silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits an example of meteoric dominated diagenesis from the triassic of central spain cauliflower shaped nodules are widespread in a single red mudstone bed in the buntsandstein triassic facies of the iberian range they consist mostly of quartz dolomite and calcite but other minerals such as barite kaolinite and iron oxyhydroxides are also present the nodules are spherical ovoid or elongate in shape and range from 1 to 8 cm across the surface of the nodules is irregular and some show a pedogenic coating of microspar the sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of ca2 and so4 2 in a vadose environment occasionally under the influence of pedogenic processes partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting as indicated by the presence of organic filaments on the quartz crystal surfaces in type a nodules the dissolution of the innermost anhydrite was complete and different types of quartz cement filled the porosity fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules in type b nodules the dissolution of the anhydrite was incomplete inhibiting quartz cementation and enabling later dolomitization of the anhydrite dolomitization appears to have been driven by sulphate reduction as indicated by the presence of bacterial bodies within the dolomite crystals dedolomitization and precipitation of barite kaolinite and calcite spar cements occurred later under the influence of meteoric solutions the nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

late quaternary hydrological dynamics in the middle kalahari forcing and feedbacks the kalahari region has become a major source of quaternary palaeoenvironmental data derived primarily from the analysis of geomorphological proxies of environmental change one suite of data from palaeolacustrine landforms has recently provided a new record of major hydrological changes in the last 150 ka burrough s l thomas d s g bailey r m 2009 mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system quaternary science reviews in press here we present an improved analysis of the drivers and feedbacks of lake level change utilising information from three main sources data from the lake system itself from analyses of other late quaternary records within the region and from climate modelling simulations using the hadley centre coupled climate model hadcm3 suggest that once triggered the lake body was large enough to potentially affect both local and regional climates surface waters and their interactions with the climate are therefore an important component of environmental dynamics during the late quaternary through its capacity to couple middle kalahari environments to distant forcing mechanisms and to itself force environmental change we demonstrate that the existence or absence of megalake makgadikgadi adds a new level of complexity to the interpretations of environmental proxy records in southern africa s summer rainfall zone

the potential of osl and tl for dating lateglacial and holocene dune sands tested with independent age control of the laacher see tephra 12 880 a at the section mainz gonsenheim abstract optically stimulated luminescence osl and thermoluminescence tl are applied in this study to date a lateglacial and holocene aeolian sediment sequence quartz and potassium rich feldspars extracted from dune sands from western germany were dated using both multiple aliquot ma and single aliquot sa protocols for luminescence dating this particular dune section was chosen for this test study because an independent age control is provided by a horizon of the laacher seetephra 12 880 a in contrast to the results for potassium feldspars which show underestimation of up to 25 the quartz fraction provides promising results with respect to the presumed geological ages

cretaceous tectonic evolution of coastal southern california insights from the catalina schist the relatively high t high p t subduction related basement in the southern california continental borderland preserves important information regarding the late mesozoic tectonic evolution of coastal southern california integrated petrologic geochemical and geochronological study of the subduction related catalina schist exposed on santa catalina island documents the formation of lawsonite blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling newly initiated subduction zone petrologic and 40ar 39ar thermochronologic data demonstrate similar perhaps shared retrograde pt histories for all but the lawsonite albite facies rocks under pumpellyite actinolite facies conditions as early as 95 100 ma the 40ar 39ar data together with the abundance of pumpellyite actinolite facies vein assemblages and lack of higher p t epidote or lawsonite blueschist over printing in high grade units are consistent with the rapid emplacement of the catalina schist into the shallow to mid crust by 95 ma by combined underplating and tectonic erosional removal of overlying rocks in the accretionary complex cf platt 1986 the catalina schist and the adjacent peninsular ranges batholith potentially represent paired metamorphic belts formed along southwest north america at 120 to 90 ma combining the evidence for the evolution of the catalina schist with the previously advanced hypotheses regarding tectonic evolution of the batholith suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone at 120 115 ma in which the catalina schist was accreted and metamorphosed

intersector element partitioning in tourmaline a potentially powerful single crystal thermometer hourglass sector zoning and related polar overgrowths are common features of metamorphic tourmaline developing as a result of variations in element preference on the different growth surfaces for sector zoned crystals three domains are present for each growth zone c c and a with compositional differences most distinct for ca and ti and among c and c sectors intersector differences vary commonly showing decreasing fractionation from core to rim attributed to increasing metamorphic grade here we show that intersector element partitioning is temperature dependent and derive empirical geothermometers based on c c and c a partitioning of ca and ti these thermometers are applicable over a range of temperatures and bulk rock compositions intersector partitioning is not affected by re equilibration and records and preserves complete t histories of individual tourmaline grains from prograde to peak and on to retrograde growth information on element mobility is preserved by tourmaline composition because intersector partitioning is independent of element concentration these factors make intersector partitioning an ideal tool to elucidate the thermal history of tourmaline grains and thus their host environment and tourmaline s refractory nature preserves these signatures even into the sedimentary record

palynological evidence for late quaternary warming episodes in southern africa abstract polleniferous sediments from southern africa associated with past warm episodes before the last glacial maximum are rare interpretation of environmental conditions during these phases are complicated by difficulties of dating in sediments from the northern areas north of 28 s the prominence of tropical woodland pollen suggests that the period between 7000 and 6500 yr b p was associated with optimal temperatures during the holocene identifying the warmest phase in the southern high lying grasslands and semi arid karoo shrublands south of 28 s is difficult because unlike the savanna areas of the north they do not yield good pollen indicators for changes in temperature such as those of frost sensitive trees however some pollen sequences from further south along the southern coast of africa 34 s and at marion island in the southern ocean 47 s suggest that the holocene temperature optimum occurred at about the same time in the sub antarctic area in the south and the subtropical regions in the north the advent of moister conditions in southern africa during the early to middle holocene is generally recorded earlier ca 7500 6500 yr b p in the north at ca 26 s than around 31 s ca 5000 yr b p this change is provisionally associated with a relative shift in seasonality from a predominance of all season precipitation to a greater proportion of summer rainfall which apparently reached the southern karoo areas two thousand years later than the northern bushveld region

rift kinematics during the incipient stages of continental extension evidence from the nascent okavango rift basin northwest botswana high resolution aeromagnetic data from the nascent okavango rift in northwest botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension crosscutting relationships between west northwest trending 180 ma karoo dikes and reactivated northeast trending proterozoic basement faults are used to document the kinematics of cenozoic faulting during the initial stages of rifting depth estimates to the top of the dikes using three dimensional euler deconvolution solutions have produced the following interpretations 1 the okavango rift is a half graben with a downthrow of 200 300 m 2 the width of the okavango rift 100 20 km is similar to that of more mature continental rifts such as the tanganyika and baikal rifts this suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin nor the amount of extension it is suggested that the cumulative downthrow sediment infill included and subsidence may be a better indicator of the relative maturity of rift basins 3 preexisting basement faults exert a major control during rifting and reactivation processes do not occur synchronously along the entire length of preexisting faults 4 the okavango rift is defined by normal faults there is no evidence of major strike slip faults thus excluding a pull apart tectonic model for this nascent continental rift stage 5 the preexisting sekaka shear zone terminates the okavango rift to the south suggesting that such shear zones represent major barriers during longitudinal propagation of rifts this probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts

diagenesis in okavango fan and adjacent dune deposits with implications for the record of palaeo environmental change in makgadikgadi okavango zambezi basin northern botswana abstract this work considers the spatial distribution and ages of western moz basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands previously published and new tl osl ages imply that extensive over washing of dune sands took place at least 100 ka ago while ages on okavango floodplains imply that the fan was formed ca 40 ka and has since undergone periods of higher and lower flood regimes sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay enhanced amorphous silica ceas which bonds weakly formed aggregates the time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using osl tl techniques hence earlier dune over washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation 100 000 years ago the relatively abundant ceas matrix in floodplain sands implies more recent semi continuous flood events again of an acidic near neutral nature leading to the formation of smectite in this case the floodplain sediments are dated as having been deposited around 40 and 11 ka when porewater content may have accelerated clay formation and silica dissolution the dual nature of the ceas in the islands reflects a changing environment from smectite dominated flooding events to sepiolite dominated desiccation events flooding may also correspond to tl osl ages over the past 40 000 years which contributed to accelerated ceas formation the sepiolite is associated with a ca rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation this work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events the implications of different stages of sand diagenesis may be significant in enhancing palaeo environmental interpretations in semi arid fluvial environments

the redistribution of b concentration and its isotopes during low grade metamorphism observations in metapelites from the central range taiwan abstract the plio pleistocene orogeny resulted in the exhumation of a regional metamorphic belt comprising low to high grade metapelites in the central range taiwan in this study we analyze the distributions of b isotopes along a prograde metamorphic profile argillite to greenschist in the western central range to constrain the effects of metamorphic fluids generated during dehydration on the redistribution of the mobile elements from west to east peak metamorphic temperatures increase from 200 c to 320 c b concentrations and 11b values generally decrease to the east with 50 loss of b and the 11b values roughly decrease from 5 to 15 the x ray diffraction xrd analysis results of these mineral compositions except for calcite show no correlation with b contents and 11b and the mixing monitoring of mineralogical compositions cannot account for the observed b contents and 11b coherent and w e distribution trend as the b isotopes may be fractionated between rock and fluid during dehydration a rayleigh distillation equation was applied into a model under similar conditions the results indicate that the measured 11b values are consistent with the modeled equilibrium of fluid rock b isotope fractionation within the constrained temperature ranges thereby supporting the hypothesis that dehydration processes act as a major control on the distribution of b isotopes in the low grade metapelites this model also estimates a 11b value of 5 8 for the integrated expelled fluids when getting totally 50 loss of b any b sequestering by tourmalines in meta sediment is an important factor to retain b to a greater depth in subducting slabs however our results suggest this factor is not significant for the low grade metapelites this study constrains the effect of dehydration on b flux and b isotope distributions during low grade metamorphism and how the heterogenetic mineralogical compositions may affect its variations and may contribute to a better understanding of b mass cycling in subduction environments

identification of cementation in overconsolidated clays the note describes the application of the generalized compressibility equations for normally and overconsolidated clays presented by nagraj srinivasa murthy 1986 based on physico chemical principles the ideas are used in a new combination as a method of identifying possible particle cementation in overconsolidation clays examination of the present method indicates that errors due to cementation of overconsolidated soils are likely the note recommends the inclusion of a step which compares the shear strength of the soil sample to that predicted for that soil based on physico chemical principles the derived sensitivity can be used to avoid cemented soils being inadvertently analyzed as uncemented overconsolidated soils

sedimentological and geochemical evidence for palaeo environmental change in the makgadikgadi subbasin in relation to the moz rift depression botswana abstract this work considers new evidence for palaeo environmental change taking place during the pleistocene in northern botswana duricrusted strandlines along the northeastern margin of sua pan provide palaeo environmental data pertaining to the makgadikgadi subbasin msb with inferences regarding the larger makgadikgadi okavango zambezi moz rift depression field xrd and geochemical data show that msb strandlines comprise calcretes lu1 type mgo rich calcretes with silica lu2 type sil calcrete lu3 type and silcrete lu4 type early freshwater episodes appear to have been followed by calcrete dominated drying phases interspersed with repeated silcretisation calcretisation through pan littoral sediments may have been both biogenically and environmentally induced calcite precipitation was in part controlled by the mg ca ratio of pore water in the pan littoral zone suggesting closed basin type evaporative conditions which were followed by a major desiccation interval phases of silcrete precipitation appear to be related to periods when the geochemistry of the lake littoral more closely resembled present day na co 3 so 4 cl type brines silica saturated acidic moderately saline groundwater preceded si precipitation which took place as the ph reduced si mobilisation occurred inter alia as a result of quartz grain dissolution enhanced by diatoms bacteria and algal growth in the moist pan littoral sio 2 rich pore waters migrated through cracked and desiccated calcrete into areas of lower salinity and lower ph resulting in preferential calcite removal and silcrete precipitation approximate tl dates imply that exposed littoral sand underwent calcretisation during the drying phases of extensive palaeo lakes which occurred prior to 110 ka 80 90 ka and 41 43 ka these wet periods compare fairly well with vostok core chronologies for southern africa

chemical and boron isotope composition of tourmaline from the kiaka orogenic gold deposit burkina faso west african craton as a proxy for ore forming processes the kiaka orogenic gold deposit burkina faso located in the paleoproterozoic domain of the west african craton is characterized by a two stage gold mineralization hosted in volcano sedimentary metamorphic rocks that was formed during the eoeburnean 2 20 2 13 ga and eburnean 2 13 2 05 ga orogenic cycles these two stages include an early disseminated low grade gold mineralization and a late vein hosted high grade gold mineralization paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline the aim of this paper is twofold i to determine the processes responsible for deposition of the economic disseminated gold mineralization and ii to identify the source of the mineralizing fluids for this purpose we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different 11b values falling within the two intervals of 25 1 to 22 0 and 19 6 to 15 1 respectively our results suggest that tourmaline formed from a distal high temperature ca 400 c reduced and low salinity hydrothermal fluid that interacted with the local host rocks based on the modeling of tourmaline fluid boron isotope fractionation we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca 2 13 ga this metamorphic fluid rock interaction model may possibly extend to other orogenic gold deposits in the west african craton

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

record in metamorphic tourmalines of subduction zone devolatilization and boron cycling boron concentrations and isotope compositions of fluids and lavas from subduction zone settings show great potential for elucidating mass flux at earth s modern convergent margins however the fluid mineral melt behaviorof b and its two stable isotopes remains relatively poorly understood boron isotope analyses of tourmaline in metasedimentary rocks subducted to 15 90 km depths 1 demonstrate the ability of this mineral to retain information regarding prograde devolatilization history in even highly retrograded rocks and 2 indicate the importance of tourmaline in affecting whole rock b loss and b isotope evolution during subduction zone metamorphism the b lost from micas during metamorphism of subducting sedimentary rocks and altered oceanic crust is isotopically more enriched in 1 1 b than the b retained in the micas beneath forearcs and volcanic arcs the b from micas is either removed from the subduction zone rocks via metamorphic fluids or sequestered by growing tourmaline in which the b can be entrained to even greater depths here we demonstrate that these metamorphic fluids could contribute to the relatively high 811b signatures observed in most arc lavas and the across arc trends of decreasing 1 1 b observed in several arcs

paleoclimatic significance of the stable isotopic composition and petrology of a late pleistocene stalagmite from botswana high resolution 18o and 13c analyses of a stalagmite from lobatse ii cave reveal late pleistocene environmental changes in botswana large shifts in 18o and 13c are observed between two main periods of deposition the first period between 51 000 and 43 000 yr b p was warm and humid and may have been associated with some c3 vegetation the second period between 27 000 and 21 000 yr b p had temperatures approximately 2 c lower and vegetation dominated by drought adapted c4 plants the intervening period between 43 000 and 27 000 yr b p bounded by two major hiatuses in stalagmite growth produced discontinuous speleothem formation probably under dry conditions new 230 th 234u mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 th 234u ages

geological relationships and u pb zircon and 40 ar 39ar tourmaline geochronology of gneisses and tourmalinites from the nevado filabride complex western sierra nevada spain tectonic implications abstract the nevado filabride complex the lowest complex in the betic hinterland forms a stack of alpine nappes the tectonic units consist of metasedimentary sequences whose ages are not well constrained gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex new radiometric data from u pb zircon dating on gneisses from the western sierra nevada confirm the presence of late carboniferous intrusive rocks in the betic hinterland these results combined with available data from the literature and a detailed structural analysis suggest that the gneisses represent a single late variscan magmatic event evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations in conjunction with geochemical data u pb zircon 314 7 304 23 ma and 40 ar 39 ar tourmaline 319 85 5 81 317 85 3 67 ma geochronology a pre late carboniferous age for the basal formation of the nevado filabride sequence can be inferred the gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites respectively superposed tectonic units rather than a continuous palaeozoic sequence is supported by the occurrence of palaeozoic rocks at the top of the nevado filabride complex the nappe tectonics as evidenced from the contractional character of the unit boundaries and the superposition of higher grade on lower grade metamorphic rocks is additionally supported by the superposition of older on younger rocks as revealed from radiometric data

exploration of driving mechanisms of equilibrium boron isotope fractionation in tourmaline group minerals and fluid a density functional theory study abstract the equilibrium boron isotope fractionations 3 4 11b tur fluid between tourmaline mineral groups and fluids at 0 5 gpa 600 1000 k are investigated using density functional theory calculations the first solvent shell controls boron isotope fractionation in solution where the values of both h3bo3 and b oh 4 decrease with increasing numbers of hydrogen bonds in supercritical fluids the weakening of hydrogen bonds and the diversity in configurations of hydrated boron species both contribute to variations in the vibrational frequencies the 1000ln tur fluid value increases with increasing mg fe mg ratios in the dravite schorl solid solution series this effect together with the crystallization sequence predicted from the gibbs free energies of tourmaline formation accounts well for the association of boron isotope and chemical zonation observed in zoned tourmaline grains the dependence of boron isotope fractionation on the b o bond length in tourmaline group minerals reflects the changes in relevant molar volumes caused by differences in the mass and charge of the atoms occupying the x y and z sites of tourmalines this study suggests that the chemical composition of the tourmaline plays an important role in controlling the boron isotope composition in tourmalines crystallized from hydrothermal systems

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

orbital forcing of climate over south africa a 200 000 year rainfall record from the pretoria saltpan late pleistocene variations in rainfall in subtropical southern african are estimated from sediments preserved in the pretoria saltpan a 200000 year old closed basin crater lake on the interior plateau of south africa we show that south african summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession as predicted by orbital precession geometry berger 1978 this south african record is out of phase with north african palaeomonsoon indices street and grove 1979 rossignol strick 1983 mcintyre et al 1989 the amplitude of the rainfall response to insolation forcing agrees with climate model estimates prell and kutzbach 1987 these results document the importance of direct orbital insolation forcing on both subtropical north and south african climate as well as the predicted antiphase sensitivity to precessional insolation forcing

boron isotope record of peak metamorphic ultrahigh pressure and retrograde fluid rock interaction in white mica lago di cignana western alps this study presents boron b concentration and isotope data for white mica from ultra high pressure uhp subduction related metamorphic rocks from lago di cignana western alps italy these rocks are of specific geological interest because they comprise the most deeply subducted rocks of oceanic origin worldwide boron geochemistry can track fluid rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones the highest b contents up to 345 g g b occur in peak metamorphic phengite from a garnet phengite quartzite the b isotopic composition is variable 11b 10 3 to 3 6 and correlates positively with b concentrations based on similar textures and major element mica composition neither textural differences prograde growth zoning diffusion nor a retrograde overprint can explain this correlation modelling shows that b devolatilization during metamorphism can explain the general trend but fails to account for the wide compositional and isotopic variability in a single well equilibrated sample we therefore argue that this trend represents fluid rock interaction during peak metamorphic conditions this interpretation is supported by fluid rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in 11b and b taking into account the local availability of serpentinites as potential source rocks of the fluids the temperatures reached during peak metamorphism that allow for serpentine dehydration and the high positive 11b values 11b 20 5 modelled for the fluids an influx of serpentinite derived fluid appears likely paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have b contents between 12 and 68 g g and 11b values that cluster around 0 11b 5 0 to 3 5 white mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks the variable b geochemistry can be successfully modelled as fluid rock interaction with low to moderate 3 fluid rock ratios where mica equilibrates with a fluid into which b preferentially partitions causing leaching of b from the rock the metamorphic rocks from lago di cignana show variable retention of b in white mica during subduction related metamorphism and exhumation the variability in the b geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones

two isotopically distinct fluid components involved in the mariana arc evidence from nb b ratios and b sr nd and pb isotope systematics nb b ratios and 11 b values of mariana lavas reveal the involvement of two isotopically distinct slab derived fluids in the magma genesis the evidence of one type of fluid is observed in lavas of the islands of anatahan alamagan agrigan and uracas in which simple mixing between fluid with very homogeneous 11 b values 5 40 0 05 and the mantle wedge controls the magma compositions in lavas of the islands of guguan asuncion and pagan however the fluids that mixed with the mantle wedge are significantly heterogeneous in 11 b 5 4 to 7 6 even within a single island the compositional difference between these two types of fluid can result from different ratios of sediment to altered oceanic crust aoc in the slabs and this hypothesis is consistent with the sr nd and pb isotope data we propose that the variable isotope compositions of the latter fluid may be derived from local changes in sediment aoc ratios of the slab that are caused by subduction of the magellan seamounts beneath the mariana arc

fault growth and propagation during incipient continental rifting insights from a combined aeromagnetic and shuttle radar topography mission digital elevation model investigation of the okavango rift zone northwest botswana 1 digital elevation models dem extracted from the shuttle radar topography mission srtm data and high resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the okavango rift zone orz northwest botswana significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive or have large throws and small scarp heights indicating waning activity faults on the outer margins of the rift exhibit either 1 large throws or significant scarp heights and are considered older and active or 2 throws and scarp heights that are in closer agreement and are considered young and active fault linkages between major fault systems through a process of fault piracy have combined to establish an immature border fault for the orz thus in addition to growing in length by along axis linkage of segments the rift is also growing in width by transferring motion to younger faults along the outer margins while abandoning older faults in the middle finally utilization of preexisting zones of weakness allowed the development of very long faults 100 km at a very early stage of continental rifting explaining the apparent paradox between the fault length versus throw for this young rift this study clearly demonstrates that the integration of the srtm dem and aeromagnetic data provides a 3 d view of the faults and fault systems providing new insight into fault growth and propagation during the nascent stages of continental rifting

geomorphological contributions to palaeolimnology on the african continent abstract article history received 19 december 2007received in revised form 5 july 2008accepted 22 july 2008available online 3 august 2008keywords quaternarybeach ridgeshorelinelakemultiproxyosl lakes are highly informative repositories of information of the terrestrial response to regional and global climatedrivers in quaternary studies lake oor records are now frequently used to establish the magnitude timing andgeographicalextentofhumid aridtransitionsovertimescalesof10 2 10 4 years whiletheresolutionof thisrecordisunparalleled much of the geochemical and sedimentary da ta is challenging to interpret and highly basin speci c geomorphologicalproxies particularlyshorelines offeranadditionaloralternativerecordoflakehistory withrecenttechnological advances facilitating both access to and the utility of this proxy record recent advances in opticallystimulated luminescence osl dating allow the determination of the age of beach ridge emplacement in theabsence of organic material while the availabilityof shuttle radar topography mission srtm data in conjunctionwith digital elevation models dem aids in the identi cation of shoreline features and the topographicreconstruction of the former extent of palaeolakes here we highlight and illustrate the use of lacustrine shorelinefeatures for unravelling a quaternary climate signal wit hin africa from otherwise inaccessible and undatablepalaeolake records in the semi arid and arid subtropics an d identify their contribution towards high resolutionmultiproxy reconstructions of tropical low latitude quaternary environments and climates 2008 elsevier b v all rights reserved

the systematics of boron isotopes in izu arc front volcanic rocks the process of fluid release from the subducting slab beneath the izu arc volcanic front izu vf was examined by measuring b concentrations and b isotope ratios in the neogene fallout tephra odp site 782a both were measured by secondary ion mass spectrometry in a subset of matrix glasses and glassy plagioclase hosted melt inclusions selected from material previously analyzed for major and trace elements glasses and radiogenic isotopes sr nd pb bulk tephra these tephra glasses have high b abundances 10 60 ppm and heavy 11b values 4 5 to 12 0 extending the previously reported range for izu vf rocks 11b 7 0 to 7 3 the glasses show striking negative correlations of 11b with large ion lithophile element lile nb ratios these correlations cannot be explained by mixing two separate slab fluids originating from the subducting sediment and the subducting basaltic crust respectively model a two alternative models models b and c are proposed model b proposes that the inverse correlations are inherited from altered oceanic crust aoc which shows a systematic decrease of b and lile with increasing depth from basaltic layer 2a to layer 3 paralleled by an increase in 11b from 1 to 10 to 24 in this model the contribution of sedimentary b is insignificant 4 of b in the izu vf rocks model c explains the correlation as a mixture of a low 11b 1 composite slab fluid a mixture of metasediment and metabasalt derived fluids with a metasomatized mantle wedge containing elevated b 1 2 ppm and heavy 11b 14 the mantle wedge was likely metasomatized by 11b rich fluids beneath the outer forearc and subsequently down dragged to arc front depths by the descending slab pb b isotope systematics indicate that at arc front depths 53 of the b in the izu vf is derived from the wedge this implies that the heavy 11b values of izu vf rocks are largely a result of fluid fractionation and do not reflect variations in slab source provenance i e subducting sediment vs basaltic crust since the b content of the peridotite at the outer forearc 7 58 ppm b mean 24 16 ppm is much higher than beneath the arc front 1 2 ppm b the hydrated mantle wedge must have released a b rich fluid on its downward path this wedge flux can explain 1 the across arc decrease in b and 11b e g izu kuriles without requiring a progressive decrease in fluid flux from the subducting slab and 2 the thermal structure of volcanic arcs as reflected in the b and 11b variations of volcanic arc rocks

geochemical mapping of carbonate sediments in the makgadikgadi basin botswana using moderate resolution remote sensing data this paper maps the carbonate geochemistry of the makgadikgadi pans region of northern botswana from moderate resolution 500 m pixels remotely sensed data to assess the impact of various geomorphological processes on surficial carbonate distribution previous palaeo environmental studies have demonstrated that the pans have experienced several highstands during the quaternary forming calcretes around shoreline embayments the pans are also a significant regional source of dust and some workers have suggested that surficial carbonate distributions may be controlled in part by wind regime field studies of carbonate deposits in the region have also highlighted the importance of fluvial and groundwater processes in calcrete formation however due to the large area involved and problems of accessibility the carbonate distribution across the entire makgadikgadi basin remains poorly understood the modis instrument permits mapping of carbonate distribution over large areas comparison with estimates from landsat thematic mapper data show reasonable agreement and there is good agreement with estimates from laboratory analysis of field samples the results suggest that palaeo lake highstands reconstructed here using the srtm 3 arc second digital elevation model have left behind surficial carbonate deposits which can be mapped by the modis instrument copyright c 2006 john wiley sons ltd

geomorphic evidence for the late quaternary palaeoclimates of the middle kalahari of northern botswana summary geomorphological studies of caves and lakes in northern botswana since 1973 have yielded a suite of 80 radiometric dates which permit palaeoenvironmental reconstruction of the landscape over the last 45 000 years the unique characteristics of the landforms when interpreted with caution yield palaeoclimatic information on both northern botswana and the catchments of the okavango river in the angolan highlands several major shifts in precipitation are indicated for the period 45 000 20 000 bp with dry conditions prevailing at 25 000 bp the late glacial was characterised by humid conditions especially from 16 000 13 000 bp whilst the early holocene was semi arid with precipitation peaks circa 6 000 5 000 bp and 2 000 bp the results contrast sharply with some southern african palaeo climatic descriptions in print where many of the inferences have been drawn from studies of the northern hemisphere inter tropical zone the implications of the study are briefly discussed in relation to palaeoclimatic models of the whole african continent

late quaternary lake level fluctuations in the mababe depression middle kalahari palaeolakes and the role of zambezi inflows abstract a systematic drilling and optical dating programme on middle kalahari beach ridge relict shoreline sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the okavango delta northern botswana this paper presents 23 ages from the mababe depression and establishes four shoreline construction phases in the late quaternary coeval with other sub basin lake high stands lake ngami these synchronous lake phases result from a coalescence of the sub basins into a unified palaeolake lake thamalakane covering an area of 32 000 km2 six additional ages are also presented from the chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as lake thamalakane phases this suggests that increased flow in the chobe and zambezi system significantly contributed to the middle kalahari lake phases in both the post glacial and holocene periods the integration of these new data and their compatibility with other regional and tropical palaeo archives is discussed in the light of understanding quaternary climate drivers within the kalahari

pan floor barchan mounds ntwetwe pan makgadikgadi botswana their origin and palaeoclimatic implications abstract the variety of forms assumed to be dunes and palaeolakes of northern botswana have long been perceived as geoproxies for the elucidation of quaternary palaeoclimates mounds on the floor of ntwetwe pan have been regarded as modified barchans pertaining to a time in the past when conditions were drier than at present the morphology hydrogeology and biology of these placed in the wider context of the palaeolacustrine hinterland indicate that they are not dunes but subaqueous features spring mounds the morphology is inconsistent with barchans conspicuous features are the elevated eyes within the enclosing arms of the crescentic forms that occur furthest from the pan edge seepages discharge from their flanks commonly about 7 m higher than the dry saline pan floor on the top of forms nearer the pan margins permanent pools of potable water occur similarly elevated above the floor hydraulic pressure to sustain the upward discharge of groundwater is provided by the excellent recharge through silicate karst depressions in the hinterland particularly those of the 936 m palaeolake floor groundwater discharge still operating today was formerly much stronger and associated with lacustrine conditions a chronological framework is provided by archaeological remains on the palaeolake floors of the hinterland these indicate that the oldest palaeolake floor pertaining to the quaternary is the 920 m level periods of spring mound formation are associated with both this and the 912 m palaeolake the implications for future luminescence dating targeting palaeoenvironmental reconstruction are discussed in the light of this new model of mound formation

late quaternary environmental change in central southern africa new data synthesis issues and prospects much of the interior of central southern africa is a sand sea within which aeolian and lacustrine landforms and sediments of local and regional extent are preserved closed cave sites are restricted to very few locations while fluvial systems traverse the margins of the interior until the early 1990s chronologies of late quaternary environmental and climatic changes developed for this region were based on only a limited number of proxy data sets derived largely from lacustrine deposits and precipitates in particular directly determined ages from aeolian deposits the most extensive suite of features in the region were absent the application of luminescence dating techniques to dune sediments and the development of further detailed chronologies from cave precipitates is now providing a more comprehensive record for the last 50 ka with some chronologies extending back a further 100 ka we present and review these data assess their contribution to enhanced understanding of late quaternary environmental changes in the region and for the first time assess them against corrected radiocarbon ages from lacustrine sites it is concluded that there is now an enhanced understanding of the spatial and temporal complexity of climate changes affecting the region in the last glacial cycle including a complex record of punctuated aridity but that many issues including data set integration and forcing mechanism controls are imperfectly understood

synthetic reconstructions of southern african environments during the last glacial maximum 21 18kyr and the holocene altithermal 8 6kyr abstract the pash database has been assembling palaeoclimatic data from the southern hemisphere for a number of years the southern african sector now contains more than 100 records spanning the last 200 000 years a number of these relate to the last glacial maximum and holocene altithermal which represent the two most recent extremes in global temperatures syntheses of these records have permitted the preparation of preliminary palaeoenvironmental maps for the two intervals the results provide insights into probable changes in atmospheric circulation as temperatures across the subcontinent rose by 6 7 c these changes are in agreement with a present day model which has been developed to explain short term variations in southern african climate the results presented here should assist in calibrating models developed to predict future climate change

evidence for a permanent lake in sua pan kalahari botswana during the early centuries of the last millennium indicated by distribution of baobab trees adansonia digitata on kubu island abstract the makgadikgadi basin in the middle kalahari desert of botswana exhibits a complex of salt pans which may partly be filled to shallow water depths during the rainy season there is good evidence that during the pleistocene and early holocene a large lake existed in the makgadikgadi basin which however became increasingly smaller during the holocene for the late holocene when the regional human population began to grow no data except for the last two centuries about lake levels and thus surface water availability have been published age estimates of selected baobab trees of two size classes growing on kubu island a granite outcrop which is located in a coastal position at the south western edge of sua pan are used to infer that at least during the early centuries of the last millennium a maximum lake level of 908 m a s l was reached during this lake phase sua pan and ntwetwe pan were interconnected and large wetlands were created large numbers of game and livestock could be sustained these data are in line with palaeoclimate data but cannot be used to differentiate the medieval warm period which was supposedly relatively wet from the subsequent little ice age which likely was comparatively dry

sedimentation on the distal reaches of the okavango fan botswana and its bearing on calcrete and silcrete ganister formation the distal reaches or flood plain of the okavango alluvial fan of northern botswana are characterized by gently undulating topography with local relief generally less than two meters the entire area is blanketed by eolian sand although the area is semiarid with evapotranspiration exceeding precipitation the area is subject to seasonal flooding by annual influx of floodwater from subtropical angola to the north distributary channels on the flood plain are poorly defined and consist of sinuous depressions lacking normal fluvial characteristics such as levees bars or incision the flood water has very little suspended load higher ground on the flood plain forms islands during the seasonal flood elevated tracts arise by displacive subsurface crystallization of carbonate and silica which is induced by trees that grow on the higher ground sedimentation on lower lying areas occurs by a combination of 1 accumulation of fine clastic material derived from the flood water and phytolithic silica both of which are mixed into the sandy substrate by illuviation and bioturbation and 2 precipitation of fine grained amorphous silica from the groundwater induced by transpiration by aquatic grasses and sedges accumulation of silica in the soil profile produces a proto silcrete this sequence grades laterally into carbonate dominated island soils the ultimate cause of this association is an abundance of water with low suspended load in an environment with a high evapotranspiration rate ganisters in ancient rocks may have a similar origin

the fate of subducted oceanic slabs in the shallow mantle insights from boron isotopes and light element composition of metasomatized blueschists from the mariana forearc abstract serpentine muds from south chamorro seamount scs drilled during odp leg 195 at site 1200 contain metamafic clasts that experienced blueschist facies metamorphism including the critical mineral assemblage pumpellyite na amphibole epidote these schists represent fragments from the actual slab mantle interface at 27 km depth their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the mariana forearc a region of melange formation as it can also be observed in onland exposures such as the catalina schist as the mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer during the subduction processes at an active convergent margin this study presents the first data of detailed b isotope 11 b and light element variations in blueschist facies minerals from the mariana arc system the primary foci are b and li concentrations and 11 b values analyzed by sims and tof sims techniques minerals such as na rich amphibole phengite and chlorite are found to be strongly enriched in li up to 70 g g be up to 8 g g and b up to 35 g g and with 11 b values of 6 4 these new data are consistent with isotopically heavy b being released into the mariana forearc mantle wedge serpentinization of dry mantle peridotite after interaction with b rich slab released high ph fluids and confirm models of significant b loss and b isotope fractionation during forearc shallow slab dehydration the elevated li be and b concentrations in minerals that comprise the bulk of the rocks namely amphibole phengite and chlorite bear a strong potential to further transport li and b as well as the isotopically light component of b to greater depths in the mantle where ongoing metamorphism is responsible for further isotope and elemental fractionation and the formation of distinct mantle reservoirs e g volcanic arc and oceanic intra plate oib magmas

use of the geochemical and biological sedimentary record in establishing palaeo environments and climate change in the lake ngami basin nw botswana sediment samples from a continuous 4 6 m profile in the dry bed of lake ngami in nw botswana were analysed for geochemistry and dated using both 14c and tl methods certain units in the profile were found to be diatom rich and these with the geochemical results were used as indicators of high and low lake levels within the basin the lake ngami sediments contain a high proportion of sio2 51 92 5 wt avg 72 4 wt and variable levels of al2o3 2 04 17 2 wt avg 8 88 wt based on elevated al2o3 and organic matter loiorgc results lacustrine conditions occurred at ca 42 ka until 40 ka and diatom results suggest that relatively deep but brackish conditions prevailed at 40 ka the lacustrine sedimentary record was terminated abruptly possibly by tectonic activity at ca 19 ka shallow aerobic turbulent conditions were prevalent but lake levels were at this time increasing to deeper water conditions up until ca 17 ka this period coincides with the late glacial maximum a period of increased aridity in the central southern africa region generally increasing sr ca ratios and decreasing loiorgc and al2o3 from ca 16 to 5 ka suggest decreasing inflow into the basin and declining lake levels based on the enrichment of lree results slightly alkaline conditions prevailed at ca 12 ka diatom results also support shallow alkaline conditions around this time these lake conditions were maintained primarily by local rainfall input as the region experienced a warmer wetter phase between 16 and 11 ka lake levels rose rapidly by 4 ka probably in response to enhanced rainfall in the angolan catchment these results indicate that lake levels in the lake ngami basin are responding to rainfall changes in the angolan catchment area and local rainfall the results confirm that the present day anti phase rainfall relationship between southern africa and regions of equatorial africa was extant during the late quaternary over the angolan highlands and nw botswana

carbonates and cementation of glacially derived cohesive soils in new york state and southern ontario cet article presente des resultats se rapportant a la teneur en carbonate de silts et d argiles naturels en fonction de la profondeur de l emplacement geographique et de l origine geologique on etudie aussi l influence de cette teneur en carbonate sur le comportement geotechnique l effet sur la pression de preconsolidation apparente est illustre en detail pour l un des sites etudies la repartition des carbonates a l interieur des varves d un depot glacio lacustre est egalement presentee pour demontrer la variabilite potentielle de la cimentation naturelle des sols les donnees en provenance de quinze sites etayent les relations proposees entre teneur en carbonate et pression de preconsolidation apparente

the boron isotopic composition of altered oceanic crust samples of drilled oceanic crust from dsdp holes 417a 417d and 418a and odp hole 735b and oceanic crust from the oman and cyprus ophiolites were analyzed for b contents and 11b composite samples from dsdp holes 417a 417d and 418a were used to represent the upper 550 m of altered oceanic crustal layer 2a whole rock samples from the troodos ophiolite cyprus and the oman ophiolite wer selected to represent crustal layer 2b dikes composite samples from odp hole 735b were used to represent crustal layer 3 the b content of the dsdp composites ranges from 7 2 ppm to 104 ppm and correlates with both 18o and k showing that it is a good indicator of the extent of low temperature alteration the 11b of the dsdp composites varies between 2 5 and 5 4 the b content of the samples from the troodos ophiolite ranges from 2 4 ppm to 8 1 ppm 11b varies from 0 9 to 7 8 the b content of the oman ophiolite samples ranges from 5 0 ppm to 11 1 ppm 11b varies from 1 6 to 16 9 the b content of the samples from odp hole 735b ranges from 1 1 ppm to 7 1 ppm 11b varies from 4 3 to 24 9 the general pattern displayed by these samples is one of greatest and most variable b enrichment at the top of the crust and least enrichment at the bottom of the section all of these samples are enriched compared to unaltered morb which is believed to have a b content of approximately 0 5 ppm the 11b values of deeper samples from layers 2b and 3 are more variable and generally higher than those from layer 2a boron contents and 11b are not correlated the data from the dsdp site 417418 composites indicate that the 11b of fluid circulating in the upper crust changes only slightly during alteration increasing by an average of 5 1 with an accompanying decrease in b concentration of 7 low temperature alteration appears to be a water dominated process resulting in minor modification of circulating seawater a minimum water rock ratio of 400 is calculated for these samples implying a minimum low temperature seawater flux through the upper oceanic crust of 3 4 1014 l y the average b content of altered oceanic crust as represented by these samples is 5 2 1 7 ppm and the average 11b is 3 4 1 1 this average isotopic composition is measurably different from the apparent average of oceanic sediments supporting the idea that 11b could be useful for identifying the source s of b in island arcs

detrital metamorphic and metasomatic tourmaline in high pressure metasediments from syros greece intra grain boron isotope patterns determined by secondary ion mass spectrometry the boron isotopic composition of zoned tourmaline in two metasediments from the island of syros determined by secondary ion mass spectrometry sims reflects the sedimentary and metamorphic record of the rocks tourmaline from a silicate bearing marble contains small 20 m detrital cores with highly variable 11b values 10 7 to 3 6 pointing to a heterogeneous protolith derived from multiple sources the sedimentary b isotopic record survived the entire metamorphic cycle with peak temperatures of 500 c prograde to peak metamorphic rims are homogeneous and similar among all analysed grains 11b 0 9 the varying 11b values of detrital cores in the siliceous marble demonstrate that in situ b isotope analysis of tourmaline by sims is a potentially powerful tool for provenance studies not only in sediments but also in metasediments a meta tuffitic blueschist bears abundant tourmaline with dravitic cores of detrital or authigenic origin 11b 3 3 and prograde to peak metamorphic overgrowth zones 1 6 fe rich rims formed during influx of b bearing fluids under retrograde conditions show strongly increasing 11b values up to 7 7 towards the margins of the grains the 11b values of metamorphic tourmaline from syros formed in mixed terrigenous marine sediments reflect the b signal blended from these two different sources and was probably not altered by dehydration during subduction

boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite abstract in order to understand the behavior of boron b and its isotope fractionation during subduction zone metamorphism b contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the sambagawa metamorphic belt central shikoku japan no systematic changes in whole rock b content and isotope composition of the metasediments were observed among the different metamorphic grades indicating the lack of a bulk fluid rock b isotope fractionation as a result of devolatilization both modal abundance and grain size of tourmaline increase with increasing metamorphic grade in contrast b contents in muscovite and chlorite decrease with increasing metamorphic grade these observations combined with mass balance calculations of b suggest the formation of tourmaline during progressive metamorphism from metamorphic fluids containing b mainly derived from muscovite and subordinately from chlorite without allowing significant net removal of b from the metasedimentary rocks tourmalines in the higher grade metasedimentary rocks have zonal structure of b isotope and major element composition with decreasing 11 b and increasing mg mg fe from the inner rim core to the outer rim the change of mg mg fe in the tourmalines with increasing grade is paralleled by similar variation in chlorite these observations suggest that the growing tourmalines record the progressive evolution of the b isotopic composition of the metamorphic fluid in the outermost rims preserving the isotope signature of peak metamorphic p t fluid conditions based on the above observations the 11 b of the tourmaline is thought to have been nearly identical to that of the metamorphic fluid resulting in the apparent b isotopic fractionation factor between metamorphic fluid and whole rock 11 b 10 b fluid 11 b 10 b whole rock which decreases from 1 007 0 003 to 1 001 0 003 from chlorite to biotite zone metamorphism such results together with the formation of tourmaline from and sequestering of b in metamorphic fluids may lead to less b isotopic fractionation as a result of subduction zone devolatilization than noted in suites containing less tourmaline this therefore makes it possible to transport b isotopic signatures which ultimately reflect earth s surface materials to the deep mantle perhaps resulting in mantle b isotope anomalies near convergent margins

origin and palaeo environments of calcareous sediments in the moshaweng dry valley southeast botswana quaternary sedimentation in the moshaweng dry valley of southeastern botswana is evaluated on the basis of geomorphological evolution and sedimentological analyses stratigraphic evidence reveals an upper surface 1095 m containing abundant sil calcrete an intermediate surface 1085 m in which sil calcrete underlies nodular calcrete and lower 1075 m surface in which sil calcrete and nodular calcrete are interbedded this subdivision is reflected in the geochemical composition of the sediments which show an overall trend of decreasing sio2 content and increasing caco3 content with depth from the highest to the lowest surface levels the calcretes and sil calcretes represent modifications of pre existing detrital kalahari group sand and basal kalahari pebbles which thinned over a karoo bedrock high modification took place during wet periods when abundant ca rich groundwater flowed along the structurally aligned valley system with the onset of drier conditions water table fluctuations led to the precipitation of nodular calcretes in the phreatic layer to a depth of about 20 m a major geochemical change resulted in the preferential silicification of the nodular calcrete deposits conditions for silica mobilization may be related to drying induced salinity and in situ geochemical differentiation brought about by pebble dissociation towards the top of the sediment pile as calcretization and valley formation progressed to lower levels silica release took place on a diminishing scale thermoluminescence dating infers a mid pleistocene age for sil calcrete formation suggesting that valley evolution and original calcrete precipitation are much older late stage dissolution of caco3 from pre existing surface calcretes or sil calcretes led to the formation of pedogenic case hardened deposits during a time of reduced flow through the moshaweng system possibly during the upper or late pleistocene copyright 2002 john wiley sons ltd

processes of silicate karstification associated with pan formation in the darwin koolpinya area of northern australia lyons lagoon one of many small pans near darwin northern australia occurs in lateritized terrain formation by structurally facilitated silicate karsti cation of the parent cretaceous sandy sediments was earlier proposed this paper presents the results of thin section microscopy and scanning electron microscopy with energy dispersive x ray study of pan sediments with a view to assessing the extent to which they re ect the geomorphological hydrogeological history severe corrosion was found to overprint the impact features of the primary sands attributed to aggressive leaching and consistent with formation of the depression by karsti cation when the water table was low formation of a smectitic matrix is considered to have followed when the water table was high upon subsequent lowering of the water level leaching was resumed altering smectite to kaolinite and resulting in incongruent dissolution of kaolinite to leave a siliceous residuum at least two cycles of high followed by low water levels are recognized such periodic changes of levels are considered essential to pan formation within freely draining lateritic terrain the processes of pan formation and alteration of pan sediments essentially residual deposits is ongoing biocorrosion being implicated as an important mechanism incongruent kaolinite dissolution to leave a silica residuum a hitherto apparently unrecognized process raises the possibility that silcrete may form as a relative accumulation which is synchronous with lateritic leaching in more elevated topographic positions copyright 2004 john wiley sons ltd

duricrust development and valley evolution process landform links in the kalahari duricrusts are an important landscape component of the kalahari region of central southern africa their exposures within the dry valleys mekgacha of the kalahari provide some of the most widespread surface outcrops of the terrestrial jurassic to holocene kalahari group sediments exposures have been extensively used in the construction of lithostrati graphic sequences on the assumption that valley systems have incised their courses through a pre existing duricrust sequence recent work however has identified the role of groundwater erosion processes in valley development which may have influenced duricrust formation studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the presence of a valley analyses of calcretes and silcretes in a series of profiles and thin sections from the letlhakeng area of botswana also indicate extensive alteration and diagenesis in association with former higher water tables sedimentary sequences within duricrust host materials can be identified but there is no evidence for correlation of duricrust cements between exposures profile studies from the auob valley in namibia however suggest that this valley has incised through a sequence of duricrusts caution is advised in future attempts to correlate duricrust types on the basis of valley exposures with the recommendation that where such exposures are used in a lithostratigraphic context only duricrust host material characteristics and not cementing materials should be considered

hourglass sector zoning in metamorphic tourmaline and resultant major and trace element fractionation a new type of sector zoning with an hourglass shape has been identified in metamorphic tourmalines that formed under a wide variety of physical and chemical conditions the two sectors in the c direction are not equivalent due to asymmetry in the crystal structure of tourmaline along the c axis the c sector is characterized by low concentrations of ti ca mg and na although al is high and has a pale commonly blue or pale green color conversely the c sector is low in mg and al and high in ca fe and ti the latter two causing the dark brown color of this sector the a sector has intermediate characteristics and probably approximates a sector free tourmaline thin sectioning of these sector zoned tourmalines perpendicular to the c axis can produce three types of apparent radial zoning patterns blue green cores dark brown cores or no distinct cores these apparent cores will further vary in relative diameter depending on the sectioning level furthermore core boundaries can be straight or ragged depending on whether the relative growth speeds for the different faces was constant or variable these textures have been used to argue for a prograde or detrital origin of tourmaline cores however sector zoning is a more appealing explanation for most of these textures and can further explain the textural resemblance among metamorphic tourmalines from highly variable bulk rock composition metamorphic history and mineral paragenesis the sector zoning that is described here develops by preferential uptake of elements on the r growth plane resulting from a combined effect of differences in surface charge and morphology of this plane in the c and c directions this leads to the preferential incorporation of more positively charged elements in the c direction and a preference for a vacant x site in the c direction because the compositional differences among the sectors are pronounced in both major and trace elements and in the same order of magnitude as growth zoning variability the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry

a joint receiver function and gravity study of crustal structure beneath the incipient okavango rift botswana rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins in order to decipher the characteristics of rifting at its earliest stage we conduct the first teleseismic crustal study of one of the world s youngest continental rifts the okavango rift zone orz where the magma has not yet breached the surface results from receiver function stacking and gravity modeling indicate that the crust mantle boundary beneath the orz is uplifted by 4 5 km and the initiation of the orz is closely related to lithospheric stretching possible decompression melting of the subcrustal lithosphere occurs beneath the orz as evidenced by a relatively low upper mantle density based on the gravity modeling

evaluating the use of dune sediments as a proxy for palaeo aridity a southern african case study abstract the dominance of dryland environments in the southern hemisphere makes the study of these regions of critical importance for the development of regional hemispheric and global models of environmental change unfortunately the wetting and drying cycles associated with semi hyperarid climates are not conducive to the preservation of traditional organic proxy data sources the last decade however has seen the development of a number of alternative archives including the application of luminescence dating techniques to dunes and other aeolian deposits in continental situations the existence of relict dune fields has long been thought to be evidence of drier conditions during the pleistocene and direct ages from these features have been interpreted almost exclusively as indicating phases of aridity however an increasing number of ages from a broader range of environments are calling into question the assumption that aeolian activity can be simply equated with aridity presented here is a comparison of dune ages that have been obtained from across southern africa with a range of proxies from both terrestrial and marine records taken as a whole three primary phases of activity can be identified at 60 40 35 20 and 17 4 ka the frequent discordance with other terrestrial records indicating coeval increases in humidity and the close correlation of these phases with wind strength proxies suggest that aridity is unlikely to be the sole or even primary forcing mechanism for aeolian activity in the region and the palaeoclimatic significance of these sedimentary archives needs to be reassessed

mapping the surface geomorphology of the makgadikgadi rift zone mrz abstract the makgadikgadi basin and wider middle kalahari region of botswana and beyond host landforms which have been attributed to quaternary environmental change including palaeolake level fluctuations and aeolian activity tectonic processes and landforms on the other hand have mostly been linked to the okavango graben and associated rift zone orz to the west of the makgadikgadi in this paper we establish the extent of tectonic surface expression associated with the makgadikgadi rift zone mrz we identify a series of parallel nne ssw normal faults and scarps expressing horst and graben structures linked to seven major blocks in the northern makgadikgadi basin using both shuttle radar topographic mission srtm and geomagnetic data subtle expression of rifting has controlled endorheic drainage topology replicated regional dune field patterns and displaced the 945 m palaoelake contour since lake desiccation these observations underscore the role of neotectonic piano key block movement in shaping surface landforms across a large expanse of the kalahari region this paper provides the first detailed map and introduction to the makgadikgadi rift zone mrz and its geomorphology

high pressure behaviour of selected boron minerals and the question of boron distribution between fluids and rocks abstract experimental results in the simplest possible systems up to generally 20 50 kbar in a few cases 100 kbar bearing on the pt stability limits of 17 end member borosilicates and borates are summarized except for the sillimanite structure type minerals grandidierite and werdingite which break down near 10 kbar the following phases are stable up to much higher pressures and no influence of boron coordination with oxygen three or fourfold can be recognized alkali free dravite b nepheline boromuscovite danburite dravite dumortierite jeremejevite oh kotoite magnesiodumortierite prismatine pseudosinhalite reedmergnerite szaibelyite sinhalite suanite the framework borosilicates reedmergnerite and danburite remain more stable towards higher pressures than their al analogues albite and anorthite respectively kornerupine stability decreases with decreasing boron content in the mineral from about 37 kbar for near end member prismatine to 12 kbar for the boron free end member with a maximum thermal stability of 1050 c at 30 kbar boron rich kornerupine prismatine is the thermally most stable borosilicate at high pressures thus all the above 15 phases can in principle occur as minerals of ultrahigh pressure metamorphic rocks boron initially enriched in crustal rocks may be subducted to depths exceeding 100 km in al rich systems b 2 o 3 generally behaves as an inert component and enters refractory minerals so that such rocks may act as sinks for boron even in the presence of h 2 o in other systems rich sio 2 na 2 o mgo however boron is more volatile and may fractionate into hydrous fluids that may cause metasomatism elsewhere altogether the behaviour of boron in natural petrogenetic systems is strongly dependent on the local chemical environment

the sedimentary cycle of the boron isotopes abstract marine sediments contain two isotopically distinct components a non desorbable fraction with 11b between 4 3 and 2 8 per mil n 10 and a desorbable component with 11b between 13 9 and 15 8 per mil n 6 the adsorption coefficient k for the uptake of b from seawater by fluvial suspended material mississippi has an experimentally determined value of 1 54 0 05 the associated isotope fractionation factor is 0 974 0 003 empirical values for k determined from the analysis of marine sediments n 6 were between 3 and 4 the amount of b adsorbed onto fluvial suspended material during estuarine mixing is about 9 109 moles yr based on pore water data adsorption experiments and comparative size fraction analyses suspended sediments from the mississippi river vs bottom deposits from the delta there is no evidence for the incorporation of b into detrital sediments at low temperature however the high b content of bauer deep metalliferous sediments must be due to incorporation of seawater b during formation of authigenic silicates the 11b of these minerals is 2 3 per mil hydrothermally altered sediments from dsdp hole 477 guaymas basin gulf of california have variable b contents from 33 ppm similar to the unaltered detritus to 1 2 ppm in the recrystallized material the latter has 11b as low as 9 0 per mil substantially lower than the unaltered material hydrothermal solutions collected from vents adjacent to the hole have boron contents elevated by about a factor of 4 relative to seawater and 11b between 16 5 and 23 2 per mil the b mobilized from the sediments is isotopically fractionated with the fluids being preferentially enriched in the heavy isotope analysis of an oxisol profile developed over granite in the guyana shield showed that boron is partially mobilized during weathering with an isotopic offset of 2 5 per mil between bedrock and soil the observed enrichment of b in shales relative to igneous rocks does not occur during weathering or during exposure to seawater at low temperature incorporation occurs only during burial diagenesis at temperatures greater than about 60 c however the enrichments cannot be attained during a single cycle of primary weathering and burial but must be cumulative over many cycles

geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice abstract hummocky or disintegration moraine is a widespread late wisconsin glacial terrain type across the glaciated great plains of mid continent north america this terrain consists of chaotic hummocks and enclosed kettle hollows that are widely underlain by clay rich till the prevailing interpretation of this terrain is one of supraglacial deposition from debris covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins recently in contrast it has been argued that hummocky moraine can form subglacially by the pressing of fine grained deformable till under the overburden pressure of stagnant ice in this paper we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil till the model confirms that hummocky topography can be the result of failure and deformation of fine grained till by differential loading below stagnant ice differential pressures at the ice bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water filled doline like depressions the growth and decay of ponds as ice down wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random linear circular elongate and other patterns of hummocks ridges and hollows formerly streamlined preferentially oriented subglacial landforms such as drumlins and rogen moraines can also be overprinted forming washboard moraine

isotopic and elemental partitioning of boron between hydrous fluid and silicate melt the fractionation of b and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 c and 100 500 mpa fluid melt partition coefficients are 1 for rhyolite melt this shows that b is not always strongly extracted from melts into hydrous fluids boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems especially at high t and is most simply explained by differences in coordination trigonal vs tetrahedral among coexisting phases combined with earlier measurements on illite water 300 350 c b isotopic fractionation defines a temperature dependent trend from 300 to 1100 c because of the large magnitude and apparent low sensitivity to bulk composition b isotopic fractionation can be readily applied to studies of diagenesis hydrothermal alteration of planetary bodies subduction zone processing and arc magma generation and magma chamber evolution

petrography and diagenesis of silcrete from the kalahari basin and cape coastal zone southern africa abstract silcrete is widely distributed in southern africa and occurs both in genetic association with deeply weathered bedrock weathering profile type and as a significant component of the cenozoic kalahari beds where it has formed through silicification of a variety of continental deposits nonweathering profile type silcrete fabrics are classified into four major types gs grain supported skeletal quartz grains in contact f floating skeletal grains floating in matrix m matrix skeletal grains 5 of volume and c conglomeratic detrital component containing pebbles f and m fabric types are subdivided on the presence absence of glaebules replacement by silica of a precursor matrix can account for the development of all four fabric types but at least some gs and c fabr c silcretes have formed by passive silica precipitation in packing voids glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes but their precise origin is uncertain authigenic silica is present as megaquartz microquartz length slow and length fast chalcedony predominantly in void fills cryptocrystalline silica and opal ct depending on the silica and ionic concentrations of pore waters ph and host sediment characteristics diagnostic petrographic features including glaebules colloform structures and chalcedonic vugh fills may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types formed in humid low ph environments or nonweathering profile types formed in high ph semiarid to arid environments

ground water silicification and leaching in sands example of the fontainebleau sand oligocene in the paris basin in the fontainebleau sand oligocene superposed lenses of quartzite are restricted to outcrops and structurally low zones of the formation the quartzite lenses result from a silicification process controlled by water table levels during the recent geomorphologic evolution of the landscape the quartzite lenses pinch out rapidly under the limestone capped plateaus and are related to the development of a bleached sand profile above the water table the sand is bleached and the former quartzite lenses are corroded along their margins moreover flint pebbles are dissolved and primary smectite and illite are weathered to kaolinite dissolved silica can thus reach supersaturated levels allowing quartz precipitation quartz grains are overgrown at the base of the profile in zones of ground water outflow related to spring lines the adjustment of the water table during the landscape incision led to formation of quartzite lenses at deeper levels this silicification process has no climatic significance but is dependent on the stability of the landscape and on its progressive dissection the rate of quartz cementation indicated by geomorphologic considerations is consistent with hydraulic and geochemical calculations it appears to be very rapid approximately 30 000 yr for a quartzite lens

siliceous duricrusts as palaeoclimatic indicators evidence from the kalahari desert of botswana abstract previous attempts to identify the environmental conditions associated with the development of siliceous duricrusts have used differences in bulk chemical compositions and cement mineralogy amongst other criteria to distinguish between samples high up to 3 00 levels of tio2 together with petrographic characteristics have been used as an indicator of past wetter climatic conditions often failing to fully emphasise the potential importance of local controls upon micromorphology and chemistry in southern africa silcretes from the cape coastal zone associated with deep weathering profiles have been attributed to formation under wetter conditions whereas kalahari silcretes are suggested to have formed under more arid alkaline conditions petrographic and geochemical analyses of silcretes from five locations within the kalahari desert of botswana one site associated with deeply weathered bedrock indicate the great micromorphological complexity of kalahari silcretes it is suggested that variations in the host material composition of kalahari silcretes are an important control of their chemistry silcretes have developed from unconsolidated kalahari sand with variations in the mineral content of the host sediment appearing to cause variation in silcrete geochemistry geochemical analysis confirms that tio2 provides the main variation between kalahari and cape coastal silcretes but the low levels of tio2 in kalahari samples are attributed to differences in host material as opposed to overriding climatic controls

cross arc geochemical variations in the kurile arc as a function of slab depth lavas from transects across the kurile islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab enrichment factors for boron cesium arsenic and antimony all elements with strong affinities for water decreased across the arc this decrease is presumably related to losses of water rich fluids during the dehydration of the subducting plate enrichments of potassium barium beryllium 10 and the light rare earth elements remained constant these species may move in silica rich fluids liberated from the slab at greater depths

diagenetic transformations and silcrete calcrete intergrade duricrust formation in palaeo estuary sediments the boteti palaeo estuary in northern botswana is located where the endoreic boteti river an overflow from the regional okavango river system enters the makgadikgadi pans the present work considers diagenetic silica and calcium carbonate dominated transformations the aims are to help identify precursor conditions for the origin of microcrystalline silcrete calcrete intergrade deposits while developing insight into pene contemporaneous silica and calcite matrix formation general precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan the pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater freshwater inflow into a saline pan drives the geochemistry of the system from freshwater to saline from neutral to high ph the geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan the source of most silica and localized caco3 is derived from the dissolution and precipitation of micro fossils while more general caco3 enrichment stems from saline pan based carbonate fractionation diagenetic change leads to colloidal then more consolidated bsio2 cao aggregate formation amorphous silica followed by transformations into opaline silica over time irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments inferring a source for local silica mobilization in intergrade deposits the distribution of calcareous micro fossils may have a similar converse effect diagenetic evidence from an intergrade deposit with a low sio2 cao ratio suggests that transformation occurred more into the pan while an intergrade deposit with a high sio2 cao ratio more likely formed closer to a land margin and was frequently inundated by freshwater pene contemporaneous silcrete calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in ph the continuing consolidation of bsio2 cao aggregates may be facilitated by the presence of increasing amounts of calcite it appears that caco3 may act as a catalyst leading to pene contemporaneous bsio2 cao aggregate formation however the processes involved require further work copyright 2013 john wiley sons ltd

evolution of compositional polarity and zoning in tourmaline during prograde metamorphism of sedimentary rocks in the swiss central alps the compositional evolution of tourmaline from diagenetic to lower amphibolite facies conditions was investigated in two metasedimentary units redbed and black shale formations along a traverse across the central alps with increasing metamorphism three distinct rim zones grew around detrital tourmaline cores simultaneously with the development of prismatic neoblasts detrital cores and all successive rim zones are preserved in the amphibolite facies rocks as indicated by whole rock data the b required for tourmaline growth was released from illite and muscovite and was not introduced from external sources the studied samples of tourmaline exhibit compositional polarity i e their compositions are different at the opposite ends of the c axis the compositional difference between the two poles can be expressed by the substitutions na mg 0 al and 2al ti mg with the positive pole always richer in al and having more x site vacancies both chemical and optical polar effects are most pronounced in the internal rim zones and become less prominent toward the external zones the systematic prograde compositional trends are depicted in a vector space for li poor tourmaline at low grade conditions the tourmaline composition is clearly controlled by the host rock composition but with increasing metamorphic grade the compositions of tourmaline from a variety of rocks converge the data show that in the central alps increasing metamorphic grade is reflected by increases in ca ca na and mg mg feh a decrease in fe3 and an increase in the occupancy of the x site of tourmaline

dunes in southeastern namibia evidence for holocene environmental changes in the southwestern kalahari based on thermoluminescence data abstract linear dunes in the southwestern kalahari provide evidence of the nature of paleoclimates they have special significance because their alignments indicate a previous circulation pattern with prevailing northwesterly winds at present the bases of the dunes are fixed by vegetation and only the dune crests are active in contrast to previous suggestions final dune stabilization took place before about 9 8 ka this corresponds to reduced wind velocities accompanied by more humid conditions this seems to be the last important environmental change recorded by significant landforms in the southwestern kalahari since about 8 ka the region remained more or less semi arid the long distance to seasonal south shifting humid tropic air masses innertropical convergence zone itcz and the climatic barrier of the namib and the karas mountains against atlantic influences allowed only minor fluctuations the holocene is a period of reduced pan development the pans are partly buried by the early holocene linear dunes as well as by young sand fields at pan 1073 southeast of grootpan intra pan dunes have formed during the last 1000 years

evolution patterns of glaucony maturity a mineralogical and geochemical approach abstract integrated mineralogical and geochemical analyses were carried out on a wide variety of glauconitic grains from 25 western european localities covering the whole range of glaucony maturity and ranging in age between the cretaceous and the pliocene a comprehensive data set documents the major evolution patterns of glaucony and the relationships between glauconitic minerals and their substrate components increasing glaucony maturity involves potassium and rubidium enrichment the progressive evolution from a k poor glauconitic smectite to a k rich glauconitic mica is accompanied by the overall decrease of distance between peaks 001 and 020 the width of basal reflection 001 however is proposed here as a more powerful tool for the discrimination of glauconitic minerals at different stages of maturity this parameter is especially efficient for the differentiation of evolved and highly evolved glaucony the range of initial substrate composition appears to control significantly major and trace element variations during glaucony maturation contrasting patterns are observed in glauconitic grains as a function of the proportion of carbonate versus siliciclastic substrate components particularly the distribution patterns of mgo and sio2 in glaucony associated with carbonate host rocks suggest progressive uptake of these elements in the structure of glauconitic minerals with increasing degree of maturation by contrast a decrease in sio2 and ai2o3 with increasing maturity may be observed when the substrate for glauconitization is represented by siliciclastic material fe shows a positive correlation with k2o in glaucony with low maturity but an opposite trend in evolved glaucony therefore fe cannot be regarded as a reliable indicator of glaucony maturity maturation of glauconitic minerals results invariably in ca and sr depletion irrespective of the original composition of the precursor material as a consequence the proportion of ca and sr may be considered as an inverse relative criterion of glauconitic mineral evolution for a given source area

the geochemical cycle of boron constraints from boron isotope partitioning experiments between mica and fluid abstract the fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3 0 gpa 500 c and 3 0 gpa 700 c for near neutral fluids 11b mica fluid 11b mica 11b fluid is 10 9 1 3 at 500 c and 6 5 0 4 at 700 c this supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas clays and melts the t dependence of this effect is approximated by the equation 11b mica clay melt neutral fluid 10 69 1000 t k 3 88 r2 0 992 valid from 25 c for fluid clay up to about 1000 c for fluid silicate melt experiments at 0 4 gpa that used strongly basic fluids produced significantly lower fractionations with 11b mica fluid of 7 4 1 0 at 400 c and 4 8 1 0 at 500 c showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated field studies have shown that boron concentrations and 11b 10b ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench thus reflecting the thermal structure of the subducting slab our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct p t range continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs

melt production beneath mt shasta from boron data in primitive melt inclusions most arc magmas are thought to be generated by partial melting of the mantle wedge induced by infiltration of slab derived fluids however partial melting of subducting oceanic crust has also been proposed to contribute to the melt generation process especially when young and hot lithosphere is being subducted the isotopic composition of boron measured in situ in olivine hosted primitive melt inclusions in a basaltic andesite from mt shasta california is characterized by large negative values that are also highly variable delta 11 b 21 3 to 0 9 per mil the boron concentrations from 0 7 to 1 6 parts per million are lower than in most other arc lavas the relation between concentration and isotopic composition of boron observed here supports a hypothesis that materials left after dehydration of the subducting slab may have contributed to the generation of basaltic andesite lavas at mt shasta

boron isotopic composition of subduction zone metamorphic rocks abstract many arc lavas contain material derived from subducted oceanic crust and sediments but it remains unresolved whether this distinctive geochemical signature is transferred from the subducting slab by aqueous fluids silicate melts or both boron isotopic measurements have the potential to distinguish between slab transfer mechanisms because 11 b fractionates preferentially into aqueous fluids whereas little fractionation may occur during partial melting previous studies have shown that 11 b values of island arc lavas 6 to 7 overlap the range of 11 b values for altered oceanic crust 5 to 25 and pelagic sediments and turbidites 7 to 11 secondary ion mass spectrometry sims analyses of minerals in subduction zone metamorphic rocks yield 11 b 11 to 3 suggesting that slab dehydration reactions significantly lower the 11 b values of subducted oceanic crust and sediments in order to explain the higher 11 b values reported for arc lavas as compared to subduction zone metamorphic rocks the b bearing component derived from the metamorphosed slab must be enriched in 11 b relative to the slab favoring an aqueous fluid as the slab transfer mechanism