directional chemical variations in diamonds showing octahedral following cuboid growth a progression from cuboid to octahedral growth has been observed in 16 natural diamonds from yakutian kimberlites x ray and cathodoluminescence topography have revealed that the change in morphology of diamonds with cloudy cuboid cores may occur without mixed habit growth but via generation of numerous octahedral apices on cuboid surfaces and subsequent gradual transformation into regular octahedral morphology nitrogen aggregation in both cuboid and octahedral domains of such diamonds suggests that they have had a long residence time under mantle conditions micro inclusions in the cuboid domains of the diamonds testify to the nucleation and growth of cuboid cores from a hydrous carbonatitic oxidized fluid the transition from cuboid hummocky growth rich in inclusions to octahedral growth without inclusions may be linked to decreasing supersaturation in the parent fluid measurements of 13c and nppm by ion microprobe show that the chemical variations observed between inner cuboid domains and outer octahedral zones commonly have a systematic character and as such they are probably not due to purely kinetic effects the peripheral octahedral zones are always enriched in 13c in comparison with inner cuboid ones and the total nitrogen content decreases with the change from cuboid to octahedral growth the octahedral outer zones show a gradual progressive increase in 13c with an overall change of up to 5 from the cuboid core 13c usually between 8 and 6 to the diamond margin 13c usually between 4 and 2 decreases in 13c of this magnitude with a gradual increase in 13c may be attributed to the rayleigh fractionation operating on a single parent fluid of close to normal mantle 13c composition with diamond precipitating by the reduction of carbonatitic fluid in a closed system however one sample shows a variation of 13c of approximately 17 to 6 and therefore suggests a possible change of fluid source composition from one containing subducted crustal organic carbon to one with common mantle carbon

an updated classification scheme for mantle derived garnet for use by diamond explorers abstract mantle derived garnets recovered in diamond exploration programs show compositional variations in cr ca mg fe and ti that reflect the chemical physical and lithological environments in which they occur occasionally together with diamond the association of diamond with mantle garnet has progressed through a number of geochemical advances most notably those of dawson and stephens 1975 and gurney 1984 which are integrated in this work with less well known petrological advances made primarily in xenolith and experimental petrology a simple robust garnet classification scheme is formulated which accommodates empirical garnet diamond relationships for peridotitic g10 g9 g12 megacrystic g1 ti metasomatised g11 pyroxenitic g4 g5 and eclogitic g3 lithologies in eight distinct garnet classes the calcium saturation characteristics of harzburgitic g10 lherzolitic g9 and wehrlitic g12 garnets are described by a ca intercept projection that also shows promise as a relative barometer for garnet lherzolite grutter and winter 1997 thermobarometric aspects of garnet diamond associations are highlighted in the scheme through the use of the minor elements mn and na though analysis by anything other than an electron microprobe is not required for classification a d suffix is added to the g10 g4 g5 or g3 categories to indicate a strong compositional and pressure temperature association with diamond the scheme remains open to improvement particularly with regard to delineation of pyroxenitic or websteritic diamond associations and to advances in ca in garnet and na in garnet thermobarometry

carbon concentrations and isotopic ratios in fluid inclusion bearing upper mantle xenoliths along the northwestern margin of north america abstract at least three and perhaps four carbon components have been identified by step heating extraction of upper mantle xenoliths from nunivak island and prindle volcano alaska fort selkirk and alligator lake yukon jacques lake and castle rock british columbia one component released mostly between 200 and 600 c comprises 30 95 of the total carbon extracted and has 13c values between 28 and 23 this carbon corresponds mostly to complex carbon compounds condensed on surface minerals and cracks and will be referred to as isotopically light carbon ilc co2 released between 600 and 800 c is derived from interstitial carbonates it has variable 13c and concentrations that generally do not exeed 24 ppm c the carbon extracted from 1000 to 1450 c is a mixture of co2 from fluid inclusions and the ilc fraction the amount of ilc released in this temperature interval never exceeds 10 ppm because part of this ilc may have been introduced from the host lava the 13c variations of this carbon extracted above 800 c is not only characteristic for the original carbon present in fluid inclusion poor xenoliths however xenoliths enriched in co2 fluid inclusions show c isotopic variations at the highest temperature steps that fall in a relatively narrow range between 10 and 4 depending upon the fluid inclusion abundance the concentration of high temperature c derived from fluid inclusions varies from 0 1 to 66 ppm anhydrous spinel lherzolites and harzburgites generally contain

subducting oceanic crust the source of deep diamonds inclusions of majoritic garnet in diamonds from the jagersfontein kimberlite formed at unusually great depths of 250 to 500 km in the asthenosphere and transition zone the original host rocks were derived from a much shallower basaltic eclogitic source the presence of negative eu anomalies in all majoritic garnets requires a crustal origin thereby linking these very deep diamond sources to subducting oceanic crust the carbon isotope values 13c of the host diamonds fall within a narrow range at 20 which is fundamentally different from the broad range 24 to 2 and bimodal distribution of carbon isotopes of jagersfontein diamonds that formed in the shallower lithosphere this indicates that majoritic garnet bearing diamonds at jagersfontein inherited their light carbon isotopic composition directly from organic matter contained in a subducting slab these diamonds were likely formed by direct conversion from graphite well within the diamond stability field

sedimentary evolution of a palaeozoic basin and ridge system the middle and upper devonian of the ahnet and mouydir algerian sahara the ahnet and mouydir regions of southern algeria are part of one of the world s largest almost undeformed exposures of palaeozoic rocks which exemplify a hitherto poorly known early variscan development of a devonian basin and ridge system this area includes a series of intracratonic basins along the northern margin of the west african craton which consists from w to e of the reggane basin azel matti ridge ahnet basin foum belrem ridge and mouydir basin the depositional and palaeogeographic interpretation is based on 71 sections in this region which for the first time were biostratigraphically calibrated by means of conodonts goniatites and brachiopods the structural evolution during devonian times was probably controlled by reactivation of ancient n s to nw se running faults in the precambrian basement which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf a hiatus during emsian times indicates widespread emergence during this interval the entire area was flooded during the earliest eifelian when the first vestiges of the azel matti ridge become evident by stratigraphic condensation the palaeogeographic differentiation is most apparent during the givetian when a shoal with reduced carbonate sedimentation was established on the azel matti ridge passing towards the west and east into basinal environments of the reggane and ahnet basins respectively the foum belrem ridge is distinguished by increased subsidence during the early givetian and by revived uplift during the late givetian in the mouydir basin further east up to 1000 m of shales were deposited during the givetian the early frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites these lithologies occur repeatedly already during the middle devonian and document intermittent anoxic conditions the basin and ridge topography is levelled by the shallowing up sequence of up to 1400 m thick upper frasnian and famennian shales which grade into a deltaic sequence of uppermost famennian tournaisian sandstones the up to now only vaguely discriminated lithostratigraphic formations of the devonian have been biostratigraphically defined in suitable type sections

anticorrelation between low δ13c of eclogitic diamonds and high δ18o of their coesite and garnet inclusions requires a subduction origin diamond is essentially impermeable and unreactive under many conditions and tiny mineral inclusions within natural diamonds can faithfully preserve information on the chemical and physical conditions during diamond growth the stable isotope ratios of carbon nitrogen oxygen and sulfur in diamonds and their mineral inclusions have been used to constrain models of diamond formation but interpretations of the data have differed dramatically the crux of the controversy lies in the interpretation of the carbon isotope ratios of eclogite suite diamonds which range well outside those expected for typical mantle materials such as peridotites basalts and carbonatites proposed explanations for these anomalous carbon isotope ratios include derivation from primordial mantle inhomogeneities fractionated mantle fl uids and subducted biogenic carbon working with samples from three continents we have analyzed the carbon isotope compositions of eclogite suite diamonds and the oxygen isotope composition of their mineral inclusions primarily by ion microprobe methods we have discovered a previously unrecognized remarkably consistent anticorrelation between these two isotopic systems in that virtually all diamonds with anomalously low carbon isotope ratios have silicate inclusions with anomalously high oxygen isotope ratios this is a fundamental observation that can only be explained by formation of eclogite suite diamonds through subduction of seafl oor altered basalt admixed with marine biogenic carbon into the fi eld of diamond stability

modelling the growth of natural diamonds abstract in the following article we have reviewed the mineralogical chemical physical and isotopic evidence concerning the growth of coated diamonds and have combined it to produce a comprehensive model for their formation we suggest that the coatings formed during a period of metasomatic activity which preceded the eruption of their host kimberlites ultrapotassic co21bh2o rich fluids resulted from the crystallisation of diamond free kimberlitic magmas intruded into the deep lithosphere the kimberlite magmas were derived from an asthenospheric source which was globally quite uniform in terms of the isotopic composition of carbon and nitrogen 13c and 15n both being close to 5 the fluids produced during crystallisation invaded overlying diamondiferous lithosphere in which different regions could contain diamond populations with different mineralogical isotopic and physical characteristics depending on the history of their source region either intrinsic to their formation or due to reaction with reduced lithosphere the fluids were super saturated with carbon although not to such an extent as to allow for the nucleation of new diamonds when these super saturated fluids encountered older diamonds new and fibrous diamond growth commenced immediately this period of metasomatic activity was followed rapidly by one or more successful kimberlite eruptions which transported the now coated diamonds to the surface in the crust carbonates within hypabyssal facies kimberlites will have 13c values which reflect the deep seated source 5 whereas those within diatreme facies may be more variable due to outgassing of co2 during eruption finally we consider isotope fractionation related to growth we suggest that approaches based on equilibrium are not appropriate to diamonds but rather that it is better to consider the nature and kinetics of the surface reactions leading to growth where diamond growth is very rapid as may be the case for fibrous diamond we propose that there will be no difference in isotopic composition between the fluid and diamond and not the 3 4 13c depletion in the latter predicted by theoretical treatments octahedral diamonds are more problematic since far less is known about their growth conditions however we show in principle how the content and perhaps isotopic composition of nitrogen within a diamond may be controlled by its growth rate we are uncertain whether 13 c would be affected

conodont thermal alteration patterns in devonian and carboniferous rocks of the ahnet and mouydir basins southern algeria abstract conodont colour alteration indices cai have been used to recognize the thermal maturation of the middle devonian to late carboniferous sedimentary infill of the ahnet and mouydir basins in southern algeria the study is based on a large set of conodont samples obtained from 69 sections and outcrops in the ahnet basin the middle and upper devonian rocks at the surface show a uniform thermal maturity level characterized by the cai values of 2 which suggests maximum burial temperatures about 50 c in contrast the carboniferous exposures yield a wide range of cai values from 1 to 3 5 besides the general tendency in maturity decreases upward in the sequence there are anomalies related to heating by small local magmatic bodies in the mouydir basin the cai values in the middle and upper devonian outcrops range from 2 to 3 5 the pattern indicates that the majority of middle devonian rocks have experienced burial temperatures of about 120 c while thermal maturation of the upper devonian strata is clearly lower with maximum heating temperatures about 50 c at the edges of the basin the observed thermal maturity has been mostly achieved during late carboniferous time prior to the variscan inversion and only locally during the mesozoic it is a product of loading by carboniferous overburden succession of which part is now removed approx 200 1300 m in the ahnet basin and up to 1200 m in the mouydir basin the analysis of palaeotemperatures suggests an average geothermal gradient of 30 c km during carboniferous time in the ahnet mouydir area in terms of hydrocarbon exploration the studied devonian succession is predominantly mature for oil and gas in contrary the carboniferous rocks are immature higher maturities are likely to occur in the subsurface

constraining the internal variability of the stable isotopes of carbon and nitrogen within mantle diamonds the carbon and nitrogen isotope values of mantle xenoliths and xenocrysts are used to trace the cycling of volatiles in the deep earth for example to place empirical constraints on the origin of diamond forming carbon in the mantle the global database for diamond shows that the 13c value of peridotitic diamonds is very narrow typically around 5 whereas eclogitic diamonds can show positive and very negative 13c values resembling crustal carbonates and crustal organic carbon 2 commonly interpreted to reflect a relationship between eclogitic diamond formation and subduction zone planet tectonics curiously diamonds from both parageneses can show positive crust like and negative mantle like 15n values from 20 most of these data are derived from single stage combustion gas sourced mass spectrometry which produces simplistic datasets by fragmenting single diamonds or using in situ ion beam techniques it is known that single diamonds can show large scale heterogeneity for their carbon isotope values and nitrogen abundances sometimes as large as entire populations of diamond across a few hundred micrometres what is less well known is the scale of nitrogen isotope heterogeneity within single diamonds and if the nitrogen isotope heterogeneity of single diamonds can provide an insight into why diamonds that show very restricted 13c values show a much large range of 15n values to investigate the scale and to determine the origin of the nitrogen isotope heterogeneity source vs fractionation during diamond formation shown for populations of mantle diamonds we have determined multiple 13c 15n values and nitrogen abundances from 14 monocrystalline mcds and 25 polycrystalline diamonds pcds using step wise oxidation gas sourced mass spectrometry these data show that the heterogeneity shown for carbon and nitrogen isotope values from single diamond samples presented here is typically 5 and 8 respectively both of which are comparable to the standard deviation for the mean mantle 13c and 15n values 3 and 4 however there are samples that show much larger heterogeneities for 13c and 15n values 23 and 33 respectively which cannot be generated by equilibrium stable isotope fractionation during or prior to diamond formation these data suggest that isotopic heterogeneity may be present within the diamond forming fluid on sub mm scale or that these diamonds formed during multiple diamond formation events from isotopically distinct sources from these 39 samples there are only 5 pcds that show a larger range of carbon isotopes relative to nitrogen isotopes but of these 5 samples only 2 show a range of 13c values outside of analytical uncertainty the remaining 34 samples show a greater isotopic heterogeneity for 15n relative to 13c values the samples with the largest carbon and nitrogen isotopic heterogeneity are also the samples with low bulk 13c values 10 whilst there is no relationship between the ranges of nitrogen isotope values for a given sample and the corresponding bulk 15n value or nitrogen content these data show that 15n values recorded in mantle diamonds are relatively heterogeneous and can show both mantle like negative and crustal sedimentary like positive 15n values within the same sample we conclude that the large range of nitrogen isotope values seen within individual diamonds means that the observation of negative mantle like nitrogen in mantle diamonds acquired by single stage bulk combustion isotope ratio mass spectrometry cannot be used as a conclusive indicator for a mantle origin for the entirety of the diamond forming carbon and vice versa also the behaviour of 15n 14n is not coupled with the behaviour of 14n 12c during diamond formation instead it appears that diamond forming fluids can have positive and negative 15n values irrespective of their 13c value s these data suggest that subduction induced nitrogen isotope heterogeneity may not be coupled with subduction induced carbon isotope heterogeneity in diamond forming fluids

evolution and facies architecture of the mesotidal guadiana river delta s w spain portugal abstract the guadiana river estuary and delta is one of the most important mesotidal fluvio marine systems of the southwestern iberian peninsula the river mouth was formed as a narrow channel excavated by fluvial incision during the pleistocene lowstand and then flooded 6500 yr ago during the flandrian transgression holocene the estuary is in an advanced state of sediment infilling in its proximal part due to its narrow morphology which prevents passage of sediment through to the open coast consequently sediment is accumulating in the river mouth causing progradation as response of the interaction between coastal and fluvial processes sedimentation is controlled by the interaction of sediment load delivered by the guadiana river and the active longshore current which also supplies sand eroded from barrier islands and friable cliffs in the portuguese algarve to the west tidal range is mesotidal 2 0 m and mean significant wave height is 0 6 m with an average period of 3 6 s morphological differences exist between the two sides of the river mouth the western side has a large spit separated from the mainland by a marsh and the eastern side shows an array of old barrier islands separated by salt marsh areas a vibracore study allows us to distinguish 12 different depositional facies in the system fluvial delta tidal inlet flood tidal delta washover beach dune estuarine channel lagoon tidal creek channel margin tidal flat and salt marsh furthermore a different architectural scheme of these facies is present on each side to the east there is a longitudinal repetition of the sandy facies corresponding to the barrier islands on the west side there is a transverse growth of a single spit containing eolian dunes these differences result from asymmetric interaction between the tidal and fluvial currents at the river s mouth and the waves and tidal currents of the open coastal zone

carbon and nitrogen isotope systematics in diamond different sensitivities to isotopic fractionation or a decoupled origin abstract using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources it has been argued that carbon and nitrogen in diamond are decoupled here we re investigate the carbon nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds n 94 from a single locality our diamond samples derived from two kimberlites in the chidliak field ne canada show large variability in 13 c 28 4 to 1 1 mode at 5 8 15 n 5 8 to 18 8 mode at 3 0 and nitrogen contents n 3800 to less than 1 at ppm in combination cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low 13 c and n to mantle like 13 c and high n overall 13 c appears to be uncorrelated to 15 n and n on both the inter and intra diamond levels co variations of 15 n log n however result in at least two parallel negatively correlated linear arrays which are also present on the level of the individual diamonds falling on these two trends these arrays emerge from the two principal data clusters are characterized by slightly negative and slightly positive 15 n about 3 and 2 respectively and variable but overall high n using published values for the diamond fluid nitrogen isotope fractionation factor and nitrogen partition coefficient these trends are perfectly reproduced by a rayleigh fractionation model overall three key elements are identified in the formation of the diamond suite studied 1 a low 13 c and low n component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event 2 repeated influx of a variably nitrogen rich mantle fluid mildly negative 13 c and 15 n 3 in waning stages of influx availability of the mantle type fluid at the site of diamond growth became limited leading to rayleigh fractionation these fractionation trends are clearly depicted by 15 n n but are not detected when examining co variation diagrams involving 13 c also on the level of individual diamonds large 5 variations in 15 n are associated with 13 c values that typically are constant within analytical uncertainty the much smaller isotope fractionation factor for carbon considering carbonate or methane rich fluids as possible carbon sources compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of 13 c values to rayleigh fractionation processes i e during fractionation a 1 change in 13 c is associated with a 10 change in 15 n as a consequence even minor heterogeneity in the primary isotopic composition of diamond forming carbon e g due to addition of minor subducted carbon will completely blur any possible co variations with 15 n or n we suggest this strong difference in isotope effects for c and n to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond

role of hurricanes in the holocene stratigraphy of estuaries examples from the gulf coast of florida abstract sarasota and little sarasota bays are shallow coastal bays located landward of a holocene barrier inlet complex on the west central microtidal gulf coast of florida sediments presently accumulating in the bay consist of 1 fine to very fine quartz sand contributed from the gulf shoreline during storm generated washover through tidal inlets and from reworking of older deposits 2 fine sand to pebble sized quartz and phosphatic sediment contributed by tertiary and pleistocene deposits 3 biogenic carbonate debris which is produced within the bays and or derived from the gulf of mexico 4 clay minerals derived from weathering of tertiary and pleistocene carbonates and clays and 5 particulate organic debris interpretations from 51 vibracores from throughout the bays have enable delineation of four major depositional sedimentary facies resulting from specific environmental conditions protected bay open bay tidal delta washover and storms bedrock in the area ranges from 0 to about 8 m msl and is largely responsible for the areal configuration of the bay and the location of the barrier islands the holocene stratigraphy of both bays has been greatly influenced by the passage of major hurricanes which carried large volumes of sand and shell gravel into the bays at least four of these storms are documented in these cores three storm units from sarasota bay have been radiocarbon dated at 2 270 1 320 and 240 years before present ybp historically documented severe hurricanes influenced this coast in 1848 and 1921 hurricanes interrupted the normal low energy slow deposition in the bays and caused inlets to open and close three storm generated sedimentary facies are identified 1 a graded storm facies which is composed of conspicuously fining upward units ranging from sandy shell gravel to slightly shelly quartz sand 2 a homogenous storm facies consisting of homogenous and relatively thin shelly sand and gravel units and 3 a fluvial storm facies which is muddy and generally free of shells representing terrigenous influx from runoff

exploration history of the palaeozoic petroleum systems of north africa subsurface exploration of the palaeozoic petroleum systems of libya algeria tunisia and morocco began in the early 1950s after the discovery of a large prospective palaeozoic basin in the sahara desert province of north africa the first exploration well to test this concept was drilled in algeria in 1952 a non commercial find in 1953 was followed by major discoveries in 1956 since that time approximately 1100 new field wildcats have been drilled to test palaeozoic and basal triassic reservoir targets in a variety of discrete petroleum systems this resulted in the discovery of approximately 330 palaeozoic sourced accumulations in the algerian libyan and tunisian portions of the sahara desert by the end of 1996 ultimate recoverable reserves in oil equivalent for these discoveries were estimated to be just over 46 billion barrels using measures of activity and reserves discovered it is seen that the history of exploration of this province has followed a normal cycle familiar to the petroleum industry which can be divided into five major periods they are pre discovery discovery post discovery boom post boom and revival it is anticipated that future advances in technology will result in new ideas and more effective exploration and ultimately in the future discovery of significant oil and gas reserves

morphodynamics of the barrier inlet system west central florida the barrier inlet system along the gulf coast of peninsular florida has one of the most diverse morphologies of any barrier system in the world the delicate balance between tidal and wave generated processes on this low energy coast permits only slight changes in either of these processes to result in significant and rapidly developing morphologic responses some of these responses are the result of natural phenomena such as hurricanes opening tidal inlets closure of inlets due to longshore transport of sediment and changes in the availability of sediment tidal prism is the primary factor in controlling inlet morphology and is greatly influenced by anthropogenic activities in the backbarrier area human activity has also modified the coast in many ways over the past several decades beginning with the construction of the first causeways in the 1920s the various modifications by development have resulted in important morphodynamic changes in the barrier inlet system these include hardening the coast on the beach and at inlets dredging and filling in backbarrier environments and construction of fill type causeways connecting the islands to the mainland construction of seawalls and jetties has inhibited normal coastal processes examples include the downdrift erosion at blind pass and big sarasota pass construction of fill type causeways between the barriers and the mainland has created artificial tidal divides that reduce the tidal prism at some inlets thereby resulting in instability or closure such as blind pass and dunedin pass this is further exacerbated by dredge and fill construction that reduces tidal prism by reducing the area of open water in the backbarrier dredging of the intracoastal waterway also results in a negative impact on selected inlets by channeling tidal flux away from some inlets impacts of these changes inhibit the barrier inlet environments from responding to open coast processes

15n isotope biogeochemistry and natural denitrification process in groundwater application to the chalk aquifer of northern france the use of 15n natural isotope tracing in an aquifer contained within chalk rocks in northern france indicates that under certain hydrogeological conditions major denitrification occurs at the boundary where the aquifer becomes confined the nitrate concentrations decrease in the direction of groundwater flow accompanied by an exponential increase in 15n expressed in 15n of the residual nitrate this is characteristic of kinetic isotope effects which accompany the reduction of the nitrate ion during denitrification hydrogeochemical and bacteriological observations confirm this process natural isotope tracing also permits this process to be distinguished from local dilution with nitrate free surface water which would entail a major drop in nitrate values without 15n isotopic enrichment a model is proposed to explain the relatively small observed magnitude of the isotopic fractionation effect

stratigraphic and microfossil evidence for late holocene tsunamis at swantown marsh whidbey island washington abstract four muddy sand sheets occur within a tidal marsh peat at swantown on the west coast of whidbey island washington the two largest sand sheets pinch out about 100 m inland and became thinner and finer grained landward all four sand sheets contain marine microfossils and have internal stratification they record repeated inundation of the marsh over a short time period by distinct pulses of sediment laden ocean water consistent with deposition by a tsunami wave train the layers have been radiocarbon dated to 1160 1350 1400 1700 1810 2060 and 1830 2120 cal yr b p the overlap in age between the two youngest layers and inferred great earthquake events at the cascadia plate boundary some 250 km to the west suggests they were emplaced by tsunamis from this source area the two older layers do not correlate with plate boundary events they may be products of tsunamis caused by earthquakes on local faults in the strait of juan de fuca or by submarine landslides in this area

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

extreme crustal oxygen isotope signatures preserved in coesite in diamond the anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor1 recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons2 early crustal growth is thought to have been enhanced by the addition of slab derived magmas leaving an eclogite residuum in the upper mantle beneath the cratons3 but the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean floor basalt and intermediate stage subduction zone eclogites and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean floor basalts we present here the oxygen isotope composition of eclogitic mineral inclusions analysed in situ in diamonds using an ion microprobe secondary ion mass spectrometer the oxygen isotope values of coesite a polymorph of sio2 inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle but are typical of subduction zone meta basalts and accordingly provide strong support for the link between altered ocean floor basalts and mantle eclogite xenoliths

post miocene stratigraphy and depositional environments of valley fill sequences at the mouth of tampa bay florida abstract post miocene sea level low stands allowed rivers and karst processes to incise the exposed carbonate platform along the gulf coast of florida few miocene to mid pleistocene deposits survived erosion along the present coast except within incised valleys since their formation these valleys have been filled and incised multiple times in response to sea level changes the thick sedimentary sequences underlying the mouth of tampa bay have been recorded as a range of depositional environments and multiple sea level incursions and excursions during pre holocene time and subsequent to the accumulation of the miocene carbonate sequences sediment analysis of cores collected from a north south transect across the mouth of tampa bay has enabled the identification of lithofacies ranging from well sorted quartz sand to dense fossiliferous phosphatic grainstone these facies were deposited in freshwater estuarine and shallow open marine environments as a result of channel development and migration within the paleovalley and cut and fill associated with individual transgressions and regressions correlation of the lithofacies does not extend across the entire transect fining upward sequences truncated by tidal ravinement surfaces that extend throughout the paleovalley can however be identified age determinations based on 14 c analysis amino acid racemization and strontium isotope analysis dating of numerous samples yield ages of miocene pliocene early pleistocene and late pleistocene as well as holocene for sequences that accumulated and were preserved in this valley fill complex numerous inconsistencies in the stratigraphic organization of the age determinations indicate that there are bad dates considerable reworking of shells that were dated or both for this reason as well as the lack of detailed correlation among the three relatively complete cores it is not possible to place these strata in a sequence stratigraphic framework

syngenetic inclusions in diamond from the birim field ghana a deep peridotitic profile with a history of depletion and re enrichment diamonds and their syngenetic mineral inclusions from placer deposits akwatia mine along the birim river ghana were studied thus providing the first detailed data collection for the west african craton inclusion contents indicate an almost exclusively peridotitic diamond suite with the vast majority being part of the harzburgitic paragenesis chemically the akwatian diamond inclusions differ from those in our 1100 sample world wide data base mainly by shifts towards lower mg fe ratios for harzburgitic olivines and orthopyroxenes extremely high ni contents in both harzburgitic and lherzolitic olivines and a higher mean cr content in chromites the inconsistency between the low mg fe ratios and the highly refractory compatible trace element signature seems best to be explained by re fertilisation of a previously depleted source similar to the metasomatic re enrichment of deformed fe ti rich and hot peridotites discussed by harte 1983 geothermometry shows akwatian inclusions to be 140 190 c hotter than the peridotitic average 1050 c given by harris 1992 since garnet opx equilibria 1100 c 50 kbar to 1370 c 67 kbar indicate a typical shield geotherm 40 42 mw m2 these elevated temperatures imply an origin of the akwatian diamonds unusually deep for a peridotitic suite this is consistent with the presence of extraordinary amounts of silicate spinel component in chromite inclusions indicative of crystallisation under higher pressures than recorded for most peridotitic suites in addition one garnet showed the highest knorringite component 66 4 mol so far observed in an inclusion in diamond the same garnet also contained a minor enstatite solid solution component which indicates crystallisation at pressures just below 80 kbar akwatian diamond inclusions therefore represent the most complete cross section through peridotitic subcontinental lithospheric upper mantle so far observed down to a maximum depth between 200 240 km

700 yr sedimentary record of intense hurricane landfalls in southern new england five intense category 3 or greater hurricanes occurring in 1635 1638 1815 1869 and 1938 have made landfall on the new england coast since european settlement historical records indicate that four of these hurricanes 1635 1638 1815 and 1938 and hurricane carol a strong category 2 storm in 1954 produced significant storm surges 3 m in southern rhode island storm surges of this magnitude can overtop barrier islands removing sediments from the beach and nearshore environment and depositing overwash fans across back barrier marshes lakes and lagoons in a regime of rising sea level accumulation of marsh lake or lagoon sediments on top of overwash deposits will preserve a record of overwash deposition we examined the record of overwash deposition at succotash salt marsh in east matunuck rhode island and tested the correlation with historical records of intense storms aerial photographs taken after hurricanes in 1954 and 1938 show overwash fans deposited at the site analysis of 14 sediment cores from the back barrier marsh confirmed the presence of these fans and revealed that 4 additional large scale overwash fans were deposited within the marsh sediments the four overwash fans deposited since the early seventeenth century at succotash marsh matches the historical record of significant hurricane induced storm surge these fans were most likely deposited by hurricanes in 1954 1938 1815 and either 1638 or 1635 radiocarbon dating of two prehistoric overwash fans indicated that these were deposited between a d 1295 1407 and 1404 1446 and probably represent intense hurricane strikes in the past 700 yr at least 7 intense hurricanes struck the southern rhode island coast and produced a storm surge that overtopped the barrier at succotash marsh

the application of c isotope measurements to the identification of the sources of c in diamonds a review abstract in the past decade the isotopic compositions of c in 600 inclusion bearing diamonds have been determined such analyses have revealed the following isotopic characteristics 1 peridotitic diamonds which typically contain garnet chromite olivine and or orthopyroxene inclusions with refractory compositions high mg cr have 13 c values predominantly between 10 and 1 with a sharp peak in the distribution near 5 2 eclogitic diamonds which commonly contain inclusions of omphacitic clinopyroxene cr poor pyrope and or eclogitic accessory minerals such as rutile kyanite coesite or sanidine have 13 c values between 34 and 3 with a smaller peak near 5 3 the isotopic compositions obtained for suites of diamonds from individual occurrences are in general unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities 4 isotopic zoning patterns and heterogeneities are found in some diamonds cores of coated diamonds tend to be depleted in 13 c relative to the rims and within single octahedral diamonds 13 c variations of nearly 6 have been reported because expected c isotope fractionations at mantle temperatures are small attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle c source have been unsuccessful nevertheless fractionation is probably responsible for some of the observed variation in 13 c values two other models have also been proposed to account for the diamond characteristics outlined above the primordial model suggests that the range and distribution of c isotope compositions are inherited from primordial c in the mantle which has an inhomogeneous isotopic composition such as that found in meteorites the subduction model suggests that subducted crustal c is the source of c in diamonds as organic and inorganic c compounds in the crust exhibit a range of 13 c values similar to that observed in diamonds this paper reviews the c isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics

shallow water marl limestone alternations in the late jurassic of western france cycles storm event deposits or both abstract the contribution of event deposits to various basin fills can be very significant higher than 90 in some cases events may lead to the formation of marl limestone alternations which can also result from cyclic changes in sea level or climate for example the marl limestone alternations of the late jurassic of western france contain abundant coarse grained accumulations that resemble storm deposits described in other western european successions the detailed analysis of facies evolution and hierarchical high frequency stacking pattern of depositional sequences of the phare de chassiron section ile d oleron western france allows the controls on marl limestone formation to be defined this section contains nearshore and shallow marine mud deposits that were exposed to high energy events elementary small and medium scale depositional sequences are defined the stacking pattern and the duration of these sequences suggest an orbital control on sedimentation precession 20 ka cycles notably controlled the formation of elementary sequences that correspond to marl limestone alternations the deposition of marly or carbonate mud occurred in this storm dominated system because of muddy sea beds the gentle slope of the shelf and the great amount of particles in suspension which reduced water energy resulting from storms sediment supply was also sufficient to limit bioturbation and favour the preservation of numerous storm deposits the production of carbonate mud was localised on positive structures and partly controlled by milankovitch scale sea level cycles transport by storms of carbonate mud to the adjacent marly depressions during high carbonate production periods led to the formation of calcareous beds marl limestone alternations in the late jurassic of western france therefore result from the combined effects of cyclic changes in carbonate production and high energy episodic events

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

multiple growth events processes and fluid sources involved in diamond genesis a micro analytical study of sulphide bearing diamonds from finsch mine rsa abstract twenty one sulphide inclusion bearing diamonds from the finsch mine south africa were analysed for nitrogen abundances and carbon isotope compositions by microbeam methods on the basis of sulphide ni contents one diamond is of peridotitic affinity the rest belongs to the eclogitic suite ftir analyses show nitrogen abundances and aggregation states from 21 to 1093 at ppm and 0 to 83 iab statistically indistinguishable from previous results for finsch eclogitic silicate inclusion bearing diamonds appleyard et al 2004 but significantly higher than observed before for diamonds of the peridotitic suite deines et al 1989 detailed analyses revealed marked variations in nitrogen characteristics within individual diamonds demonstrating a complex mantle residence consistent with multiple episodes of diamond growth over time linked to the growth stratigraphy of the diamond sims micro analyses show variations in 13 c from 8 90 to 2 80 with a mean value of 5 54 1 80 1 standard deviation closely overlapping the typical worldwide value the c isotopic variability within individual diamonds ranges up to 3 26 sims based nitrogen abundances are 3 2221 at ppm with heterogeneous distribution within individual diamond from the 13 c n co variations within individual diamonds three major processes of diamond growth for sulphide inclusion bearing samples at finsch are proposed 1 some diamonds were precipitated during a single event of open system isotopic fractionation in fluids that varied from oxidised carbonatitic to reduced ch 4 rich in this growth scenario nitrogen is either compatible or incompatible during diamond growth 2 other diamonds show abrupt 13 c n changes indicative of diamond growth involving mixing of several fluid sources 3 some diamonds grow from a combination of the two previous processes the models are consistent with metasomatic diamond growth involving single and multiple fluid sources multiple growth stages for individual diamonds may have taken place over extended time periods and which have important implications for diamond dating studies

the calcareous tufa in the tadrart acacus mt sw fezzan libya an early holocene palaeoclimate archive in the central sahara article i nfo the palaeoclimatic signi cance of calcareous tufa deposits found in the presently hyperarid tadrart acacus massif central sahara sw libya is discussed on the basis of u th datings stable isotope analyses and the regional hydrogeological context these deposits formed mostly between c 9600 and 8100 yr bp the isotopic signature 18 o and 13 c of this tufa indicates that the water forming these carbonates originated as isotopically depleted rainwater as a result of the northward expansion of the sw african monsoon system during the early holocene carbonate sedimentation in the tadrart acacus decreased shortly before c 8000 yr bp attributed to a drop in precipitation in the course of the well known 8 2 kyr event a correlation between tufa development and climate change in n and e africa and in the mediterranean basin during the early holocene is discussed the effect of early holocene climate change on both the landscape and the dwellers living in the tadrart acacus region is also evaluated

linking tsunami deposits submarine slides and offshore earthquakes abstract the recognition that many tsunamis are associated with coastal sedimentation has been of great value in the study of tsunamis prehistorically geological investigation of such sediments has resulted in the identification of a series of palaeotsunamis that appear to have taken place in different areas of the world in most cases however it has proved difficult to link former tsunamis to specific source mechanisms studies of modern tsunamis have also faced difficulties in the recognition of the specific source mechanisms for example offshore earthquakes may trigger submarine slides that combine to produce complex patterns of tsunami flooding at the coast

empirical evidence for the fractionation of carbon isotopes between diamond and iron carbide from the earth s mantle we have studied two samples of mantle diamond containing iron carbide inclusions from jagersfontein kimberlite south africa syngenetic crystal growth is inferred using morphological characteristics these samples provide an opportunity to investigate the isotopic partitioning of 13c in a terrestrial natural high pressure and high temperature hpht system the difference for the 13c values between the diamond and coexisting iron carbide averaged 7 2 1 3 these data are consistent with available data from the literature showing iron carbide to be 13c depleted relative to elemental carbon i e diamond we infer that the minerals formed by crystallization of diamond and iron carbide at hpht in the mantle beneath the kaapvaal craton it is unclear whether crystallization occurred in subcratonic or sublithospheric mantle in addition the source of the iron is also enigmatic nonetheless textural coherence between diamond and iron carbide resulted in isotopic partitioning of 13c between these two phases these data suggest that significant isotopic fractionation of 13c 12c 13c up to 7 can occur at hpht in the terrestrial diamond stability field we note that under reducing conditions at or below the iron iron wustite redox buffer in a cratonic or deep mantle environment in earth the cogenesis of carbide and diamond may produce reservoirs of 13c depleted carbon that have conventionally been interpreted as crustal in origin finally the large 13c for diamond iron carbide shown here demonstrates 13c for silicate metallic melts is a parameter that needs to be constrained to better determine the abundance of carbon within the earth s metallic core

paleoecological record of hurricane disturbance and forest regeneration in nicaragua studying infrequent phenomena e g hurricanes and slow processes e g forest regeneration greatly challenges the ecological techniques of real time studies by combining the two relatively new approaches of paleotempestology and fine resolution palynology this study provides insight into the impacts of hurricanes and the post hurricane regeneration of forests i analyzed a 5 m sediment core from a swamp lagoon on the caribbean coast of nicaragua that covered the entire 8000 yr history of the swamp urquhart g r 1997 disturbance and regeneration of swamp forests in nicaragua evidence from ecology and paleoecology ph d dissertation the university of michigan ann arbor mi usa x rays revealed a sand layer dating to c 3300 bp of the type deposited by hurricanes pollen analyses showed this sand layer was followed by major changes in vegetation and fires this pattern is identical to the wake of hurricane joan which struck the area in 1988 and left 90 000 hectares of damaged swamp forest that burned shortly after after the prehistoric hurricane forest vegetation did not return until 500 yr later due to repeated burning this parallel event of the past illustrates a possible course for modern forest regeneration as a counterpart to direct ecological analysis fine resolution paleoecological study can provide great insight for the study of rare events and slow processes