40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

metallogeny in the capricorn orogen western australia the result of multiple ore forming processes abstract the capricorn orogen includes metamorphosed tectonic units intrusive igneous rocks volcano sedimentary and sedimentary basins situated between the pilbara and yilgarn cratons these include the gascoyne complex ashburton blair bresnahan yerrida bryah padbury and earaheedy basins and edmund and collier former bangemall basins mineral deposits and occurrences in these units comprise stratabound base metals including the giant abra pb cu ba edmund basin and magellan pb earaheedy basin and numerous hydrothermal veins and precious and or base metal lodes some of the more important orogenic precious metal lodes are those of the peak hill group in the bryah padbury basins and the egerton field in the gascoyne complex the metallogeny of the capricorn orogen reflects syn to post capricorn orogenic events linked to either protracted convergence tectonics between the pilbara and yilgarn cratons or to a two stage convergence history one during the capricorn orogeny ca 1 83 ga pilbara yilgarn convergence the other during the yapungku orogeny ca 1 79 north australian west australian convergence the metallogeny of the capricorn orogen was further sustained by continental mafic magmatism at about 1 45 and 1 07 ga during which heat advection may have caused widespread hydrothermal circulation particularly in the edmund collier earaheedy basins and adjacent tectonic units an interesting but as yet unexplained aspect of the capricorn metallogeny is the regional pb ba enrichment a conceptual model that attempts to explain the billion year long history of the metallogeny in the capricorn orogen is proposed mineral exploration in the capricorn orogen is somewhat limited and not as intensive as is in the brown fields of the archaean terranes of western australia

u th he chronology of the robe river channel iron deposits hamersley province western australia abstract channel iron deposits cid supply 40 of australia s iron ore but their genesis is still the subject of debate two well characterised samples of goethite hematite cid from a diamond drill core in mesa j of the robe river area in western australia were dated using u th he methods in order to constrain the timing of iron oxide formation and thereby provide a temporal context for cid genesis u th he ages he ages range from 25 7 0 6 to 7 0 0 8 ma and despite a high degree of scatter they corroborate relationships expected from the internal ooidal stratigraphy for individual ooids the hematitic core is older than or indistinguishable from the age of the surrounding goethitic cortex the goethitic cortices are in turn older than the ferruginised wood fragments recovered from the cementing goethitic matrix the data suggest the following paragenesis i hematitic cores in ooids formed in the early to middle miocene as documented by ages of 14 3 3 7 ma and 18 3 3 5 ma measured in the shallower 8 2 m deep and deeper 32 8 m sample respectively ii goethitic cortices of both samples formed in the late middle to early late miocene at 11 6 3 0 ma iii wood fragments form a prominent component of the matrix and were ferruginised during the late miocene he ages ranging from 9 4 0 5 to 8 2 0 4 ma in the deeper core and 8 4 0 9 to 7 0 0 8 ma in the shallower core the data suggest that the unique environmental conditions conducive to cid formation existed during the miocene and that a typical robe river cid sequence likely took 4 to 8 myr to accumulate a methodological implication of this study is that it confirms the previous observation of vasconcelos et al 2013 suggesting that the temperature utilised for he extraction from iron oxides has a critical impact on the mobility of parent nuclides the typical 1000 c laser heating used for crystalline minerals like apatite or zircon induces loss of u and th and results in erroneously old ages modest extraction temperature

neogene weathering and supergene manganese enrichment in subtropical south china an 40ar 39ar approach and paleoclimatic significance deep weathering profiles and associated supergene mn oxides deposits are widely distributed in south china however data on their ages are not available until most recently ar 40 ar 39 laser incremental heating analysis of potassium bearing supergene mn oxides collected from four weathering profiles in the qinzhou fangcheng mn belt guangxi province south china provides for the first time numerical constraints on timing of weathering and supergene mn enrichment thirty eight of 42 cryptomelane grains yield well defined plateau 36 grains or pseudo plateau 2 grains ages ranging from 17 6 0 5 ma to 4 71 0 08 ma 2 sigma the results when combined with data from mn oxides deposits from the adjacent guangdong and hunan provinces document a protracted history of weathering and supergene mn enrichment spanning from the earliest miocene to the latest pliocene in south china this indicates that warm and humid climatic conditions conducive to intense weathering and secondary mineralization prevailed across south china during the whole neogene the climatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and fauna associations from cenozoic inland and marginal sea basins of south china the weathering geochronology data also provide temporal constraints on the initiation and intensification of the east asian monsoon that has been the cause for the south china s humid climate c 2007 elsevier b v all rights reserved

triassic magnetic overprints related to albitization in granites from the morvan massif france the morvan massif located in the northern part of the french massif central shows widespread unusual deep alteration profiles in the palaeozoic basement rocks gneisses granites and rhyolites they are characterised by overall albitization chloritisation and haematisation albite and chlorite constitute their characteristic mineral paragenesis haematite pigmentation comes with this paragenesis and gives an overall pink coloration to the weathered rocks a paleomagnetic analysis was conducted after detailed petrographical and mineralogical studies of the alteration that lead to the albitized profiles special attention was devoted to the occurrence of haematite the main paleomagnetism carrier with the though to date this palaeoweathering event petrographical checking after heating treatments showed that deshydroxylation of chlorite also led to the formation of secondary haematite this haematite formed by recrystallization acquired remanence properties and thus superimposed and hided the original magnetic signal this problem has prevented the extraction of the original magnetic signal especially in the sections made up of rhyolites virtual geomagnetic poles vgps obtained from the characteristic directions carried by haematite have been compared to the european apparent polar wander path apwp an early triassic magnetic overprint has been recorded in the albitized basement rocks of the morvan massif the age obtained for this albitization phenomena strengthens the hypothesis that albitization developed in relation with palaeoweathering processes linked with the triassic palaeosurface therefore large scale continental weathering could be the main mechanism for late permian early triassic remagnetizations which are widespread throughout the palaeozoic formations of western europe as far as we know this is the first time that deep palaeoweathering phenomena affecting hard and non disaggregated weathered rocks of the basement have been directly dated palaeomagnetism appears as a precious tool to date alteration events developed in widely and long lasting emerged areas of the basement

u th pb isotope data indicate phanerozoic age for oxidation of the 3 4 ga apex basalt abstract the occurrence of ferric oxides in archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient earth recognizing that post archean weathering may also produce oxide minerals drill cores have been used to obtain samples beneath the depth of phanerozoic weathering the first core of the archean biosphere drilling project abdp 1 documented hematite as alteration products in 3 4 ga basalts from the marble bar area of the pilbara craton nw australia and this has been used to infer the presence of an o2 bearing atmosphere in the archean it is possible however that despite recovery of samples from 100 m depth oxidation of the basalts occurred much younger than the depositional age in this study the age of oxidation of the apex basalt from the abdp 1 drill core at marble bar is constrained by u th pb geochronology lead and u concentrations of the basalts from the abdp 1 drill core vary greatly between

flow of bottom water in the northwestern weddell sea the weddell sea is known to feed recently formed deep and bottom water into the antarctic circumpolar water belt from whence it spreads into the basins of the world ocean the rates are still a matter of debate to quantify the flow of bottom water in the northwestern weddell sea data obtained during five cruises with r v polarstern between october 1989 and may 1998 were used during the cruises in the weddell sea five hydrographic surveys were carried out to measure water mass properties and moored instruments were deployed over a time period of 8 5 years to obtain quasi continuous time series the average flow in the bottom water plume in the northwestern weddell sea deduced from the combined conductivity temperature depth and moored observations is 1 3 0 4 sv intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed the variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large scale circulation taking into account entrainment of modified warm deep water and weddell sea deep water during the descent of the plume along the slope between 0 5 sv and 1 3 sv of surface ventilated water is supplied to the deep sea this is significantly less than the widely accepted ventilation rates of the deep sea if there are no other significant sources of newly ventilated water in the weddell sea either the dominant role of weddell sea bottom water in the southern ocean or the global ventilation rates have to be reconsidered

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

40ar 39ar geochronological constraints on the evolution of lateritic iron deposits in the quadrilatero ferrifero minas gerais brazil weathering profiles overlying the sapecado pico and andaime iron ore deposits quadrilatero ferrifero qf minas gerais brazil reach depths of 150 400 m and host world class supergene iron orebodies in addition to hosting supergene ore bodies of global economic significance weathered banded iron formations at the quadrilatero ferrifero and elsewhere e g carajas hamersley are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth laser incremental heating 40ar 39ar results for 69 grains of hollandite group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the sapecado pico and andaime deposits reveal ages ranging from ca 62 to 14 ma older mn oxides occur near the surface while younger mn oxides occur at depth however many samples collected at the weathering bedrock interface yield ages in the 51 41 ma range suggesting that the weathering profiles in the quadrilatero ferrifero had already reached their present depth in the paleogene the antiquity of the weathering profiles in the quadrilatero ferrifero is comparable to the antiquity of dated weathering profiles on banded iron formations in the carajas region brazil and the hamersley province western australia the age versus depth distributions obtained in this study but not available for other regions containing similar supergene iron deposits suggest that little further advance of the weathering front has occurred in the quadrilatero ferrifero lateritic profiles during the neogene the results suggest that weathering in some of these ancient landscapes is not controlled by the steady state advance of weathering fronts through time but may reflect climatic and geomorphological conditions prevailing in a remote past the geochronological results also confirm that the ancient landsurfaces in the quadrilatero ferrifero probably remained immune to erosion for tens of millions of years deep weathering mostly in the paleogene combined with low erosion rates account for the abundance and widespread distribution of supergene iron manganese and aluminum orebodies in this region

the paroo station mine supergene lead deposits western australia geological and geochemical constraints abstract the paroo station mine lead project is situated near wiluna in the central part of western australia it is located in an outlier of the paleoproterozoic earaheedy group overlying the southeast yerrida basin the total measured and indicated mineral resource is currently as at 31st of december 2014 estimated at 31 8 mt of ore at 4 4 pb the deposits are hosted by deeply weathered carbonate and clastic sediments of the paleoproterozoic yelma formation the lead only mineralisation contains only traces of sulfides and is composed of few supergene minerals predominantly cerussite and anglesite with minor pyromorphite and plumbogummite the paroo station mine demonstrates a high degree of supergene pb mobility which has resulted in almost complete destruction of primary pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below the deposits are interpreted to have formed from mississippi valley type mvt stratabound relatively low grade sulfide pb zn mineralisation located within dolomites of the sweetwaters well member of the yelma formation the primary mineralisation based on modelling the pb distribution was controlled by a network of northeast northwest and meridional oriented fractures and small faults lead isotope study yielded from 1 80 to1 88 ga ages of the mineralisation based on the stacey and kramers 1975 two stage model the mineralising event is related to the capricorn orogeny this data is well in line with previous u pb dating of monazites interpreted to be related to the mineralisation 1 82 ga muhling et al 2012 high 238u 204pb values 10 26 10 58 are indicative of the archean basement contribution and or more likely with sediments originally derived from that basement the paroo station mine mineralisation is thought to be similar in style and formed synchronously with the mvt prospects in the teague area in the earaheedy basin approximately 120 km to the northeast the mineralisation at the paroo station mine was significantly upgraded through a multi stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels from top to bottom 1 the base of the collapse quartz clay breccia unit marking the dolomite weathering front 2 the contact of siltstone and sandstone packages coincident with the feldspar kaolinite weathering front and 3 the mid to lower parts of the sandstone unit close to the base of weathering the lowest enrichment zone extends as a continuous blanket at a significant up to 500 m distance at a 1 pb cut off beyond the upper mineralisation projection this enrichment is interpreted to be formed by lateral pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones this active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation

timing and duration of supergene mineralization at the xinrong manganese deposit western guangdong province south china cryptomelane 40 ar 39 ar dating supergene mn oxide deposits are widely distributed in guangxi guangdong yunnan and hunan provinces south china accounting for 18 of the total mn reserves in the country direct dating of supergene mn enrichment however is lacking in this paper we present high resolution 40ar 39ar ages of mn oxides from the xinrong mn deposit western guangdong to place numerical constraints on the timing and duration of supergene mn enrichment a total of ten cryptomelane samples spanning a vertical extent of 67 m were dated using the 40ar 39ar laser incremental heating technique with seven samples yielding well defined plateau or pseudo plateau ages ranging from 23 48 0 91 to 2 06 0 05 ma 2 one sample yields a staircase spectrum that does not reach a plateau the spectrum however indicates the presence of two or more generations of mn oxides in the sample whose ages are best estimated at 22 34 0 31 and 10 2 0 86 ma respectively the remaining two samples gave meaningless or uninterpretable results due to significant 39ar recoil and contamination by old phases the 40ar 39ar data thus reveal a protracted history of weathering and supergene mn enrichment that started at least in the end of the oligocene or beginning of miocene and extending into the latest pliocene staircase apparent age spectra resulting from banded or botryoidal samples yield an average growth rate of mn oxides at 0 6 0 7 10 3 mm kyr 1 the values indicate that a 1 mm grain of mn oxides may host minerals precipitated during a time span of ca 1 5 m y and accumulation of mn oxides to form economic deposits under weathering environments may take millions of years the distribution of weathering ages shows that the oldest mn oxides occur on the top of the profile whereas the youngest minerals are found at the bottom suggesting downward propagation of weathering fronts however two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations such a complexity of age distribution is interpreted in terms of preferential penetration of mn rich weathering solutions along more permeable fault zones or as a result of multi stages of dissolution and re precipitation of mn oxides a synthesis of geochronological and geological data suggests that formation of the xinrong deposit was a consequence of a combination of favorable lithological climatic and structural conditions because the climatic and structural conditions are similar among the provinces of south china during the cenozoic the geochronological results obtained at xinrong may also have implications for the timing of supergene mn enrichment throughout south china

timing of weathering induced alteration of manganese deposits in western australia evidence from k ar and 40 ar 39 ar dating under particular climatic conditions tetravalent k mn oxides can be formed during terrestrial chemical weathering of mn ores in this study the results of k ar and 40 ar 39 ar dating of weathering related k mn oxides mainly cryptomelane k ba mn 8 o 16 xh 2 o from supergene mn deposits in the pilbara and peak hill mn provinces nw australia and from scattered occurrences of k mn oxides found in deeply weathered rocks in western australia are presented in ideal circumstances k ar and 40 ar 39 ar ages will reflect the time since the formation of k mn oxides however 40 ar 39 ar and k ar dating of k mn oxides which formed as a result of chemical weathering may be complicated by admixture of mica inherited from bedrock and by mixing of different generations of weathering related k mn oxides to correct for admixture of inherited mica a two stage k ar dating technique was applied some samples of dated cryptomelane had finely banded textures and therefore might represent several generations the age differences between these generations were investigated using microsampling and laser microprobe 40 ar 39 ar dating total fusion age determinations of several groups of growth bands of a late oligocene densely layered cryptomelane from the woodie woodie mn deposit nw australia suggest an averaged growth rate of 0 8 or 0 3 mm m y however the growth rate calculated for much of the band sequence is 1 5 mm m y which is five times greater than for the remaining youngest group of bands 0 3 mm m y thus suggesting uneven growth rates the low growth rate might be the result of very slow growth of cryptomelane and or the presence of hiatuses in cryptomelane precipitation total fusion ages measured on void filling banded and colloform cryptomelane in samples from a vertical profile through a mesalike deposit near horseshoe nw australia show peaks at 52 to 48 44 to 43 40 to 39 35 30 to 29 and 7 ma these results are interpreted as reflecting formation of weathering products in the deposit over a remarkably extended period possibly in an episodic manner related to alternation of more humid and drier climatic periods during the tertiary the results from this study show some evidence of episodic formation of weathering related manganese oxides within the deposits studied although modification of age patterns by local hydrological and geomorphological factors allows only a limited degree of correlation of these periods on a regional scale

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

dating of chemical weathering processes by in situ measurement of u series disequilibria in supergene fe oxy hydroxides using la mc icpms abstract constraints on the timing of weathering processes in the northern australian regolith have been obtained by in situ measurement of u series disequilibria in u rich supergene fe oxy hydroxides using a laser ablation mc icpms technique this approach has permitted the measurement of 234 u 238 u and 230 th 238 u activity ratios in finely crystalline fe oxy hydroxides from the ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes 230 th 238 u systematics in the finely crystalline fe oxy hydroxides yield 230 th ages ranging from 60 to 350 ka the most reliable 230 th ages cluster between 122 and 216 ka suggesting fe oxy hydroxides formation and associated weathering peaked during the previous 2 interglacial periods this is supported by the 230 th ages and isotope composition of pisolith fe oxy hydroxide pedogenic nodules cores which are demonstrated to behave as closed systems the u isotopic composition of the fe oxy hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite secondary overprinting is evident in some samples as a large range in 234 u 238 u our results suggest that weathering intensity varies with global climate cycles and that together with weathering events dated by 40 ar 39 ar of mn oxides elsewhere in northern australia feng y x and vasconcelos p 2001 quaternary continental weathering qeochronology by laser heating 40 ar 39 ar analysis of supergene cryptomelane geology 29 7 635 638 weathering rates in this region are orbitally forced

quaternary continental weathering geochronology by laser heating 40ar 39ar analysis of supergene cryptomelane incremental laser heating analyses of supergene cryptomelane clusters extracted from three distinct weathering profiles from the mary valley region southeast queensland australia yield reproducible and well defined plateau ages ranging from 346 15 to 291 14 ka 2 a precipitation of supergene cryptomelane in this period implies that relative humid climate prevailed in southeast queensland from 340 to 290 ha a result consistent with oxygen isotope analyses of marine sediments from ocean drilling program site 820 and with regional pollen and spore records these results the first report on the precise ar 40 ar 39 dating of quaternary supergene cryptomelane indicate that ar 40 ar 39 analysis of pedogenic minerals provides a reliable geochronometer for the study of quaternary surficial processes useful in the study of soil formation rates continental paleoclimates and archaeological sites devoid of datable volcanic minerals

40ar 39ar and u th he 4he 3he geochronology of landscape evolution and channel iron deposit genesis at lynn peak western australia u th he geochronology of authigenic goethite cements from the lynn peak channel iron deposit cid hamersley province western australia reveals a history of mineral precipitation ranging from ca 33 to 14 ma massive goethites from nearby weathering profiles at roy hill north a possible source of detrital material during the aggradation of the lynn peak channels yield u th he results as old as ca 64 ma the combination of u th he geochronology with incremental outgassing 4he 3he studies on proton irradiated samples reveals that lynn peak goethites host radiogenic 4he in low retentivity lrd and high retentivity hrd domains and that the hrds account for most of the sample mass and have lost very little of their original 4he over geologic time such high retentivity is especially notable given the goethites were collected from the surface where they were subject to significant heating by solar irradiation minor contamination by detrital fragments of potentially 4he rich primary phases e g rutile ilmenite zircon occurs in some samples fortunately the 4he 3he method permits characterization of this extraneous 4he component which is small 10 wt of the total 4he in the goethite and can be corrected out in estimating the goethite formation age these results indicate that the lynn peak channel was already aggraded and undergoing goethite cementation by ca 33 ma the history of aggradation and channel cementation independently measured through 40 ar 39 ar geochronology is consistent with that obtained from the u th he and 4he 3he record laser incremental heating 40 ar 39 ar geochronology of detrital and authigenic mn oxides primarily cryptomelane kmn 8o 16 xh 2o from the same locality in the lynn peak channel reveals that detrital oxides are older than ca 44 ma and as old as ca 65 ma and authigenic oxides are younger than ca 35 ma and as young as ca 16 ma authigenic cryptomelane precipitation and channel cementation occurred throughout the miocene with a particularly strong period at around 20 ma the 40 ar 39 ar geochronological results suggest that regional weathering profiles developed before 44 ma and possibly as early as ca 65 ma were incised and partially eroded in the 44 35 ma interval semi arid conditions promoted the transport of large volumes of sediments aggrading the regional drainage system metasomatic reactions in the aggraded channels caused the ferruginization and in some places manganese replacement of the cids goethite and k mn oxide cementation continued throughout the oligocene and miocene post miocene aridification contributed to the preservation of the cemented channel sediments forming some of the largest readily mineable iron ore deposits on earth

sims microanalysis of the strelley pool formation cherts and the implications for the secular temporal oxygen isotope trend of cherts abstract the significance of oxygen isotope ratios in archean chert has long been debated cherts from the c 3 4 ga strelley pool formation spf pilbara craton western australia host some of the oldest stromatolite and microfossil evidence for life but the genesis and timing of silica cements has been unclear field relations petrography and a combination of laser fluorination and in situ sims measurements of 18o in quartz show that bedded cherts of the spf were originally precipitated as carbonates and were later widely replaced by quartz three localities were studied and analyzed for 18o qz in chert 1 camel creek foliated metamorphosed bedded cherts and meter scale black chert veins 2 unconformity ridge and abdp8 drill core stromatolitic and bedded chert overlying basal detrital quartz sandstone and 3 the trendall locality bedded stromatolitic chert replacing original dolomite low temperature hydrothermal quartz and mm to decimeter scale chert quartz veins laser fluorination mm scale values of 18o qz range from 14 2 to 18 2 vsmow at camel creek 9 3 to 18 9 at unconformity ridge and 13 7 to 25 7 at trendall values of 18o qz in cm to decimeter scale hydrothermal chert veins cutting bedded carbonates at trendall range from ca 15 to 16 whereas bedded cherts are 17 to 26 these laser data include the highest 18o values reported for cherts in paleoarchean sediments and are up to 4 higher than the upper limit of 22 reported in other studies in apparent contrast to the long standing secular temporal trend which shows such high 18o only in younger chert however analysis by laser fluorination at the 1 mm scale cannot resolve microtextures seen petrographically in contrast in situ sims analyses can resolve petrographic microtextures and show 18o qz at 10 m scale have an even greater range of 7 31 in bedded cherts at the trendall locality up to 9 above the secular temporal trend textures observed optically at the trendall locality were classified as microquartz mesoquartz chalcedony megaquartz veins and cavity megaquartz sem cl imaging shows two generations of meso and megaquartz bright cl with well developed growth zoning and dark cl with massive or mottled texture microquartz is the earliest textural generation of quartz and has a maximum 18o qz of 22 by sims dark cl mesoquartz has similar 18o to microquartz and is interpreted to also be early bright zoned cl mesoquartz which formed post archean has even higher 18o up to 29 vein megaquartz crosscuts most quartz generations and has a restricted range of 18o mostly from 16 to 19 chalcedony pseudomorphs rhombic cavities and fractures lines the edges of veins and has similar 18o to veins 16 19 late cavity megaquartz is bright and zoned by cl grows into late open cavities and has the highest 18o qz values reported from the pilbara up to 31 3 thus the highest 18o quartz cements at the trendall locality are the youngest and may be related to weathering early silicification and the formation of microquartz chalcedony and low 18o mesoquartz occurred during low temperature hydrothermal activity in the archean none of the spf quartz examined is interpreted to have formed as a direct precipitate from paleoarchean seawater thus values of 18o qz do not record either water chemistry or temperature of archean oceans in situ sims analysis shows that high 18o qz values above 22 are only found in late forming cavity megaquartz and bright zoned cl mesoquartz at the trendall locality the spf results from our sample suite demonstrate the ability to resolve complex history using detailed petrography and sims analysis similar studies may show equal complexity of 18o qz data for other localities that are interpreted to show secular temporal trends for chert the apparent increase of 18o qz through time may reflect differences in diagenesis and or an inherent and previously unrecognized sampling bias that compares fundamentally different populations of quartz such as archean hydrothermal chert from volcanic greenstone belts with unrelated phanerozoic biogenic quartz

chronology of pleistocene weathering processes southeast queensland australia 40ar 39ar dating of mn oxides from the upper kandanga uka mn deposit southeast queensland australia shows that the weathering profile overlying the deposit is younger than 1 ma seventy plateau ages obtained for 81 grains from 17 samples fall into three groups i 1000 800 ka n 2 ii 630 510 ka n 2 and iii 400 50 ka n 66 the last age group can be further resolved using the mixture modelling solution by sambridge and compston sambridge m s compston w 1994 mixture modeling of multi component data sets with application to ion probe zircon ages earth planet sci lett 128 373 390 into two major age clusters sub groups iiia and iiib with peak values at 313 4 ka n 43 with an outlier and 213 7 ka n 22 respectively the results suggest that a supergene blanket in a mn deposit may develop relatively fast if weathering and erosional conditions are appropriate the age peaks measured in this study correspond to warm periods identified in specmap oxygen isotope records of deep sea sediments and local paleoclimatic data the correlation of weathering age peaks with oceanic climatic indicators suggests a climatic control on continental chemical weathering where warm and humid conditions are conducive to increasing weathering rates the clustering of supergene mn oxide precipitation ages suggests that weathering rates vary with time supporting an episodic weathering history for the profiles investigated

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

episodic weathering in southwestern amazonia based on u th he dating of fe and mn lateritic duricrust abstract this study presents the results of mineralogical analysis and u th he dating of micrometric grains of hematite goethite cryptomelane and hollandite from six samples of bauxite fe mn mn al fe mn al and mn al p duricrusts located in southwestern amazonia brazil the samples were collected at different elevations to constrain temporal epochs of lateritization and to provide insights into the cratonic landscape evolution of proterozoic and paleozoic terrains in brazil three main phases of duricrust formation were recognized indicating a long period of lateritic weathering the first phase preserved as a fragment in fe duricrust on the paleozoic sedimentary rocks began in the permian 290 ma and lasted until the late cretaceous 65 ma the second and best preserved phase formed mn duricrusts on the paleoproterozoic apui lateritic surface 150 250 m a s l and on the aripuana lateritic surface

sea level changes and buried islands in a complex coastal palaeolandscape in the south of western australia implications for greenfield mineral exploration abstract weathering intensity changes due to climatic variability across tectonically stable portions of continental crust can generate a thick and extensive weathered cover resulting in regolith dominated terrains rdts mineral exploration in rdts is challenging because of the lack of bedrock outcrop and the difficulty of linking surface regolith geochemistry to the geology at depth complex weathering obscures the expression of the basement geochemistry in the regolith and therefore the footprints of mineral systems are difficult to detect the southeast of the yilgarn craton and the albany fraser orogen afo in the south of western australia is an rdt that extends along the coastline and the eucla basin this study proposes a landscape evolution model of the afo driven by transgression regression sea level changes that resulted in the formation of numerous islands and development of estuarine zones this model contrasts with the river system dominated landscape evolution present in the yilgarn craton this difference has significant implications for mineral exploration and geochemical interpretation of the regolith in this region weathering profiles developed on inland and on island are thicker and more mature than those developed in sea inundated areas even if in the yilgarn craton local areas display reworking of weathering profiles and other complexities from permian non marine tertiary sediments and quaternary fluvial and aeolian sediments at a regional scale if vertical geochemical mobility of elements has occurred on inland and on island are more reliable for understanding geochemical anomaly basement relationships whereas the marine inundated areas require a more detailed investigation because of the role of marine reworking of weathering profiles and possible mixture of sediments from different provenances landscape changes from the topographically high dissected yilgarn environment with thick saprolite development and uneven basement topography to the nearly flat regions dominated by sand dunes and thin saprolite development at the coastline these regions are the result of the erosional and depositional effects of successive sea level transgression regression cycles within this framework the following four different regolith settings have been identified in a progressive change from yilgarn craton environments to the modern coastline 1 albany 2 kalgoorlie norseman 3 esperance and 4 neale mapping the palaeocoastlines islands and estuarine zones as well as the region of influence of marine limestones and sediments can significantly improve the understanding of how surface geochemistry relates to the landscape and how it links with the geology at depth and therefore how it may reflect the presence of mineral systems understanding the difference in the landscape evolution between the afo and yilgarn craton is essential to properly calibrate mineral exploration protocols in both regions

distribution of 230th in the labrador sea and its relation to ventilation abstract measurements of dissolved and particulate230th and232th by thermal ionization mass spectrometry were made on samples collected from the labrador sea and the denmark strait and iceland scotland overflow waters the large scale feature of low and invariant230th evident in deep waters of the labrador sea and previously observed in the deep northern atlantic can be reproduced using a reversible scavenging model that includes the effect of ventilation advective transport also must be important in the redistribution of230th in other regions of the atlantic and for other long lived tracers such as231pa 26al and10be

geochronology of the baye mn oxide deposit southern yunnan plateau implications for the late miocene to pleistocene paleoclimatic conditions and topographic evolution 40ar 39ar laser incremental heating analyses of supergene k mn oxides from weathering profiles at the baye mn deposit southern yunnan plateau sw china were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution weathering profiles in the baye mn mine are dominated by 20 30 m thick saprolites which are locally covered by unconsolidated alluvial deposits we analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations in most cases different grains from the same hand sample and different samples from the same site have reproducible 40ar 39ar plateau ages the plateau ages of individual grains are also consistent with the respective isochron integrated and ideogram ages this age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles sixty four grains from 32 hand samples collected from weathering profiles at four sites a to d yield well defined plateau or pseudoplateau ages ranging from 2 98 0 07 to 0 16 ma 2 the age results show that mn oxides become progressively younger towards lower elevations samples collected from the highest elevation site 1855 m a s l have the oldest ages at 2 98 0 07 to 2 83 0 13 ma samples from intermediate elevations 1821 to 1815 m a s l yield ages ranging from 2 91 0 10 to 2 08 0 11 ma and those from the lowest sites 1768 to 1753 m a s l are younger than 1 25 0 08 ma the age versus elevation relationship indicates fast downward propagation of weathering front since the late pliocene and permits estimating the weathering rates at 3 3 3 8 6 6 1 2 and 11 1 1 9 m myr 1 during the time periods of 2 98 2 83 2 91 2 08 and 1 25 to 0 16 ma respectively the average river incision rates estimated from the surface exposed ages range from 235 10 to 416 19 m myr 1 during the past 3 million years the incision rate calculated from minimum exposed ages at sites b d is 33 2 m myr 1 for the time period of 2 91 1 25 ma which is consistent with the slow incision of streams on the ridges over the yunnan plateau since the early pleistocene three mn oxide pebbles contained in the alluvial deposits overlying the saprolite dominated weathering profiles at the two highest sites provided additional information on the timing of weathering four grains from two pebbles yield plateau ages of 6 32 0 19 to 5 27 0 10 ma whereas the other two grains from the third pebble indicate the minimum formation ages of 8 2 0 4 and 9 3 0 3 ma these ages confirm the existence of older weathering profiles now dismantled in the region manganese oxide 40ar 39ar ages of the baye deposit when combined with results from other localities indicate that lateritic weathering and supergene mn enrichment and by inference warm and humid climates conducive to intense weathering have prevailed over the plateau since the middle to late miocene the climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records confirming that supergene mn oxides can be used as a useful proxy for past climate age clusters of mn oxides at 2 9 2 4 1 2 0 8 and 0 6 0 4 ma are broadly coincident with and thus likely reflect intensification events of the indian summer monsoon that brings moisture and abundant precipitation to the yunnan plateau these clusters also coincide with the periods of significant surface uplift in the yunnan plateau demonstrating a causal link between topographic evolution plateau uplift and intensification of the monsoonal climate

using 234u 238u to assess diffusion rates of isotope tracers in ferromanganese crusts abstract ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography interpreting crust isotope profiles however requires an assessment of the rate of diffusion of each element within the crust we address this issue using u and th isotope profiles coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts a depth profile of 230thxs 234u for a north atlantic crust alv539 yields a growth rate of 3 05 mm myr consistent with an existing 10be 9be record but the growth rate implied by a 234u 238u profile in the same crust is 6 times faster due to diffusion of u in the crust the difference between the observed 234u 238u profile and that expected from 230thxs 234u enables the effective diffusivity of u to be calculated as 1 2 10 6 cm2 year recalculated literature data indicate that other crusts have diffusivities which range down to 4 7 10 8 cm2 yr this variation is to be expected given the range of porosity observed for ferromanganese crusts concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust these distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of u such diffusivities are assessed for crust alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts li os and sr have diffusivities of 2 10 4 3 10 5 and 2 10 5 cm2 year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions th nd pb and be have diffusivities less than 10 9 cm2 year and are highly immobile in crusts a reassuring result for th and be crust chronologies and for nd and pb isotope tracing of past water masses using crusts hf has a diffusivity of 3 10 8 in alv539 fast enough to perturb but not destroy its isotope history for studies of hf in this and other crusts diffusion should therefore be considered measuring u isotope profiles provides a potential way of assessing such diffusion for hf or for other elements with relatively low distribution coefficients into ferromanganese crusts

metalliferous manganese oxide mineralization associated with the boleo cu co zn district mexico apart from its large reserves of cu co zn mineralization contained within stratiform ore beds or mantos the boleo district of central baja mexico also contains significant concentrations of mn oxides typically 2 3 wt mn interspersed with hydrothermal sulfides and both hydrothermal and weathering related metal oxide minerals within the mantos mn oxides occur as concentric nodules disk shaped concretions laminae granules or aggregates veins dendrites and various replacements the oxides are composed of varying amounts of hollandite cryptomelane todorkite romanechite pyrolusite and x ray amorphous mn oxide chemically the manto oxides are unique owing to their extraordinarily high abundances of co cu pb and zn typically 2 5 wt but up to 23 wt existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of mn oxides within the mantos a new mno 2 fe 2 o 3 t mgo diagram is proposed in order to discriminate among highly metalliferous mn oxides based on the new classification scheme manto mn oxides record a complex interaction between hydrothermal and diagenetic processes to better understand the mechanism of oxide deposition radiogenic isotopes sr and pb and ree geochemistry were used to delineate fluid and metal sources isotopic and ree constraints indicate that the boleo mn oxides are largely hydrothermal in origin however they show varying degrees of modification resulting from diagenetic processes hydrothermal mn oxides are syngenetic synsedimentary to syndiagenetic in origin and were precipitated in association with sulfides reflecting localized variations in redox conditions within the manto sequence due to the downward infiltration of metal bearing brines apart from morphological and stratigraphic evidence e g occurrence of nodular crusts along the upper contact of the manto a syngenetic origin is supported by a single k ar date of 7 0 0 2 ma which is within the estimated age range for deposition of boleo formation clastic sediments interaction between brine and seawater or brine and pore fluid as it infiltrates the sedimentary pile produced hydrothermal precipitates with certain hydrogenetic attributes i e positive ce anomaly lree enrichment on the other hand oxides with hydrogenetic and with mixed hydrothermal hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids by far the greatest degree of remobilization occurred in response to compactional dewatering of the manto where the dewatering pore waters enriched the uppermost manto strata in mn fe zn co and to lesser degrees cu later diagenetic modifications coincided with the development of a supergene overprint which typifies boleo formation strata at elevations above the present day water table