signatures of their continental sources and hence can be used to constrain the composition of crust eroded in the barberton area in the paleoarchean period based on sm nd ratios we estimate that this crust was more mafic than today with an average sio 2 content of 60 5 2 wt this composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic ultramafic and felsic rocks this implies that the archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth neodymium and hf crustal residence ages show that the eroded crust was on average 300 400 ma older than the deposition age of the sediments which highlights the importance of intracrustal reworking of older crust at 3 2 ga in the barberton area the si rich sediments have slightly positive e nd t 3 23ga but extremely radiogenic e hf t 3 23ga up to 11 based on analyses of 3 4 ga buck reef cherts we suggest that the radiogenic hf isotopic signature of the si rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks the latter have extremely high lu hf ratios such that their e hf values would increase dramatically by about 100 epsilon units every 100 ma in the ca fe rich sediments one sample contains carbonate that preserves the typical rare earth element features of seawater precipitates the initial nd isotopic composition of this sample e nd t 3 23ga 1 7 is within the range of previous estimates for archean anoxic seawater

carbonates and cherts as archives of seawater chemistry and habitability on a carbonate platform 3 35 ga ago insights from sm nd dating and trace element analysis from the strelley pool formation western australia abstract carbonates and cherts in the 3 35 billion year old strelley pool formation fm australia host stromatolites that are among the oldest remnants of life on earth however it is still not entirely clear whether these mineral phases are authigenic precipitates and whether they represent reliable geochemical archives of early earth environments here we present major trace element and nd isotope data of stromatolitic carbonates associated crystal fan carbonates and cherts in the strelley pool fm i to assess the reliability of these chemical sediments as geochemical archives of the fluids from which they precipitated ii to date the time of formation of carbonate and silica phases and iii to trace the sources of elements prevailing in microbial habitats 3 35 ga ago stromatolitic carbonates plot together with the stratigraphically underlying marble bar cherts on a sm nd regression line yielding 3253 320 ma in contrast associated crystal fan carbonates together with altered marble bar cherts yield 2718 220 ma suggesting that their sm nd isotope system was reset after deposition both types of carbonates as well as white cherts show shale normalized rare earth element and yttrium patterns reysn with the exception of redox sensitive ce and eu and heavy reysn to middle reysn depletion that are parallel to those of modern seawater indicating a predominantly seawater derived origin positive eusn anomalies 2 1 2 4 combined with heterogeneous nd3 35ga values between 3 2 and 5 8 within individual alternating stromatolite laminae further support that the dissolved fraction of seawater on the ancient carbonate platform was variably affected by both continental weathering and high temperature hydrothermal fluids contributing elements of both young mafic or older felsic rocks in conclusion trace element and nd isotope data presented here match well with the depositional environment as characterized based on lithological geochemical and stratigraphic relationships on an early continent showing at least episodic emergence above the sea level supporting microbial life on a shallow marine platform

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

changes of ge si ree y and smnd isotopes in alternating fe and si rich mesobands reveal source heterogeneity of the 2 54 ga sijiaying banded iron formation in eastern hebei china abstract the north china craton ncc is one of the most important regions hosting abundant banded iron formations bifs the 2 54 ga sijiaying bif the best preserved and most extensive deposit in eastern hebei is intercalated and closely associated with meta volcanic rocks of the luanxian group in this context major and trace element and sm nd isotopic analyses of individual mesobands of a sijiaying bif specimen were conducted to characterize the source and depositional environment over a transient period low al 2 o 3 tio 2 and high field strength elements hfses concentrations show that the bif is relatively detritus free shale normalized rare earth and yttrium distributions ree y of individual bif mesobands exhibit positive la anomalies enrichment in hree relative to lree and mree and suprachondritic y ho ratios which are typical features of marine waters throughout the archean and proterozoic the presence of consistently positive eu anomalies indicates a significant high t hydrothermal input while the absence of true ce anomalies suggests deposition from an anoxic water column by comparison with other typical bifs e g the isua bif from greenland the kuruman bif from south africa the sijiaying bif is depleted in hree and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity these features coupled with sm nd isotopic relations and ge si distributional patterns point to two decoupled sources controlling the depositional environment of the bif and thus reveal source heterogeneity for silica and iron of the sijiaying bif high fe fluxes were associated with seafloor vented hydrothermal fluids which received their sm nd isotopic signature from alteration of depleted oceanic crust whereas significant amounts of silica were associated with ambient seawater whose ree signature was controlled by solutes derived from weathering of nearby mesoarchean continental landmasses the old up to 3 0 ga nd t dm model ages of si rich mesobands of the bif support such a scenario

meso and neoarchean banded iron formations and genesis of high grade magnetite ores in the anshan benxi area north china craton a reply