petrology of mn carbonate silicate rocks from the gangpur group india abstract metamorphosed mn carbonate silicate rocks with or without oxides assemblage i and mn silicate oxide rocks with minor mn carbonate assemblage ii occur as conformable lenses within metapelites and metacherts of the precambrian gangpur group india the petrology of the carbonate minerals rhodochrosite kutnahorite and calcite that occur in these two assemblages is reported early stabilisation of spessartine aegirine quartz and carbonates in a wide solid solution range was followed by pyroxmangite tephroite rhodonite through decarbonation reactions subsequently jacobsite hematite braunite hollandite and hausmannite have formed by decarbonation oxidation processes during prograde metamorphism textural characteristics and chemical composition of constituent phases suggest that the mineral assemblages reflect a complex relationship between protolithic composition variation of xco2

compositional variations in manganese carbonate micronodules from the lower proterozoic nsuta deposit ghana product of authigenic precipitation or post formational diagenesis abstract detailed petrographic and microprobe studies of micronodules identified in the manganese carbonate orebody and adjacent mn rich rocks at the 2 2 ga nsuta deposit ghana indicate distinct lithologic control on nodule structure and chemical composition nodules in the mn rich rocks adjacent to the manganese carbonate orebody conform with the well banded nature of the rock and are distinctly zoned into alternating bands compositionally dominated by mg kutnahorite and mg ca rhodochrosite individual nodules are partially or completely cemented by thin dark silica and carbonates of ca mg mn solid solution nodules in the orebody on the contrary are generally massive homogeneous consist predominantly of rhodochrosite and are only partially cemented by dark silica based on the stratigraphic relationship of the nodule bearing rocks to the manganese carbonate orebody and the variations in texture and composition observed in the nodules it would appear that the mn carbonate nodules precipitated diagenetically in response to changing pore water chemistry and or metal mn and ca supply the overall mineralogical and geochemical features indicate in addition that the nodules may have preserved original carbonate compositions both metastable and stable during burial diagenesis and low grade regional metamorphism

evaluation of the application of rare earth elements to paleoceanography anomalous departures of rare earth elements ree from similarity of chemical behaviour are most evident in a comparison of ce with its neighbors under reducing conditions ce3 acts like other ree but oxidized ce4 is adsorbed on mn fe oxyhydroxides or organic particulates and consequently the remaining solution will tend to exhibit a negative ce anomaly in addition more subtle fractionations may emphasize the light or the heavy or the middle ree a pattern of ree may be recorded by immediate diagenesis on a suitable chemical substrate such as apatite fish debris conodonts carbonate or chert this record was originally thought to reflect the paleoredox conditions of primary deposition however any such primary record may be overprinted by late diagenetic alteration early analyses of ree in conodonts from a wide range of sediments suggested a prevalence of reducing conditions no ce anomaly in pre carboniferous oceans and more common oxidation negative ce anomaly in the late paleozoic however recent research by other investigators indicates that variations in cean are more diverse in origin and in particular many are distorted to an unknown extent by late diagenesis in any case it is now clear that geological age does not show a consistent relationship to the level of the ce anomaly the ce anomaly apparently does not play any regular role in the isotopic events that accompany prominent mass extinctions further research is required to understand and thus make useful the controls on the ce anomaly

aluminous granulites of the archean craton of kasai zaire petrology and p t conditions aluminous granulites of the archean 2 8 ga kasai craton zaire consist of two main mineral assemblages grt opx and sil grt crd rocks the high grade metamorphic conditions as deduced from grt opx and grt opx pl qtz equilibria are 720 c 6 7 kb considering the zoning of the same minerals the slope of the p t path is estimated at 15 b c thermobarometry involving crd is consistent with those p t conditions three cordierite forming reactions have been observed petrographically grt qtz crd opx r1 grt sil crd spl r2 grt sil crd mag r3 these equilibria are continuous reactions end member reactions have slopes less than 15 b c they are decompression reactions occurring after the metamorphic climax using available thermodynamic data r3 fixes the oxygen fugacity to a value below the qfm buffer log 10f o2 17 6 at 720 c 6 7 kb and xh2ofl 0 3 in the graphite stability field the absence of graphite in the rocks shows that the end of the granulite facies metamorphism did not occur under important co 2 streaming the polymetamorphic history of this archean craton is considered 1988 oxford university press

paleoproterozoic manganese and base metals deposits at kisenge kamata katanga d r congo the kisenge kamata manganese deposit katanga d r congo belongs to a group of paleoproterozoic sedimentary manganese ore occurrences that have been recognized for various parts of africa it is a relatively small deposit with total estimated reserves of similar to 12 mt ore but it is of major importance for understanding key aspects of the geology of the kasai block of the congo craton the available constraints indicate that the deposit formed in the 2 0 1 9 ga time interval in the orosirian period the manganese rich facies of the kisenge kamata deposits range in composition from manganese carbonate rocks to non calcareous deposits dominated by manganese rich garnet associated rocks are mainly grey graphitic shales the kisenge kamata sediments are significantly enriched in cu co ni zn and other accessory metals v mo ga field constraints and geochemical data suggest that the kisenge kamata sediments were probably deposited in a relatively small intracratonic basin graben within the kasai block with an inlet allowing periodical access for ocean water influx carrying mn ree and other metals primary carbonate ore has relatively low delta c 13 3 to 6 parts per thousand and high delta o 18 13 to 20 parts per thousand values confirming their derivation from marine fluids

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

geodynamic and climate controls in the formation of mio pliocene world class oxidized cobalt and manganese ores in the katanga province dr congo the katanga province democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves they originated from sediment hosted stratiform copper and cobalt sulfide deposits within neoproterozoic metasedimentary rocks heterogenite the main oxidized cobalt mineral is concentrated as cobalt caps along the top of silicified dolomite inselbergs the supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world class manganese cap deposits in western katanga i e the black earths that are exploited by both industrial and artisanal mining here we provide constraints on the genesis and the timing of these deposits ar ar analyses of oxidized mn ore and in situ u pb sims measurements of heterogenite yield mio pliocene ages the ar ar ages suggest a multi phase process starting in the late miocene 10 5 ma when the metal rich substratum was exposed to the action of meteoric fluids due to major regional uplift further oxidation took place in the pliocene 3 7 2 3 ma and formed most of the observed deposits under humid conditions co and mn caps on metal rich substrata and coeval fe laterites on barren areas these deposits formed prior to the regional shift toward more arid conditions in central africa arid conditions still prevailed during the quaternary and resulted in erosion and valley incision which dismantled the metal bearing caps and led to ore accumulation in valleys and along foot slopes

a sm 1bnd isotopic study of heterogeneous granulites from the archean kasai lomami gabbro norite and charnockite complex zaire africa abstract sm 1bnd model ages t dm of acid granulites from the kasai complex agree with previous rb 1bsr and u 1bpb ages of 2 8 ga for the early and main stage of granulite facies metamorphism these sm 1bnd results give an upper limit of 3 1 ga for the age of the protoliths of the charno enderbites and metapelitic gneisses which represent the majority of the acid rocks of the complex a sm 1bnd study whole rock and internal isochrons and t dm ages of the associated basic granulites meta gabbro norite massif and metadolerite dykes crosscutting the acid granulites demonstrates that the second stage of granulite metamorphism corresponding to regional deformation of all the rocks is nearly coeval with the emplacement of the basic rocks at 2 4 ga

manganese deposits in the global lithogenetic system quantitative approach abstract a computerized data base including 330 localities and believed to represent at least 95 of the presently recorded spot accumulations of ore grade mn on land is the basis for a quantitative analysis of terrestrial mn resources this file is reprinted in full appendix the present subaerially exposed global ore mn resources are calculated as 17 9 109 t mn 1 in this figure are included the actually mineable straight mn deposits 8 7 109 t mn potentially mineable land based mn accumulations 9 2 109 t mn and actual or potential mn that could be extracted as a byproduct of mining other metals 0 6 109 t mn this distribution is strongly influenced by giant accumulations where the single exceptional kalahari mn field contains over 50 of the presently economic mn ore reserves or 23 42 of the global land mn resources a set of attributes has been selected to treat the global ore mn population in terms of genesis geotectonic and environmental setting and lithologic associations in terms of genesis precipitation from aqueous solutions was responsible for at least 99 of the contemporaneous and probably also the past mn accumulations now exposed on land weathering of mn orebodies has left its mark on 93 of the mn localities and 24 are now represented entirely by supergene assemblages less than 0 01 of the ore mn resources however are formed by weathering related accumulation over silicate rocks ultramafics in terms of geotectonic environments the bulk of the land based mn deposits 97 formed in intraplate and stable continental margin settings 3 1 formed along pacific type and rift type continental margins and only 0 00045 of the deposits formed in an oceanic setting this is in contrast with the outstanding mn accumulating capacity of the present ocean and is a consequence of the low preservation potential of the oceanic domain in terms of lithologic associations 96 of the mn in land based deposits is present in marine sedimentary associations 70 of mn is in banded iron formations 14 4 is in detrital and 11 1 is in carbonate dominated associations chert and jasper limestone sandstone shale and banded iron formation are statistically the most common immediate hosts to mn ores with recorded hosting frequencies of 79 50 45 37 and 35 respectively in terms of geological history the lower proterozoic accounts for 58 9 of the prreserved ore mn on land followed by oligocene 17 2 jurassic 6 2 and middle proterozoic 4 5 in terms of the intensity of mn accumulation per one million years of geological time oligocene 110 106 t mn ma is two orders of magnitude greater than the nearest time periods jurassic 8 9 106 t mn ma and lower proterozoic 6 5 106 t mn ma the historical distribution pattern of the land based mn deposits seems to indicate that accumulation of the bulk of the present ore grade mn is the result of repeated recycling with a land ocean trend abruptly initiated at the time of early cratonization about 2 5 ga this has been supplemented by a substantially less significant but remarkably steady reverse trend of addition of juvenile mn released from the mantle into the crust mafics and particularly basalts are the most important intermediaries in the cumulative secular increase of liberated and accumulated mn in the crust direct to indirect proven to hypothetical spatial coincidence of basalts and mn ores can be demonstrated on at least 169 localities out of 330 51 evaluated

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

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

coticules of the belgian type area stavelot venn massif limy turbidites within the nascent rheic oceanic basin abstract coticule is the name originating in belgium for a fine grained metasedimentary yellowish rock mainly composed of quartz spessartine and mica which is repeatedly interstratified in the hematite rich purple slates of the lower ordovician of the ardenne region it was described for the first time in the nineteenth century in the stavelot venn massif ardenne which is accordingly the type area for this peculiar lithology since then numerous occurrences were described all over the world in metamorphic rocks of various grades and ages the exact nature of the belgian coticule protolith remained hypothetical although intensively discussed for a long time in the literature after an extensive review of all the stratigraphical sedimentological and geochemical results and new field geochemical major and trace elements and nd isotopic investigations we propose a significantly improved and sustained genetic model the thin coticule layers were deposited offshore in a deep oceanic basin as limy mud turbidites during the early floian c 477 ma early ordovician density currents came from the north from the shelf which bordered the emergent brabant massif at that time and flowed down the slope to the deep basin plain the purple shales enclosing the coticule layers represent the normal pelagic sedimentation in the basin these shales are exceptionally rich in fe and mn because of the hydrothermal activity of the nearby and young rheic ocean ridge indeed the rheic ocean opened in the early ordovician c 482 ma during diagenesis the strongly oxidizing depositional environment favoured the mobility of mn 2 as opposed to fe 3 that remained insoluble and immobile this allowed for the migration of divalent manganese from the pelagic shales and for replacing calcium in the turbiditic carbonate fraction to form rhodochrosite later epizonal metamorphism transformed the clay quartz rhodochrosite paragenesis of the protolith into the muscovite quartz spessartine paragenesis of the coticule hence in the type area coticule genesis needed a peculiar environment including a continental shelf with limy mud deposits a continental slope generating periodical turbidites and a nearby oceanic ridge here the nascent rheic ocean delivering hydrothermal iron and manganese

continuous growth of hydrogenetic ferromanganese crusts since 17 myr ago on takuyo daigo seamount nw pacific at water depths of 800 5500 m ferromanganese crusts cover all outcrops on takuyo daigo seamount traversed during remotely operated underwater vehicle rov dives except in places covered by foraminifera sand takuyo daigo is a cretaceous seamount located in the northwest pacific ocean geological and bathymetric mapping provide the framework for this study chemical and mineralogical analyses of the hydrogenetic ferromanganese crusts show temporal and spatial variations typical of those found in previous studies outcrops from 800 to 5500 m water depths are covered with ferromanganese crusts up to 105 mm thick beryllium isotope dating shows that the crusts have apparently been growing continuously at all water depths even through the modern oxygen minimum zone omz contrary to some earlier models for deposition growth rates vary from 2 3 to 3 5 mm myr with fe or mn fluxes of 0 07 0 11 g cm2 myr since the early middle miocene co mn ratios decrease with water depth while fe mn and other metallic elements increase or show no change based on the analysis of the uppermost crust surface this is probably because co is the most abundant redox sensitive element derived from seawater that occurs in crusts

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

contrasting parageneses in the manganese silicate carbonate rocks from parseoni sausar group india and their interpretation mn silicate carbonate rocks at parseoni occur as conformable lenses within metapelites and calc silicate rocks of the precambrian sausar group india the host rocks are estimated to have been metamorphosed at uppermost p t conditions of 500 550 c and 3 4 kbar the mn rich rocks contain appreciable fe reflected in the occurrence of magnetite 1 mno 1 magnetite 2 mno 15 and magnetite 3 mno 10 two contrasting associations of pyroxmangite with and without tephroite developed in the mn silicate carbonate rocks under isothermal isobaric conditions the former assemblage formed in relatively fe rich bulk compositions and equilibrated with a metamorphic fluid having a low xco2 0 2 and the latter equilibrated with a co2 rich fluid rhodochrosite magnetite 1 quartz protoliths produced the observed mineral assemblages on metamorphism partitioning of major elements between coexisting phases is somewhat variable fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism oxygen fugacity during metamorphism was monitored at or near the qfm buffer in tephroite bearing domains and the fluid composition was buffered by mineral reactions in respective domains as compared to other metamorphosed mn deposits of the sausar group the mn silicate carbonate rocks at parseoni were therefore metamorphosed at much lower fo2 through complex mineral fluid interactions

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

malachite an indicator of major pliocene cu remobilization in a karstic environment katanga democratic republic of congo abstract malachite cu3 oh 2 co3 is a common mineral in the central african copperbelt where it constitutes a high grade 57 cu copper ore and is exploited industrially and by artisanal miners the genesis of this supergene ore in katanga has yet received little attention here we focus on the genetic processes giving rise to this regionally important mineral malachite is secondary to primary copper sulfides which indicates that it ultimately formed by oxidation of the latter in fine grained siliciclastic host rocks it is often partly impregnative commonly with variations in malachite content parallel to the bedding this type may form either by absolute enrichment or by in situ replacement of sulfides within voids of various types including dissolution cavities in carbonate host rocks it precipitates in distinctive forms such as thick laminated crusts and speleothemes up to several tens of centimeters in length ree geochemistry suggests that malachite formed in groundwater environments through in situ recombination of carbonate ions originating from dissolving host rock carbonates and downward percolating cu2 leached from copper sulfides thermodynamic modeling confirms this genetic environment and shows that chrysocolla cu al 2h2si2o5 oh 4 nh2o coexists with malachite when si content increases in the groundwater fluids the importance of karstic features in katanga is also re evaluated karstic features appear to be rather frequent and to constitute ideal site traps for the formation of secondary base metal ores

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

the age of supergene manganese deposits in katanga and its implications for the neogene evolution of the african great lakes region abstract supergene manganese deposits commonly contain k rich mn oxides with tunnel structure such as cryptomelane which are suitable for radiometric dating using the 39ar 40ar method in africa mn deposits have been dated by this method for localities in western and southern parts of the continent whereas only some preliminary data are available for central africa here we present new 39ar 40ar ages for mn oxide samples of the kisenge deposit in southwestern katanga democratic republic of the congo the samples represent supergene mn oxide deposits that formed at the expense of primary paleoproterozoic rhodochrosite dominated carbonate ores main phases of mn oxide formation are dated at c 10 5 ma 3 6 ma and 2 6 ma for a core that crosses a mineralized interval the latter shows a decrease in age with increasing depth recording downward penetration of a weathering front surface samples of the kisenge deposits also record a c 19 2 ma phase as well as c 15 7 ma 14 2 ma and 13 6 ma phases the obtained ages correspond to distinct periods of paleosurface development and stability during the mio pliocene in katanga because katanga is a key area bordered to the north by the congo basin and to the east by the east african rift system these ages also provide constraints for the geodynamic evolution of the entire region for the mio pliocene the kisenge deposits record ages that are not systematically found elsewhere in africa although the 10 5 11 ma event corresponds to a roughly simultaneous event in the kalahari manganese field south africa the rest of the katanga paleosurface record differs somewhat from records for other parts of africa for which older eocene ages have been obtained this difference is most probably related to the specific regional geodynamic context uplift of the east african plateau with associated erosion and the opening of the east african rift system at c 25 ma are events whose effects in the study area interfere with those of processes responsible for the development of continent wide paleosurfaces

comparison between datangpo type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on rare earth elements abstract datangpo type sedimentary manganese deposits which are located in northeastern guizhou province and its adjacent areas are mn carbonate type deposits hosted in black carbonaceous shale that represent a series of medium to large deposits containing a huge tonnage of reserves paas normalized rare earth element distribution patterns of manganese ores record hat shaped rey ree y plots characterized by pronounced middle rare earth element enrichment evident positive ce anomalies weak to strong positive eu anomalies and negligible negative y anomalies these rey geochemical characteristics are different from those of country rocks and record the processes and features of sedimentation and diagenesis manganese was precipitated as mn oxyhydroxide particles in oxidized water columns with the sorption of a certain amount of rare earth elements subsequently transforming from mn oxyhydroxides to rhodochrosite and redistributing rey in reducing alkaline pore water during early diagenesis a number of similarities can be observed through a comparison of datangpo type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on their rare earth elements the precipitation of datangpo type manganese ores is similar to that of hydrogenetic crusts and nodules based on their positive ce anomalies and relatively higher total rey concentrations however several differences also exist compared to hydrogenetic crusts and nodules datangpo type manganese ores record smaller positive ce anomalies lower total rey concentrations unobvious fractionation between y and ho and weak to strong positive eu anomalies these were caused by quicker sedimentary rates in the oxic water columns of the shallower basin after which pore water became strongly reducing and alkaline due to the degradation of organic matter in the early diagenetic stage in addition compared to typical deposits in the world datangpo type manganese ores are similar to hydrogenetic deposits and different than hydrothermal deposits all of these characteristics of manganese ores indicate that datangpo type manganese ores the principal metallogenic factors of which include oxidation conditions during deposition and reducing conditions during early diagenetic stages represent hydrogenetic deposits

oligocene early miocene antarctic nearshore diatom biostratigraphy lower oligocene ca 31 ma to lower miocene ca 18 5 ma biosiliceous microfossils recovered from the cape roberts project crp 2 2a drill cores provide both paleoenvironmental and biostratigraphic information enhancing our understanding of the geological history of the victoria land basin antarctica the biochronostratigraphic record obtained provides key data with which time space facies models may be developed assemblages of neritic and pelagic microfossils such as those from the crp 2 2a drill cores provide links between the primarily pelagic microfossil derived chronostratigraphy of the southern ocean and the facies models of the neritic zone the crp 2 2a holes drilled in the victoria land basin antarctica during 1998 barrett p j et al eds 2001 studies from the cape roberts project ross sea antarctica scientific results of crp 2 2a parts i and ii terra antartica vol 7 4 5 665pp include several sections that contain well preserved and relatively abundant biosiliceous microfossils scherer et al 2001 oligocene and lower miocene siliceous microfossil biostratigraphy of cape roberts project core crp 2 2a victoria land basin antarctica in barrett p j ricci c a eds studies from the cape roberts project ross sea antarctica scientific report of crp 2 2a terra antartica vol 7 4 pp 417 442 produced a biostratigraphic zonal framework consisting of 10 biozones two of the zonal boundaries were correlated with the magnetostratigraphically calibrated southern ocean diatom biozonation many of the taxa observed and utilized in the above framework could not be assigned categorically to existing taxa leading to use of informal nomenclature we present an updated biostratigraphy and zonation scheme based on the latest taxonomic concepts and on new quantitative and qualitative siliceous microfossil data from the crp 2 2a drill core including formal description of four new diatom taxa used in the definition of antarctic nearshore diatom zonal boundaries new taxa formally proposed are fragilariopsis truncata cymatosira palpebraforma rhizosolenia fidicularis and hemiaulus angustobrachiatus

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

oxygen isotope perspective on crustal evolution on early earth a record of precambrian shales with emphasis on paleoproterozoic glaciations and great oxygenation event abstract we present stable isotope and chemical data for 206 precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0 5 to 3 5 ga with a dense coverage for 2 5 2 2 ga time interval when earth experienced four snowball earth glaciations and the irreversible rise in atmospheric o2 we observe significant downward shift of several and a smaller range of 18 o values 7 to 9 in shales that are associated with the paleoproterozoic and potentially neoproterozoic glaciations the paleoproterozoic samples consist of more than 50 mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering their pervasively low 18 o and d down to 85 values provide strong evidence of alteration and diagenesis in contact with ultra low 18 o glacial meltwaters in lacustrine deltaic or periglacial lake sikussak type environments associated with the paleoproterozoic glaciations the d silicate values for the rest of precambrian shales range from 75 to 50 and are comparable to those for phanerozoic and archean shales likewise these samples have similar ranges in 13 c org values 23 to 33 pdb and corg content 0 0 to 10 wt to phanerozoic shales precambrian shales have a large range of 18 o values comparable to that of the phanerozoic shales in each age group and formation suggesting similar variability in the provenance and intensity of chemical weathering except for the earliest 3 3 3 5 ga archean shales which have consistently lower 18 o values moreover paleoproterozoic shales that bracket in age the great oxidation event goe overlap in 18 o values absence of a step wise increase in 18 o and d values suggests that despite the first order change in the composition of the atmosphere weathering cycle was not dramatically affected by the goe at 2 4 2 3 ga shales do not show comparable 18 o rise in the early phanerozoic as is observed in the coeval 18 o trends for cherts and carbonates there is however a sharp increase in the average 18 o value from the early archean to the late archean followed by a progressively decelerating increase into the phanerozoic this decelerating increase with time likely reflects declining contribution of mantle extracted normal 18 o crust and lends support to crustal maturation and increasing 18o sequestration into the crust and recycling of high 18 o and 87sr 86sr sedimentary rocks this secular increase in the 18 o composition of the continental crust could have also had a mild effect on seawater 18 o composition

exsolution lamellae of kirschsteinite in magnesium iron olivine from an angrite meteorite exsolution phenomena of kirschsteinite cafesi04 in olivine have been studied by single crystal x ray diffraction techniques and scanning electron microscopy sem of oriented polished thin sections pts of three single crystals separated from the antarctic angrite lew86010 supplemented by micro area x ray diffraction with the laue method mxl by synchrotron radiation sr for pts of a rock chip of lew8601o the cell dimensions of the host olivine and exsolved kirschsteinite are a 4 79 3 b 10 39 5 and c 6 06 3 a and a 4 87 5 b 11 14 10 and c 6 36 5 a respectively from the precession photos the pts of olivine single crystals oriented parallel to 100 show exsolution lamellae of kirschsteinite up to i 0 m in width the two sets of lamellae are symmetrically related and parallel to 031 and 031 electron microprobe analysis gave si02 33 1 ti02 0 07 al203 0 03 feo 49 4 mno 0 61 mgo 13 4 cao 2 2 cr203 0 02 v203 0 01 nio 0 05 sum 99 4 wt for the host olivine and si02 33 3 ti02 0 03 feo 31 5 mno 0 39 mgo 5 4 cao 28 5 cr203 0 02 nio 0 05 sum 99 2 wt for the exsolved kirschsteinite the results from mxl for the olivine crystals on the rock pts are compatible with the observation on the single crystals that the lamellae are parallel to 031 and 031 the 031 and 031 planes have been known to be twin planes for olivine and the twinning is by reticular pseudomerohedry based on a quadruple lattice although other reported exsolved precipitates in meteoritic olivines exist as inclusions kirschsteinite in lew86010 olivine takes the form of lamellae our explanation is that lew86010 olivine is fe rich and that lamellar precipitates are more easily formed than inclusions because exsolution lamellae along 031 in fe rich olivine maintain lattice coherency

a multiple approach radiometric age estimate for the rotoiti and earthquake flat eruptions new zealand with implications for the mis 4 3 boundary abstract pyroclastic fall deposits of the paired rotoiti and earthquake flat eruptions from the taupo volcanic zone new zealand combine to form a widespread isochronous horizon over much of northern new zealand and the southwest pacific this horizon is important for correlating climatic and environmental changes during the last glacial period but has been the subject of numerous disparate age estimates between 35 1 2 8 and 71 6 ka all errors are 1 s d obtained by a variety of techniques a potassium argon k ar age of 64 4 ka was previously determined on bracketing lavas at mayor island volcano offshore from the taupo volcanic zone we present a new more precise 40ar 39ar age determination on a lava flow on mayor island that shortly post dates the rotoiti earthquake flat fall deposits of 58 5 1 1 ka this value coupled with existing ages from underlying lavas yield a new estimate for the age of the combined eruptions of 61 0 1 4 ka which is consistent with u th disequilibrium model age data for zircons from the rotoiti deposits direct 40ar 39ar age determinations of plagioclase and biotite from the rotoiti and earthquake flat eruption products yield variable values between 49 6 2 8 and 125 3 10 0 ka with the scatter attributed to low radiogenic ar yields and or alteration and or inheritance of xenocrystic material with inherited ar rotoiti earthquake flat fall deposits occur in new zealand in association with palynological indicators of mild climate attributed to marine isotope stage mis 3 and thus used to suggest an age that is post 59 ka the natures of the criteria used to define the mis 4 3 boundary in the northern and southern hemispheres however imply that the new 61 ka age for the rotoiti earthquake flat eruption deposits will provide the inverse namely a more accurate isochronous marker for correlating diverse changes across the mis 4 3 boundary in the southwest pacific