ferromanganese oxide deposits from the central pacific ocean i encrustations from the line islands archipelago abstract chemical and mineralogical analyses of a well controlled suite of ferromanganese encrustations from the line islands archipelago central pacific suggest that they represent purely hydrogenous deposits i e they have formed through the slow accumulation of trace metal enriched oxides directly from the water column mineralogically they consist predominantly of mno2 and amorphous feoohxh2o compositionally they are similar to mno2 nodules from adjoining basinal areas but are enriched in both mn mean 20 4 max 29 3 and co mean 0 55 max 1 57 mno2 is the most important trace metal bearing phase strong associations are noted between it and co mo ni zn and cd whilst only be is associated specifically with feooh v sr and pb are partitioned between the authigenic oxide phases whilst ti most probably occurs as tio2xh2o cu is contained in both aluminosilicate contaminant phases and fe oxide phases these relations are considered to reflect the differing scavenging behaviour of mn and fe oxides in the water column crusts from 1 2 km are enriched in mn and the mn related elements and exhibit higher mn fe ratios than deeper crusts which are compositionally constant the higher mn fe ratios may result from a supply of mn from continental borderland sediments at these depths which is transported horizontally by advective diffusive processes since manganophile elements are enriched relative to mn in the 1 2 km crusts it is considered that the supply of mn is scavenged by existing oxides is oxidised and effectively occludes them a higher proportion of oxide particles thus exhibit mn oxide scavenging properties in the 1 2 km depth zone the increased vertical flux of mn resulting from the supply at 1 2 km is not reflected by higher mn fe ratios in deeper crusts so that the vertical flux of oxides is not simply related to the standing crop the mn fe ratios of the crusts thus reflect the composition of suspended oxides at similar depths

lower cambrian paleosols from the cantabrian mountains northern spain a comparison with neogene quaternary estuarine analogues abstract the lower cambrian barrios beds cantabrian mountains consist of a siliciclastic succession deposited in deltaic environments punctuated by intertidal and fluvial systems in the barrios de luna area this succession displays a progradational depositional systems tract that contains linkages between upper offshore tidal sand bars fluvial channels and floodplain environments the uppermost fluvial channels are encased within floodplain or interfluvial deposits some of which develop muddy paleosols the detailed micromorphological scanning electron microscopy sem geochemical and mineralogical studies of their pedogenic profiles indicate the presence of three well differentiated horizons fragmentation and shrinking of the upper horizons originated a network of channel like to vuggy porosity occluded by microlaminated clays cyclically emplaced by tidal floods and seasonal oxidation the middle horizons have concretionary to vermicular textures associated with microbioturbation and bacterial colonization and the lower ones are mottled containing numerous preserved lithorelics the barrios sulfidic ustifluvent paleosols represent short time pedogenic events under semiarid subtropical conditions and support previous studies of cambrian microbial life on subaerially exposed microenvironments

physical speciation of trace metals in fe mn concretions from a rendzic lithosol developed on sinemurian limestones france abstract because of their high content in heavy metals fe mn concretions present in soils can be a source of release of trace metals in the environment metal rich concretions were isolated from a top horizon of a rendzic lithosol developed on limestones in france the distribution of metallic elements cr co ni cu zn pb was studied at different scales field sampling isolated concretions and individual particles methods of investigation combined bulk chemical analysis optical microscopy x ray diffraction xrd and microanalysis using scanning and transmission electron microscopes the concretions are constituted by concentric rings and contain quartz feldspars kaolinite mica goethite and ti oxide as crystalline phases a strong heterogeneity in the chemical composition is evidenced at all different scales the total composition of different populations of fe mn concretions depends on field sampling metals are detected at the micrometer and nanometer scales with variable frequencies occurrences reflecting the abundance of metal bearing components and the size of metal clusters statistical treatments of microanalytical data show marked trends a strong positive correlation of mn and to a lesser extent of ca with ni zn and to a lesser extent with co and cu indicates trace metals to be associated with poorly crystallized mn ca rich areas the correlation of pb with fe and p suggests its incorporation in an fe rich phosphate component no correlation of metals with well crystallized silicates fe sesquioxides or sulfate minerals is established cr was diffusely distributed

monsoon fluctuations in two 20 000 yr b p oxygen isotope pollen records off southwest india abstract two marine pollen diagrams for sediments off the southwest coast of india provide a 20 000 yr history of the nearby continental vegetation determined by the monsoon climate within the framework of the isotopic stratigraphy two important phases of the evolution of the monsoon climate are a very arid period about 22 000 18 000 yr b p and a very humid period culminating at 11 000 yr b p the very arid period corresponds to the lowest pollen representation of mangrove vegetation which is used here as an indirect indicator of monsoonal runoff this aridity is due to a very weak southwest airflow a great reduction of summer monsoonal rainfall and reduced runoff of the western ghats rivers the extension of the monsoon over india after the last glacial maximum has been a gradual process following the northward progression of the intertropical convergene zone the very humid period corresponding to the highest pollen representation of mangrove vegetation and the heaviest summer rainfall is a climatic response to the maximum summer insolation of the northern hemisphere at 11 000 yr b p

the distribution of rare earth and minor elements in manganese nodules and sediments from the equatorial and s w pacific abstract 62 manganese nodules and 17 associated sediments from the equatorial and s w pacific have been analyzed for a number of elements including the rare earth elements ree by instrumental neutron activation sc co as hf th and ree occur in higher concentrations in s w pacific than in equatorial pacific nodules reflecting the higher iron content of the s w pacific nodules these elements are probably incorporated nondiscriminantly into the iron oxyhydroxide structure by direct sorption from seawater because they are too large to be incorporated into the manganese oxide structure the highest ce la ratios are also found in the s w pacific nodules this may result from the fact that these nodules lie beneath the path of the fast flowing well oxygenated antarctic bottom water aabw which facilitates oxidation of ce to the tetravalent state if so this supports the idea that the ce la ratio of manganese nodules is an important redox indicator negative cerium anomalies were noted for nodules from areas f and g can be interpreted on the same model the ree contents in equatorial pacific siliceous oozes and s w pacific red clays are similar on a carbonate free basis the ce la ratio of the s w pacific red clays is however much higher than that of the equatorial pacific sediments possibly reflecting the preferential oxidation of ce to the tetravalent state beneath the aabw there appears to be no systematic relationship between the ree abundance in the nodules and their associated sediments although absolute ree abundances are higher in nodules than in their associated sediments nodules with the highest ce la ratios are however found on sediments with the highest ce la ratios the principal difference in the ree distribution pattern of the nodules and sediments lies in the well known cerium anomaly which is positive to slightly negative for nodules and always negative for sediments

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

mineralogy of atmospheric microparticles deposited along the greenland ice core project ice core the mineralogical composition of atmospheric dust particles changes along the greenland ice core project grip ice core in phase with other paleoatmospheric records showing that climatic changes affect the location the relative extent and the surface characteristics of the dust source areas fifty grip ice samples from the eemian to the last glacial maximum were analyzed using a scanning electron microscope coupled with x ray energy dispersive system microprobe more than 20 mineral types or groups were identified and their relative abundance was evaluated a different mineralogical composition was systematically observed in cold and warm time periods quartz illite chlorite micas and feldspars prevail in the mineral assemblage associated with cold stages or phases marine isotope stage mis 4 mis 2 cold part of mis 3 fast variations while kaolinite and fe hydr oxides dominate in warm episodes mis 5e5 warm parts of fast variations the first dust mineral assemblage is related mainly to the mechanical weathering prevailing in arid and cold mid high latitude areas while the second dust mineral assemblage is related mainly to deep chemical weathering in warm and humid low latitude areas k c and o q ratios may be assumed as indicative of the relative importance of low latitude versus arid and mid high latitude source areas it is worthwhile to mention that the first warmer part of eemian 5e5 differs in dust mineralogy from the second part 5e4 5e1 and that also the fast variations are accompanied by clear mineralogical changes of the dust microparticles the changes of the atmospheric dust mineral background are possibly related to changes of the atmosphere dynamics meridian pressure gradient polar front shift storm track changes etc and or to changes in the relative extent of the deflation source areas

composition diagenesis and weathering of the sediments and basement of the callabonna sub basin central australia implications for landscape evolution abstract the evolution of the callabonna sub basin central australia is described in terms of sediment provenance stratigraphy sedimentology and postdepositional mineral and chemical alteration six mineral exploration drillholes in the mundi mundi plain were studied using a combination of qualitative and quantitative xrd pimatm portable infrared mineral analyzer xrf normative analysis and sem the sedimentary cover consists of 160 210 m of diagenetically modified fluvio lacustrine tertiary clay silt and sand comprising sandy mottled and gray clay units overlain by quaternary soil the basement consists of weathered proterozoic schist gneiss siltstone and shale quartz kaolinite smectite and muscovite are the dominant minerals within the sediments whereas the weathered bedrock contains quartz feldspar kaolinite illite and muscovite characteristic wavelength absorption features from pimatm show that the youngest sandy clay unit exhibits the lowest reflectance intensities because of its lower diagenetic maturity weathering of primary minerals within the basement produced an illite kaolinite rich saprolite whose clays exhibit strong reflectance intensities and well developed crystallinity the micromorphology of all secondary products reflects their environment of crystallization low porosity and anoxic conditions within the sediments resulted in poorly developed clays of variable textures colors and compositions conversely the weathered bedrock developed over protracted periods of in situ weathering resulting in better developed secondary products this study distinguishes between preburial weathering of the basement detrital mineralogy postdepositional alteration of the sedimentary cover and subsequent weathering of previously formed mineral assemblages this has widespread implications for paleotopographic weathering reconstitution and landscape evolution thus 1 the extent of weathering of basement relative to diagenesis and maturity of sediments and gradational nature of their unconformity suggests that most of the alteration of basement occurred prior to and was sustained after deposition of sediments and 2 morphotectonic evolution of the region controlled the composition of transported materials by changing the style and patterns of fluvial systems further influencing the distribution and preservation of sediments and weathered bedrock and hence regional landscape morphology

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

elemental partitioning into mn and fe oxides derived from dolomitic shale hosted pbzn deposits northwest queensland australia abstract exploration for dolomitic shale hosted base metal mineralization in northwest queensland is largely dependent upon geochemical evaluation of ironstones fe oxides because of their abundance in the weathered zone often quantitatively account for most of the base metal content of an ironstone mn oxides are less well developed but provide particularly good hosts for ba zn and pb and hence assume importance where present the mn fe coatings found in stream coatings vigorously scavenge and concentrate solution transported base metals and hence have to be very carefully evaluated when used during exploration however the present survey of base metal contents in gossans and ironstones in northwest queensland has shown that the ba cu p pb ni s and zn contents of the mn and fe oxides can be used to accurately indicate their parent material only ce and co are possibly preferentially incorporated into mn oxides but neither element is a pathfinder for pb zn mineralization development of the mn oxides chalcophanite todorokite lithiophorite and the cryptomelane group incorporating k ca mg ba co zn and pb reflects acid conditions during sulfide oxidation when an ironstone consistently contains mn oxides with greater than 1 pb or 10 zn it can be regarded as derived from pb or zn rich mineralization as ba is often associated with sulfides and facilitates the incorporation of pb into cryptomelane ba without accompanying base metals in a mn oxide implies derivation from base metal barren pyrite and or pyrrhotite cu pb zn al phosphate sulfate and arsenate are all incorporated into fe oxides however only pb which is more strongly partitioned into hematite and al and phosphate which are preferentially incorporated into goethite show clear partitioning between the fe oxide phases the preferential incorporation of phosphate rather than sulfate into an fe oxide makes areas showing higher sulfate contents particularly good targets knowledge of the nature of the distributions of pathfinder elements helps understanding of the associations seen in whole rock analyses and leads to rational application of geochemical data during exploration

morphological and interlayer geochemical studies on manganese nodules from the southwestern carlsberg ridge abstract mixed facies of manganese nodules from the southwestern carlsberg ridge have been analysed for mn fe ni cu co and zn we present 71 analyses including the averages for outer layer inner layer and near core material morphological studies show seven types of nodules of which medium size spheroidal ellipsoidal and intergrown nodules are most abundant only spheroidal and ellipsoidal nodules with rough surfaces show higher concentrations of mn fe cu ni co and zn towards the outer layer whereas the smooth surface nodules other than the spheroidal type show higher concentrations of these elements towards the core region the other nodule types show a decreasing content of these elements in all the layers analysed copper and ni are present in low concentrations and are associated with zn and mn the mineralogy indicates diagenetic processes involving metal remobilisation and the chemistry suggests the precipitation of metals mostly from seawater variations in the source of supply of the elements are responsible for the observed variations in the chemical composition of the nodules

radiation induced defects in kaolinites indirect assessment of radionuclide migration in the geosphere abstract defect centres are demonstrated in natural kaolinites by means of electron paramagnetic resonance the irradiation origin of these paramagnetic defect centres pdc is demonstrated by artificial irradiation four types have been used x rays rays he and pb2 ion beam implantation of kaolinites representing different formation conditions hydrothermal weathering synthesis several types of pdc designated a a and b centres are clearly identified and can be separated with respect to their relative stability one a centre is stable over geological periods moreover it is shown that defect centres in kaolinites reflect effectively the cumulative radiation dose the potential use of tracing radionuclide migration through pdc in kaolinite has been tested on three low temperature alteration systems some associated with u concentrations 1 a hydrothermal alteration system nopal i chihuahua mexico and 2 two weathering systems in tropical areas latentes from brazil and cameroon the kaolinites investigated show a significant concentration of pdc which corresponds with background natural irradiation the different types of pdc detected may form a record of successive irradiations of kaolinite linked directly to geochemical conditions prevailing during and after kaolinite growth because of their different thermal stability these different centres can be considered as being formed at different periods this makes a unique tool for tracing the dynamics of the transfer of radionuclides and allows the use of kaolinite as a very sensitive in situ dosimeter because some of the geological settings chosen for nuclear wastes are located in environments containing kaolinites pdc dosimetry could be directly useful to a safety assessment of these wastes

pedogenic lithogenic or anthropogenic origin of cr ni and v in soils near a petrochemical facility in southeast mexico abstract large cr v and ni contents were found in soils near cd pemex in tabasco mexico their origin was hypothesized to be either anthropogenic derived from emissions of a petrochemical facility or lithogenic from the volcanic ash deposited by the el chichon volcano or accumulated residually by pedogenesis we proved these possible origins by analyzing composite samples taken from the soil surface at different distances from the facility as well as samples from two soil profiles located close and far away from the facility within the same land unit i e acrisols developed on miocene marine terraces the sites located far away from the facility were closer to the volcano and therefore supposedly more affected by the ash deposition all samples were analyzed for their total metal content and profile samples for their different metal fractions additionally the soil microstructure mineralogy and elemental composition of particular pedogenic features were studied in thin sections from the different soil horizons results evidenced that the emissions from the petrochemical facility do not contribute significantly to the total concentration of cr ni and v in the adjacent soils nickel was found in the soil in concentrations between 39 and 318 mg kg 1 particularly large contents of this element were associated to volcanic ash additions visualized in the thin sections micromorphological analyses of nodule like pebbles found in both profiles evidenced that these features are laterite fragments transported from preexisting soil the matrix of these fragments is enriched with cr and v 3000 7000 mg kg 1 of cr and 1000 3000 mg kg 1 v and these elements are actually translocated into relatively newly formed iron concretions found within these laterites and reaching concentrations of up to 10 000 17 000 mg kg 1 cr the fractionation showed that cr and v were dominantly associated to crystalline iron oxides and to the residual fraction water extractable exchangeable and edta extractable fractions were below detection limits we conclude that the studied elements are of lithogenic and pedogenic origin they were enriched dominantly through residual accumulation by lateritization probably of mesozoic ultramafic rocks from the margins of the caribbean plate and later fragmented and transported by water to the present location only ni was also partly added by the recent volcanic activity

glacial to holocene fluctuations in hydrography and productivity along the southwestern continental margin of india oxygen isotope and sedimentological records of a sediment core from the southwestern continental margin of india are used to reconstruct the fluctuations in sea surface hydrography and productivity during the last deglaciation the delta o 18 records of globigerinoides ruber and globigerinoides sacculifer exhibit high amplitude oscillations during the early deglaciation and holocene significant variations in delta o 18 during the holocene and a high glacial interglacial amplitude in delta o 18 values delta delta o 18 similar to 2 1 o suggest large fluctuations in sea surface hydrography related to monsoonal precipitation on land our results suggest that although the last deglacial events are coherent in time with the regional and global climatic events fluctuations in sea surface salinity during holocene are unique to this region maximum monsoonal precipitation appears to have occurred after similar to9 8 ka bp unlike the records from the western arabian sea biological productivity was maximum between similar to 18 and 15 ka bp and substantially diminished between similar to 13 and 6 ka bp the increased strength in the winter monsoon during the glacial period could have enhanced convective mixing and nutrient injection leading to high primary production reduced surface productivity during the early holocene may be attributed to the complex dynamics of the circulation system and hydrological cycle along the southwest coast of india c 2001 elsevier science b v all rights reserved

pedogenic manganese oxide coatings calcium buserite on fracture surfaces in tortonian upper miocene red mudstones southern turkey abstract fracture surfaces of red mudstones overbank deposits within the kuzgun formation of late miocene tortonian age are coated by thin black colored manganese oxide beneath a thin soil cover in the mersin area of southern turkey these manganese oxide coatings have been investigated by a range of analytical techniques including x ray diffraction xrd infrared spectral ir differential thermal analysis thermal gravimetric dta tg scanning electron microscopy and energy dispersive x ray sem edx analyses the basal reflections at 10 00 7 22 5 00 3 35 2 50 2 29 and 2 10 a sharp infrared spectral lines at 469 and 519 cm 1 and typical dehydration of interlayer water

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

internal microfeatures of manganese nodules from the central indian ocean abstract the internal features of manganese nodules from abyssal hills ranging in height from 600 to 880 m and the nodules from abyssal plains in the central indian ocean have been studied nodules from the abyssal hills show mainly parallel laminations whereas nodules from the abyssal plains show prominent well developed columnar structures features such as botryoids and fractures filled with fe and mn oxides are more common in the nodules of abyssal plains whereas the clay content and clastic grains are more common in abyssal hill nodules compositional studies of layers within the nodules show that mn ni cu and zn increase from the nucleus to the outer layer while fe and co are inclined to decrease the innermost layer adjacent to the nucleus is hydrogenetic mn fe ratio of mn fe ratio of 4 2 5 4 and contain mainly todorokite with minor mno2 phillipsite and quartz a mixing model shows that the contribution of interstitial water to the outer diagenetic layers was about 75 and that of sea water was about 25

paleoenvironmental analyses of an organic deposit from an erosional landscape remnant arctic coastal plain of alaska abstract the dominant landscape process on the arctic coastal plain of northern alaska is the formation and drainage of thaw lakes lakes and drained thaw lake basins account for approximately 75 of the modern surface expression of the barrow peninsula the thaw lake cycle usually obliterates lacustrine or peat sediments from previous cycles which could otherwise be used for paleoecological reconstruction of long term landscape and vegetation changes several possible erosional remnants of a former topographic surface that predates the formation of the thaw lakes have been tentatively identified these remnants are characterized by a higher elevation a thick organic layer with very high ground ice content in the upper permafrost and a plant community somewhat atypical of the region ten soil cores were collected from one site and one core was intensively sampled for soil organic carbon content pollen analysis and 14c dating the lowest level of the organic sediments represents the earliest phase of plant growth and dates to ca 9000 cal bp palynological evidence indicates the presence of mesic shrub tundra including sedge birch willow and heath vegetation and microfossil indicators point to wetter eutrophic conditions during this period carbon accumulation was rapid due to high net primary productivity in a relatively nutrient rich environment these results are interpreted as the local response to ameliorating climate during the early holocene the middle holocene portion of the record contains an unconformity indicating that between 8200 and 4200 cal bp sediments were eroded from the site presumably in response to wind activity during a drier period centered around 4500 cal bp the modern vegetation community of the erosional remnant was established after 4200 cal bp and peat growth resumed during the late holocene carbon accumulation rates cars were greatly reduced in response to the combined effects of declining productivity associated with climatic cooling and increased nutrient stress as paludification and permafrost aggradation sequestered mineral nutrients

mineralogy and composition of concentric layers within a manganese nodule from the north pacific ocean abstract the minor element composition of concentric layers within a single ferromanganese nodule from the eastern north pacific exhibits strong correlations with fe and mn contents but appears to be independent of pronounced mineralogic variations on the basis of these correlations the elemental composition of individual layers apparently is controlled by the relative contribution of two sources seawater and interstitial water of associated sediment in contrast the mineralogy of the nodule consisting of birnessite in the outer few layers and todorokite in the inner layers is considered to be a function of nodule diagenesis

dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling we have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity volatile concentration and diffusivity on the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems we conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2 4 orders of magnitude our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions an analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics r t1 2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship r log t is more appropriate during active bubble growth under certain conditions a second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression model results for degassing of initially water saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent on the basis of decompression history explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates 1 5 m s 1

application of in situ produced cosmogenic 10be and 26al to the study of lateritic soil development in tropical forest theory and examples from cameroon and gabon abstract depth profiles of in situ produced cosmogenic nuclides including 10 be t 1 2 1 5 10 6 years and 26 al t 1 2 0 73 10 6 years in the upper few meters of the earth s crust may be used to study surficial processes quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations in this paper we discuss the fundamentals of the method and apply it to two lateritic sequences located in african tropical forests

mn2 bearing kaolinites from lateritic weathering profiles geochemical significance electron paramagnetic resonance epr spectra of kaolins from several lateritic profiles from cameroon show the presence of trace amounts of mn2 ferruginous indurated nodules differentiated in the intermediate zone of the profiles present the highest mn2 content there is no relation between the concentration of mn2 and the total manganese content of the samples most mn2 ions are associated with kaolinite with only a small part being substituted in residual muscovite electron paramagnetic resonance parameters are consistent with the presence of mn2 outer sphere hydrated complexes several chemical treatments deferration oxidation na saturation and heating up to 400 c do not influence the mn2 epr spectra and show that these cations are trapped within kaolinite particles possible trapping sites may be structural defects between adjacent kaolinite layers and or interleaved smectite layers confined within kaolinite particles the presence of stable mn2 outer sphere complexes within kaolinite particles is interpreted as a direct witness of past fluctuations of a water table during the formation of the lateritic profile by operating as a redox front this water table has controlled the differentiation of the intermediate nodular zone of the lateritic profiles

on the development of highly viscous skins of liquid around bubbles during magmatic degassing we report experiments which show that the development of high viscosity volatile depleted skins of liquid around bubbles has a major effect on bubble growth rates and thus on volcanic conduit flow dynamics decompression experiments were performed at room temperature in a cylindrical shock tube using viscous solutions of gum rosin and acetone gra after an initial transient the propagation of the flow front follows an empirical exponential law covering most of the final expansion theories of magma flow dynamics that rely on a constant liquid viscosity conclude that most of the degassing takes place in a diffusion limited decelerating bubble growth regime by contrast we assume that viscous resistance to bubble growth is controlled by the high viscosity of the liquid skins around bubbles and find that in both explosive gra and magma flows bubble growth lies mostly in a diffusion driven but viscosity limited accelerating regime we infer that in explosive events diffusion limited degassing is unlikely to occur before fragmentation

mineralogy of fe mn nodules in albeluvisols in the carpathian foothills poland fe mn nodules are common components occurring in various soils and especially in soils exhibiting imperfect drainage the formation distribution chemical composition size and shape of fe mn nodules are the subject of many studies and the research literature contains a lot of information concerning these issues on the other hand data regarding the mineral composition of fe mn nodules in the soil environment are limited to only a few studies the main aim of the present study was to determine the mineral composition of the fe mn nodules found in albeluvisols in the carpathian foothills in poland using quantitative x ray diffraction qxrd fourier transform infrared spectroscopy ftir and mossbauer spectroscopy ms the obtained results show that fe mn nodules from albeluvisols in the carpathian foothills in poland are composed mainly of silicates quartz k feldspars plagioclases and phyllosilicates dioctahedral mica smectite chlorite and kaolinite which are impregnated with iron and manganese oxides iron minerals are represented by goethite and probably lepidocrocite both iron minerals are characterized by very small particles and poor crystallinity goethite occurring in all of the studied fe mn nodules clearly prevails over lepidocrocite which exhibits a higher concentration only in nodules from the lower part fragipan of the soil profile located on a footslope and showing the strongest stagnic color pattern manganite and todorokite represent manganese minerals however todorokite was observed only in nodules obtained from illuvial horizons btx btg of the studied albeluvisols the fe mn nodules obtained from eluvial horizons eg show higher amounts of silicates quartz k feldspars plagioclases than the nodules obtained from illuvial horizons on the other hand fe mn nodules obtained from illuvial horizons fragipan and argillic exhibit higher amounts of phyllosilicates i e clay minerals in comparison with the eluvial horizon the differences in the quantitative mineral composition of the fe mn nodules between eluvial and illuvial horizons are related to the translocation of clay minerals from upper soil horizons into the lower part of the soil profiles this indicates that the nodules from e horizons were formed after translocation of major portion of clay fraction down the soil profile the similar mineral composition of the fe mn nodules and bulk soil material indicates that the nodules were formed in situ because of cyclic reduction and oxidation of fe and mn oxides

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

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

vesiculation in rhyolite at low h2o contents a thermodynamic model we present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly h2o oversaturated rhyolitic melts the high temperature 900 1100 c experiments involve heating of rhyolitic obsidian from hrafntinnuhryggur krafla iceland to above their glass transition temperature tg 690 c at 0 1 mpa for times of 0 25 24 h during experiments the rhyolite cores increase in volume as h2o vapor filled bubbles nucleate and expand the extent of vesiculation as tracked by porosity is mapped in temperature time t t space at constant temperature and for a characteristic dwell time the rhyolite cores achieve a maximum volume where the t t conditions reach thermochemical equilibrium for each t t snapshot of vesiculation we use 3 d analysis of x ray computed tomographic xct images of the quenched cores to obtain the bubble number density bnd and bubble size distribution bsd bnds for the experimental cores are insensitive to t and t indicating a single nucleation event all bsds converge to a common distribution independent of t melt viscosity or initial degree of saturation suggesting a common growth process we use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of and thermochemical affinity a two computable parameters that are dependent on t pressure and h2o content the model reproduces the experimental data set and data from the literature to within experimental error and has application to natural volcanic systems where bubble formation and growth are not diffusion limited e g lavas domes ignimbrites conduit infill

water movement through an aggregated gravelly oxisol from cameroon abstract increasing population pressures have caused increased utilization of the gravelly stone line soils common to the hilly landscapes of equatorial africa this study was conducted to determine if macropore water flow and immobile water regions should be considered in describing solute movement while developing nutrient management strategies break through curves btc s from miscible displacement experiments with tritiated water were measured from 70 cm long by 9 6 cm internal diameter water saturated undisturbed soil columns simulations produced by theoretical transport models were fitted to the btc s to determine the magnitude of dispersivity and immobile water regions gravel separates composed of kaolinite gibbsite goethite and manganese oxides had porosities ranging from 0 13 to 0 32 m 3 m 3 with a composite sample porosity of 0 20 m 3 m 3 adsorption coefficients of tritium ranged from 0 031 to 0 052 ml g for the three horizons in the soil columns columns containing gravel 30 by volume and 62 by weight gave asymmetrical btc s a convective dispersioe cd transport model was unable to simulate observed btc s accurately the mobile immobile mim water model provided close agreement to btc s obtained at flow rates ranging from 2 71 to 111 cm d the water saturated soil columns had about 50 of all water in immobile regions soil water dispersivity was 3 3 cm 2 n d n 1 with empirical constant n 1 3 from a curvilinear plot of the dispersion coefficient and the mobile pore water velocity parameters estimated from one column were applied to the btc s of a similar soil column the mim model showed close agreement between the measured and the independently estimated btc s these soil characteristics can contribute to the rapid deep transport of a limited quantity of solute and to the storage and or slow diffusive mass transport of the remaining solute from within immobile regions

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

experimental fragmentation of crystal and vesicle bearing silicic melts we experimentally investigate the effect of crystals on the fragmentation behavior of a three phase melt gas crystals system under rapid decompression starting materials are cylinders of hydrated haplogranite melts containing alumina crystals that are placed at 6 30 mpa and 600 800 c in a fragmentation bomb for foaming subsequently these bubble and crystal bearing melts are rapidly decompressed within 30 mpa for phenocryst and microlite bearing samples

field and laboratory experiments on weathering rates of granodiorite separation of chemical and physical processes to study the rate and mechanism of granitic rock weathering field weathering experiments using granodiorite tablets diameter 3 5 cm height 1 1 cm weight 30 g were conducted at a catchment over 10 yr the tablets were exposed at three positions having different weathering conditions ground surface above aquifer and in aquifer the weight of the tablets decreased linearly with time 0 022 wt yr 1 for the ground surface 0 013 wt yr 1 above the aquifer and 0 42 wt yr 1 in the aquifer only minor changes of the tablet surface were observed after 10 yr of weathering at the ground surface and above the aquifer in contrast the tablets in the aquifer were remarkably weathered and weathering of biotite was particularly noticeable whereas that of plagioclase k feldspar quartz and hornblende was minor or moderate the weathering of granodiorite is inferred to proceed by initial dissolution of mineral grain boundary chemical process and subsequent detachment of the mineral grain physical process to evaluate the amount of weathering caused only by the chemical process a laboratory dissolution experiment was conducted on the granodiorite tablet using a flow through reactor at 20 c and ph 6 7 the obtained rates 0 0078 0 010 wt yr 1 were 50 times slower than the field rate in the aquifer this result shows that the contribution of the physical process to granodiorite weathering in the aquifer is very large compared to that of the chemical process and the relative intensities of the two processes during the 10 yr weathering are estimated to be chemical physical 1 50

gold mass balance in lateritic profiles from savanna and rain forest zones abstract the gold mass balance was studied in two areas gold mineralization one in a savanna zone in south mali the other in a rain forest zone in french guiana in the ferricrete profile of the savanna zone the gold mass balance is strongly negative within the ferricrete itself where almost all the primary gold stock is leached beneath the ferricrete an absolute accumulation of fine secondary gold is found in the mottled zone the gold cycle is controlled by the migration of fe and associated with the neogenesis of iron oxides particularly hematite the lateritic profile in the rain forest zone is truncated and a soil is developing from the underlying saprolite in equilibrium with the equatorial conditions an absolute accumulation of gold is found in the soil with gold migration and reprecipitation being controlled by the evolution of the macroporosity the global mass balance of gold in the guiana deposit shows a leaching rate of 40 in the weathered part of the mineralization this process occurs mostly at the weathering front in the two examples gold shows opposite behaviors due to different pedogenic and paleoclimatic conditions this might influence the interpretation of geochemical soil surveys in various intertropical environments

periodic behavior in lava dome eruptions lava dome eruptions commonly display fairly regular alternations between periods of high activity and periods of low or no activity the time scale for these alternations is typically months to several years here we develop a generic model of magma discharge through a conduit from an open system magma chamber with continuous replenishment the model takes account of the principal controls on flow namely the replenishment rate magma chamber size elastic deformation of the chamber walls conduit resistance and variations of magma viscosity which are controlled by degassing during ascent and kinetics of crystallization the analysis indicates a rich diversity of behavior with periodic patterns similar to those observed magma chamber size can be estimated from the period with longer periods implying larger chambers many features observed in volcanic eruptions such as alternations between periodic behaviors and continuous discharge sharp changes in discharge rate and transitions from effusive to catastrophic explosive eruption can be understood in terms of the non linear dynamics of conduit flows from open system magma chambers the dynamics of lava dome growth at mount st helens 1980 1987 and santiaguito 1922 2000 was analyzed with the help of the model the best fit models give magma chamber volumes of 0 6 km3 for mount st helens and 65 km3 for santiaguito the larger magma chamber volume is the major factor in explaining why santiaguito is a long lived eruption with a longer periodicity of pulsations in comparison with mount st helens

weathering trends and parent material characteristics of polygenetic oxisols from minas gerais brazil i mineralogy in geologically stable areas in the tropics climatic changes and geomorphic cycles give origin to polygenetic soils polygenesis involves new soil formation phases taking place on preweathered materials from previous phases resulting in soils with rather similar chemical and mineralogical properties polygenetic soils from minas gerais brazil were investigated with the purpose of distinguishing mineralogical aspects in relation to polygenesis the soil materials were studied by micromorphology and electron microscope and analyzed by xrd all soils are strongly weathered and even show weathered grains of ilmenite and quartz the soils show two phases of kaolinite formation pseudomorphs after mica and precipitation from solution and of accumulation of iron compounds desilication causes gibbsite formation from kaolinite occasionally even in the deep saprolite superposed processes can be due to progressively deeper weathering or to changes of the external environment

radiocarbon dating of palaeosol components in moraines in lapland northern sweden abstract accurate geochronologies are the key for comparison of palaeoclimate records in order to clarify problems concerning the geochronology of glacier fluctuations we dated palaeosols in moraines in the kebnekaise mountain region of swedish lapland these palaeosols already have been dated several years ago using bulk samples of the palaeosols and yielded ages of 6180 5790 and 2750 2100 cal yr bp in an attempt to evaluate the sources of contamination associated with bulk samples these soils were re dated by using coleoptera fragments cenococcum geophilum spores humic acids and woody plant tissues the new dates reveal that the two palaeosols found within the moraines represent much longer periods of time than was previously assumed soil formation periods can be distinguished for periods 7800 7580 6300 4080 2450 2000 and 1170 740 cal yr bp these age groups point to differentiated soil formation processes resedimentation bioturbation and or cryoturbation the results show that dated terrestrial macrofossils from glacial moraines indicate climate changes that are in accordance with other proxy records in northern and central scandinavia and in certain times off phase with glacier fluctuations in southern maritime norway this throws light on possible changing interaction of forcing factors during the holocene

unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites an integrated study from yaou french guiana an integrated study of o and h isotopes in the lateritic profile of yaou french guiana was undertaken to investigate the usefulness of stable isotopes as tracers of climatic changes in continental environments the studied profile is composed of a 27 m thick saprolite mostly developed in the past under wet and dry tropical climate in association with a duricrust overlain by a 3 m thick yellow latosol formed more recently under present equatorial hot and humid climate 18o d values determined for weathering goethite pseudomorphs after pyrite and kaolinite microcrystalline clay groundmass throughout the 30 m deep profile reflect formation temperatures consistent with present 25 c and realistic past climatic temperatures 20 c 30 c indicating that weathering minerals formed in isotopic equilibrium with their genetic environment and were not subjected to significant isotope exchange after formation a distinct shift downward 2 for 18o 15 for d from low to high 18o d values occurs around 20 m depth in the saprolite it is interpreted as recording the change from the past tropical to the present equatorial climate goethite and kaolinite in the 5 10 m thick saprolite interval immediately above the active basement weathering front are in isotopic equilibrium with modern water and must have formed under present equatorial humid conditions in contrast goethite and kaolinite found higher up in the saprolite and in the duricrust formed in the past under tropical wet and dry climate from waters distinctly depleted in 18o and d relative to modern water the marked depletion of paleo meteoric water at yaou most likely reflects a more contrasted or monsoonal character of the ancient tropical climate the present study shows that ancient weathering minerals in lateritic profiles preserve their 18o d values and carry a time signal the time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post formational remobilization such as the goethite pseudomorphs after pyrite occurring at yaou groundmass kaolinite is more susceptible to partial remobilization through successive dissolution precipitation reactions which may obliterate the paleoclimatic signal unraveling the climatic record carried by weathering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions

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

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

controls on conduit magma flow dynamics during lava dome building eruptions 1 lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity we develop a transient model of the magma flow in a volcanic conduit from an open system magma chamber with continuous replenishment the model incorporates gas exsolution bubble growth gas escape through the magma and decompression induced crystallization and considers variations in magma temperature water content conduit diameter phenocryst size chamber volume and magma rheology calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime when discharge rate is low and crystals grow efficiently leading to high magma viscosity to another stable state when discharge rate is high and crystallization is negligible the difference in discharge rates between these regimes can be several orders of magnitude periods are similar to the observed timescales and mainly depend on the chamber volume the system shows strongly nonlinear responses to the variation of governing parameters if magma has a bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of newtonian rheology large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature water content phenocryst size distribution or conduit diameter the system can fluctuate between low and high discharge rates with transitions to explosive activity

rates and time scales of clay mineral formation by weathering in saprolitic regoliths of the southern appalachians from geochemical mass balance rates of clay formation in three watersheds located at the coweeta hydrologic laboratory western north carolina have been determined from solute flux based mass balance methods a system of mass balance equations with enough equations and unknowns to allow calculation of secondary mineral formation rates as well as the more commonly determined primary mineral dissolution rates was achieved by including rare earth elements ree in the mass balance rates of clay mineral formation determined by mass balance methods have been used to calculate the time needed for a 5 50 g kg 1 change in relative clay abundance in the saprolite at coweeta this corresponds to the response time of the clay mineral to for example a change in climate the 5 change in relative clay abundance is the smallest change that can generally be detected using x ray diffraction xrd response times range from tens of thousands to hundreds of thousands of years extrapolating the coweeta clay formation rates to other southern appalachian regoliths the time required to form measured clay abundances production times in eastern blue ridge and inner piedmont regolith have been calculated the production times of clay mineral assemblages range from 2 k y to 2 m y with mean values ranging from 50 k y to 1 m y the results of this study are consistent with the arguments of thiry 2000 1 that the best resolution of the paleoclimatic record in marine clay rich sediments and mudrocks is 1 or 2 m y 1 ref 107

atypical ferromanganese nodules from pelagic areas of the central indian basin equatorial indian ocean abstract two box core samples one each from red clay and siliceous ooze sediment provinces from the central indian basin indian ocean were studied both cores contained ferromanganese nodules at their tops the red clay sediment also hosts buried nodules at a depth of 82 cm chemical characterization of the sediments indicates that early diagenetic remobilization of transitional metals is pronounced in the siliceous ooze sediment while in the red clay no such effect is discernible this is further corroborated by the abundance and dissolution patterns of microorganisms and ferromanganese micronodules occurring in the two sediment columns bulk chemical analyses indicate that all the nodules are of the hydrogenetic b type vernadite and todorokite are the major minerals both of which constitute laminated nondirectional colloform and cuspate microstructures petrographic studies yield evidence of recrystallization of todorokite and partial transformation of vernadite to todorokite the former process being more advanced in buried nodules microprobe analyses revealed that primary todorokite has a lower mn fe ratio and higher ni content than recrystallized todorokite in the same sample the mineralogical reconstitution in the nodules is attributed to postdepositional intranodule diagenesis the present study does not reveal the correlation between mineralogy and microstructure in the nodules which has been suggested for the nodules from the pacific neither can any mineral or microstructure be considered specific for any particular genetic model the studied nodules are therefore atypical the mineralogical and chemical reordering in these ferromanganese nodules raises the question of the viability of radiometric methods in determining growth rates of such nodules

an experimentally validated numerical model for bubble growth in magma abstract volcanic eruptions are driven by the growth of gas bubbles in magma the timing and rate of bubble growth are important because they determine whether enough gas pressure can develop to fragment the melt bubbles grow in response to decompression and diffusive transport of dissolved volatiles predominantly h2o that exsolve into the bubbles growth is resisted by the viscosity of the melt both melt viscosity and h2o diffusivity have non linear dependence on the concentration of h2o dissolved in the melt which necessitates a numerical approach to modelling bubble growth several bubble growth models have previously been published and applied but none of them has been validated against continuous in situ experimental data or provided as a user friendly tool here we present a numerical bubble growth model implemented in matlab which allows for arbitrary temperature and pressure pathways and accounts for the impact of spatial variations in dissolved h2o concentration on viscosity and diffusivity we validate the model against two sets of experimental data 1 new continuous data for gas volume fraction as a function of time collected using optical dilatometry of vesiculating hydrous obsidian samples which were heated from 930 c to 1000 c at atmospheric pressure this dataset captures isobaric isothermal bubble growth under strongly disequilibrium conditions 2 discrete data from published decompression experiments at 825 c and pressures from 200 mpa to 5 mpa with decompression rates from 0 1 mpa s 1 to 10 mpa s 1 these experiments represent isothermal decompression driven bubble growth spanning equilibrium to strongly disequilibrium conditions the numerical model closely reproduces the experimental data across all conditions providing validation against contrasting bubble growth scenarios the validated model has a wide range of potential volcanological applications including forward modelling of bubble growth phenomena and inverse modelling to reconstruct pressure temperature time pathways from textures and volatile contents of eruptive products

study of two alteration systems as natural analogues for radionuclide release and migration abstract u deposit hosted in hydrothermally altered tuffs in mexico together with weathering profiles from cameroon were studied as natural analogues of radionuclide release and migration using petrological and spectroscopic methods infrared and electron paramagnetic resonance we have distinguished successive secondary mineral parageneses and the behaviour of radionuclides in the u deposit the mineral parageneses show that uranium migration is mainly controlled by the redox potential and silica activity of the altering solutions the high silica content of the solutions is caused by the intense alteration of volcanic rocks two types of secondary clay mineral parageneses are evidenced a kaolinization intense where uranium is accumulated in the welded tuffs and a smectitization mainly developed in the underlying weakly welded tuffs several types of kaolinite have been defined according to their genesis fillings in fissures and feldspar pseudomorphs their location relative to a breccia pipe where uranium has accumulated core and rim of the pipe surrounding rhyolitic tuffs and particle morphology structural order and substitutional fecontent it is shown that the variations of the concentration of paramagnetic defect centres always more than ten times as important than those measured in weathering kaolinites are only correlated to the location of the kaolinites the highest values correspond to the breccia pipe kaolinites e g kaolinites located in the uranium accumulation zones moreover one or two main defects centres are detected depending on the intimate association of kaolinites with uranium bearing minerals besides in weathering kaolinites from u depleted laterites defect centre concentrations are correlated to the total fe203 content in bulk samples this means that the defect centre acts as a memory of the travel of uranium when this element was sorbed onto iron gels in the first stage of weathering it is concluded that paramagnetic defect centres in kaolinites might allow an efficient fingerprint of successive irradiations in the natural analogues under study and could be an useful tool to control radionuclides migration through kaolinite containing clayey materials such as those used for waste repository a better understanding of radiation efficiency as well as accurate dose ratekaolinite containing clayey materials such as those used for waste reposit estimation are needed for a quantitative tracing of the migration ofa better understanding of radiation efficiency as well as accurate dose ratekaolin radionuclide elements with this aim a simulation has been undertaken withestimation are needed for a quantitative tracing of the migration of various radiations sources we have determined for each irradiation the parameters of the paramagnetic centres created in order to understand the way they are forming the knowledge of the parameters governing the formation and the stability of the radiation centres in kaolinites allow to use this mineral as a natural dosimeter

trace element accumulation in mn fe oxide nodules of a planosolic horizon the aim of this work was to determine the importance of nodule formation on the dynamics of major and trace elements tes along a planosol toposequence developed in metamorphic parent material at la ch tre massif central france the different horizons were sampled within three pits and analysed for major and trace element contents the nodule rich horizon was studied more closely a simplified sequential extraction scheme x ray diffraction xrd and microscopic approaches were used in order to determine the individual phases containing te in nodules along the slope the nodule rich horizon varies in thickness is composed of different oxide fractions and has different scavenging efficiencies according to the te considered iron was found to accumulate in the middle of the slope while mn accumulated at the base the scavenging effect is only evident for ni in profile 1 for fe and cu it is maximal in profile 2 where the nodule rich horizon is the thickest for pb and mn maximal scavenging effect is recorded for both profiles 2 and 3 in the lowest part of the slope cr is not accumulated at all this was related to the water dynamic and the hydromorphic conditions prevailing along the slope results obtained by sequential extractions and associated x ray diffraction on the different nodule size fractions and those obtained by electron microprobe allow inference of the te distribution in nodules nodules were mainly composed of three to four types of cements surrounding grains of quartz feldspars micas and accessory minerals iron rich cements si and al rich cements mn rich cements and ti rich cements in places the iron rich cements consist of poorly crystalline goethite and possibly some ferrihydrite ferrihydrite is associated with cr as demonstrated by extractions goethite contained mn and most of the te extracted except for ni and pb fine grained si and al rich cements were also observed they contain variable amounts of ti and mn mn rich cements were not present in all the nodules and were mainly linked to the dark zones of the nodules the nature of these mn oxides could not be determined they were found to contain co ni cu and probably pb

experimental observations of the effect of crystals and pre existing bubbles on the dynamics and fragmentation of vesiculating flows the effect of crystals and pre existing gas phases on the dynamic degassing and fragmentation of magmas in a volcanic conduit have been simulated by analogue experiments a series of decompression experiments of viscous solutions of gum rosin and acetone with and without the incorporation of particles silicon carbide glass beads organic seeds or pre existing air bubbles have been performed in a transparent shock tube the addition of internal bubbles or solid particles in the starting solution shifts the fragmentation conditions towards lower acetone volatile contents and lower decompression ratios the effect of solid particles depends on their number density and spatial distribution in the starting solution a small number of sinking particles has little impact on the flow dynamics however a floating layer of particles provides a high concentration of surface bubbles that leads to an early fragmentation pulse the observations also imply that the presence of crystals if able to generate a high number density of bubbles will lead to an earlier and deeper fragmentation of the magma in the volcanic conduit

growth of diagenetic ferromanganese nodules in an oxic deep sea sedimentary environment northeast equatorial pacific abstract to study the accretion mechanism of diagenetic ferromanganese nodules formed in an oxic deep sea sedimentary environment manganese nodules sediments and pore waters were sampled in the northeast equatorial pacific diagenetic ferromanganese nodules are ellipsoidal to discoidal in shape rough in surface texture enriched in cu ni and 10 a manganite and are ubiquitous on the abyssal plain hydrogenetic nodules are polynucleated irregular in shape smooth in surface texture enriched in co and mno 2 and occur dominantly on seamounts numerous microlayers with diagenetic or hydrogenetic characteristics occur successively in the nodules concentration of nitrate in sediment pore waters indicates that overall redox conditions are oxic however mn oxide grains on the walls of burrows and worm tubes are interpreted to be reprecipitated metal ions supplied from oxygen depleted microenvironments that formed in the oxic sediment column by decomposition of organic matter active mixing of bottom sediments by benthic fauna and bottom currents is indicated by ubiquitous burrows worm tubes and tracks and trails high values in sediment mixing coefficients 1 cm 2 yr and scattering of data points in depth profiles of pore water nitrate concentrations we suggest that diagenetic ferromanganese nodules in an oxic deep sea sedimentary environment grew from remobilized metal ions as well as reprecipitated mn oxide grains which were supplied to the nodules episodically during the stirring of bottom sediments by benthic fauna and intermittent strong bottom current flow

formation and transformation processes of iron duricrust systems in tropical humid environment abstract formation and transformation processes of iron duricrust developed under tropical humid conditions in the southeastern part of the central african republic have been investigated in terms of geomorphological petrological and geochemical pathways the study of five typical weathering profiles of the most extensive iron duricrust system now known in the world indicates that saprolitization and ferruginization are sequentially involved in such processes as reflected by kaolinite gibbsite and hematite goethite replacements alkali earths rare earths metals and elements belonging to chemically inert minerals segregate during saprolite and iron duricrust formation and degradation mass balance calculations performed on a typical weathering system enable us to quantify net mass element transfers deriving from lateritic weathering processes volumetric change collapse or dilation as well as al and ti clearly discriminate between the saprolitization and ferruginization processes saprolite development preserves the geochemical inheritance of parent rocks whereas ferruginization depletes it and mostly reflects through mineralogical pathways pedoclimatic conditions and global climatic changes the global iron duricrust system of the southern part of central africa derived from previous hematite rich crust and not from ancient bauxites as it is generally believed for iron crusts formed on west african shields

a new model of volatile bubble growth in a magmatic system isobaric case 1 the nucleation growth and ultimately coalescence of volatile bubbles in a silicate melt magma chamber play a crucial role in the processes leading to volcanic eruptions a new diffusion limited nonlinear growth and nucleation model of volatile bubbles in such a system is proposed here in contrast to previously existing models the present one treats the competitive effects of the other randomly located bubbles on the growth dynamics in presence of a hydrodynamic coupling with the melt advection field through its viscous resistance i consider here cases where the fluid pressure is kept constant numerical results pertaining to a basaltic and a rhyolitic melt suggest that for small volatile supersaturations and for small times bubble growth occurs essentially as in the single bubble problem however for longer times the influence of the other bubbles is important i find that bubble growth rate decreases exponentially with time and propose an approximate expression for the decay rate finally for larger volatile supersaturations the bubble growth is subjected to transitions to and from an inflationary regime whereby the bubble radius increases very rapidly during a small time interval

chemical weathering in the drainage basin of a tropical watershed nsimi zoetele site cameroon comparison between organic poor and organic rich waters abstract this study deals with the weathering processes operating at the scale of a small catchment nsimi zoetele cameroon and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters samples of river spring and groundwaters from nsimi zoetele were filtered through membranes of decreasing pore size 0 22 m 0 025 m or 300 000 da 5000 da to separate colloidal fractions from the truly dissolved one major and trace elements and dissolved organic carbon doc were analysed in each fraction two kinds of waters can be distinguished in the catchment clear and coloured waters clear waters exhibit low concentrations of major and trace elements and doc elements are carried in these solutions in a true dissolved form except al and rare earth elements rees by contrast the higher abundances of al fe and trace elements in coloured waters are controlled by the colloidal fraction thermodynamic equilibrium calculations show that clear waters are in equilibrium with kaolinite and iron oxi hydroxide which are major minerals in the weathered soil for coloured waters the aqueous speciation of ca mg cu fe al la and th was calculated taking into account the complexes with humic acids speciation calculations for cu fe al la th show a strong complexation with humic acids in good agreement with the results of the filtration experiments by contrast although filtration experiments show a strong control of major cations by organic matter for example 75 for ca speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements this discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure finally both filtration experiments and speciation calculations show that organic matter plays an important role in natural doc rich waters organic acids increase significantly the dissolution rates of silicates and oxi hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment

geochemical differentiation in laterite and bauxite formation abstract the geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predominantly formed by residual enrichment of iron and aluminium residual enrichment is corroborated by various arguments and clearly proved for laterite formation on ultramafic rocks tropical weathering of the most widespread acidic rocks leads in spite of their high al fe ratio generally to formation of laterites showing a high enrichment of iron whereas aluminium is not or only slightly enriched this chemical variation indicates that high amounts of al are removed additionally to si in soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite hematite and kaolinite as main constituents dissolved free aluminium exist in equilibrium with kaolinite only in minimal contents hence the removal of si and al probably takes place in form of freshly formed colloidal or subcolloidal x ray amorphous particles with kaolinitic or similiar composition the frequently observed occurrence of two generations of secondary kaolinite in laterites corroborates this conclusion there is general agreement that bauxites are formed by stronger leaching compared with laterites increased leaching lowers the concentration of dissolved silica thus enabling gibbsite instead of kaolinite stability the concentration of dissolved free aluminium however is increased in equilibrium with gibbsite free aluminium shows in contrast to iron an effective solubility atph 5 giving rise to downward leaching and re precipitation at higher ph below a laterite crust which covers the bauxite zone in most of the bauxite deposits in spite of the stronger leaching bauxite formation implies a lower mass loss mainly si during weathering than the formation of laterites loss ofsi al related to their highly increased iron content

mineralogy and crystal chemistry of mn fe co ni and cu in a deep sea pacific polymetallic nodule minor element concentrations in marine ferromanganese nodules are primarily controlled by the mineralogy which itself depends on redox conditions at the sediment water interface results are presented for the first in depth x ray microstructural and microspectroscopic investigation of a mixed hydrogenetic diagenetic nodule which is representative of ferromanganese deposits on abyssal plains the measurements were conducted by micro x ray diffraction and x ray absorption spectroscopy both xanes and exafs on hydrogeneous and diagenetic regions of the nodule the hydrogenetic diagenetic interface was imaged by x ray microfluorescence after which regions of interest were chosen to represent mineralogical and chemical transformations that occurred at the early stage of suboxic diagenesis in the hydrogenetic nodule oxic environment mn is speciated as fe vernadite a nanocomposite material composed of intergrown feroxyhite feooh and monodispersed phyllomanganate layers having no interlayer mn vernadite in the diagenetic nodule suboxic environment mn is speciated dominantly as mg rich 10 a vernadite which consists of a random intergrowth of vernadite and its transformation product todorokite the authigenic 10 a vernadite precipitated from the components of vernadite in fe vernadite that were dissolved in suboxic microenvironments of the sediment direct evidence supporting a redox driven dissolution reaction is provided by the valence composition of mn as measured by micro xanes which is 0 69mn4 0 24mn3 0 07mn2 average 3 62 0 04 v u for fe vernadite and 0 61mn4 0 23mn3 0 16mn2 average 3 28 0 04 v u for 10 a vernadite ni and cu derived mainly from dissolved vernadite and oxidized organic matter replace structural mn3 4 in both the mno2 layer and todorokite domains of 10 a vernadite pure todorokite in highly diagenetic regions of the nodule has an average formula of mg2 0 167 mn4 0 783 mn3 0 099co3 0 002ni2 0 076cu2 0 040 o2 n h2o with an atomic ratio of cu ni co mn 0 13 which is slightly lower than 0 167 1 6 the maximum metal uptake capacity reported for marine nodules by analogy with synthetic todorokites we infer that mg2 which has a hydrated diameter close to that of the 3 3 tunnel size of todorokite and mn3 and cu2 which prefer jahn teller distorted octahedra play a crucial role in templating the topotactic transformation of 10 a vernadite to todorokite and stabilizing todorokite in suboxic marine sediments

bubble growth in rhyolitic melts experimental and numerical investigation bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically rhyolitic melts were hydrated at 150 mpa 780 850 c to uniform water content of 5 5 5 3 wt the pressure was then dropped and held constant at 15 145 mpa upon the drop bubbles nucleated and were allowed to grow for various periods of time before final rapid quenching of the samples the size and number density of bubbles in the quenched glasses were recorded where number densities were low and run duration short bubble sizes were in accord with the growth model of scriven 1959 for solitary bubbles however most results did not fit this simple model because of interaction between neighboring bubbles hence the growth model of proussevitch et al 1993 which accounts for finite separation between bubbles was further developed and used to simulate bubble growth

bubble growth during decompression of magma experimental and theoretical investigation a model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt the model accounts for the interplay of three dynamic processes decompression rate of the magma deformation of the viscous melt around the growing bubble and diffusion of volatiles into the bubble generally these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically for a better understanding of the physics of the processes we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance diffusion or linear decompression rate we show that the solutions are a function of time and two dimensionless numbers which are the ratios of either the diffusive or viscous time scales over the decompression time scale the conditions for each growth regime are provided as a function of the two governing dimensionless parameters analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts the model solutions including the division to the growth regimes as function of the two parameters provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure supersaturation and gas volume fraction the model results are in agreement with the conditions of plinian explosive eruption e g mount st helens 18 may 1980 where high gas overpressure is expected the conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions mostly in equilibrium degassing and partly in overpressure conditions we show that in a situation of quasi static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble insensitive to the diffusivity profile

age calibration of carbonate rind thickness in late pleistocene soils for surficial deposit age estimation southwest usa abstract carbonate rinds have been used for cross correlation of landforms as well as a quantitative indicator of soil age using the measured rind thickness of clasts found within a deposit whose age has been independently determined allows the construction of a calibrated surface age proxy measurements were taken at sites within the mojave desert the northwestern sonoran desert the southern great basin and the western colorado plateau these sites are all within about 300 km of the intersection of the borders of the states of california arizona and nevada in the study area elevation varied from 200 to 1200 m map was from 95 to 195 mm and mat was from 18 4 to 23 3 c the calibrated proxy while not accounting for the effects of parent material or climate on rind development does show a strong correlation r 2 0 74 p

dissolution of kaolinites and accumulation of iron oxides in lateritic ferruginous nodules mineralogical and microstructural transformations abstract in situ geochemical and structural microanalysis of non disturbed samples of laterite and mineralogical identifications of microsamples demonstrate an orderly succession of mineralogical and structural transformations these transformations result in the formation of fe nodules they start from a micaceous phase whose micropores are occupied by hematite because the source of iron is external to micas this hematite corresponds to an absolute accumulation in the meaning of brewer 1964 the following transformations constitute a succession of three mineral phases corresponding to relative accumulations with the removal of some components and in situ reutilization of residual materials 1 phase of fe kaolinite accumulation during which the structures inherited from micas are preserved 2 phase of al hematite accumulation resulting from a dissolution of fe kaolinite with development of pedoplasma and vesicular porosity 3 phase of accumulation of more or less aluminous goethite which ultimately results in the crystallization of geodes at the borders of voids

mechanism and geochemical implications of the late holocene organic carbon burial event an example from the southwest coast of india carbonaceous clay found in the palaeo deposits of meenachil river basin was analysed for the total organic carbon nitrogen and textural characters radiocarbon dating of this clay formation and the embedded carbonized wood fragments was also done the values obtained for the organic carbon nitrogen and c n ratio of the carbonaceous clay exhibited a very remarkable range suggestive of different environmental conditions of formation it is evident from the present study that since the lignin and cellulose are the dominant components of terrestrial higher plants allochthonous and submerged organic matter have high c n ratio the radio carbon dating of the sediment and wood samples suggested that the palaeo deposits of meenachil river basin were formed as result of many transgrassive regressive episodes which had started even before the formation of vembanad lake the formation of carbonaceous clay as found sandwiched in palaeo deposits was due to the global eustatic sea level fluctuations and tectonism

an experimental facility for the investigation of magma fragmentation by rapid decompression a special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression the facility operates in a regime similar to that of shock tubes and at temperatures up to 950 c and pressures up to 200 bar cylindrical samples diameter ca 17 mm length ca 50 mm undergo rapid decompression in a high temperature high pressure section of the facility following the disruption of a diaphragm separating that section from a low pressure low temperature section actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization the results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature this new fragmentation facility is presented as a tool for experimental volcanology under well constrained conditions