spatial distribution of zircon with shock microtwins in pseudotachylite bearing granite gneisses vredefort impact structure south africa abstract mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and or shear stress however so far zircon with microtwins has rarely been described in situ the present study of pseudotachylite bearing granitic gneisses from the vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features zircon hosted by granitic gneiss does not reveal extensive shock deformation with decreasing distance to pseudotachylite zircon progressively develops sub planar fractures planar deformation bands low angle boundaries rarely thin 112 microtwin lamellae and closest to the vein it is fragmented in contrast zircon hosted by pseudotachylites contains thick and closely spaced microtwin lamellae which form up to three sets within individual grains microtwin lamellae are associated with planar 112 low angle boundaries and with overprinting crystal plastic and brittle deformation we suggest that planar 112 low angle boundaries in zircon are indicative of shock deformation and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter as opposed to endogenic seismically induced friction melts locally elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon

petroleum origin and accumulation in basin evolution a quantitative model basin data geologic geophysical geochemical hydrodynamic and thermodynamic can be combined for quantified hydrocarbon prediction a three dimensional deterministic dynamic basin model can be constructed to calculate all the measurable values with the help of mass and energy transport equations and equations describing the physical and or physicochemical changes in organic matter as a function of temperature input data consist of heat flux initial physical and thermal properties of sediments paleobathymetric estimates sedimentation rate and amount and type of organic matter subsequently the model computes pressure temperature physical and thermal properties of sediments maturity of organic matter and the hydrocarbon potential of any source rock as a func ion of space and time thus the complex dynamic processes of petroleum formation and occurrence in a given sedimentary basin can be quantified for example hydrocarbon potential maps for any given source rock and any geologic time slice of the basin evolution can be provided as computer printouts the computer model can be applied to any stage of an exploration campaign and updated as more information becomes available

mineralogy and geochemistry of the clay fraction of sediments from the namibian continental margin and the adjacent hinterland the continental shelf off namibia can be divided into two sectors at latitude 18 40 s based on the distinctive morphologies of the two regions and on differences in composition of their surficial sediments the kunene shelf is narrow shallow and covered mainly by terrigenous sediment whereas the walvis shelf is wide deep and supports predominantly biogenic sediment the walvis shelf also possesses a number of distinguishing features its outer region has a concave profile which is attributed to upbuilding of the inner shelf during periods of lowered sea level its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast the seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water and to the influx of aeolian terrigenous detritus especially illite close to shore long belts of terrigenous mud extend southwards from both the kunene and orange rivers the currents responsible for transporting this sediment are poleward directed tongues of the angola benguela front in the north and mixed poleward moving south atlantic central water in the south the kunene river carries more smectite to the sea than the much larger orange the distribution of clay minerals on the continental shelf follows that of the adjacent hinterland with smectite and to a lesser extent kaolinite being significant in the north and illite predominating in the south along the upper slope the concentrations of smectite and kaolinite increase due to their importation from the kunene river by poleward directed antarctic intermediate water the geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins compared with marine sediments these clays are enriched in eight of the fourteen elements investigated namely al fe mg co mn pb rb and zn in contrast the offshore element distribution patterns show little resemblance to the patterns of individual drainage basins clays in the marine sediments are enriched relative to the river sediments in ca k cd ni and sr five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal calcium carbonate phosphorite glauconite and terrigenous components average element concentrations of individual lithofacies are enriched relative to the river clays in cd diatomaceous facies ca cd ni and sr calcareous and facies k and ni slightly glauconitic facies cd and sr terrigenous facies regionally the clay fractions of unconsolidated marine sediments contain high concentrations of al fe mg k mn and rb on the kunene margin due to aluminosilicates being imported by the kunene river ca and sr reach high values on the walvis outer margin in response to upwelling along the coast maximum concentrations of cd are associated with diatomaceous mud on the walvis inner shelf cu ni and zn reach high levels at a few spot locations where organic matter morg has suffered little dilution from other components especially opal and pb is concentrated with glauconitic sediment on the kunene shelf semi consolidated sediments are exposed intermittently on the outer margin they provide evidence that the palaeoclimate became much drier in post miocene times but in the north this picture is complicated by the introduction of sediment from the kunene river during the late tertiary palaeo oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased

calculation of petroleum masses generated and expelled from source rocks abstract an algebraic scheme for calculating the masses of petroleum formed in mature source rocks has been constructed the scheme exploits routine geochemical measurements and a basic understanding of the behaviour of sedimentary organic matter at elevated temperatures under both laboratory and subsurface conditions application of these techniques to data from a wide range of source rock types has led to the following general conclusions 1 1 major oil and gas generation from kerogen occurs between 120 and 150 c in the subsurface gas generation continues to temperatures 160 c 2 2 oil prone source rocks with average initial potential 5 kgt 1 of rock and average organic carbon contents 1 5 expel 60 90 of the total oil generated 3 3 oil expulsion from leaner source rocks is less efficient most of the oil generated remains in the source rock it is subsequently converted to lighter components at higher temperatures and expelled in the vapour state 4 4 the major loss of organic carbon from rich oil prone source rocks during generation and expulsion means that the petroleum potential and organic carbon content of mature source rocks are much less than their initial immature values hence the algebraic scheme must be used to calculate the initial values before expelled petroleum masses can be determined

the expulsinator versus conventional pyrolysis the differences of oil gas generation and expulsion simulation under near natural conditions abstract the expulsinator provides a new technique to simulate generation and expulsion of hydrocarbons under near natural conditions this is realized by application of lithostatic pressure onto an intact source rock disc with undisturbed mineral matrix and kerogen network and by hydrous pyrolysis conducted in open flow through mode conventional pyrolysis methodologies serve well in generation kinetics assessment but are only of limited use for investigation of primary migration and expulsion due to destructive sample preparation inappropriate pressure regimes and pyrolysis in closed mode and or in the absence of water however comparison of results generated with conventional pyrolysis with expulsinator experiments is important to assess the performance of the generation and expulsion simulation carried out with the expulsinator thus the generation and expulsion behaviour of this new methodology has been compared with established pyrolysis methodologies like hypy and closed small vessel pyrolysis csvp and has been assessed by rock eval pyrolysis expulsinator experiments released higher amounts of bitumen than csvp this has been attributed to increased cross linking reactions of bitumen in case of the classic pyrolysis favouring formation of pyrobitumen differences in yields and composition of n alkanes were controlled not by generation kinetics exclusively but also by expulsion effects in particular delayed expulsion caused by geochromatography expulsinator conversion rates of toc exceeded 81 whereas those of hydrous csvp remained at 65 higher expulsinator toc conversion was accompanied by lower gas generation compared to csvp this is attributed to the open setup of the expulsion experiment which by rapid removal of products generated prevents their secondary alteration particularly polymerization to pyrobitumen and oil to gas cracking at higher temperatures hypy of expulsinator pyrolysis residues verified a low generation potential remaining after the expulsion experiment the expulsinator device delivers oil and gas generation data which can be applied in numerical modelling exercises

misorientation analysis and the formation and orientation of subgrain and grain boundaries in contrast to the behaviour of individual grains both inter and intra granular boundaries within rocks have received much less attention however many geological processes particularly during deformation e g yielding dislocation creep recrystallisation superplasticity and various fracture mechanisms and petrophysical properties depend to some extent on the nature of boundaries present in a rock in this contribution we consider the role of intergranular and intragranular crystal boundaries a precise characterisation of such boundaries depends on defining the crystallographic and dimensional orientations of the boundary and the misorientation between the adjacent regions i e grains subgrains etc separated by the boundary although several theoretical descriptions of boundary configuration are available practical precision is lacking and approximations are necessary we describe two specific approximations for boundary formation and orientation obtained using the sem electron channelling technique the first is a geometrical interpretation of electron channelling patterns ecp in terms of the likely formation and orientation of the intervening boundary the second considers the misorientation between adjacent regions across a boundary this involves a model which assumes a simple geometrical relationship between crystal slip systems responsible for the rotation and misorientation between adjacent regions and the formation and orientation of the resulting boundary these approximations are capable of a identifying trends in the dispersion of crystallographic directions during deformation b identifying active slip systems c calculating the relative schmid factors for each crystal slip system and therefore the most likely system to be activated d modelling synthetic misorientations and predicting the crystal slip systems and boundary configurations to be expected and e comparing real data with synthetic models our analyses are illustrated via natural examples of dynamic recrystallisation in quartzite and a theoretical simulation of the behaviour of an individual quartz grain during deformation

electron backscatter diffraction analysis of zircon a systematic assessment of match unit characteristics and pattern indexing optimization quantitative microstructural analysis of zircon using electron backscatter diffraction ebsd requires a comparison of empirically collected electron backscatter patterns with theoretical patterns or match units derived from known crystallographic parameters there are 23 possible crystallographic data sets for zircon and associated match units derived from natural and synthetic zircon and from theoretical calculations over a range of pressures and different rare earth element ree compositions a systematic assessment of these match units has been undertaken by ebsd analysis of each of four zircons from a range of geological environments combined with principal components analysis and self organizing map networks comparison of the different match units shows a systematic relationship across all samples that are related to changes in unit cell dimensions associated with pressure and compositional variations systematic variations in the data generated from 96 ebsd maps each comprising 10 000 electron backscatter patterns indicate that match units associated with increasing pressure or ree dopants yield poorer quality ebsd data the match units from low pressure undoped natural zircon consistently yield the best ebsd results and are recommended for natural zircon ebsd studies irrespective of the zircon source or u content the results provide a clear strategy for optimizing the acquisition and analysis of ebsd data from zircon from both crustal and mantle sources in addition the developed approach to match unit analysis may be applied to all other crystalline materials potentially optimizing ebsd analyses from a range of materials

continental shelf upwelling and benthic ostracoda in the benguela system southeastern atlantic ocean abstract the distribution of benthic ostracoda micro crustacea on the continental shelf off southwestern africa is controlled by sea floor physical and chemical parameters which in turn can be correlated with the positions of quasi permanent upwelling cells of the benguela system the linkage between benthic and surface physico chemical environments and consequently between benthos and surface parameters is not direct however being modulated by equatorward motion in the upper water column benthic poleward water motion fluvial input and cross shelf advection of off shelf water these phenomena and in particular the intensity of surface upwelling show a sharp distinction about 27 s so that the ostracod faunas in the northern benguela region are relatively sparse and dominated by palmoconcha walvisbaiensis hartmann and cytherella namibensis dingle while in the southern benguela region the faunas are more diverse and dominated by pseudokeijella lepralioides brady and ruggieria cytheropteroides brady when minor taxa are also considered each cell in the benguela upwelling system is shown to be associated with a unique ostracod fauna which can be related to its in particular environmental characteristics

plastic deformation of metamorphic pyrite new evidence from electron backscatter diffraction and forescatter orientation contrast imaging application of new scanning electron microscope techniques to the study of deformed metamorphic pyrite reveals evidence for plastic deformation not readily recognised by more traditional methods specifically use of forescatter solid state detectors in conjunction with tilted polished specimens of pyritic ore produces high quality crystallographic orientation contrast images which map the distribution of deformation domains within grains use of electron backscatter diffraction allows quantification of the crystallographic misorientations shown by the orientation contrast images combination of these techniques shows that the pyrite studied deforms by slip on 100 and more rarely 110 systems slip is often associated with distributed rotation of up to 20 about and more rarely axes pyrites may have simple histories involving rotation about a single axis or more complex histories involving rotation about different axes and more rarely in different domains of the same pyrite grain or sequential rotations about quite different systems typically distributed rotation about followed by discrete rotation about a non crystallographic axis

pressure solution in carbonate source rocks and its control on petroleum generation and migration abstract based on detailed geochemical and petrographic studies of conventional cores of a triassic age carbonate source rock formation from three well sites at increasing maturity stages it is shown that the effects of carbonate diagenesis especially pressure solution during relatively shallow burial stages exercise major controls on the processes of petroleum generation and migration during deeper burial pressure solution leads to an internal redistribution of organic matter within this source rock formation in the form of local concentrations as solution seams and stylolites they provide very favourable conditions for effective petroleum generation and migration due to their internal structure and composition three dimensional arrangement of kerogen surrounded by carbonate cemented rims as tight seals the geochemical and petrographic effects of petroleum generation and expulsion are documented e g the expulsion of petroleum occurred as a separate phase fluid at high efficiencies 60 80 for the normal and isoprenoid alkanes and was facilitated by fractures presumably resulting from pore fluid overpressuring a five stage conceptual model is proposed to explain how the effects of pressure solution achieved during relatively shallow burial stages exercised controls on the extent and efficiency of petroleum generation and expulsion during deeper burial stages

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

stability of zircon in a low grade ultramylonite and its utility for chemical mass balancing the shear zone at mieville switzerland at mieville switzerland granitic basement rocks of the aiguilles rouges massif are cut by a subvertical shear zone the variscan shearing event produced a low grade ultramylonite zone which is up to 50 m wide and several kilometers long investigations of accessory zircons from the undeformed wall rock and from the most highly deformed ultramylonite show only minor alterations and mechanical damage of zircon crystals even in an extreme state of deformation this outstanding stability of zircon allows one to consider the element zr as immobile and therefore to use it as a passive marker for calculations of mass and volume changes during deformation processes application of this method confirms that the deformation at mieville proceeded under simple shear constant volume and isochemical conditions this is in agreement with the results of classical geochemical mass balancing in addition uncertainties inherent in the geochemical approach can be minimized by the new method

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea ii molecular composition of alkylated naphthalenes phenanthrenes benzo and dibenzothiophenes abstract variations with depth in the quantitative distributions of di and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the upper jurassic age kimmeridge clay formation brae field area north sea reveal in part the effects of depletion with hydrocarbon expulsion trends of increasing depletion with approach to the nearest shale sandstone contacts as documented by a decrease in carbon normalized yields are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes but less drastic than previously observed for long chain alkanes cf part i leythaeuser et al 1988 lowest yields consistently were encountered at the outermost 1 2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for c15 to c30n alkanes 80 95 than for c2 and c3 alkylnaphthalenes 30 40 in contrast c2 and c3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post expulsion selective replenishment from the adjacent oil accumulation expulsion was not associated with any detectable fractionation among c1 and c3 alkylnaphthalenes or c1 and c2 alkylphenanthrenes as evidenced by a mass balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early mature through to mature stages of thermal evolution it was verified that the methylphenanthrene index mpi 1 and methyldibenzothiophene ratio mdr of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum

rapid palaeoceanographic changes in the benguela upwelling system for the last 160 000 years as indicated by abundances of planktonic foraminifera abstract two sediment cores retrieved from the continental slope in the benguela upwelling system geob 1706 19 33 7 s 11 10 5 e and geob 1711 23 18 9 s 12 22 6 e reveal striking variations in planktonic foraminiferal abundances during the last 160 000 years these fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off namibia four species make up over 95 of the variation within the core and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages the fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment the cold water planktonic foraminifer neogloboquadrina pachyderma sinistral n pachyderma s dominates the modern day coastal upwelling centres and neogloboquadrina pachyderma dextral and globigerina bulloides characterize the fringes of the upwelling cells globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional reduced nutrient levels of the subtropical waters in the fossil record episodes of high n pachyderma s abundances are interpreted as evidence of increased upwelling intensity and the associated increase in nutrients the n pachyderma s record suggests temporal shifts in the intensity of upwelling and corresponding trophic domains that do not follow the typical glacial interglacial pattern periods of high n pachyderma s abundance describe rapid discrete events dominating isotope stages 3 and 2 the timing of these events correlates to the temporal shifts of the angola benguela front jansen et al 1997 situated to the north of the walvis ridge absence of high abundances of n pachyderma s from the continental slope of the southern cape basin indicates that southern ocean surface water advection has not exerted a major influence on the benguela current system the coincidence of increased upwelling intensity with the movement of the angola benguela front can be interpreted mainly by changes in strength and zonality of the trade wind system

pleistocene deep sea ostracods from the oki ridge sea of japan iodp site u1426 and condition of the intermediate water the sea of japan also termed the east sea has a circulation system isolated from the pacific ocean and east china sea the east asian winter monsoon drives the circulation system and cools the tsushima warm current twc to form the japan sea intermediate proper water jsipw the intermediate water conveys oxygen to deep sea floors which is available for benthic animals during the pliocene 3 5 2 8 ma temperate intermediate water tiw was formed under the weak winter monsoon and extinct ostracod tiw taxa were found little is known about early pleistocene intermediate water and the extinction mode of benthic ostracods we studied radiolarians and ostracods from deep sea sediments between 2 0 and 1 3 ma marine oxygen isotope stage mis 77 to mis 41 at integrated ocean drilling program site u1426 sea of japan the ostracod faunas contained tiw and jsipw taxa the radiolarian subtropical water taxa and the jsipw ostracods indicate a small influx of the twc and the jsipw the tiw occasionally expanded to the middle bathyal zone by analogy with the relationship between the modern jsipw and winter monsoon weak winter monsoon possibly caused gentle temperature gradients in the water column and the expansion of the tiw the jsipw taxa expanded their ranges into the deep sea during interglacial periods

benthic ostracoda in the benguela system se atlantic a multivariate analysis abstract q mode factor analysis of ostracod faunas from 127 modern sediment samples associated with the benguela system over the continental margin off namibia and south africa identifies ten factor associations of species fa these fa account for 93 of the variance and can be related to nine sea bottom variables temperature salinity dissolved oxygen total orgainc matter elemental iron terrigenous component calcium carbonate glauconite sand and mud transfer functions for these variables are calculated for future palaeo environmental analyses on the modern continental slope fahenryhowella melobesioides andkrithe capensis are controlled primarily by cross shelf temperature gradients while on the uppermost slope and outer shelf dissolved oxygen values restrict the distribution of facytherella namibensis oxygen depleted andruggieria cytheropteroides normal oxygen levels in the north and south respectively inner and middle shelf faunal associations are controlled by high salinities north of 26 s fapalmoconcha walvisbaiensis while substrate variables terrigenous glauconite mud and organic matter are the main determinants farther south fabensonia knysnaensis knysnaensis pseudokeijella lepralioides bairdoppilata simplex ambostracon keeleri flabellicostata andparacypris lacrimata these faunal hydrographic relationships are linked to various dynamic oceanographic features such as upwelling cross shelf advection and shelf parallel bottom currents

quantification of the effect of carbonate redistribution by pressure solution in organic rich carbonates abstract a mathematical model is presented for the quantification of the effects of carbonate dissolution and volume reduction on the basis of mass balance type comparisons between sediment volumes before and after effective carbonate redistribution due to pressure solution application of the model to a carbonate source rock formation from italy allowed the quantification of the degree of organic matter enrichment in solution seams or stylolites and the extent of organic matter dilution in adjacent matrix layers the diagenetic unmixing of carbonate versus non carbonate materials has resulted in a heterogenous distribution of organic matter in this source rock formation this effect needs to be considered in hydrocarbon exploration for assessment of source potential

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system this study documents the chemical and textural responses of zircon in the elba igneous complex with particular reference to the 7 to 7 8 ma old monte capanne pluton in relation to its coeval volcanic counterpart capraia using bse imaging and quantitative electron microprobe analyses the monte capanne pluton displays multiple field and geochemical evidence for magma mixing the samples we have investigated including monzogranitic mafic enclave and dyke samples display similar zircon textures and are associated with an extremely large range of trace and minor element hf y hree th u compositions which contrast with relatively simple textures and zoning patterns in zircons from a capraia dacite we have used a relatively simple textural classification patchy zoning homogenous cores oscillatory zoning and unzoned zircon as the basis for discussing the chemical composition and chemical variation within zircons from the monte capanne pluton based on these data and other works dini et al 2004 in lithos 78 101 118 2004 it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the monte capanne magma chamber in particular mixing was responsible for the development of the patchy zoning texture in the zircon cores which was associated with reactions between other accessory phases including monazite apatite allanite which we infer to have significantly affected the th distribution in zircon zircons from the mc pluton displaying homogeneous cores have chemical affinities with zircons in the coeval capraia volcanic system consistent with the participation of a capraia like mantle end member during mixing further zircon growth in the mc pluton produced the oscillatory zoning texture which records both long term crystal fractionation and transient recharge with both silicic and mafic magmas events in a hybrid magma chamber it is inferred that hf and the th u ratio cannot be used alone to infer magmatic processes due to their dependency on temperature nor are they a diagnostic feature of xenocrystic grains this study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system

some garnet microstructures an illustration of the potential of orientation maps and misorientation analysis in microstructural studies the microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction in both cases there is a very strong crystallographic preferred orientation with measurements clustered round a single dominant orientation each garnet grain is divided into domains with similar orientations limited by boundaries with misorientations of 2 or more in both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains one sample is a deformed garnet that shows considerable distortion within the domains lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes the domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery the second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes these boundaries probably formed by coalescence of originally separate garnets we suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy the data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material

metasomatism and the crystallization of zircon megacrysts in archaean peridotites from the lewisian complex nw scotland zircon megacrysts are locally abundant in 1 40 cm thick orthopyroxenite veins within peridotite host rocks in the archaean lewisian gneiss complex from nw scotland the veins formed by metasomatic interaction between the ultramafic host and si rich melts are derived from partial melting of the adjacent granulite facies orthogneisses the interaction produced abundant orthopyroxene and within the thicker veins phlogopite pargasite and feldspathic bearing assemblages two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins patterns of zoning rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host both zircon types have u pb ages of ca 2464 ma broadly contemporaneous with granulite facies events in the adjacent gneisses zircon megacrysts locally form 10 of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts their unusual size is linked to the suppression of zircon nucleation and increased zr solubility in the si undersaturated melts the metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs as such the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas

rare earth and trace element mobility in mid crustal shear zones insights from the mont blanc massif western alps the behaviour of rare earth elements ree during fluid rock interaction in mid crustal shear zones has received little attention despite their potential for mass balance calculation and isotopic tracing during deformation in this study several cases of large ree mobility during alpine fluid driven shear zone development in the pre alpine granitic basement of the mont blanc massif are considered on a regional scale the undeformed granite compositions range within 5 wt sio2 70 5 75 3 wt and magmatic chemical variations are of the order of 10 20 ascribed to minor effects of crystal fractionation major and trace element mobility observed in shear zones largely exceeds these initial variations shear zones developed a range of mineral assemblages as a result of shearing at mid crustal depths at not vert similar0 5 gpa 400 c five main shear zone assemblages involve muscovite chlorite epidote actinolite and calcite respectively as major phases in most cases selective enrichments of light or heavy ree and y ta hf are observed ree mobility is unrelated to deformation style cataclastic mylonitic the intensity of strain and to the shear zone s major metamorphic mineral assemblages instead the changes in ree concentrations are ascribed to the alteration of pre existing magmatic ree bearing minerals during deformation related fluid rock interaction and to the syntectonic precipitation of metamorphic ree bearing minerals mainly monazite bastnasite aeschynite and tombarthite minor proportions 2 of these accessory phases with grain sizes mostly 20 m account for enrichments of up to 5 1 compared to the initial granite whole rock ree budget the stability of the ree phases appears to be largely dependent on the altering fluid composition ree mobility is ascribed to changes in ph and to the availability of co32 po42 and so42 ligands in the fluid such processes are likely to influence the mobility of ree y hf and ta in shear zones

stress state interpretation in light of pressure solution creep numerical modelling of limestone in the eastern paris basin france abstract this paper presents an interpretation of the present stress state in the eastern paris sedimentary basin france based on considerations that regard the long term limestone rheology a comprehensive stress profile was acquired in this region and it was determined that the minimum horizontal stress in an argillite layer is much larger than that in the under and overlying stiffer limestone layers while the major horizontal stress seems almost insensitive to lithology in a previous study the genesis of this intriguing stress state was modelled by considering the limestone to be elastic however the calculation implied the use of unrealistic mechanical characteristics for this rock extremely low long term elastic modulus and poisson s ratio from the study it was concluded that some dissipative viscous process of unknown origin might be at work in the limestone herein we propose that the deformation micro mechanism that is responsible for this time dependent behaviour is pressure solution creep psc we show that this hypothesis is mechanically realistic by adapting an existing model of psc which is based on the description of mass transfer within grain to grain contacts dissolution diffusion and precipitation we deduce a set of long term rheological characteristics for the limestone and use them in the numerical model of the stress genesis in the paris basin the results show that psc is a possible explanation for the present stresses and total strains in this region the fact that limestone should be considered as visco elastic over geological time scales with a viscosity ranging between 10 17 and 10 21 pa s and not as purely elastic is at the core of this work although it is probably ubiquitous in nature psc is notoriously difficult to assess in the laboratory thus our effort of coupling small scale processes in a quantitative manner to large scale deformation and in situ measurements provides a useful tool for gaining insight into the role of psc in the long term behaviour of carbonated rocks and its effect on sedimentary basin evolution

inclusion localised crystal plasticity dynamic porosity and fast diffusion pathway generation in zircon abstract a population of oscillatory zoned igneous zircon grains in a javanese andesite contains fluid and mineral inclusions up to 10 m across trapped during zircon growth orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal plastic deformation overprints growth zoning and has localized around 1 10 m pores and inclusions cumulative crystallographic misorientation of up to 25 around pores and inclusions in zircon is predominantly accommodated by low angle 010 slip a distinctly different population of sub micron pores is present along subgrain boundaries and their triple junctions these are interpreted to have formed as a geometric consequence of dislocation interaction during crystal plasticity dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of cl active rare earth elements the extent of the modification suggests deformation related fast pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion this enhanced diffusion is interpreted to represent a combination of fast diffusion pathways associated with creep cavitation dislocations and along low angle boundaries these new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange these pathways may provide links to the zircon grain boundary thus negating the widely held assumption that inclusions in fracture free zircon are geochemically armoured once they are physically enclosed

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

hydrocarbon generation in southern sicily a three dimensional computer aided basin modeling study abstract a three dimensional computer aided basin modeling study was performed in order to determine timing and spatial distribution of hydrocarbon hc generation in southern sicily the entire information concerning the geologic evoulution such as age thickness lithology and porosity of individual sedimentary layers estimates on paleobathymetry and sediment water interface temperatures initial physical and thermal properties of various lithologies heat flow and particularly type and amount of organic matter was used to construct a conceptual model and to determine the necessary input parameters the simulation of basin evolution resulted in a very detailed and comprehensive definition of the thermal history of the study area subsequently the thermal history of the noto source rock throughout the entire study area was used to define the hc generation potential the latter was not elaborated with the help of maturity data but utilizing a kinetic model the simulation results showed that the main phase of oil generation was during the last 5 ma when a foredeep basin was created in front of the overthrust sheets here very thick pliocene pleistocene sediments were deposited the rapid burial caused a quick temperature increase i e high heating rates which initiated the onset of oil generation at relatively low maturity levels furthermore it also caused an early expulsion of immature heavy oil from the noto source rock

organic matter maturation under the influence of a deep intrusive heat source a natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock toarcian shale northern germany four shallow boreholes were drilled in the hils syncline northern germany in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen type ii bearing source rock the holes penetrated the carbonceous lias shales posidonia shale lower toarcian and part of the adjacent dogger and lias mudstones the maturity of the organic matter in the cores recovered ranges from immature 0 48 r0 to overmature 1 45 r0 due to location of the hils syncline in the vicinity of the vlotho massif which is deep seated intrusive body facies variations of the lias within the short geographical distances in the study area are negligible organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the lias shales kerogen bitumen etc and on the evidence of a uniform initial composition of these sediments in the study area dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations about 50 of the initial kerogen was transformed into oil gas and inorganic compounds during the vitrinite reflectance increase from 0 48 to 0 88 ro and only marginally more during the maturity increase from 0 88 to 1 45 ro only a small portion of the generated material remained in the source rock even at a relatively early stage of generation 0 68 ro expulsion efficiency of oil plus gas reached a value of 86 at the end of the main generation stage 0 88 ro

framework of hydrocarbon generation and destruction in eastern smackover trend laminated lime mudstones of the lower member of the jurassic smackover formation are significant source rocks for crude oil across mississippi alabama and florida the source facies was deposited in an anoxic and perhaps hypersaline environment that preserved algal derived kerogen the distribution of kerogen along laminations of depositional origin and along stylolites of diagenetic origin resulted in efficient expulsion of crude oil with increasing thermal maturity crude oil initially emplaced in reservoirs was cracked to yield gas condensate and then methane rich in nonhydrocarbon gases such as hydrogen sulfide carbon dioxide and nitrogen early destruction of methane was driven by thermochemical sulfate reduction the thermal maturity framework of the smackover rend explains the distribution of hydrocarbon discoveries and suggests areas previously overlooked by exploration

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea i gross composition of c15 soluble organic matter and molecular composition of c15 saturated hydrocarbons in mature oil prone source rocks of the upper jurassic kimmeridge clay in the brae field area north sea the yield and the gross composition of solvent extractabte pctroleum likc compounds as well as the molecular proportions of c15 saturated hydrocarbons are primarily controlled by the effects of hydrocarbon expulsion during primary migration within sample series extending from shale centres towards adjacent reservoir sandstones regular and pronounced yield decreases have been documented as well as marked compositional changes for c15 soluble organic matter and its compound class fractions saturated hydrocarbons aromatic hydrocarbons nso compounds and for normal and isoprenoid alkanes relative migration losses indicating relative expulsion efficiencies were determined for the c15 alkanes throughout the outer five meters of thick shales relative expulsion efficiencies increase regularly and drastically for all compounds considered e g exceeding 90 for the c15 n alkanes in the outermost shale samples samples from thin 5 50 cm shale layers interbedded between reservoir sandstones reveal similarly high expulsion efficiencies for n alkanes the degree of depletion remains uniform throughout the molecular range c15 to c30i e no compositional fractionation occurred during primary migration which is interpreted to reflect expulsion of oil as a single phase fluid a simple conceptual model is proposed to explain the mechanisms and relationships of generation and expulsion processes in time and space a geochemically relevant conclusion is that the c15 soluble organic matter recovered by solvent extraction of mature source rocks represents only a portion of the total non volatile petroleum like material originally generated

evolution of zircon deformation mechanisms in a shear zone lanzo massif western alps abstract magmatic zircons within two sheared gabbroic dykes from the peridotitic massif of lanzo western alps italy revealed evolution of deformation from crystal plasticity to rigid body rotation during shear zone evolution this is the first time that multiple zircon grains have been analysed in a kinematic context in a shear zone zircon grains recorded crystal plastic deformation activating the commonly inferred 001 and 100 glide systems to the newly identified 110 glide system the exact selection of glide system could be dependant of deformation conditions such as pressure temperature and strain rate moreover the activation of one or several glide systems within a single grain could be favoured by the primary orientation of the grains combined with a high strain rate in these sheared gabbros the deformation mechanisms evolve from plastic deformation at low strain rate conditions to increase strain strain softening and localisation of deformation the progressive shear zone development and the softening of the matrix relative to the zircon has lead to a switch from crystal plasticity to rigid body rotation of zircon the zircon grains rigid body rotation involved that their long axes became parallel to the lineation of the shear zone causing reorientation and dispersion of the misorientation axes away from kinematic y

three dimensional morphology and connectivity of stylolites hyperactivated during veining abstract the limestone of the palaeozoic jack formation in north queensland australia contains bedding parallel stylolites that locally increase in amplitude adjacent to calcite veins the veins formed in response to inhomogeneous strain locally superposed on the tightly folded strata stylolites were locally hyperactivated during vein formation and provided calcite for vein filling three dimensional reconstructions of the morphology of the stylolites based on detailed serial profiles demonstrate that the side faces of columnar stylolites form an interconnected fluid pathway through the rock the connective morphology defines a drainage network comprising highly tortuous major trunks of high amplitude side faces linked locally by minor pathways of variable amplitude the permeability pathway anastomoses in three dimensions making it impossible to image in two dimensions alone

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

interpreting u pb data from primary and secondary features in lunar zircon in this paper we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon u pb isotopic systems and the significance of these features for understanding lunar processes lunar zircons can be classified according to i textural relationships between zircon and surrounding minerals in the host breccias ii the internal microstructures of the zircon grains as identified by optical microscopy cathodoluminescence cl imaging and electron backscattered diffraction ebsd mapping and iii results of in situ ion microprobe analyses of the th u pb isotopic systems primary zircon can occur as part of a cogenetic mineral assemblage lithic clast or as an individual mineral clast and is unzoned or has sector and or oscillatory zoning the age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data a secondary set of microstructures superimposed on primary zircon include localised recrystallised domains localised amorphous domains crystal plastic deformation planar deformation features and fractures and are associated with impact processes the first two secondary microstructures often yield internally consistent and close to concordant u pb ages that we interpret as dating impact events others secondary microstructures such as planar deformation features crystal plastic deformation and micro fractures can provide channels for pb diffusion and result in partial resetting of the u pb isotopic systems

extrapolation of the kinetics of oil and gas formation from laboratory experiments to sedimentary basins it is generally accepted that the formation of oil and gas from sedimentary organic matter or kerogen is adequately described by kinetic models based on first order kinetics with rate constants obeying an arrhenius equation these models which are extensively used in oil exploration to assess the petroleum potential of sedimentary basins are based on geological observations1 2 3 4 5 6 but are formally quite similar to models designed to simulate oil shale pyrolysis7 8 9 or coal processing10 11 12 the idea of blending these two approaches is not new see for example ref 2 but until now the most sophisticated attempts reproduced only the gross features of kerogen cracking on a geological timescale5 or were uncontrolled by geochemical measurements13 14 here we show that the kinetic parameters determined from the laboratory pyrolysis of an immature kerogen sample apply to the conditions of sedimentary basins altough the time and temperature scale are completely different

experimental simulation of chemomechanical processes during deep burial diagenesis of carbonate rocks chemomechanical processes involved in the deep burial diagenesis of carbonate petroleum reservoirs are still poorly understood to better understand these processes and explain how porosity and permeability can be preserved at the great depth of dbrs deeply buried reservoirs we developed an experimental device allowing both the simulation of high pressure stresses temperature conditions 80 c 60 mpa of confining pressure and differential stress up to 40 mpa of dbr and the circulation of different fluids in rock samples we tested triaxial multistep creep tests four core samples of a cemented limestone and analyzed creep deformations fluids chemistry and petrographical and petrophysical properties of samples different flow conditions no flow and flow through and chemical compositions natural meteoric water with and without phosphate ions were considered our study showed that the precipitation of calcite on free pore walls of micrites blocks the microporosity between micrite crystals thus rendering the microporosity inaccessible to fluids hence the connected porosity decreased strongly after experimentation this is due to the psc pressure solution creep which is the main process implied in the porosity reduction of a carbonate rock during deep burial the preservation of macropores during psc allows the preservation of permeability in addition calcite solubility is positively dependent on mechanical parameters axial compaction and axial stress thus suggesting that calcite can precipitate during decompression of deep basinal fluids resulting in changes in porosity a comparison of experimental results with theoretical calculations showed that the integration of the psc process into calculation databases would greatly improve the modeling of dbr

primary migration by oil generation microfracturing in low permeability source rocks application to the austin chalk texas fracturing of low permeability source rocks is induced by pore pressure changes caused by the conversion of organic matter to less dense fluids oil and gas these fractures increase the permeability and provide pathways for hydrocarbon migration an equation for the pressure change is derived using four major assumptions 1 the permeability of the source rock is negligibly small 0 01 d 10 20 m2 so that the pore pressure buildup by the conversion is much faster than its dissipation by pore fluid flow 2 the stress state is isotropic so that horizontal and vertical stresses are equal the source rock fails when the pore pressure equals the overburden pressure 3 the properties of the rock organic matter and fluids remain constant during oil generation this assumption is valid when the change in depth i e pressure and temperature is small 4 only two reaction rates are required for the conversions a low temperature reaction rate for the kerogen oil conversion e approx 24 kcal mol a approx 1014 m y and a high temperature reaction rate for oil gas conversion e approx 52 kcal mol a approx 5 5 1026 m y the equations for generation rate and pressure change are applied to the austin source rock by adjusting the several variables to fit geochemical data core saturations and observed levels of oil and gas production this application demonstrates that the equations are easily applied in calculating depths of primary migration for low permeability source rocks

comparison of quaternary coastal upwelling proxies off central peru abstract three apc advanced hydraulic piston core holes of odp leg 112 from the shelf sites 681 and 680 and upper slope site 686 off the central peruvian coast contain a productivity and coastal upwelling record of the last 400 000 yrs marine planktonic diatom sediment assemblages are used as primary productivity proxies their downcore record is compared to variations in other proxies weight percent organic carbon percent large diatoms in the coarse fraction 13c of the endobenthic foraminiferbolivina seminuda and the geochemical parameter uk37 unsaturation index of c37 alkenonones low values of calculated primary paleo productivity are matched by low values of organic carbon but the percentages of diatoms in the coarse fraction do not follow this trend levels of 13c correspond to neither the diatom productivity proxy nor the organic carbon proxy the geochemical index uk37 seems to indicate high surface water temperatures whereas the calculated primary productivity indicates increased upwelling and thus lower temperatures

geological and geochemical characterization of the lower cretaceous pearsall formation maverick basin south texas a future shale gas resource as part of an assessment of undiscovered hydrocarbon resources in the northern gulf of mexico onshore mesozoic section the u s geological survey usgs evaluated the lower cretaceous pearsall formation of the maverick basin south texas as a potential shale gas resource wireline logs were used to determine the stratigraphic distribution of the pearsall formation and to select available core and cuttings samples for analytical investigation samples used for this study spanned updip to downdip environments in the maverick basin including several from the current shale gas producing area of the pearsall formation the term shale does not adequately describe any of the pearsall samples evaluated for this study which included argillaceous lime wackestones from more proximal marine depositional environments in maverick county and argillaceous lime mudstones from the distal lower cretaceous shelf edge in western bee county most facies in the pearsall formation were deposited in oxygenated environments as evidenced by the presence of biota preserved as shell fragments and the near absence of sediment laminae which is probably caused by bioturbation organic material is poorly preserved and primarily consists of type iii kerogen terrestrial and type iv kerogen inert solid bitumen with a minor contribution from type ii kerogen marine based on petrographic analysis and pyrolysis carbonate dominates the mineralogy followed by clays and quartz the low abundance and broad size distribution of pyrite are consistent with the presence of oxic conditions during sediment deposition the pearsall formation is in the dry gas window of hydrocarbon generation mean random vitrinite reflectance values ro 1 2 2 2 and contains moderate levels of total organic carbon average 0 86 wt which primarily resides in the inert solid bitumen solid bitumen is interpreted to result from in situ thermal cracking of liquid hydrocarbon generated from original type ii kerogen that was prevented from expulsion and migration by low permeability the temperature of maximum pyrolysis output tmax is a poor predictor of thermal maturity because the pyrolysis s2 peaks from rock eval analysis are ill defined vitrinite reflectance values are consistent with the dry gas window and are the preferred thermal maturity parameter a maverick basin pearsall shale gas assessment unit was defined using political and geologic boundaries to denote its spatial extent and was evaluated following established usgs hydrocarbon assessment methodology the assessment estimated a mean undiscovered technically recoverable natural gas resource of 8 8 tcf of gas and 3 4 and 17 8 tcf of gas at the f95 and f5 fractile confidence levels respectively significant engineering challenges will likely need to be met in determining the correct stimulation and completion combination for the successful future development of undiscovered natural gas resources in the pearsall formation

geochemical effects of petroleum migration and expulsion from toarcian source rocks in the hils syncline area nw germany for mature oil prone source rocks of toarcian age in the hils syncline area nw germany the geochemical effects of primary migration of petroleum are shown expulsion has occurred at high efficiencies primarily via a network of macro and micro fractures the effects of this fracture controlled expulsion with respect to gross and molecular composition of c15 soluble organic matter are documented samples from fractured intervals have expelled total c15 soluble organic matter more efficiently than adjacent unfractured intervals a systematic comparison of samples taken at cm intervals away from macro fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures extracts of shale samples directly at fracture walls are enriched in nso compounds and highly polar asphaltene like fractions the composition of residual petroleum fluids extracted from the fracture fill materials is variable in some cases there is a close match with that of the shale at some distance away from the fractures in other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes rapid heating by a nearby intrusive body at shallow depths when the toarcian sediments still contained about 15 20 pore water has played an important role overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature toarcian sections finally the closely interdependent relationships between petroleum generation and petroleum migration processes are documented especially the evolution of these relationships with maturity progress from immature 0 48 rm to mature 0 68 and 0 88 rm and overmature 1 45 rm stages is shown for example in the maturity interval from 0 68 to 0 88 rm most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum for some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities

geochemical studies of crude oil generation migration and destruction in the mississippi salt basin abstract the main source for crude oil in the mississippi salt basin is the laminated lime mudstone facies of the lower smackover crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0 55 vitrinite reflectance short range lateral migration of crude oil was focused to upper smackover and norphlet reservoirs but vertical migration also charged some overlying cotton valley rodessa lower tuscaloosa and eutaw reservoirs following migration from the lower smackover thermal maturity history of reservoir rocks controls the preservation of crude oil gas condensate and methane slow thermal cracking of crude oil occurred in deep upper smackover reservoirs resulting in formation of gas condensate and precipitation of solid bitumen the maximum thermal maturity for preservation of condensate is equivalent to about 1 3 vitrinite reflectance only methane pyrobitumen and nonhydrocarbon gases including hydrogen sulfide persist at higher levels of thermal maturity early destruction of methane in deep upper smackover reservoirs near the wiggins arch is driven by thermochemical sulfate reduction lesser availability of sulfate in norphlet reservoirs in lower mobile bay could account for methane preservation at higher levels of thermal maturity one basic geochemical strategy for further exploration of the mississippi salt basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation another strategy is to avoid drilling traps with overmature reservoir rocks

migration of hydrocarbons in carbonate source rocks of the stasfurt member ca2 of the permian zechstein borehole aue 1 germany the role of solution seams abstract the effects of petroleum migration from carbonate source rocks through preferential expulsion of hydrocarbons from stylolites is investigated this type of expulsion is associated with compositional fractionation saturated hydrocarbons and nso compounds are expelled preferentially pristane and phytane show higher relative expulsion efficiencies than n c 17 and n c 18 alkanes aromatic hydrocarbons especially trimethylnaphthalenes are fractionated by the expulsion processes triterpanes and steranes are not fractionated by expulsion from solution seams and could be valuable tools for oil source rock correlation in carbonate rocks

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

a novel approach for recognition and quantification of hydrocarbon migration effects in shale sandstone sequences a detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m 1 050 ft through sequences of interbedded source rock type shales 0 84 rm maturity and reservoir sandstones allowed recognition and quantitation of a number of migration effects detailed gas chromatography mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum range hydrocarbons to a much higher degree than the centers of thick shale units for the alkanes expulsion occurs with pronounced compositional fractionation effects shorter chain length n alkanes are expelled preferentially and isoprenoid alkanes are expelled to a lesser degree than their straight chain isomers based on material balance calculations expulsion efficiencies were determined and found to be very high in certain instances for thin interbedded shales they decrease from about 80 around c15 to near zero in the c25 region there is no evidence for significant redistribution of steranes and triterpanes in the two sequences compared to c15 to c25 n alkanes they appear relatively immobile the composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length hence consideration of bulk expulsion efficiencies gives an unrealistic picture furthermore the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk oil migration and without significant fractionation the degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction and the primary migration process appears to have occurred with chromatographic separation the migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes the composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock by this mechanism the origin of accumulations of light oils and gas condensates in low mature sequences bearing predominantly terrestrial derived organic matter can be explained finally the migration effects documented in this study have some consequences for interpretation of geochemical data e g the pristane n c17 ratio a commonly accepted maturity parameter has been shown to be also controlled by the degree of hydrocarbon expulsion

palaeo oceanographical implications of early middle miocene subtropical ostracod faunas from the continental shelf of the se atlantic ocean abstract early aquitanian and burdigalian and middle langhian miocene marine ostracoda were studied from the continental shelf off the orange river sw africa eighty one species belonging to 44 genera were recognised comparison with contemporary east african faunas indicate strong similarities simpson coefficient of generic affinity sc 38 this suggests that the shelf off the orange river at that time experienced sub tropical conditions with water temperatures 20 c in contrast there were barriers to faunal exchange with areas farther north along the west african shelf sc for gabon 24 previously published odp data suggest that by late miocene time significant upwelling of cool water had begun over the northern benguela region of the walvis ridge abutment and by late pliocene time intense frontal turbulence was established in the southern benguela region it is postulated that these phenomena isolated the orange shelf ostracod faunas from further influence by the east coast agulhas current a comparison of the orange shelf ostracods with those from adjacent se atlantic deep water regimes indicates no simultaneous faunal changes in response to major neogene palaeo oceanographic events

the effect of crystal plastic deformation on isotope and trace element distribution in zircon combined bse cl ebsd feg empa and nanosims study plastically deformed zircon grains from granulite facies ivrea verbano zone southern alps italy and amphibolite facies tauern window eastern alps austria shear zones have been investigated the main focus was on the effects of crystal plastic deformation on the distribution of trace elements and their isotopes zircon grains reveal crystal plastic deformation patterns in form of i gradual bending of the lattice high density of free dislocations ii highly deformed margins revealing a combination of low and high angle boundaries and gradual bending of the lattice and iii a low angle boundary lab network chemical and isotopic maps and profiles show that trace elements in zircon are re distributed in all plastically deformed domains changes in trace element composition along labs are sometimes revealed by brighter cl and darker bse signal labs and domains of high free dislocation density have depletion in u y yb and enrichment in ce la and nd while ti and p are either enriched depleted or remain unaffected and hf demonstrates stability y and yb are decreased in concentration across labs and have oscillating concentration in domains of high free dislocation density our observations confirm that crystal plastically deformed domains in zircon act as effective pathways for trace cations the pb isotopic system is disturbed by crystal plastic deformation microstructures as indicated by relative 207 pb 206 pb ages showing significant discordance in plastically deformed zircon domains in deformed domains a positive correlation between dislocation density and the degree of isotopic age distortion is observed fractured or porous domains and domains with high density of sub grain boundaries are enriched in common pb from the matrix and thus show significantly older relative 207 pb 206 pb ages than pristine domains in contrast pb loss occurs in domains with high free dislocation density at a distance of 5 10 m from the grain boundary caused by out diffusion of radiogenic pb this study adds to a growing database on the consequences of crystal plastic deformation for trace elements and isotopic systems in zircon we provide additional evidence that such deformation cannot be neglected and may have important implications for zircon geochronology and geochemistry

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

influence of hydrocarbon injection on the compaction by pressure solution of a carbonate rock an experimental study under triaxial stresses abstract the preservation of good petrophysical properties high porosity high permeability at great depth in carbonate rocks may lead to the existence of a deeply buried reservoir dbr a target of interest for the oil industry one of the key processes controlling diagenesis of the burial environment is pressure solution creep psc an efficient compaction process responsible for the evolution of porosity and permeability in many carbonate reservoirs in this experimental study we examine the effect of i the presence of oil in the pore space and ii its timing of injection on the psc process and the petrophysical properties of a carbonate rock the experiments were performed using a flow through high pressure cell allowing the simulation of the pressure stresses and temperature conditions of a dbr multi disciplinary data mechanical chemical petrographical and petrophysical demonstrate that without oil in the pore space the main diagenetic process is the psc a process reducing by three the initial porosity but having no influence on intrinsic water permeability an early injection of oil prior to water circulation causes the inhibition of psc by the coating of the grains leading to the preservation of porosity conversely a late injection of oil does not preserve initial porosity the dataset obtained from these experiments show the importance of the timing of oil charging in a reservoir in the preservation of initial porosity at great depth by the inhibition of psc however the coating of grains by hydrocarbons may also inhibit further diagenetic processes leading to a creation of secondary porosity at depth

determination of gross kinetic parameters for petroleum formation from jurassic source rocks of different maturity levels by means of laboratory experiments abstract solvent extracted core samples of lower toarcian carbonate rich shales posidonia shale lias of the hils syncline area lower saxony basin northern germany covering a wide maturity range vitrinite reflectance rr 0 48 0 88 and 1 45 and a kimmeridge mudstone rr 0 45 are pyrolyzed at three different heating rates 0 1 0 7 and 5 0 k min by application of non isothermal first order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal mol and pre exponential arrhenius factors are obtained by an iteration method hypothetical hydrocarbon generation curves are calculated for the lias assuming three geological heating rates 0 5 5 and 50 k 106a peak hydrocarbon generation temperatures range from 135 to 165 c

linking shock textures revealed by bse cl and ebsd with u pb data la icp ms and sims from zircon from the araguainha impact structure brazil

sedimentological and diagenetic controls of gas in lower and early upper carboniferous sediments ne germany abstract fine grained organic rich paleozoic sediments are proven source rocks for conventional gas systems in nw europe and large unconventional gas reservoirs are assumed in lower to early upper carboniferous black shales however despite numerous boreholes exploring for hydrocarbons the shale gas potential remains uncertain this paper comprises detailed information on the sedimentary structure and lithofacies associations along with spatial distribution thickness thermal maturity organic and inorganic petrology as well as geochemistry and mineralogy of lower to early upper carboniferous black shales in ne germany to unravel the evolution of shale gas relevant properties sediments of the kohlenkalk formation carboniferous limestone in the rugen island area were deposited on an extensive deep water carbonate shelf transgressive regressive cycles led to the formation of an up to 2000 m thick intercalation of mud and limestones high total organic carbon contents 2 4 7 0 wt of a mainly type ii iii kerogen have been deposited under dysoxic to anoxic conditions and attained favorable present day thermal maturities 0 8 1 8 vrr indicating a shale oil potential in the upper part of the succession and a shale gas potential in the deeper parts sediments of the altmark north brandenburg culm synorogenic flysch formation were deposited as a turbidity current derived intercalation of a sand silt mud interbedded and black mudrock lithofacies in a deep marine siliciclastic slope to basinal setting the 1900 m thick succession has toc contents of 0 5 2 wt and present day thermal maturities of 2 3 3 2 vrr indicating a possible hybrid gas accumulation in areas where the succession is buried

microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in toarcian source rocks of different maturities abstract this study is part of a comprehensive research program on the generation and migration of petroleum from the organic rich posidonia shale lower toarcian jurassic of the hils syncline northern germany four cores of the complete posidonia shale with varying amounts of underlying pliensbachian and overlying aalenian mudstones provided the sample base depending upon their distance from a presumed subjacent heat source the vlotho massif the cores display progressive levels of increasing thermal maturation with r 0 values 0 48 0 68 0 88 and 1 45 which span the limits of the oil generation window primary stratigraphic lithologic and petrographic attributes of the four sections are all very similar each includes a lower marlstone member about 5 m thick overlain by 11 31 m of calcareous clay shale clay minerals calcite pyrite kerogen and quartz are the principal constituents thus there is no evidence that depositional conditions of the posidonia shale varied significantly over the sampled area the omnipresence of framboidal pyrite clearly formed from the activity of sulphate reducing bacteria at or near the depositional interface is strong evidence that uniform reducing anoxic conditions were persistent at all localities this conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry organic macerals and other properties are directly related to the variable levels of thermal maturation kerogen from the least mature section r o 0 48 is type ii and composed of alginite a alginite b liptodetrinite bituminite and some terrestrial sporinite vitrinite and inertinite macerals with increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties alginite of the least mature section displays a strong green fluorescence at 0 68 r o alginite macerals exhibit yellow orange fluorescence and are structurally well preserved although geochemical data indicate generation and loss of large amounts of oil at 0 88 r o hydrocarbon generation is almost complete and few primary alginites remain they have been transformed into secondary liptinites designated as metaalginite which have similar fluorescence colours at the highest maturation level 1 45 r o metaalginites are only weakly fluorescent with a shift toward red colours calcite and or bitumen filled bedding plane micro and macrofractures occur exclusively in the organic rich posidonia shale at maturity levels of 0 68 r o and above such features are totally absent in the immature posidonia section and in the adjacent organic poor mudstones this observation indicates that the fractures developed in direct response to overpressuring i e pore pressure exceeding lithologic load related to oil and gas generation in the source rock the fractures undoubtedly provided important expulsion conduits for generated products

effects of phosphate ions on intergranular pressure solution in calcite an experimental study abstract intergranular pressure solution ips is a coupled chemical mechanical process of widespread importance that occurs during diagenesis and low temperature deformation of sedimentary rocks laboratory experiments on ips in halite quartz and calcite have largely concentrated on the mechanical aspects of the process in this study we report the effects of pore fluid chemistry specifically varying phosphate ion concentration on the mechanical compaction by ips of fine grained calcite powders at room temperature and 1 to 4 mpa applied effective stress phosphate was investigated because of its importance as a biogenic constituent of sea and pore waters increasing the pore fluid phosphate concentration from 0 to 10 3 mol l systematically reduced compaction strain rates by up to two orders of magnitude the sensitivity of the compaction strain rate to phosphate concentration was the same as the sensitivity of calcite precipitation rates to the addition of phosphate ions reported in the literature suggesting that the rate of ips in phosphate bearing samples was controlled by calcite precipitation on pore walls the results imply that ips and associated porosity permeability reduction rates in calcite sediments may be strongly reduced when pore fluids are enriched in phosphates for example through high biologic productivity or a seawater origin future modeling of ips related processes in carbonates must therefore take into account the effects of pore fluid chemistry specifically the inhibition of interfacial reactions

u pb isotopic evidence for the accretion of different crustal blocks to form the lewisian complex of northwest scotland single zircon and titanite u pb shrimp data presented for tonalite trondhjemite granodiorite ttg suite gneisses and an ultramafic rock from the northern and central regions of the lewisian complex of northwest scotland show that protolith ages of tonalitic gneisses in the northern region 2800 2840 ma are significantly younger than those in the central region 2960 3030 ma further evidence of a major 2490 2480 ma metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body a dioritic gneiss from the northern region has also been dated at c 2680 ma the northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the lewisian which were based upon this concept need replacing it is instead proposed that two distinct crustal blocks now the northern and central regions were tectonically juxtaposed along a boundary corresponding to the laxford front juxtaposition would appear to have occurred in proterozoic times as it must have postdated the 2490 2480 ma inverian metamorphism recorded only in the central region and the emplacement of granite sheets restricted to the northern side of the boundary the first recorded event common to both regions is resetting of titanite ages associated with c 1750 ma laxfordian amphibolite facies metamorphism zircon inheritance in rocks of both regions is scarce within one zircon from the northern region a c 3550 ma core was found this represents the oldest known material from the region

fluid evolution in fracturing black shales appalachian basin opening mode veins in cores drilled from the mudrocks overlying and underlying the major silurian salt decollement in the appalachian plateau tioga and lawrence counties pennsylvania have mineralogic and isotopic compositions generally matching those of their host mudrocks suggesting opening and filling amid little cross stratal fluid motion calcite and most trace minerals probably entered the veins via dissolution reprecipitation from nearby host rock consistent with this interpretation are the observations that 1 trace minerals within the veins including quartz pyrite and dolomite are invariably also present within the layers hosting the veins with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock and 2 carbon and oxygen isotopic compositions of vein filling calcite are similar to those of calcite within the host rock with vein filling 18o slightly depleted and 13c slightly enriched modeling the fluid isotopic evolution assuming vein opening and filling amid immobile connate formation water accounts for these minor but systematic differences which are attributable to increasing temperature and hydrocarbon maturation an exception to the above trend is barite which despite its low solubility is systematically enriched in veins with respect to the host rock it is unclear whether barite precipitation resulted from the influx of external fluids perhaps deriving from silurian salt or from barium mobilized at depth from local clays or organic material

planar microstructures in zircon from paleo seismic zones pseudotachylytes resulted from frictional melts associated with ultramylonites in high grade metapelitic rocks from the ivrea verbano zone in the southern alps northern italy were studied with focus on the deformation microstructures in zircon the aims were to investigate the characteristics of zircon deformation in seismic zones and to recognize specific microstructures generated in zircon during earthquakes which could be useful for mineral dating of paleo seismic events helps to understand how seismic energy is released at depth and interacts with metamorphic processes the interior of polished zircon grains ranging from 30 to 150 m in length were investigated with optical microscope and scanning electron microscope sem techniques including secondary electron se backscattered electron bse forward scattered electron fse cathodoluminescence cl imaging and crystallographic orientation mapping by electron backscatter diffraction analysis ebsd grains were studied in situ and as separated fractions embedded in epoxy disks among different cataclastic and crystal plastic deformation microstructures in zircon we identified characteristic planar deformation bands pdbs planar fractures pfs and curviplanar fractures cfs planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain which varies from 0 4 to 2 7 pdbs are usually parallel to 100 crystallographic planes have width from 0 3 to 1 m and average spacing of 5 m in 2d sections planar deformation bands appear as contrast lamellae in orientation contrast images and in ebsd maps and in rare cases can be observed with the optical microscope pdbs form in specifically oriented grains due to high differential stresses high temperatures and high strain rates generated in seismically active environment and or due to shearing in the vicinity of frictional melts discovered structures represent a result of crystal plastic deformation of zircon grains with operating dislocations having 010 glide system and misorientation axis therefore they can be classified as a new type 4 lattice distortion pattern according to the existing classification for zircon piazolo et al 2012 kovaleva et al 2014 we have demonstrated that formation of planar fractures in zircon takes place not only during impacts but also in seismically active zones we observe at least two cases of formation of pfs with 100 orientation 1 as a result of evolution of pdbs and 2 as micro cleavage this study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events but also as a result of seismic events at depth due to unusually high differential stress strain rate and temperature according to the new findings pdbs in zircon from the deep crust are supposed to represent newly recognized evidence of seismicity

production and preservation of organic matter during deposition of the bakken formation late devonian and early mississippian williston basin abstract the organic rich up to 35 toc lower and upper black mudstone members of the bakken formation late devonian and early mississippian accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic palaeoceanographic and palaeoclimatic conditions during bakken deposition the semi isolated williston basin was located close to the equator 5 to 10 n in a zone of prevailing east to west winds a pattern of estuarine like marine circulation between the williston basin and open ocean conditions at the western craton edge of north america is proposed surface water discharged from the basin was replaced by nutrient rich deeper water sourced from the equatorial undercurrent in the pacific basin undercurrent flow into the williston basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition in response to increased productivity that led to a high rate of organic sedimentation and quiescent deep water conditions due primarily to the geographic isolation of the basin and the restriction of estuarine like circulation to the upper water column anoxic bottom water developed on the basin floor as sea level rose during lower and upper black mudstone deposition anoxic bottom water expanded across the basin enhancing organic matter preservation conditions in the williston basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic rich mud deposition these results have potentially important implications for the accumulation of similarly organic rich deposits across the north american interior during late devonian and early mississippian time

mineralogy and organic geochemistry of the terrestrial lacustrine pre salt sediments in the qaidam basin implications for good source rock development abstract terrestrial pre salt sediments are not only good lithologic reservoirs similar to marine pre salt carbonates but also are supposed to have high hydrocarbon generation potential because the overlying terrestrial thick bedded salts are generally preserved in basin centers however their status as source rock has attracted much less attention than as reservoir in this paper we choose the qaidam basin as an example and study the minerology organic geochemistry and organic petrology of the pre salt sediments in a hydrocarbon rich unit where abundant petroleum resources have been discovered under halite beds lithoscanner logging of six wells and x ray diffraction results of 378 samples show that terrestrial pre salt sediments are characterized by mixed siliciclastic carbonate sulphate rocks and their relative proportions are laterally varied quartz feldspar and clay minerals are of allochthonous origin and classified as argillaceous endmember while dolomite and ankerite are of autochthonous origin as carbonate endmember organic geochemistry analysis of 118 samples show great variability in toc s2 and tmax values and this variability is not related to mineral composition heterogeneity as hydrogen index is not well correlated with detritus carbonate and sulphate minerals contents indicating the interacting controls of source rocks by both exogenetic and endogenetic processes in brackish mesosaline lakes strong bacterial reworking of organic matter results in the dominant macerals of pre salt sediments as bituminite and oil generation at low maturity levels sealed by impermeable halite beds and in close proximity to reservoirs the generating hydrocarbon by pre salt good source rocks is effectively accumulated which entails hydrocarbon exploration under halite beds in other terrestrial salt bearing basins

principles of geochemical prospect appraisal our current understanding of the geochemical processes responsible for the creation of commercial accumulations of oil and gas can be reduced to simple rules and equations to allow the petroleum geologist to use geochemistry to estimate before drilling the likely volumes and compositions of oil and gas delivered to the trap in most sedimentary basins oil is expelled from source rocks between 120 and 150 c whereas most gas and gas condensate are released between 150 and 230 c when the initial potential of source rocks exceeds 10kg mt oil expulsion efficiencies are between 60 and 90 expelled oil and gas migrate as petroleum rich phases driven by fluid potential gradients most flow is laterally updip in beds with effective horizontal permeabilities greater than 1 md in lower permeability rocks the petroleum fluids move vertically up or down along the path of least resistance i e the most permeable or thinnest beds separating them from a high permeability lateral carrier bed the residual saturations of petroleum left behind along the migration pathway are of the order of a few percent of the porosity geochemical analysis of source rocks helps determine the yields and compositions of petroleum fluids expelled in the catchment area of a prospect these fluid masses are converted to volumes from knowledge of the phase relations and densities of petroleum fluids evaluation of the total volume of rock through which the petroleum potentially can migrate and the mean porosity of the rocks that constitute the migration pathway allow migration losses to be assessed these losses constrain both the range of migration of petroleum fluids away from the mature source rock and the volumes of petroleum remaining for entrapment in the prospect

mechanisms of strain accommodation in plastically deformed zircon under simple shear deformation conditions during amphibolite facies metamorphism abstract this study documents the strain accommodation mechanisms in zircon under amphibolite facies metamorphic conditions in simple shear microstructural data from undeformed fractured and crystal plastically deformed zircon crystals are described in the context of the host shear zone and evaluated in the light of zircon elastic anisotropy our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral crystal plastically deformed zircon grains have axis oriented parallel to the foliation plane with the majority of deformed grains having axis parallel to the lineation zircon accommodates strain by a network of stepped low angle boundaries formed by switching between tilt dislocations with the slip systems 010 and 1 10 110 and rotation axis 001 twist dislocations with the rotation axis 001 and tilt dislocations with the slip system 001 and rotation axis 010 the slip system 1 10 110 is newly described for zircon most misorientation axes in plastically deformed zircon grains are parallel to the xy plane of the sample and have 001 crystallographic direction such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology deformation mechanisms and dominant slip systems in zircon young s modulus and p wave velocity have highest values parallel to zircon 001 axis indicating that zircon is elastically strong along this direction poisson ratio and shear modulus demonstrate that zircon is also most resistant to shearing along 001 thus 001 axis is the most common rotation axis in zircon the described zircon behaviour is important to take into account during structural and geochronological investigations of poly metamorphic terrains geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone s large scale stresses in the crust and possibly the timing of deformation if the isotopic systems of deformed zircon were reset

effects of elemental sulfur during programmed pyrolysis of kerogen abstract the effects of elemental sulfur during programmed pyrolysis of eocene green river shale and common rock matrix minerals were assessed experimentally mixtures of elemental sulfur and green river shale with calcium carbonate silicon dioxide calcium sulfate and zinc sulfide were analyzed with the weight fraction green river shale held at near constant values the addition of elemental sulfur resulted in substantial decreases in hydrogen index and substantial increases in oxygen index the effect is so significant that thermally immature type i kerogen can yield a pyrolysis response typical of type iii kerogen moreover elemental sulfur appears to influence estimations of thermal maturity during programmed pyrolysis only minor differences in pyrolysis response were noted for the different matrix compounds without elemental sulfur this study suggests that programmed pyrolysis results on rocks containing unusually high contents of elemental sulfur should be interpreted with caution

the deep crust beneath island arcs inherited zircons reveal a gondwana continental fragment beneath east java indonesia abstract inherited zircons in cenozoic sedimentary and igneous rocks of east java range in age from archean to cenozoic the distribution of zircons reveals two different basement types at depth the igneous rocks of the early cenozoic arc found along the southeast coast contain only archean to cambrian zircons in contrast clastic rocks of north and west of east java contain cretaceous zircons which are not found in the arc rocks to the south the presence of cretaceous zircons supports previous interpretations that much of east java is underlain by arc and ophiolitic rocks accreted to the southeast asian margin during cretaceous subduction however such accreted material cannot account for the older zircons the age populations of archean to cambrian zircons in the arc rocks are similar to gondwana crust we interpret the east java early cenozoic arc to be underlain by a continental fragment of gondwana origin and not cretaceous material as previously suggested melts rising through the crust feeding the early cenozoic arc picked up the ancient zircons through assimilation or partial melting we suggest a western australian origin for the fragment which rifted from australia during the mesozoic and collided with southeast asia resulting in the termination of cretaceous subduction continental crust was therefore present at depth beneath the arc in south java when cenozoic subduction began in the eocene

resolution of impact related microstructures in lunar zircon a shock deformation mechanism map abstract the microstructures of lunar zircon grains from breccia samples 72215 73215 73235 and 76295 collected during the apollo 17 mission have been characterized via optical microscopy cathodoluminescence imaging and electron backscatter diffraction mapping these zircon grains preserve deformation microstructures that show a wide range in style and complexity planar deformation features pdfs are documented in lunar zircon for the first time and occur along 001 110 and 112 typically with 0 1 25 m spacing the widest pdfs associated with 112 contain microtwin lamellae with 65 misorientation relationships deformation bands parallel to 100 planes and irregular low angle misorientation axes this geometry is consistent with a dislocation glide system with 010 during dislocation creep nonplanar fractures recrystallized domains with sharp irregular interfaces and localized annealing textures along fractures are also observed no occurrences of reidite were detected shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon pdfs form along a limited number of specific hkl planes that are perpendicular to directions of high young s modulus suggesting that pdfs are likely to be planes of longitudinal lattice damage twinned 112 pdfs also contain directions of high shear modulus a conceptual model is proposed for the development of different deformation microstructures during an impact event this shock deformation mechanism map is used to explain the relative timing conditions and complexity relationships between impact related deformation microstructures in zircon

a critical evaluation of depositional parameters controlling the variability of organic carbon in arabian sea sediments abstract organic carbon distribution in the surficial sediments of the arabian sea to some extent mirrors the surface productivity being both high in the peripheral portions and low in the central region suggesting that the productivity is a primary source of organic matter however a critical examination reveals that the organic enrichment on the slope of the indian margin is two to four higher max 16 71 than on the slope of the arabian peninsula max 7 54 while the productivity in the former region is three to four times lower than the latter this observation suggests that the influence of productivity on organic enrichment is of secondary importance high concentrations 4 of organic carbon exactly coincide with the oxygen minimum zone 150 1500 m water depth evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon incidently the organic rich band along the western indian slope is wide long and highly concentrated as compared to that of the slope of the arabian peninsula and no organic enrichment is found on other continental slopes of the arabian sea although an equally intense oxygen minima impinge on the floor of the slope such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments rates of fine terrigenous sediment deposition shelf width slope gradient bottom currents and adsorption capacity of individual clay minerals all these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the degree of preservation

response of cathodoluminescence to crystal plastic deformation in zircon geochemical and geochronological studies of zircon are commonly supplemented by cathodoluminescence cl imaging because it provides a means of recognizing different generations of zircon growth at high spatial resolution crystal plastic deformation of zircon can have significant effects on zircon geochemistry detailed analyses from electron backscatter diffraction mapping combined with panchromatic cl imaging and hyperspectral cl mapping of several crystal plastically deformed grains from different geological settings are used to establish the relationships between crystal plastic deformation and cl in zircon results show a strong spatial association between deformation microstructures and cl response that lead to modification of cl that commonly cross cuts primary zoning variable contributions from two fundamental deformation related processes result in a variety of cl characteristics a defect control on panchromatic cl intensity particularly at low angle subgrain boundaries and changes in spectral cl response due to deformation related modification of cl active ree geochemistry a framework is provided for the recognition of deformation related microstructures using cl and the usefulness of cl imaging in the discrimination of these microstructures is critically evaluated

the platycopid signal of oxygen depletion in the ocean a critical evaluation of the evidence from modern ostracod biology ecology and depth distribution abstract the use of relative abundances of filter feeding platycopid ostracods in estimating past levels of dissolved oxygen in the ocean the platycopid signal hypothesis is examined paying particular attention to evidence from faunistic biological and ecological studies the degree of support for the hypothesis has been overstated available data on ostracod depth distribution show that previous claims for platycopid dominance in the oxygen minimum zone in different oceanic regions are misleading and based on only a few studies observations and experimental studies on living platycopids are limited in scope and should be treated with caution we conclude that previously published and widely applied calibrations of platycopid abundance against dissolved oxygen levels are inadequately justified by the available data any further application of the platycopid signal technique should necessarily address these criticisms

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

deformation related microstructures in magmatic zircon and implications for diffusion an undeformed glomeroporphyritic andesite from the sunda arc of java indonesia contains zoned plagioclase and amphibole glomerocrysts in a fine grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation a suite of xenocrystic zircon records proterozoic and archaean dates whilst a discrete population of zoned euhedral igneous zircon yields a shrimp u pb crystallisation age of 9 3 0 2 ma quantitative microstructural analysis of zircon by electron backscatter diffraction ebsd shows no deformation in the inherited xenocrysts but intragrain orientation variations of up to 30 in 80 of the young zircon population these variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c axis 001 which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in ebsd maps less common misorientation axes account for minor components of crystallographic dispersion these observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of 100 and 010 slip systems the restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population and the absence of deformation within the host igneous rock and inherited zircon grains indicate that zircon deformation took place within a low melt fraction 5 melt mid lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression tectonic stresses within the compressional sunda arc at the time of magmatism are considered to be the probable driver for low strain deformation of the cumulate in the late stages of initial crystallisation these results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal plastic microstructures in zircon is not restricted to high strain rocks such microstructures have previously been shown to enhance bulk diffusion of trace elements u th and ree in zircon the development of deformation microstructures and therefore multiple diffusion pathways in zircon in the magmatic environment has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates

on the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone abstract we present novel microstructural analyses of zircon from a variety of strained rocks for the first time multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation based on careful in situ observations for sampling we selected zircon bearing rocks that were deformed by simple shear samples covered a range of p t conditions and lithologies including various meta igneous and meta sedimentary gneisses microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of i the long axes of plastically deformed zircon crystals ii the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and iii the foliation and lineation directions of the respective samples we assume that zircon crystals did not experience post deformation rigid body rotation and thus the true geometric link can be observed the relationships are the following a plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane b crystals with axes oriented at an angle 15 to the foliation plane are undeformed or fractured zircon crystals that have axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the 001 crystallographic direction zircon grains with the axis aligned at 30 60 to the lineation within the foliation plane often develop either two low miller indices misorientation axes or one high miller indices misorientation axis host phases have a significant influence on deformation mechanisms in a relatively soft matrix zircon is more likely to develop low miller indices misorientation axes than in a relatively strong matrix these relationships are independent of zircon s grain size and shape and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal plastic deformation behavior of zircon and on the geometry of its slip systems we describe previously unknown rheological and crystallographic properties of zircon which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation additionally relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology if deformation events result in zircon distortion and as a consequence partial or complete resetting of the zircon isotopic system the age of deformation can be accurately dated