high efficiency of natural lamellar remanent magnetisation in single grains of ilmeno hematite calculated using mossbauer spectroscopy rocks from large remanent magnetic anomalies on earth have been found to contain exsolved rhombohedral oxides which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on mars theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite here single grains 250 m of titanohematite with ferri ilmenite exsolution lamellae from mesoproterozoic metamorphic rock samples in southwest sweden and the adirondack mountains usa are studied using room temperature mossbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism mossbauer spectra of synthetic samples of titanohematite fe0 95ti0 05o3 and fe0 9ti0 1o3 were collected for comparison and showed a dominant six line magnetic spectrum due to fe3 in titanohematite with a weak sextet due to fe2 fe3 charge transfer mossbauer spectra of the natural ilmeno hematite grains are similar to those for synthetic titanohematite but contain additionally two paramagnetic doublets corresponding to fe2 and fe3 in ilmenite however several grains also contain an additional weak broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model similar to the previous mossbauer results for natural hemo ilmenite there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri ilmenite lamellae and no evidence for single domain or superparamagnetic magnetite the compositions of titanohematite and ferri ilmenite in the individual ilmeno hematite grains calculated from the mossbauer area ratios show that compositions are closer to end member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains consistent with observations that lamella thicknesses are as small as a few nm bulk compositions of ilmeno hematite grains calculated from the mossbauer data confirm that the swedish samples are significantly more ilmenite rich than the adirondack samples and agree with an estimation of bulk composition through point counting of electron backscatter images the mossbauer data allow a quantitative estimation of contact layer abundance which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies contact layer abundance determination by mossbauer spectroscopy provides a valuable new tool for mineral based magnetic anomaly interpretation on earth and other planetary bodies

the namibian and brazilian southern south atlantic petroleum systems are they comparable analogues tectonic reconstructions made across the southern south atlantic ocean indicate a diversity of rift and drift basin characteristics on the conjugate margins that define them as different stratigraphic and structural entities in terms of petroleum systems the basins are not as unlike as some characteristics suggest given the lack of significant hydrocarbon discoveries to date south of the walvis ridge doubts have been cast on the presence in this area of the prolific lower cretac eous lacustrine and marine source rock systems which are well known in the greater campos basin and offshore angola oils and condensates from the basins south and north of the walvis ridge exhibit geochemical similarities suggesting that comparable source rock systems are present in both areas the condensate geochemical analysis results from the kudu field in namibia are compared with oils from marine and lacustrine sources in brazil indicating that the kudu condensates are derived from at least two different source rocks these results suggest that the underexplored basins offshore namibia contain thermally mature lower cretaceous lacus trine and marine source rocks offering a new frontier for petroleum exploration in africa s southern south atlantic

effects of strait mixing on ocean stratification the density distribution in the abyssal atlantic ocean suggests that mixing associated with overflows across deep sills may account for substantial amounts of deep mixing estimates of the strait mixing are made from published estimates of the overflows and the difference between antarctic bottom water densities across the vema channel and the romanche fracture zone to demonstrate that the strait mixing is an order of magnitude larger than the abyssal mixing estimated for a standard diffusivity of 1 times 10 ndash 4 m2 s ndash 1

magnetic phases in hemo ilmenite insight from low velocity and high field mossbauer spectroscopy 1 we have studied natural samples of hemo ilmenite ilmenite with hematite exsolution lamellae by use of mossbauer spectroscopy the samples are from the allard lake intrusion in quebec the stoichiometric compositions of the ilmenite host and the hematite lamellae are approximately fe2 0 84mg2 0 14ti4 0 98fe3 0 04o3 and fe2 0 17mg2 0 01ti4 0 18fe3 1 64o3 respectively from low velocity mossbauer measurements the fe2 and fe3 components of the fe2 substituted hematite and fe3 substituted ilmenite formed by incomplete exsolution of hematite and ilmenite have been suggested identified high field mossbauer measurements obtained at low temperature indicate the presence of a minor ferrimagnetic component

effects of nanoscale exsolution in hematite ilmenite on the acquisition of stable natural remanent magnetization abstract to investigate the acquisition mechanism of high and stable natural remanent magnetization nrm in rocks of the russell belt adirondack mountains new york we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties the samples contain titanohematite with ilmenite lamellae end member hematite without lamellae and rare magnetite as potential carriers for the nrm transmission electron microscopy tem observations and element mapping by energy filtered tem eftem of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness 2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick the ilmenite lamellae and titanohematite hosts with the compositions of ilm 90 100 hem 10 0 and ilm 7 16 hem 93 84 respectively share 001 planes and the abundant fine ilmenite lamellae have coherent sharp structural and compositional interfaces with their titanohematite hosts comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae these results are consistent with the lamellar magnetism hypothesis suggesting that the acquisition of high and stable nrm is related to the interfacial area between fine ilmenite lamellae and their host titanohematite end member hematite with a multi domain magnetic structure only contributes a minor amount to the nrm in these samples

uncommon steranes in brazilian marginal crude oils dinoflagellate molecular fossils in the sergipe alagoas basin brazil comprehensive two dimensional gas chromatography with time of flight mass spectrometry gc gc tofms was used to identify unusual steranes in branched cyclic hydrocarbon fractions of crude oils from sergipe alagoas basin brazil and infer the geochemical settings of their cretaceous source rocks hopane sterane and sterane ratios and the presence of gammacerane and carotane in the oil samples indicates a saline source rock depositional environment and predominantly marine organic matter om input steroidal hydrocarbons were represented by alkyl steranes and several sterane series including abundant short chain steranes c21 and c22 regular steranes c27 c29 and diasteranes c21 c23 c27 c29 the ring a methyl sterane series contained 3 methyl steranes c22 and c23 c28 c30 abundant 3 methyldiginane c22 and 3 methyldiacholestanes c28 including 4 methyl steranes with abundant c30 4 23 24 trimethylcholestanes in the branched cyclic fraction of these crude oils the c29 sterane isomers 23 24 dimethylcholestane and 24 ethylcholestanes were resolved by gc gc tofms and a c29 dinosterane derivative was identified for the first time in brazilian crude oils the 23 24 dimethyl and 4 23 24 trimethyl steroids can be related to dinoflagellate om and abundant dinosteranes suggests dinoflagellate blooms during source rock deposition in this brazilian marginal basin additionally the detection of c28 c32 3 alkyl cholestanes c29 c33 3 alkyl 24 methylcholestanes and c30 c34 3 alkyl 24 ethylcholestane with ethyl to pentyl groups at c 3 resulted in a complex sterane fingerprint as expected for petroleum generated from source rocks deposited under predominantly marine saline conditions with high microalgal input these biomarker results provide additional insight into the character and geologic processes responsible for the source rocks of crude oils in the brazilian marginal basins this information will be a valuable exploration tool that can be used to constrain the distribution of different oil types in these basins

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

convectively driven mixing in the bottom boundary layer closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing parcels of fluid were observed adjacent to the bottom that were warm by several millikelvin relative to fluid immediately above on average the vertical gradient of potential temperature in the superadiabatic statically unstable bottom layer was found to be 21 7 3 1024 km 21 or 6 0 3 1025 kg m24 in potential density turbulent dissipation rates increased toward the bottom but were relatively constant over the dimensionless depth range 0 4 1 0z d where d is the mixed layer height the rayleigh number ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 1011 much larger than the value of approximately 10 3 required to initiate convection in simple laboratory or numerical experiments an evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is the greater the turbulent dissipation rate in the neutral layer away from the bottom will be the vertical structures of averaged profiles of potential density potential temperature and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection the rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom it is suggested that the concentration of offshore ekman transport near the bottom of the ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils

lamellar magnetism and exchange bias in billion year old titanohematite with nanoscale ilmenite exsolution lamellae i mineral and magnetic characterization

the relationship between exsolution and magnetic properties in hemo ilmenite insights from mossbauer spectroscopy with implications for planetary magnetic anomalies 1 remanence properties of ilmenites with exsolved hematite have recently been the object of much study because their high natural remanent magnetization nrm is greater than predicted for these two minerals x ray diffraction xrd and mossbauer spectroscopy in conjunction with electron microprobe and nrm measurements were used to explore the possibility that single domain sd hematite lamellae in the ilmenite host could explain the observed remanence and to determine if magnetite is present xrd results show the presence of ilmenite and hematite mossbauer data show that only species assigned to hematite ilmenite and a small amount of tetrahedral fe3 are present magnetic properties at high and low t also indicate that the only magnetic minerals are ilmenite and hematite magnetic data suggest that ultra fine hematite lamellae are magnetically ordered and their resultant remanent magnetic anomalies may contribute significantly to magnetism on terrestrial planets even those without present day magnetic fields

a closer look at remanence dominated aeromagnetic anomalies rock magnetic properties and magnetic mineralogy of the russell belt microcline sillimanite gneiss northwest adirondack mountains new york a large distinct negative aeromagnetic anomaly of over 2000 nt associated with microcline sillimanite quartz gneisses in the russell area northwest adirondack mountains was previously shown to be remanence dominated although the carriers of remanence were not well documented russell belt gneisses have a strong natural remanent magnetization with steep remanence directions d 263 i 58 an average intensity of 3 6 a m and typical susceptibilities of 10 4 si the remanence is thermochemical in origin acquired during cooling from peak metamorphic conditions of 650 750 c during the ottawan orogen 1050 1080 ma the reversed polarity of remanence reflects a reversed paleofield rather than self reversed contrary to earlier suggestions the gneisses contain up to 3 oxide predominantly metamorphic titanohematite which accounts for the low susceptibility values and highly stable remanence optical observations show titanohematite grains with multiple generations of ilmenite pyrophanite rutile and spinel exsolution lamellae microprobe analyses confirm titanohematite compositions ranging from 72 to 97 fe2o3 with hematite83 being most typical in rare samples inclusions of magnetite were identified the ubiquitous presence of titanohematite and the rare occurrence of magnetite is supported by thermal and alternating field demagnetization studies saturation magnetization measurements hysteresis properties temperature hysteresis studies and low temperature remanence measurements numerous crustal granulites have titanohematite as part of the oxide assemblage and this may contribute a strong remanent component to what have previously been considered to be solely induced anomalies

nonlinear preisach maps detecting and characterizing separate remanent magnetic fractions in complex natural samples natural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques here a non linear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent preisach maps using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved the method is applied to synthetic and natural standard samples containing magnetite and hematite as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed it is shown that certain offset high coercivity patterns in remanent preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite and that in some magnetite bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range experimental results and theoretical considerations indicate a minimal coercivity of about 10 mt for sd magnetite such that observation of lower coercivities implies psd or md grain sizes a diagnostic hematite pattern with a peak downwards offset of 17 2 of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field

risks inherent in the cenozoic stratigraphic plays in basins of the uruguayan continental margin abstract the genesis and evolution of the uruguayan continental margin are linked to the geological processes that resulted in the breakup of the western gondwana and the opening of the atlantic ocean intense hydrocarbon exploration over the last decade has identified several plays in the mesozoic cenozoic sedimentary basin infill the main exploration milestone was the drilling of the raya x 1 well in the pelotas basin which targets an oligocene turbidite deposit the well encountered no hydrocarbon accumulations but has contributed to the knowledge of a frontier area and has raised questions regarding the geological risks associated with the cenozoic prospects in the uruguayan continental margin in this study the exploratory potential of a set of stratigraphic plays identified in the cenozoic sedimentary succession of the uruguayan continental margin basins was analysed through the interpretation of 2d seismic sections a significant risk appears to be related to migration and charge of the cenozoic reservoirs from cretaceous source rocks because of the lack of major faults and the development of thick transgressive sedimentary packages especially in the pelotas basin the best areas for vertical charge of the cenozoic reservoirs resides on the southwest of the uruguayan continental margin distal part of the punta del este basin and in a portion of the central region southernmost part of the uruguayan pelotas basin

influence of enhanced abyssal diapycnal mixing on stratification and the ocean overturning circulation abstractthe meridional overturning circulation moc is composed of interconnected overturning cells that transport cold dense abyssal waters formed at high latitudes back to the surface turbulent diapycnal mixing plays a primary role in setting the rate and patterns of the various overturning cells that constitute the moc the focus of the analyses in this paper will be on the influence of sharp vertical variations in mixing on the moc and ocean stratification mixing is enhanced close to the ocean bottom topography where internal waves generated by the interaction of tides and geostrophic motions with topography break it is shown that the sharp vertical variations in mixing lead to the formation of three layers with different dynamical balances governing meridional flow specifically an abyssal bottom boundary layer forms above the ocean floor where mixing is largest and hosts the northward transport of the heaviest waters from the southern channel to the closed basins a deep layer forms above the b

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

role of bottom water transport and diapycnic mixing in determining the radiocarbon distribution in the pacific 1 the mechanisms controlling the distribution of radiocarbon over the deep pacific are examined using firstly a simplified one and a half layer model and secondly an isopycnic circulation model with parameterized radiocarbon sources and sinks two mechanisms control the radiocarbon at depth relatively fast lateral transport of bottom waters determining the horizontal distribution and a slower balance between advection diffusion and radio decay in the vertical in the isopycnic model there is a strong topographic control of the bottom water spreading which is more complex than in the idealized model sensitivity studies reveal that altering the bottom intensified diapycnic mixing leads to significant changes in the radiocarbon distribution through both the direct diapycnic transfer and indirectly by modifying the bottom water transport and northward penetration of young radiocarbon waters

application of 24 norcholestanes for constraining source age of petroleum abstract two c26 sterane ratios the 24 nordiacholestane ratio ndr and the 24 norcholestane ratio ncr are useful for assessing petroleum source age values for these parameters generally increase in the cretaceous when diatoms were rapidly radiating and further in the oligo miocene due to widespread high latitude deposition of diatomaceous sediments sources with shale lithologies tend to exhibit the highest ratios while carbonate and terrigenous rich lithologies generally show lower ratios on the north slope of alaska elevated ratios are observed for cretaceous hrz oils and mixed oils containing cretaceous input compared to older triassic and jurassic derived oils tertiary sourced oils from romania and bulgaria have much higher ratios relative to jurassic sourced oils in the black sea region oleanane was observed in comparable levels in some black sea tertiary and jurassic oils and source rocks ndr and ncr coupled with other age diagnostic indicators such as oleanane and dinosterane provide powerful tools for differentiating age of source and organic facies for oils from the black sea and sub andean basins of peru and ecuador respectively oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence do not display high values regardless of age

comparison of the rift and post rift architecture of conjugated salt and salt free basins offshore brazil and angola namibia south atlantic abstract this study presents a regional comparison between selected 2d seismic transects from large conjugated salt and salt free basins offshore southern brazil campos basin santos basin pelotas basin and southwest africa kwanza basin northern and southern namibe basin walvis basin tectonic stratigraphic interpretation of the main rift and post rift units free air gravity data and flexural isostatic backstripping were used for a comprehensive basin to basin documentation of key mechanisms controlling the present day differences in conjugated and neighbouring south atlantic basins a significant variation in the tectonic sedimentary architecture along strike at each margin and between the conjugated basins across the south atlantic reflects major differences in 1 the structural configuration of each margin segment at transitional phase between rifting and breakup as emphasized in the highly asymmetric settings of the large santos salt basin and the conjugated salt free southern namibe basin 2 the post breakup subsidence and uplift history of the respective margin segment which caused major differences for example between the campos and espirito santo basins and the conjugated northern namibe and kwanza basins 3 variations in the quantity and distribution of post breakup margin sediments which led to major differences in the subsidence history and the related present day basin architecture for example in the initially rather symmetric siliciclastic pelotas and walvis basins and 4 the deposition of aptian evaporites in the large rift and sag basin provinces north of the rio grande rise and walvis ridge highly contrasting the siliciclastic basins along the margin segments south of the ridges the resulting present day architecture of the basins can be generally classified as i moderately symmetric salt free and magma rich in the northern part of the southern segment i highly asymmetric salt bearing and magma poor vs salt free and magma rich in the southern part of the central segment and iii moderately symmetric to asymmetric salt rich and magma poor in the central part of the central segment of the south atlantic

boundary mixing and topographic blocking on the mid atlantic ridge in the south atlantic abstract it is commonly observed in hydrographic sections crossing midocean ridges that the isopycnals on the ridge flanks slope downward toward the crests although the observed vertical scales of isopycnal dipping are not consistent with steady diffusive boundary layers on slopes the cross flank density gradients can nevertheless be caused by diapycnal mixing acting on timescales of several years the corresponding pressure gradients are usually inferred to be associated with cyclonic along flank flows recent observations of southward flow along the highly corrugated western flank of the mid atlantic ridge near 20 s are inconsistent with this conceptual picture however data from seven zonal cross ridge sections and from four meridional along ridge sections were used to analyze the hydrography on the flanks of the mid atlantic ridge between 2 n and 30 s the majority of the hydrographic stations were occupied over deep cross flank canyons which extend over a total length of 90 000 km in the tropica

organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in brazilian marginal basins abstract an investigation of bulk isotopic and molecular features of a wide selection of source rocks and oils from brazilian marginal basins ranging from lower cretaceous to tertiary in age provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo environments the methods employed include evaluation of organic carbon contents rock eval pyrolysis vitrinite reflectance measurements determination of carbon isotope ratios elemental and visual kerogen analysis assessment of oils and solvent extracts by liquid and gas chromatography and quantitative biological marker investigations using gc ms for aliphatic hydrocarbons and uv vis spectrophotometry for metalloporphyrins selected data were evaluated using principal component analysis the results enable the recognition and differentiation of seven depositional regimes lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate marine deltaic marine highly anoxic with a predominance of calcareous mudstone lithology and marine anoxic with predominance of siliciclastic lithology diagnostic features in the biological markers from these depositional environments include the carbon number distributions of n alkanes pristane phytane ratios absolute concentrations of carotane higher acyclic isoprenoids c 30 hopane and steranes and the abundance of the latter relative to 4 methylsteranes the occurrence and abundance of particular biological markers such as gammacerane tricyclic and tetracyclic terpanes 18 h oleanane 28 30 bisnorhopane and 25 28 30 trisnorhopane and the relative abundances of nickel and vanadyl porphyrins

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

oil and gas habitats of the south atlantic introduction the south atlantic hydrocarbon province defined for the purposes of this volume the south atlantic comprises the atlantic ocean south of a direct line from tangier morocco to the guyana venezuela border and north of a direct line from cape town south africa and cape horn argentina at the end of 1997 this region contained 4 3 of the world s total discovered oil reserves and 2 4 of the world s total discovered gas reserves petroconsultants 1998 on the basis of the 1997 figure six countries brazil argentina nigeria gabon congo brazzaville and angola host 96 3 of the region s oil reserves and 92 1 of the region s gas reserves figure 1 illustrates the geographical distribution of the reserve splits table 1 gives the details one hundred years of discoveries voluminous basin margin asphalts in west africa cote d ivoire to angola argentina brazil and surinam attracted the serious attention of entrepreneurs from the late 1800s as discussed in more detail by dolan shallow wells were drilled soon afterwards on seep locations though with little commercial success more systematic explorations began on both margins in the 1930s and in 1941 the first large discovery the candeias field in the reconcavo basin of brazil was made the reserves of this field are now placed at 120 mmbo success did not materialize in west africa until 1955 when the small benfica field was discovered in the kwanza basin of angola thereafter following the first giant field discovery in the niger delta in 1958 west african reserves rose

application of tetracyclic polyprenoids as indicators of input from fresh brackish water environments abstract c30 tetracyclic polyprenoids tpp are most prominently observed in samples derived from low salinity i e fresh to brackish lacustrine environments and are generally present in low levels in samples derived from saline i e marine and saline lacustrine environments a near shore facies of the chonta formation peru that has no marine palynomorphs but abundant chlorococcalean green algal nonmarine palynomorphs has high levels of tpp suggesting green algae or chlorophyta are a possible source for the tpp compounds the ratio between a c30 tpp compound and 27 norcholestanes is useful for assessing this nonmarine algal input moderate elevations of tpp above what is common in marine derived samples were found in ostensibly marine source rocks and oils from certain basins of western and northern south america middle magdalena colombia maracaibo venezuela and trinidad basins this is likely due to transport from the nonmarine to the marine environment because of an influx of fresh water into the near shore marine environment alternatively oils from these basins may have inputs from near shore shallow marine algae with chemistry similar to that found in lacustrine settings the tpp ratio in conjunction with other environmental indicators such as 4 methyl steranes or hopane sterane ratio is useful for differentiating marine and nonmarine influences in pre salt oils and source rocks of west africa the tpp ratio used with other environmental indicators gammacerane c29 steranes c30 steranes and age diagnostic biomarkers dinosteranes 4 methylsteranes can be useful in differentiating among nonmarine source facies for example in the turpan hami basin china permian saline lacustrine and jurassic lacustrine deltaic facies can be discriminated

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

oligo miocene lacustrine microbial and metazoan buildups from the limagne basin french massif central the limagne basin french massif central is an extensive continental lacustrine system accommodating microbial and metazoan buildups from chattian to aquitanian age a description of these buildups and their associated biotic components in grand gandaillat and crechy quarries provides insights into their spatio temporal distribution patterns flats cauliflowers domes cones and coalescent columnar morphologies have been identified with a main laminated mesofabric and laminated columnar filamentous and caddisfly coated microfabrics two low gradient margin models emerged based on the changes in the distribution morphology and size of the microbial and metazoan rich deposits through time the first model applies to the chattian deposits in the grand gandaillat quarry and the second to the aquitanian deposits in the crechy quarry this study emphasizes the heterogeneity of marginal lacustrine carbonate siliciclastic cycles the lacustrine palustrine cycles of the limagne basin record i lacustrine deposits composed of microbial and metazoan buildups and organic matter rich marls indicating periods of high accommodation and ii palustrine deposits composed of mudstone and clayey paleosoils indicative of periods of low accommodation the cycles differ with thin and symmetrical deposits that dominate the chattian cycles whereas thicker and asymmetrical deposits mark the aquitanian cycles in addition the chattian buildups are exclusively microbial and only a few centimeters thick whereas the aquitanian ones are multiple meters in height and are composed of both microbes and metazoans climatic tectonism volcanic and local parameters are involved in the deposition of buildups and their organization in each different cycle climate may control the long term arid humid sedimentary succession tectonic may explain the difference in the deposit thickness between two cycles of different ages and localizations as well as the specific distribution of the buildups throughout the basin volcanism may control the chemical lacustrine conditions and local physiography may impact the soil removal ability of the lake margin

conjugate divergent margins an introduction the main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the south central and north atlantic conjugate margins from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays these themes are complemented by geodynamic concepts based on physical and numerical models and by comparisons with present day embryonic margins which are succinctly discussed in some papers

a phase of transient subsidence sediment bypass and deposition of regressive transgressive cycles during the breakup of iberia and newfoundland abstract seismic outcrop and well data from west iberia and newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence in west iberia two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present day continental shelf this character is similar to newfoundland where correlative strata comprise lower cretaceous cenomanian coarse grained siliciclastics accumulated around local sediment source areas the interpreted data reveal that the two breakup sequences 1 materialise sediment bypass onto continental slope depocentres that experienced important tectonic subsidence during continental breakup but without showing typical syn rift growth packages 2 generate specific forced regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin subsidence and sediment stacking patterns in both west iberia and newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere to their mantle breakup stages on both margins coarse grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep water basins this work also confirms that in a setting dominated by a significant sediment influx yet lacking the burial rates of continental slope basins in newfoundland west iberia comprised accommodation driven basins during continental breakup not necessarily sediment starved as a corollary of our analysis we classify breakup sequences around the world based on the characteristic lithologies of their regressive transgressive depositional cycles

enrichment of diamondoids and 13c in condensate from hudson canyon us atlantic abstract gas condensate migrated vertically to upper jurassic and lower cretaceous reservoir rocks in the hudson canyon hc area of the baltimore canyon trough us atlantic condensate samples are enriched in diamondoids 4742 4774 ppm and 13c 13c 23 7 to 24 6 pdb several lines of evidence suggest that the main source rock is within the middle to lower jurassic section of hudson canyon condensates from hudson canyon are similar to condensates generated by the upper jurassic smackover formation of the us gulf coast a laminated lime mudstone with shale facies the petrologic fabric of the lime mudstone favors early generation of diamondoids hydrogen rich kerogen and clay catalysts are concentrated within thin laminations of depositional origin separated by thicker barren carbonate layers

seismic stratigraphy and subsidence analysis of the southern brazilian margin campos santos and pelotas basins abstract the campos santos and pelotas basins have been investigated in terms of 2d seismo stratigraphy and subsidence the processes controlling accommodation space e g eustacy subsidence sediment input and the evolution of the three basins are discussed depositional seismic sequences in the syn rift barremian to the drift holocene basin fill have been identified in addition the subsidence uplift history has been numerically modeled including i sediment flux ii sedimentary basin framework iii relation to plate tectonic reconfigurations and iv mechanism of crustal extension although the initial rift development of the three basins is very similar basin architecture sedimentary infill and distribution differ considerably during the syn rift sag to the drift basin stages after widespread late aptian early albian salt and carbonate deposition shelf retrogradation dominated in the campos basin whereas shelf progradation occurred in the santos basin in the tertiary these basin fill styles were reversed since the paleogene shelf progradation in the campos basin contrasts with overall retrogradation in the santos basin in contrast long term cretaceous paleogene shelf retrogradation and intense neogene progradation characterize the pelotas basin its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation these temporally and spatially varying successions were controlled by specific long term subsidence uplift trends onshore and offshore tectonism in the campos and santos basins affected the sediment flux history distribution of the main depocenters and occurrence of hydrocarbon stratigraphic structural traps this is highlighted by the exhumation and erosion of the serra do mar serra da mantiqueira and ponta grossa arch in the hinterland as well as salt tectonics in the offshore domain the pelotas basin was less affected by changes in structural regimes until the eocene when the andean orogeny caused uplift of the source areas flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

turbulent mixing and hydraulic control of abyssal water in the samoan passage 1 we report the first direct turbulence observations in the samoan passage sp a 40 km wide notch in the south pacific bathymetry through which flows most of the water supplying the north pacific abyssal circulation the observed turbulence is 1000 to 10 000 times typical abyssal levels strong enough to completely mix away the densest water entering the passage confirming inferences from previous coarser temperature and salinity sections accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence specifically the flow accelerates substantially at the primary sill within the passage reaching speeds as great as 0 55 m s 1 a strong hydraulic response is seen with layers first rising to clear the sill and then plunging hundreds of meters downward turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill in addition to the primary sill other locations along the multiple interconnected channels through the samoan passage also have an effect on the mixing of the dense water in fact quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers suggesting that abyssal mixing depends sensitively on bathymetric details on small scales

abyssal ocean overturning shaped by seafloor distribution the abyssal ocean is broadly characterized by northward flow of the densest waters and southward flow of less dense waters above them understanding what controls the strength and structure of these interhemispheric flows referred to as the abyssal overturning circulation is key to quantifying the ocean s ability to store carbon and heat on timescales exceeding a century here we show that north of 32 s the depth distribution of the seafloor compels dense southern origin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2 5 kilometres unless ventilated from the north the overlying mid depths 1 to 2 5 kilometres deep host comparatively weak mean meridional flow backed by analysis of historical radiocarbon measurements the findings imply that the geometry of the pacific indian and atlantic basins places a major external constraint on the overturning structure

abyssal upwelling and downwelling driven by near boundary mixing abstracta buoyancy and volume budget analysis of bottom intensified mixing in the abyssal ocean reveals simple expressions for the strong upwelling in very thin continental boundary layers and the interior near boundary downwelling in the stratified ocean interior for a given amount of antarctic bottom water that is upwelled through neutral density surfaces in the abyssal ocean between 2000 and 5000 m up to 5 times this volume flux is upwelled in narrow turbulent sloping bottom boundary layers while up to 4 times the net upward volume transport of bottom water flows downward across isopycnals in the near boundary stratified ocean interior these ratios are a direct result of a buoyancy budget with respect to buoyancy surfaces and these ratios are calculated from knowledge of the stratification in the abyss along with the assumed e folding height that characterizes the decrease of the magnitude of the turbulent diapycnal buoyancy flux away from the seafloor these strong diapycnal upward and downwa

northward abyssal transport through the samoan passage and adjacent regions a conductivity temperature depth hydrographic survey in january february 1994 and a 17 month deployment of current meter moorings from september 1992 to march 1994 were carried out to determine the volume transport water mass characteristics and diathermal fluxes of northward flowing abyssal waters in the samoan passage and adjacent regions of the south pacific ocean geostrophic calculations relative to 1 2 c potential temperature indicated northward transport of 7 8 sv in the samoan passage 1 1 sv through a gap in robbie ridge and 2 8 sv along the eastern flank of the manihiki plateau all of the total of 11 7 sv of northward geostrophic transport was in waters colder than 1 1 c the northward transport distribution was bimodal in temperature with a cold mode of 3 6 sv in the range 0 65 0 70 c occurring entirely in the samoan passage and a warm mode of 3 0 sv in the range of 0 80 0 85 c occurring mainly along the manihiki plateau within the samoan passage 7 1 sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3 day period the 17 month mean transport from the moored array was 6 0 sv 0 5 by using the observed temporally varying flow within the samoan passage together with the hydrographic snapshot across the region an estimate of the total mean northward transport of 10 6 sv 1 7 was obtained estimates of the flow across near bottom potential temperature surfaces indicate extraordinarily high rates of mixing with heating of the abyssal layer up to 20 w m 2 corresponding to diffusivities up to 10 1 m2 s 1

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

nanoscale haematite ilmenite lamellae in massive ilmenite rock an example of lamellar magnetism with implications for planetary magnetic anomalies summary massive nearly pure haemo ilmenite layers from historic ore deposits in rogaland norway contain very few silicates or other oxides and typically produce remanence dominated magnetic anomalies these rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains a typical bulk composition ilm 84 exsolved at high temperature to produce host ilmenite ilm 94 and micron sized haematite lamellae ilm 23 as measured by electron microprobe emp subsequent undercooling of the ilmenite and the micron scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit cell scale leaving depleted hosts between lamellae with compositions of ilm 98 and ilm 15 13 as measured by tem edx samples have high coercivities and average nrm values of 25 a m 1 which typically show 2 per cent saturation in the nrm state the amount of magnetization in these samples is too high to be solely accounted for by a spin canted af moment in the haematite based on monte carlo simulations of haematite ilmenite interfaces at the atomic scale and on measured rock magnetic properties we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine scale titanohaematite and ilmenite exsolution lamellae

quantifying antarctic bottom water and north atlantic deep water volumes 1 a near global census of antarctic bottom water aabw and north atlantic deep water nadw is essayed through a nonnegative least squares analysis of conservative and quasi conservative seawater properties aabw thickness generally decreases from south to north modulated by ocean bathymetry likewise nadw thickness generally decreases from north to south in the atlantic ocean nadw dominates below the thermocline of the atlantic ocean at least as far south as the subtropical gyre of the south atlantic with a lesser but still significant influence around the entire antarctic circumpolar current in the indian ocean and in the pacific ocean however in the pacific and indian oceans aabw dominates below the thermocline in addition measurable quantities of aabw reach into the abyssal north atlantic on both sides of the mid atlantic ridge the census results suggest that aabw occupies roughly twice the volume of nadw in the three main oceans and that aabw is in contact with roughly twice the area of the deep main ocean floor compared with nadw however these results are somewhat sensitive to choices of water masses their values of seawater properties and the weightings of the seawater properties used in the analysis

significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data how and where the ocean tides dissipate their energy are long standing questions1 that have consequences ranging from the history of the moon2 to the mixing of the oceans3 historically the principal sink of tidal energy has been thought to be bottom friction in shallow seas4 5 there has long been suggestive evidence6 7 however that tidal dissipation also occurs in the open ocean through the scattering by ocean bottom topography of surface tides into internal waves but estimates of the magnitude of this possible sink have varied widely3 8 9 10 11 here we use satellite altimeter data from topex poseidon to map empirically the tidal energy dissipation we show that approximately 1012 watts that is 1 tw representing 25 30 of the total dissipation occurs in the deep ocean generally near areas of rough topography of the estimated 2 tw of mixing energy required to maintain the large scale thermohaline circulation of the ocean12 one half could therefore be provided by the tides with the other half coming from action13 on the surface of the ocean

exchange bias identifies lamellar magnetism as the origin of the natural remanent magnetization in titanohematite with ilmenite exsolution from modum norway abstract large and stable negative magnetic anomalies in southwestern sweden southern norway the adirondacks usa and quebec canada are related to rock units with a magnetic fraction consisting primarily of ilmeno hematite or hemo ilmenite it has been suggested that the unusual magnetic stability of these rocks results from lamellar magnetism this is a type of magnetic remanence carried by uncompensated magnetic layers at interfaces between nanoscale exsolution structures of antiferromagnetic afm hematite and paramagnetic ilmenite here we present the first direct proof that this lamellar magnetism indeed is responsible for the natural remanent magnetization nrm of a rock from modum norway our argument expands a previous observation that in mineral grains from this rock the cooling of a positive induced remanence from room temperature to 5 k which is well below the ordering temperature of ilmenite 57 k leads to a large negative shift of the low temperature lt hysteresis loop this can only be explained by exchange bias due to exchange coupling across the hematite ilmenite interfaces in a different experiment we now have cooled the original nrm of untreated grains to 5 k and then measured the hysteresis loop again in several separate grains we observed large shifts of the hysteresis curves this shows that exchange bias develops also from the untreated nrm this observation proves that the moments which carry the nrm also participate in the exchange coupling at the hematite ilmenite interfaces therefore the nrm is not carried by defect moments or stress induced moments which occur in normal bulk hematite a closer look at the nrm induced lt loops shows that exchange bias acts in both field directions though one direction is clearly predominant this observation can be interpreted as a frozen equilibrium of different proportions of oppositely directed lamellar moments a key feature of the original lamellar magnetism hypothesis we discuss lamellar aggregation and the formation of exchange coupled clusters to explain the observed high efficiency of lamellar nrm we also conclude that remanence carried by lamellar moments should not be used for paleointensity estimates of terrestrial or extraterrestrial material

turning ocean mixing upside down abstractit is generally understood that small scale mixing such as is caused by breaking internal waves drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes however the observational evidence that the strong turbulent fluxes generated by small scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small scale mixing converts light waters into denser ones thus driving a net sinking of abyssal waters using a combination of theoretical ideas and numerical models it is argued that abyssal waters upwell along weakly stratified boundary layers where small scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom the abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters driven by small scale mixing in the stratified interior above the bottom boundary layers and a slightly larger net upwelling driven b

lamellar magnetism and exchange bias in billion year old metamorphic titanohematite with nanoscale ilmenite exsolution lamellae ii exchange bias at 5 k after field free cooling of nrm and after cooling in a 5 t field

observations of the flow of abyssal water through the samoa passage abstract during the fall of 1994 a conductivity temperature depth hydrographic survey was carried out as part of the world ocean circulation experiment one time survey line p15n the survey included standard water properties required by woce line p15n extended southward from the aleutian islands along 165 w into the vicinity of the samoa passage the line was adjusted to follow the axis of the passage and time was found to complete a cross section survey across the passage this paper will present geostrophic computations of flow velocities through the gap including transport estimates and will present longitudinal plots of properties the longitudinal plots show evidence of hydraulic control at the sill in the samoa passage a best estimate of northward transport of water colder than 1 2 c potential temperature is determined to be 8 4 sv

effect of fine scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks southwest sweden implications for crustal magnetism mid proterozoic granulites in sw sweden having opaque minerals hematiteilmenite with minor magnetite and occurring in an area with negative aeromagnetic anomalies have strong and stable reversed natural remanent magnetization 9 2 a m with 100 remaining after demagnetization to 100 mt samples were characterized by optical microscopy electron microprobe emp transmission electron microscopy tem and rock magnetic measurements earliest oxide equilibrium was between grains of titanohematite and ferri ilmenite at 650 600 c initial contacts were modified by many exsolution cycles hematite and ilmenite ilm hosts and lamellae by emp are ilm 24 25 ilm 88 93 like titanohematite and ilmenite above 520 c on burton s diagram 1991 finer hosts and lamellae by tem are ilm16 3 and ilm 88 4 like coexisting antiferromagnetically ordered af hematite and ilmenite below 520 c on burton s diagram this may be the first example of analytical identification in one sample of former hematite now finely exsolved and af hematite tem microstructures consist of gently curving semicoherent ilmenite lamellae within hematite flanked by precipitate free zones and abundant ilmenite disks down to unit cell scale 1 2 nm strain contrast of disks suggests full coherence with the host and probable formation at the reaction titanohematite af hematite ilmenite at 520 c magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite richer than ilm 28 that apparently exsolved without becoming magnetized down to 520 c where hematite broke down to af hematite plus ilmenite producing abundant af hematite below its neel temperature intensity of magnetization is greater than possible with hematite alone and tem work suggests that ultrafine ilmenite disks in af hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces

biomarker similarities between the saline lacustrine eocene green river and the paleoproterozoic barney creek formations abstract the paleoproterozoic barney creek formation which is currently interpreted as a restricted deep marine paleoenvironment plays a disproportionate role in our understanding of proterozoic ocean chemistry and the rise of complex life the barney creek formation hosts several unusual biomarker features specifically its methylhopane and carotenoid signatures herein we demonstrate that the saline lacustrine eocene green river formation shares a similar distribution of methylhopanes and carotenoids which is characteristic of saline lacustrine organic matter more generally these distinct methylhopane and carotenoid patterns are not observed together in marine organic matter of any geologic age these results imply a saline lacustrine depositional environment for the barney creek formation which agrees with earlier but now abandoned depositional models of this formation as a result models of proterozoic ocean chemistry and emergence of complex life that rely on a marine barney creek formation should be reexamined alternatively if paleoproterozoic marine biomarker signatures resemble those of younger saline lacustrine systems then this must be recognized to accurately interpret geologic biomarker and paleoenvironmental records

magnetism at depth a view from an ancient continental subduction and collision zone recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle two approaches to constraining magnetizations are developed providing minimum 0 01 si and maximum 0 04 si susceptibility estimates given some assumed thickness 15 km here these values are higher than are found in many continental rocks are there rocks deeper in the crust or upper mantle that are more magnetic than expected or are the model assumptions incomplete what is the magnetic behavior of deep crustal and upper mantle rocks when slightly cooler than the curie or neel temperatures of their magnetic minerals after being exhumed from locations of high grade metamorphism at greater depth different sets of equilibrium metamorphic minerals can be considered that would form under different conditions results on 1501 samples from the western gneiss region wgr norway mainly from mafic and ultramafic bodies subducted to depths of 60 200 km and temperatures of 750 up to 950 c at the very highest pressures show that rocks did not fully equilibrate to the dominant metamorphic facies conditions there is a large variation in petrophysical properties oxide minerals and mineral assemblages in wgr samples though they cannot explain the broad high amplitude deep seated anomalies measured in this region the presence of magnetite and exsolved titanohematite and hemo ilmenite in samples shows those magnetic phases are preserved even at eclogite facies conditions in part because complete eclogite facies equilibrium was rarely achieved

deformation of the continental crust the legacy of mike coward this special publication in memory and celebration of the work of professor mike coward is about the deformation of the continental lithosphere the collected papers discuss geometry structural principles processes and problems in a wide range of tectonic settings and thereby reflect the breadth of coward s interests they encompass the evolution of precambrian basement gneiss terrains the geometry and evolution of thrust systems basement involvement and structural inheritance in basins syn orogenic extension salt tectonics the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization examples are drawn from the lewisian and moine thrust belt of nw scotland the italian apennines nw himalayas the cyclades oman zagros mountains colombian cordillera carpathians north sea offshore brazil regional studies of the irumide belt central africa taurus mountains turkey greater south america and from the witwatersrand basin of south africa and the antler orogeny of sw usa

magnetic petrology of high fe ti eclogites from the ccsd main hole implications for subduction zone magnetism 1 the high fe ti eclogites with exsolved lamellar in 530 600 m depths from the chinese continental scientific drilling ccsd main hole in the sulu ultra high pressure uhp metamorphic belt eastern china record an anomalously high susceptibility natural remanent magnetization nrm and koenigsberger ratio q nrm ji ji is induced magnetization this provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks in this paper we have measured systematically magnetic properties and mineral assemblage and structure for three special samples no 83 no 86 and no 89 results show that these fe ti rich eclogites are the result of the fine grained titano magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo ilmenite we found that the dominant fe bearing oxide minerals in samples studied are ilmenite hematite hem ilm up to 25 little titano magnetite and pyrite the ferromagnetic susceptibility is mainly controlled by fine grained titano magnetite and nrm is closely related to the exsolved lamellar structure we hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno hematite during cooling and decompression processes these fe ti rich eclogites might be one of the sources of high magnetic anomalies observed in the sulu subduction zone eastern china

magnetization of exsolution intergrowths of hematite and ilmenite mineral chemistry phase relations and magnetic properties of hemo ilmenite ores with micron to nanometer scale lamellae from allard lake quebec hemo ilmenite ores from allard lake quebec were first studied over 50 years ago interest was renewed in these coarsely exsolved oxides based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations nrms 32 to 120 a m reported here to understand the magnetism and evolution of the exsolution lamellae the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy tem phase chemistry and relations between mineral chemistry and the hematite ilmenite phase diagram cycles of exsolution during slow cooling resulted in lamellae down to 1 2 nm thick combined electron microprobe tem and x ray diffraction xrd results indicate that hematite hosts reached a composition approximately ilmenite ilm 14 4 and ilmenite hosts ilm 98 the bulk of the very stable nrm which shows thermal unblocking 595 620 c was acquired during final exsolution in the two phase region canted antiferromagnetic r3c hematite r3 ilmenite hysteresis measurements show a very strong anisotropy with a stronger coercivity normal to than parallel to the basal plane orientation of the lamellae magnetic saturation m s values are up to 914 a m compared to 564 a m predicted for a modally equivalent spin canted hematite corrected for 15 r 2 tio 3 substitution low temperature hysteresis ac susceptibility measurements and mossbauer results indicate a neel temperature t n of the geikielite substituted ilmenite at 43 k the low temperature hysteresis and ac susceptibility measurements also show a cluster spin glass like transition near 20 k below t n of ilmenite an exchange bias occurs with a 40 mt shift at 10 k

nature and origin of lamellar magnetism in the hematite ilmenite series grains consisting of finely exsolved members of the hematite ilmenite solid solution series such as are present in some slowly cooled middle proterozoic igneous and metamorphic rocks impart unusually strong and stable remanent magnetization tem analysis shows multiple generations of ilmenite and hematite exsolution lamellae with lamellar widths ranging from millimeters to nanometers rock magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths yet is stronger than can be explained by canted antiferromagnetic caf hematite or coexisting paramagnetic pm fe ti ordered r 3 ilmenite alone in alternating field demagnetization to 100 mt many samples lose little remanence indicating that the nrm is stable over billions of years this feature has implications for understanding magnetism of deep rocks on earth or on planets like mars that no longer have a magnetic field atomic scale simulations of an r 3 ilmenite lamella in a caf hematite host based on empirical cation cation and spin spin pair interaction parameters show that contacts of the lamella are occupied by contact layers with a hybrid composition of fe ions intermediate between fe2 rich layers in ilmenite and fe3 rich layers in hematite structural configurations dictate that each lamella has two contact layers magnetically in phase with each other and out of phase with the magnetic moment of an odd non self canceling fe3 rich layer in the hematite host the two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments a lamellar ferrimagnetism made possible by the exsolution because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella lamellar magnetism is unique and quite distinct from conventional ferrimagnetism simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent af hematite layers and are parallel to the basal plane 001 thus lamellar magnetism is created at the temperature of chemical exsolution and is a chemical remanent rather than thermal remanent magnetization however in thermal demagnetization experiments too short for lamellar resorption demagnetization temperatures are those of the caf hematite considerably higher than temperatures of original lamellae formation internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution a thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host and that once in place neither the location nor the magnetic moment will be easily disturbed in an ilmenite host the external magnetic field cannot control the chemical location of a hematite lamella which is dictated by the enclosing ilmenite but once lamellae have formed the field can dictate their magnetic moments these moments however are not locked chemically to the host resulting in lower coercivity the effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field in grains with 001 oriented parallel to the field it would be effective in producing in phase magnetic moments and very strong remanence in grains with 001 normal to the field the field would be less effective in producing in phase magnetic moments hence producing weak remanence the most intense lamellar magnetism per formula unit occurs with in phase magnetization high lamellar yields and the largest number of lamellae per unit volume i e smallest lamellar size compared to the magnetic moment per formula unit m pfu and magnetic moment per unit volume m v of end member magnetite m pfu 4 b m v 480 ka m and hematite m pfu 0 0115 b m v 2 1 ka m results for some atomic models reasonably tied to natural conditions are m pfu 0 46 1 36 b and m v 84 250 ka m

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

on the origins of deep and bottom waters of the indian ocean the characteristics of the deep and bottom waters of the indian ocean when illustrated on potential density anomaly surfaces indicate that the waters enter from both the atlantic and pacific oceans the paths of spreading are constrained by the complex topography and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments especially in the northern indian ocean the weddell sea contributes to the densest waters found in the western basins and the ross sea and adelie coast to the densest waters found in the eastern basins both dense water varieties are altered by and incorporated in the less dense water above initially water carried by the circumpolar current then water from the north atlantic and finally by deep water whose characteristics are derived in the northern indian ocean contact with the sediments increases the silica content of the bottom water in the southern ocean in the northern indian ocean the sediments alter the silica of the water at the bottom and together with enhanced salinity from diffusion of saline overflows from the marginal seas above imprint unique markers to the deep water that flows back to the south at middepths the series of ridges between madagascar and australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern indian ocean characteristics southward within the circumpolar current low oxygen deep water from the pacific is carried across the atlantic and into the indian ocean south of africa part flows around the cyclonic weddell sea gyre and part extends across the southern ocean water from another pacific source can be seen near 2000 m extending westward from the tasman sea south of aus tralia and across the indian ocean and perhaps to the aghulas current region southeast of africa in this study we show some maps of deep and bottom water characteristics of the indian ocean which may be used in a qualitative sense to infer pathways of spreading of water below a depth of about 2000 m the indian ocean has numerous ridges and basins figure 1 that severely restrict possible pathways of deep and bottom wa ter flow in the following we will begin by looking at char acteristics at the bottom of the ocean to reveal the path ways of spreading of the densest bottom waters from basin to basin then a series of six potential density anomaly surfaces at progressively shallower depths from about 4000 m up to about 2000 m will be shown to see what character istics influence the deep waters and how pathways of flow are constrained by the bathymetry of the indian ocean the choice of the specific density levels table 1 was based primarily on various property extrema shown by the 18os atlantis ii cruise 93 colored sections that accompany warren s 1981 detailed discussion of the major features of the central indian ocean between madagascar and aus tralia the sections approximately midway between antarctica and the northern indian ocean boundaries are well positioned to show features originating from both the north and the south as well as some brought into the indi an ocean by the antarctic circumpolar current the full ocean area isopycnal maps provide essential evidence on the origins and fates of features seen in the sections and