contrasting arc and morb like assemblages in the paleoproterozoic flin flon belt manitoba and the role of intra arc extension in localizing volcanic hosted massive sulphide deposits the flin flon belt trans hudson orogen manitoba and saskatchewan is the largest paleoproterozoic volcanic hosted massive sulphide vms district in the world with 118 7 million tonnes mt of z

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

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota the loss of fe from some pre 2 2 ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2 2 ga i have examined the behavior of fe in both pre and post 2 2 ga paleosols from depth profiles of fe 3 ti fe 2 ti and fe ti ratios and fe 3 ti vs fe 2 ti plots this new approach reveals a previously unrecognized history of paleosols essentially all paleosols regardless of age retain some characteristics of soils formed under an oxic atmosphere such as increased fe 3 ti ratios from their parental rocks the minimum oxygen pressure p o 2 for the 3 0 2 2 ga atmosphere is calculated to be about 1 5 of the present atmospheric level which is the same as that for the post 1 9 ga atmosphere the loss of fe common in paleosol sections of all ages was not due to a reducing atmosphere but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere this reductive dissolution of ferric hydroxides occurred either 1 after soil formation by hydrothermal fluids or 2 during and or after soil formation by organic acids generated from the decay of terrestrial organic matter terrestrial biomass on the early continents may have been more extensive than previously recognized

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

apatite weathering and the phanerozoic phosphorus cycle despite widespread debate in the literature there is still considerable uncertainty concerning which nutrient ultimately controls marine net ecosystem production nep over geologic time geochemical arguments suggest that phosphorus is the culprit the weathering of apatite the primary phosphorus sink in earth s exosphere controls long term phosphorus availability if phosphorus is the ultimate controlling nutrient over geologic time scales then long term marine nep is coupled to the release of phosphorus from apatite weathering the most abundant apatite compositions found in nature are igneous fluorapatite and marine sedimentary carbonate fluorapatite sparse data exist on how these compositions dissolve under earth s surface conditions to demonstrate a need for these data and their application we present a kinetic treatment of existing data augmented by new results we then use these results in a weathering model designed to illustrate the control exerted by temperature via activation energy and surface area on the phosphorus flux from apatite dissolution during the phanerozoic our conclusion is that activation energy and hence temperature and apatite surface area are important parameters governing the phosphorus flux from apatite weathering and therefore marine nep during phanerozoic time

tropical laterites life on land and the history of atmospheric oxygen in the paleoproterozoic the ca 2 2 ga hekpoort paleosol of the transvaal supergroup in southern africa has been considered a type example and the youngest iron depleted paleosol formed under a reducing atmosphere in the early precambrian however the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron depleted pallid lower zone and an iron enriched lateritic upper zone previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds the laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass revised stratigraphic correlation indicates that the hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of wolhaarkop in griqualand west this information indicates that the oxygen evolution curve based on loss or retention of iron in paleosols should be reexamined

relationships between clay mineralogy hydrothermal metamorphism and topography in a western cascades watershed oregon usa this study investigates variation in clay mineralogy and its relation to hydrothermal metamorphism hillslope processes and topography in the western cascade mountains the study area is the drainage basin of dorena lake a medium sized 686 km2 watershed located near cottage grove or the bohemia mining district is on the southeastern rim of the watershed in a hydrothermally metamorphosed region associated with a set of granodiorite plutons to characterize large scale patterns of clay mineral distribution within the watershed suspended sediments were collected from 43 stream locations samples of several metamorphosed and unaffected volcanic and volcaniclastic rocks were collected to help clarify metamorphic reaction processes one active earthflow was also sampled x ray diffraction methods were used to determine the mineralogy of the clay sized 2 m fraction of the samples clay mineralogy varies systematically across the watershed and the three major stream tributaries carry sediment with distinct mineralogical signatures discrete minerals include kaolinite smectite chlorite and illite interstratified kaolinite smectite and chlorite vermiculite cv are also present the active earthflow and unmetamorphosed rock samples primarily contain smectite in contrast metamorphosed rock samples are composed of some combination of illite interstratified illite smectite cv and chlorite examination of clay mineral distribution reveals the effects of hydrothermal metamorphism in the mining district on clay mineralogy hillslope processes and landscape development compared with most of the watershed the mining district has steeper slopes and higher elevations and lacks smectite almost entirely analyses of metamorphosed bedrock units indicate that smectite originally present in the rocks was converted to nonexpandable clay minerals during metamorphism induration of bedrock and loss of expandable clays resulted in thin soils and steep topography debris slides and flows are the dominant mass movement processes in this area earthflows are noticeably absent probably because thick smectitic soils are lacking elsewhere in the watershed where smectite is abundant slopes are more gentle and both shallow and deep seated mass movements are common the clay mineralogy of bedrock and soils is thus integrally tied to the hillslope processes operating within the watershed and therefore affects the geomorphology of the landscape high relief hydrothermally metamorphosed volcanic centers surrounded by weaker rocks in adjacent lowlands are common features in the western cascade mountains these massifs were likely formed by differential erosion processes similar to those operating in the dorena lake watershed

timing constraints of orogeny to cratonization thermochronology of the paleoproterozoic trans hudson orogen manitoba and saskatchewan canada abstract a comprehensive thermochronometric investigation across a 600 km transect of the paleoproterozoic trans hudson orogen tho in canada has resolved two first order thermal events 1 widespread peak metamorphism at 1 82 1 79 ga associated with terrane amalgamation and 2 a late collisional metamorphic episode at ca 1 77 ga that is characterized by significant strike slip deformation and rapid cooling geophysical observations of the lithosphere beneath the tho reveal crust that is markedly different on either side of the crustal scale tabbernor sturgeon weir tsw strike slip fault system which offsets lower crustal reflectors the tho east of the fault system records u pb titanite and total pb monazite domain ages that are relatively indistinguishable from 40ar 39ar hornblende and mica cooling ages indicating rapid cooling and perhaps exhumation following 1 77 ga metamorphism seismic imaging indicates the eastern orogen is an irregular package of stacked crustal blocks and our thermochronometric data demonstrate it behaved as a single coherent element shortly 20 30 m y following cessation of convergence in contrast rocks located west of the tsw fault system cooled distinctly later preserving 40ar 39ar muscovite and biotite ages between 1 74 and 1 72 ga this crustal architecture delineated from seismic surveys illustrates a more homogeneous slightly 5 km thinner crust in the case of this orogen apparent stabilization and cratonization occurred ca 100 m y after peak terrane amalgamation and notably the orogen was likely cratonized as two blocks east and west of the tsw fault system

characteristics of surface current flow inferred from a global ocean current data set a seasonal global ocean current data set ocds digitized on a 5 sup 0 grid from long term mean shipdrift derived currents from pilot charts is presented and described annual zonal means of v component currents show subtropical convergence zones which moved closest to the equator during the respective winters in each hemisphere net annual v component surface flow at the equator is northward zonally average u component currents have greatest seasonal variance in the tropics with strongest westward currents in the winter hemisphere an ensemble of ocean currents measured by buoys and current meters compares favorably with ocds data in spite of widely varying time and space scales the ocds currents and directly measured currents are about twice as large as computed geostrophic currents an analysis of equatorial pacific currents suggests that dynamic topography and sea level change indicative of the geostrophic flow component cannot be relied on solely to infer absolute strength of surface currents which include a strong ekman component comparison of ocds v component currents and meridional transports predicted by ekman theory shows agreement in the sign of transports in the midlatitudes and tropics in both hemispheres ekman depths required to scale ocds v component currents to computed ekman transports are reasonable at most latitudes withmore layer depths deepening closer to the equator less

serpentinized troctolites exposed near the kairei hydrothermal field central indian ridge insights into the origin of the kairei hydrothermal fluid supporting a unique microbial ecosystem the vent fluids of the kairei hydrothermal field khf on the central indian ridge near the rodriguez triple junction are atypical they have a very high h2 concentration a relatively high si concentration and a remarkably low ch4 h2 ratio recently particular attention has been paid to the khf because the hydrothermal fluids are suggested to support a hydrogen based hyperthermophilic subsurface lithoautotrophic microbial ecosystem hyperslime which is considered to be a likely modern analogue for the early earth ecosystems prior to photosynthesis despite the increasing interest in the fluid chemistry and associated biota the origin of the unusual chemistry of the hydrothermal fluids is still uncertain here we suggest interaction of seawater with troctolite recently discovered from small hills near the khf provide a possible explanation for the composition of the khf fluids dives with the manned submersible shinkai 6500 recovered plagioclase dunite troctolites and olivine gabbros which generally constitute the lower oceanic crust microscopic observations revealed that olivine crystals in the samples were partly to completely replaced by serpentine and magnetite indicating the generation of h2 by serpentinization reactions between olivine and hydrothermal fluids theoretical model calculations predict that the high h2 and high si concentrations of the hydrothermal fluids can be attributed to serpentinization of the troctolites and subsequent hydrothermal reactions with basaltic wall rocks under the khf the unique geological setting of the kairei hydrothermal system where deep crustal rocks are emplaced in the shallower part of oceanic crust is considered to be responsible for the unusual chemistry of the kairei hydrothermal fluids driving the occurrence of the hyperslime

a zonally symmetric model for volcanic influence upon atmospheric circulation we consider the effects of volcanic activity upon zonal wind flow in a model atmosphere a low latitude volcanic eruption could lower the tropospheric pole to equator temperature difference and thereby affect the atmospheric motions when the temperature contrast decreases the zonal wind velocities at high altitudes are reduced to conserve angular momentum the velocities in the lower atmosphere near the surface must increase thus providing a momentum source for ocean currents we suggest that this momentum source may have played a role as a trigger for inducing the 1982 83 anomalous el nino and possibly other climate changes

mass balance reconstruction of a modern vertisol implications for interpreting the geochemistry and burial alteration of paleo vertisols abstract utilizing identical sampling and analytical techniques the morphological and chemical characteristics of a modern vertisol houston black series central texas can be directly compared with an upper mississippian paleo vertisol from the appalachian basin pennington formation east central tennessee mass balance reconstructions suggest retention of primary pedochemical patterns in the paleo vertisol including patterns of soil volume change strain and transport functions translocations of many major and trace elements retention of primary pedochemical patterns suggests that vertisols constitute nearly closed systems during burial diagenesis chemical and mineralogical changes associated with burial diagenesis of the paleo vertisol include oxidation of organic carbon oc illitization of smectites dehydration and recrystallization of fe mn oxyhydroxides and dolomitization of pedogenic calcite significant differences in the chemical behavior of gilgai microhigh and microlow pedons in modern vertisols have implications for interpretation of geochemical data obtained from paleo vertisols overall wetter soil conditions and variable redox potential under gilgai microlows promote greater depths of leaching and mobility of redox sensitive trace elements including co cr cu mn ni and v gilgai microhighs behave as evaporative wicks that draw moisture and soluble phases towards the soil surface resulting in precipitation of metal hydrosylate complexes and sulfates gypsum at the capillary fringe and shallower depths of leaching and fixation of trace elements paleoprecipitation estimates from paleosols based on the depth to the top of the pedogenic carbonate horizon should therefore utilize field petrographic and geochemical data for characterizing maximum depths of leaching loss or gain of exchangeable bases and calcification rather than relying solely upon field data

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

the possible link between net surface heating and el nino three diagnostic analyses are described which strongly suggest the importance of local net surface heating in the life history of the large scale air sea phenomenon centered in the eastern tropical pacific ocean commonly called el nino these analyses rely upon monthly marine weather summaries for the period 1957 to 1976 in the first correlations and coherence spectra were calculated which show a strong link between the net surface heat flux and sea surface temperature variations over the eastern equatorial pacific the second analysis also based upon the use of coherence spectra indicates a sea temperature precursor in the eastern ocean near 25 degrees s which precedes sea temperature changes near the el nino focus near 5 degrees s since the link between the two regions would require ocean advective velocities that appear to be unreasonably large this analysis also suggests the importance of atmospheric forcing through the surface heat flux in the third analysis a proxy variable is described that seems to be a reasonable indicator of the overall effect of ocean dynamics on the temperature of the el nino core region a composite analysis of the four el nino events of 1957 1965 1972 and 1976 suggests that local surface heating is important during the early part of an event whereas dynamical factors dominate later

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota comment and reply the major points of disagreement between holland and rye and ohmoto concern 1 the behavior of ferric iron 2 the mechanisms of fe loss 3 the identification of real paleosols and 4 the ages of some paleosols

the sturgeon falls paleosol and the composition of the atmosphere 1 1 ga bp a paleosol is exposed along the north bank of the sturgeon river some 25 km sw of baraga michigan the paleosol was developed on hydrothermally altered keweenawan basalt and is overlain by the jacobsville sandstone textures mineralogy and chemical composition change gradually upwards from unweathered metabasalt through the paleosol to the contact of the paleosol with the jacobsville sandstone many of these changes are similar to those in modern soils developed on basaltic rocks however k has clearly been added to the paleosol probably by solutions which had equilibrated with k feldspar in the jacobsville sandstone the keweenawan basalt was oxidized quite extensively during its conversion to greenstone during weathering the remaining fe2 was oxidized to fe3 and was retained in the paleosol the composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the o2 pressure to the co2 pressure in the atmosphere during the formation of the paleosol formula see text free o2 must have been present in the atmosphere 1 1 ga ago but its partial pressure could have been 10 3 times lower than in the atmosphere today

geochemical studies in the proterozoic metamorphic terrane of the guaxupe massif minas gerais brazil a discussion on large ion lithophile element fractionation during high grade metamorphism abstract the trace element composition of high grade metamorphic rocks in the guaxupe massif is investigated the large ion lithophile element composition is compared in both the granulite facies rocks and the associated amphibolite facies gneisses the k rb th contents in the granulite facies rocks from other parts of brazil show that the depletion of these elements is variable the covariance of k rb ratios with k suggest the importance of mineralogical control protolith composition nature of fluid etc the distribution of u and th appears to indicate the protolith composition of the rock types

pedogenic translocation of fe in modern and ancient vertisols and implications for interpretations of the hekpoort paleosol 2 25 ga abstract many interpretations of oxygen levels in the precambrian atmosphere use interpretations of fe distributions measured in paleosols this article addresses the current lack of knowledge concerning baseline fe translocations in modern soil analogs for the 2 25 ga hekpoort paleosol at waterval onder south africa as well as the lack of understanding of diagnostic petrographic features related to fe translocation preserved in the precambrian paleosols petrographic features related to fe translocations include redox related fe depletions and enrichments and possibly fe mn nodules which compare favorably with similar pedogenic features in paleozoic paleo vertisols and modern analog vertisols additionally preservation of sepic plasmic microfabrics in the hekpoort paleosol which are characteristic of clay soils experiencing shrink swell supports previous paleo vertisol interpretations total fe losses in vertisols are 10 50 losses increase with increasing soil age and with increasing mean ann

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

evidence of anoxic to oxic atmospheric change during 2 45 2 22 ga from lower and upper sub huronian paleosols canada paleoweathered zones in four underground drill holes in quirke ii mine and one drill hole by denison mines in elliot lake area ontario canada were studied for petrography mineralogy and whole rock chemistry these paleosols formed beneath 2 45 ga old lower huronian formations are characterized by upward depletion of fetotal and fe3 in the whole rock by upward depletion of fetotal fe mg ratios in chlorites and fine grained muscovite and by the presence of fe2 minerals e g pyrite pyrrhotite ilmenite in contrast the paleosols underlying upper huronian strata 2 22 ga old exhibit an upward increase in fe3 and fetotal and contain hematite and goethite uraninite and pyrite bearing paleoplacers occur in lower huronian strata whereas the upper huronian formations contain red beds and are devoid of detrital uraninite and pyrite these differences between the sub huronian paleoweathering profiles and mineralogical characteristics of the lower huronian strata overlying the paleosols support the previously postulated oxyatmoversion theory and restricts the time interval for its occurrence to between 2 45 and 2 22 ga assuming that few tens of millions of years were required for the deposition of lower huronian groups oxyatmoversion likely occurred at about 2 4 ga

early proterozoic deformation along the southern margin of the kisseynew gneiss belt trans hudson orogen a 30 ma progressive deformation cycle fold axes and lineations across the southern margin of the kisseynew gneiss belt were progressively reoriented from northwesterly to northeasterly during an early proterozoic southwest directed compressive deformation cycle partitioning of deformation resulted in a concentration of non coaxial strain along the boundary between the kisseynew gneiss belt and flin flon snow lake belt the structural evolution was complicated by streamlining of the movement vector around the comparatively rigid flin flon snow lake belt the relationship of metamorphic assemblages to kinematic fabrics indicates that deformation was coeval with a prograde retrograde metamorphic cycle progressive deformation occurred during a period of 30 ma which is consistent with phanerozoic orogenic time scales

pedogenic processes and domain boundaries in a vertisol climosequence evidence from titanium and zirconium distribution and morphology abstract the occurrences of titanium ti and zirconium zr within eight vertisols formed in a climosequence on the upper beaumont formation of the texas gulf coastal plain were investigated in order to determine processes responsible for ti and zr redistribution during pedogenesis discontinuities defined by significant shifts in particle size distribution and the content in volume percent of zr are present at 160 to 260 cm depth in each pedon the discontinuities are interpreted to be functional boundaries i e physico chemical expressions of pedogenic domains between an upper soil domain dominated by open system pedogenesis and a lower more closed system domain dominated by chemical weathering the depth at which the functional boundary occurs is dependent on physical and hydrogeochemical influences which are largely a function of available water soil materials above the discontinuities are slightly coarser textured and enriched in zr whereas below the sediments are finer textured and have lower and more constant zr contents the zr is associated almost exclusively with zircon and zr contents correlate positively to the weight percent sand coarse silt with negligible zr present in the profile volume loss gain i e soil strain e a mass balance calculation that assumes zr or ti immobility during pedogenesis indicate ezr values nearly four times greater than eti large values of ezr within the upper soil domains are due primarily to sand and coarse silt additions to the vertisols and preclude use of zr as a basis of mass balance calculations in these soils despite its relative chemical stability by comparison ti is generally conserved within the clay rich soil profiles and is therefore better suited for mass balance calculations of volume change and mobile element translocation during pedogenesis

age and source of detrital zircons from the missi formation a proterozoic molasse deposit trans hudson orogen canada the missi formation in the flin flon basin forms part of a discontinuous series of molasse type sediments found throughout the early proterozoic trans hudson orogen in northern saskatchewan and manitoba the flin flon basin contains a sequence of proximal fan to braided stream fluvial conglomerates and sandstones which unconformably overlie subaerially weathered amisk group volcanic rocks stratigraphic way up indicators have been preserved even though these rocks have undergone greenschist facies metamorphism and polyphase deformation the sedimentary rocks are crosscut by intrusive rocks which provide a minimum age of sedimentation of 1840 7 ma detrital zircons from each of the six stratigraphic subdivisions of the flin flon basin were analyzed using the single zircon pb evaporation technique euhedral to slightly rounded zircons dominate each sample and these zircons give ages of between about 1854 and 1950 ma the missi sediments were thus deposited between 1840 and 1854 ma possible sources for

the ciw index a new chemical index of weathering abstract a new chemical index of weathering that can be applied to modern soils and precambrian paleosis is proposed ciw al2o3 the value of this index increases as the degree of weathering increases and the difference between ciw index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material

petrography and geochemistry of a 2000 2200 ma old hematitic paleo alteration profile on ongeluk basalt of the transvaal supergroup griqualand west south africa abstract a 4 m thick paleo alteration profile thought to represent one of the oldest known ferruginized hematitic paleoweathering zones is described from a depth of 720 m in a drill core on the farm drakenstein 263 some 20 km northwest of hotazel in griqualand west south africa the paleo alteration profile is developed on 2240 57 ma ongeluk basalt along the angular unconformity that separates transvaal strata from 2070 95 ma basal red beds of the olifantshoek group basaltic ongeluk lava was subjected to greenschist facies metamorphism represented by a mineral assemblage of fe pumpellyite chlorite albite and quartz before paleosol development the subsequent paleo alteration is characterized by concentration of fe2o3 in the form of hematite iron rich chlorite and abundant sericite the abundance of sericite results in a large excess of k2o relative to the amount of al2o3 that could have been derived from weathering of the parent basalt the origin of the excess potassium is problematical and could have been derived from basin water groundwater or metamorphic fluids field relationships favour introduction from basin water during deposition of overlying olifantshoek red beds elements such as sodium calcium and magnesium were leached from the alteration profile whereas ree became concentrated within it the most interesting aspect of the ree distribution is the preferred concentration of the lighter lanthanides la and nd in the middle part of the alteration profile relative to heavy ree and the development of a marked negative ce anomaly in the paleo weathered material hydrothermal vein alteration took place after deposition of the overlying olifantshoek red beds

chromium isotope fractionation during oxidative weathering implications from the study of a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports detailed stable cr sm nd and rb sr isotope data for a ca 1 9 ga old subaerial weathering profile at schreiber beach ontario canada from which detailed major and trace element signatures and 18 o values were previously reported the weathering profile developed on neoarchean 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation the aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough in a period following the great oxygenation event goe 2 45 ga ago and prior to the deposition of the gunflint cherts at 1 82 ga these iron rich cherts belong to one of the last regionally extensive banded iron formations bifs deposited before the 1 billion year long mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of bifs the gradual textural mineralogical and geochemical changes from unweathered basalts to strongly weathered hematite bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions the 53 cr value of unweathered pillow basalt cores 0 19 0 02 2 is within the range of mantle inventory values whereas weathered brown to green basalts soils exhibiting up to 30 lower cr concentrations compared to unweathered pillow cores are isotopically lighter 53 cr 0 35 0 11 in contrast red hematite rich basalts and hyaloclastites underlying the brown to green basaltic soils with the highest 18 o enrichment in the profile are isotopically heavier 53 cr 0 05 0 15 rb sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574 24 ma mswd 13 which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence burial of the gunflint basin the non correlation of k enrichment factors with 53 cr 18 o and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering instead correlations between 53 cr and 18 o and between 53 cr ce ce u and v indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re deposition at depth from high 18 o freshwaters partial re precipitation of heavy cr at depth can be explained on the basis of the weak positive correlation between 53 cr and fe enrichment factors it is seen as a consequence of the reduction of mobile cr vi and co precipitation as mixed fe iii cr iii oxhydroxides during contemporaneous oxidation of the fe 2 bearing groundwaters at depth a process similar to the one governing the deposition of most precambrian bifs redistribution of certain elements during the weathering process in this case rees is furthermore indicated by a correlation line in a sm nd isochron diagram defined by the schreiber beach data with a slope corresponding to an age of 1 93 0 19 ga mswd 26 an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality an oxidative atmosphere at 1 9 ga as implied by the results from the schreiber profile is furthermore supported by positively fractioned cr isotopes 53 cr from 0 06 to 0 39 recorded in the iron rich gunflint cherts directly above the palaeo weathered horizons at schreiber beach these values are interpreted to stem from a positively fractionated shallow seawater chromium composition at 1 88 ga potentially reflecting a continental run off characterized by positive 53 cr and are in accordance with cr isotope signatures in worldwide bifs including the gunflint iron formation our results show the potential of cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in earth s history

formation and diagenesis of a sub huronian saprolith comparison with a modern weathering profile an early proterozoic paleosol that preserves archean granite protolith textures occurs beneath the lorrain formation in the early proterozoic huronian supergroup near ville marie quebec textural mineralogical and geochemical comparisons between this saprolith and a modern weathering profile developed on the devonian toorongo granodiorite in eastern australia indicate similarities that can be attributed to the effects of primary weathering reactions the comparison also reveals anomalous features at ville marie that are caused by diagenetic reactions between the weathered granite and meteoric waters k metasomatism in the form of sericite replacement of kandites is ubiquitous in the ville marie saprolith zone and is ascribed to reaction of low na k meteoric waters with the weathered profile as they moved along a relatively permeable unconformity surface earlier diagenetic reactions may have produced calcite alteration occurring in a thin zone at the base of the profile thought to represent the po

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

volcanic reconstruction of paleoproterozoic arc volcanoes the hidden and louis formations flin flon manitoba canadathis is a companion paper to dewolfe y m gibson h l and piercey s j 2009 petrogenesis of the 1 9 ga mafic hanging wall sequence to the flin flon callinan and triple 7 massive sulphide deposits flin flon manitoba canada canadian journal of earth sciences 46 this issue the hanging wall to the flin flon callinan and triple 7 volcanogenic massive sulphide deposits of the flin flon district is composed of the hidden and louis formations the contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism the formations form a composite volcanic edifice that was erupted and grew within a large volcanic tectonic subsidence structure hosting the deposits that developed within a rifted arc environment the formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows sills and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks the hidden formation is interpreted to represent a small shield volcano and the louis formation a separate shield volcano that developed on its flank both the hidden and louis volcanic edifices were constructed by continuous low volume eruptions of pillow lava a gradual change from a dominantly extensional environment during the formation of the footwall flin flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the hidden and louis formations are unlikely to host significant volcanogenic massive sulphide type mineralization however synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization indicating their control on ore formation longevity and reactivation as magma and fluid pathways during the growth of the hidden and louis volcanoes

chronology of crustal growth and recycling in the paleoproterozoic amisk collage flin flon belt trans hudson orogen canada u pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the flin flon greenstone belt now recognized as a paleoproterozoic tectonic collage

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

geology and major and trace element chemistry of late archean weathering profiles in the fortescue group western australia implications for atmospheric po2 abstract we have identified two thick weathering profiles between flows of the mt roe basalt unit of the fortescue group in the pilbara area of western australia the age of weathering is constrained by u pb zircon dating to lie between 2765 and 2715 ma the profiles herein referred to as the mt roe 1 and 2 paleosols were mapped in detail for 1500 m and 760 m along strike respectively samples were collected along traverses across both profiles where they were well developed and well exposed the profiles which may be structurally offset parts of the same weathering horizon have well developed upper sericite zones 1 15 m thick these grade downward into chlorite rich zones which are chemically and physically heterogeneous the chlorite zones grade downward into unweathered basalts the mt roe 1 profile is overlain by 2 6 m of bedded probably lacustrine sediments these in turn are covered by succeeding lava flows evidence of a surficial origin for the paleosols includes mud cracks ripple marks and rip ups of the paleosol material in the post paleosol sediments soft sediment deformation and probable dewatering structures are present along the paleosol sediment contact the mt roe 2 profile is overlain directly by the succeeding lava flow with little or no intervening sediments some brecciation and pillowing is observed along the base of the overlying basalt flow in both cases probably due to emplacement of lava into a shallow lake or onto wet soil in the sericite zones of both profiles nearly all of the fe mn mg and zn and a substantial fraction of the ca pb na ni and si have been leached the chlorite zones have elevated fe pb ni and zn contents and appear to be repositories of elements lost from the sericite zone al ti zr and th were immobile in the mt roe 1 paleosol and are concentrated by factors of 2 5 2 6 over the unweathered basalt v u and cr were also nearly immobile major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen k is strongly enriched toward the upper contact of the sericite zone and was introduced during post weathering metasomatism together with rb and sr na and ca are also enriched near the top of each profile and small amounts of each may have been introduced during regional low grade metamorphism of the hamersley basin at 2168 10 ma

white mica domain formation a model for paragonite margarite and muscovite formation during prograde metamorphism scanning transmission electron microscopy images of the 00 l white mica planes in crystals from central switzerland and crete greece reveal that domains of paragonite margarite and muscovite are ordered within the basal plane energy dispersive x ray analyses show that both cations in the interlayer and in the 2 1 layer have ordered on the scale of tens to hundreds of nanometers domain boundaries can be both sharp and crystallographically controlled or diffuse and irregular a model outlining the domain formation process is presented that is consistent with x ray powder diffraction and transmission electron microscopy data the domain model incorporates aspects of a mixed layered and a disordered compositionally intermediate phase models the main feature of the model is the formation of mica species that segregate within the basal plane and contradict the notion of homogeneous layers within mixed layer phases implications for the formation of all diagenetic and very low grade metamorphic 2 1 sheet silicates are discussed

petrogenesis of the 1 9 ga mafic hanging wall sequence to the flin flon callinan and triple 7 massive sulphide deposits flin flon manitoba canada a detailed study of the geochemical and isotopic characteristics of the volcanic rocks of the hidden and louis formations which make up the hanging wall to the volcanogenic massive sulphide deposits at flin flon manitoba was carried out on a stratigraphically controlled set of samples the stratigraphy consists of the lowermost dominantly basaltic hidden formation and the overlying dominantly basaltic louis formation of importance petrogenetically is the 1920 unit a basaltic andesite with nb thmn 0 54 0 62 end 1 9ga 3 6 5 9 ehf 1 9ga 8 5 9 6 and 204pb 206pb 23 9 the basaltic flows that dominate the hidden formation have nb thmn 0 16 0 29 end 1 9ga 1 7 4 4 ehf 1 9ga 7 0 11 8 and 204pb 206pb 16 9 18 6 the carlisle lake basaltic andesite top of hidden formation is characterized by nb thmn 0 16 0 14 and 204pb 206pb 21 4 the rhyodacitic tower member bottom of louis formation has nb thmn 0 23 end1 9ga 4 6 ehf1 9ga 9 3 and 204pb 206pb 22 2 the

the flin flon paleosol revisited a ca 1 85 ga paleosol developed on seafloor altered mafic rocks is preserved beneath continental sandstones and conglomerates of the missi group near flin flon manitoba previous work has focused

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion