effects of experimental leaching on rbsr and kar isotopic systems and ree contents of diagenetic illite leaching with 1 n hcl 1 n nh4cl acetone humic acid extracts cation exchange resin and nh4 edta have negligible effects on the rb sr and k ar systems of different size fractions 0 4 and 0 8 2 m of a 1m diagenetic illite none of these treatments including the 1 n hcl treatment induced preferential removal of radiogenic 87sr and 40ar from mineral structures however the 1 n hcl treatment removed significant amounts of sr and ree as compared to the treatments by nh4 edta and cation exchange resin however much of these hcl derived elements resided in non clay minerals such as carbonate and fe oxide phases the k ar and rb sr ages of the samples were not affected by the hcl treatment any more than by the nh4 edta or the cation exchange resin because of potential loss of solid material during elutriation of differentially treated samples the hcl treatment is preferred over other methods allowing sufficient controls on the isotopic and mass balances between the leachate and the corresponding residue of an untreated sample for rb sr isotopic dating purposes of clay minerals from sedimentary and metasedimentary rocks leaching experiments give information about the ambient environment of crystallization analysis of both the leachate and residue under carefully controlled conditions is recommended

petrologie du gisement lateritique manganesifere d azul bresil at azul state of para brazil the lateritic weathering of organic shales with rhodochrosite and phyllosilicates mainly muscovite has led to one of the largest maganese deposits of brazil 45 mt of reserves with 42 65 mn

chemical weathering of mn garnets under lateritic conditions in northwest ivory coast west africa abstract lateritic profiles developed on mn 2 bearing metamorphic rocks in the ziemougoula area northwest ivory coast permit the observation of chemical weathering of garnets essentially spessartites two main stages are found in the process of weathering in the lower part of profiles spessartites alter principally to birnessite and the al content of the parent garnet is leached out of the zone higher in the profile garnets alter directly to lithiophorite in this geochemical environment al is no longer mobile in such a layer the earlier formed birnessite is transformed into nsutite along with minor cryptomelane these latter minerals can locally evolve into ramsdellite or pyrolusite the associated nsutite and lithiophorite are the principal phases of the hard manganese crust capping the hillrocks of the lateritic landscape of the ziemougoula area

sr and ar isotope studies of detrital smectites from the atlantic ocean d s d p legs 43 48 and 50 mineralogical morphological and isotopic rb sr and k ar determinations were made on some detrital smectites of palaeocene and cenomanian ages from d s d p drillings in the atlantic ocean these minerals are not inert in their depositional environment authigenic laths grow on detrital sheets with sharp borders this authigenesis could occur slightly after deposition in a closed system for some of these smectites it has been tentatively quantified by the rb sr and k ar isotopic methods which seem also well suited to evaluate the chemical extent of this authigenesis at least no preferential loss of 40ar vs 87sr could be detected in the minerals even in those which are smaller than 0 2 m

40ar 39ar dating of west african lateritic cryptomelanes abstract dating weathering processes is an innovative approach to understanding climate dependant modifications of the earth s surface potassium bearing manganese oxides are a ubiquitous feature of weathering mantles developed from mn bearing parent rocks recent studies have demonstrated the possibility of dating k rich manganese oxides from the hollandite group such as cryptomelane k 1 2 mn 8 o 16 nh 2 o using 40 ar 39 ar argon ion laser probe method this work applies that approach to cryptomelanes formed in the lateritic manganese ore deposit of tambao northern burkina faso a present unexploited deposit among the cryptomelane richest in the world two samples of massive mn rich concretions 10mm and 4mm in thickness were collected from the massive 80 m high mn rich hill at tambao a detailed petrographical study shows that these cortical formations are composed of successive na rich cryptomelane layers with na na k ba up to 20 seven and five subsamples from these two concretions were studied with a 40 ar 39 ar laser probe the age spectra indicate ages that are coherent with the general organization of the concretions showing a regular decrease from cores towards rims within the two concretions ages range from 49 8 0 4 ma to 48 1 0 1 ma and from 47 5 0 3 ma to 44 5 0 4 ma respectively the youngest ages being obtained for the most external layers the integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm ma these ages clusters show that weathering at tambao was functioning during the early eocene and thus suggest that there is a relationship between the formation of the bauxites of the african surface and the mn fe laterites

k ar and rb sr ages of celadonites from the troodos ophiolite cyprus k ar dating and rb sr isochrons of two celadonite veins from the troodos ophiolite cyprus consistently yield precipitation ages of 77 80 and 87 92 ma respectively their k ar bulk rock age data 84 7 2 3 ma combined with the oldest celadonite age and recently published radiolarian ages suggest that the troodos ophiolite was formed between 85 and 92 ma providing an estimate for the duration of celadonite formation of at least 7 m y but possibly up to 15 m y these data are consistent with data from deep sea drilling project holes suggesting that precipitation of the most significant portion of vein minerals in the oceanic crust occurs within about 12 m y but possibly up to 19 m y after crustal formation leaching experiments suggest that k ar rb and sr in exchangeable and nonexchangeable sites of the structure of stoichiometric celadonite maintain their isotope characteristics and are resistant to change from diagenetic processes or surface alteration

40ar 39ar laser probe dating by step heating and spot fusion of phengites from the dora maira nappe of the western alps italy 40ar 39ar laser probe dating of phengitic micas has been carried out by step heating and spot fusion procedures these micas represent successive deformation stages in the structural evolution of the internal dora maira nappe western alps single phengites from a gneiss affected by a single ductile strain under retrogressive conditions sample 99 1 display complete isotopic resetting with nearly homogeneous intracrystalline ar distribution and yield plateau ages of about 40 ma small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist sample ptx3 they show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a concentric age zoning from 68 ma on the rim to 87 ma in the core on one cleavage plane this zonation is fully consistent with the laser derived discordant age spectrum which ranges from 40 to 90 ma from low to high temperatures according to the deformation history of both samples these preliminary data suggest a deformation control on ar migration during recrystallization processes and they are consistent with the timing of the collisional evolution previously reported for southern dora maira units this study shows that the 40ar 39 ar continuous laser probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

the effect of fe spin crossovers on its partitioning behavior and oxidation state in a pyrolitic earth s lower mantle system abstract geophysical interpretations of the earth s interior and its dynamics are significantly influenced by phase transitions of the constituting minerals and their chemical compositions pressure induced fe spin crossovers in the main mineral phases of the earth s lower mantle mg fe silicate perovskite and ferropericlase have been suggested to influence fe partitioning resulting in separate layers with distinct physical properties however previous results remain ambiguous regarding the exact effect of fe spin crossovers and the actual transition pressures we observe here a continuous decrease of the fe 2 mg partition coefficient k d between silicate perovskite and ferropericlase from 25 gpa to 79 gpa in a pyrolitic earth s lower mantle system at about 97 gpa the k d significantly increases with an accompanied decrease of the fe 3 fe ratio in perovskite which therefore leads to an amplified change in the fe 2 k d we conclude that the fe 2 high spin to low spin crossover in ferropericlase and the fe 2 high spin to intermediate spin crossover in perovskite at mid lower mantle pressures 30 80 gpa exert no control on k d but the fe 2 intermediate spin to low spin crossover in silicate perovskite at about 100 gpa preferentially partitions fe into silicate perovskite and reduces its fe 3 content the change in oxidation state and partitioning behavior of fe will increase thermal conductivity and probably could induce a thermal boundary layer at this depth

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

chemical and physical erosion rhythms of the west african cenozoic morphogenesis the 39 ar 40 ar dating of supergene k mn oxides chemical weathering and mechanical erosion are first order processes of long term tropical morphogenesis which is still poorly deciphered for lack of time constraints we address this issue by laser probe 39ar 40ar dating of generations of cryptomelane k1 2mn8o16 nh2o from the manganese ore deposit of tambao in northern burkina faso this mn deposit results from the supergene weathering of carbonate and silicate mn protores underneath lateritic palaeolandsurfaces it consists of an upper cryptomelane rich domain and a lower domain where pyrolusite b mno2 is the dominant mn oxide the oldest 39ar 40ar ages 59 45 ma are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts 3 4 2 9 ma apparent correlations of 39ar 40ar age groups with d18o and eustatic curves allow definition of the different stages of morphogenesis paleocene eocene ages 59 45 ma bracket a greenhouse period propitious to bauxitic weathering the lack of significant ages between 45 and 29 ma characterizes a period dominated by mechanical erosion during which detrital sediments including lateritic materials were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface two major weathering periods separated by a second erosion episode 24 18 ma are also depicted at the end of oligocene 29 24 ma and lower to mid miocene 18 11 5 ma in the upper domain during which newly shaped land surfaces conspicuously weathered the shorter weathering and erosion episodes recorded in the lower domain from 18 to 2 9 ma led to the final geomorphic changes that were conducive to the formation of glacis the preservation of old cryptomelane 59 45 ma in the upper part of the ore deposit indicates a cenozoic denudation limited to the erosion of previous bauxites and partly of ferricretes

estimated solubility products and fields of stability for cryptomelane nsutite birnessite and lithiophorite based on natural lateritic weathering sequences

dating constituent formation in soils to determine rates of soil processes a review abstract rates of soil processes are poorly known one approach to determine them could be through the radiogenic isotopic dating of soil constituents such methods have been recently used to date minerals in soils and weathering profiles thanks to the development of high resolution techniques such as 40ar 39ar u series of micro drilled samples u series by laser ablation and u th he and to the development of suitable methodology expanding application of the techniques to a variety of supergene minerals e g fe and mn oxihydroxides carbonates etc in this paper we identify i the main soil constituents that can be dated through these isotopic methods that are mainly fe oxides mn oxides pedogenic carbonates and organic matter ii the main pedogenic processes that leads to their formation in soils and iii the different soil settings that allow transforming soil constituent ages into rates of soil processes namely growth bands transformation fronts and chronosequences

k ar and 40ar 39ar geochronology of supergene processes in the atacama desert northern chile tectonic and climatic relations new k ar and 40 ar 39 ar data obtained for 29 samples of supergene alunite jarosite and hollandite group minerals from the central depression of northern chile document almost continuous supergene oxidation from 33 to 9 ma the downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m ma 1 these calculated rates are sensitive to pre existing topography and show that the present relief observed in the central depression represents a modified miocene landscape the results presented here are in good agreement with previously published geochronological data for the atacama desert the combination of all published data including the results presented here defines a continuous skewed distribution with a maximum between 21 and 14 ma the relative paucity of ages greater than 30 ma reflects the removal of supergene profiles by erosion during and immediately after the incaic compressive deformation from 29 to 9 ma supergene oxidation profiles were generated under semi arid conditions throughout the atacama region ages between 9 and 5 ma on the northern and southern margins of the desert mark the end of supergene oxidation processes consistent with a pliocene onset of present day hyperaridity

alteration supergene des protores a grenats manganesiferes dans quelques gisements d afrique de l ouest l alteration meteorique de quartzites a grenat spessartite precambriens conduit sous des climats tropicaux humides a la constitution de minerais oxydes secondaires ceux ci resultent de la superposition dans un ordre variable de divers processus elementaires en particulier hydrolyse des grenats mn ca fe sont elimines et une phase amorphe silico alumineuse subsiste seule neoformation de lithiophorite du manganese importe se combine avec l aluminium des produits amorphes silico alumineux lessivage un vide apparait au centre du grenat altere et s agrandit jusqu a ne laisser subsister que la matrice de quartz piquetee de cavites polyedriques epigenie par la cryptomelane la gangue de quartz est remplacee par la cryptomelane les grenats pouvant rester inalteres

detachment faulting in a bivergent core complex constrained by fault gouge dating and low temperature thermochronology abstract the central menderes massif in western turkey is a bivergent metamorphic core complex characterised by two detachment faults that operated with opposite sense of shear here we present k ar ages and hydrogen isotope data for fault gouge as well as fission track and u th he ages for apatite and zircon from bedrock samples to constrain the timing of detachment faulting comparison of the k ar ages and low temperature thermochronologic ages indicates brittle faulting at temperatures between 240 and 70 c at the western termination of the gediz detachment k ar ages between 12 and 5 ma suggest rather continuous faulting since the mid miocene a pliocene phase of rapid slip is mainly recorded in the eastern part of this detachment in contrast k ar ages from the buyuk menderes detachment cluster at 22 16 and 9 3 ma and provide evidence for two phases of faulting which is in accordance with new and published low temperature thermochronologic data hydrogen isotope ratios of 76 to 115 indicate infiltration of meteoric water during detachment faulting and gouge formation the youngest k ar ages and cooling histories for hanging and footwall rocks imply that the activity of both detachments ended 3 ma ago when the currently active grabens were established

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

lateritic concentrations of manganese oxyhydroxides and oxides latentes result from the weathering of a variety of rocks under humid tropical conditions when developed at the expenses of mn2 bearing parent rocks laterite consists of a thick mantle made up of manganese oxides and hydroxides two main examples are described from tropical africa the first example displays a lateritic profile on metamorphic rock the second example deals with lateritic profiles developed on a precambrian sedimentary rock from the base to the top of the studied profiles different mn hydroxide and oxide sequences have differentiated their composition depends on i the constitutive parent minerals with vary ing weathering rates ii the intensity of weathering which progresses towards the profile top permitting pseudomorphic replacement of mn4 oxides for early formed mn3 hydroxides most of the manganese ore result from a relative accumulation however mn can also be mobilized in different places of weathering profiles leading to accumulations in other places mainly at the base of weathering profiles that absolute accumulation can occur in non mn bearing weathering horizons kaolinitic horizons to form mn nodules in which kaolinite is dissolved this process provides al which combines to mn3 forming al mn hydroxides lithiophorite

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

some observations on the interpretation of 40ar 39ar age spectra several recent applications of the 40ar 39ar age spectrum technique have utilized the ability of this technique to resolve the internal ar isotope distribution in crystals for the purpose of placing constraints on the timing of thermal events additionally a few of these studies have attempted to use certain properties of the age spectrum to estimate the temperature and duration of heating experienced by the sample during the disturbance although these successful applications are encouraging the interpretation of age spectra of disturbed samples is in many ways still an art form with the beholder apparently relatively free of constraints that might limit an assessment in fact there are a number of restrictions on interpretation imposed by 1 multiple episodic loss 2 slow cooling 3 mixed phases 4 resolution of the age spectrum 5 excess 40ar 6 recoil artifacts 7 grain distribution and size and 8 phase changes while in the vacuum extraction system these effects are discussed with a view to understanding possible ambiguities in ar release patterns in most cases only with independent geochronological knowledge an understanding of the release systematics and complete sample characterization can these constraints be successfully dealt with in a quantitative framework

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

first 40ar 39ar geochronology of lateritic manganiferous pisolites implications for the palaeogene history of a west african landscape abstract a pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in west africa using 40 ar 39 ar laser probe analysis on different generations of cryptomelane from fe mn pisolites and their embedding lateritic matrices prior to the 40 ar 39 ar geochronological analysis a detailed petrographical study has allowed the identification of successive assemblages from mn pisolites to fe mn pisolites embedded in mn rich and fe rich matrices respectively the age spectra obtained for pisolites including cores cortices and matrices enables the definition of three age clusters around 56 59 44 47 and 24 27 ma the first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced mn leaching and fe al accumulations due to warmer and wetter climatic conditions in the age interval 56 47 ma that characterizes the tertiary greenhouse effect period propitious to bauxite formation in west africa the absence of 40 ar 39 ar ages between 44 and 27 ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled but could still be favourable to fe and or al oxy hydroxides formation before drier climatic conditions became rather effective at the oligocene period to sustain mechanical erosion rather than chemical weathering processes the age cluster 24 27 ma indicates however a reactivation of the manganiferous lateritic weathering late oligocene the 40 ar 39 ar dating results are discussed in terms of climatic condition changes during the palaeogene that were favourable to the genesis of either al and fe or mn oxy hydroxides in the course of development and evolution of the pisolitic formation the 40 ar 39 ar dating also provides a new highlight to large scale geomorphological patterns of west africa

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