diurnal shear instability the descent of the surface shear layer and the deep cycle of equatorial turbulence a new theory of shear instability in a turbulent environment is applied to eight days of velocity and density profiles from the upper equatorial pacific this period featured a regular diurnal cycle of surface forcing together with a clear response in upper ocean mixing during the day a layer of stable stratification and shear forms at the surface during late afternoon and evening this stratified shear layer descends leaving the nocturnal mixing layer above it using high resolution current measurements the detailed structure of the descending shear layer is seen for the first time linear stability analysis is conducted using a new method that accountsfortheeffectsofpreexistingturbulenceoninstabilitygrowth shearinstabilityfollowsadiurnalcycle linked to the afternoon descent of the surface shear layer this cycle is revealed only when the effect of turbulence is accounted for in the stability analysis the cycle of instability leads the diurnal mixing cycle typically by 2 3h consistent with the time needed for instabilities to grow and break late at night the resulting turbulence suppresses further instabilities lending an asymmetry to the mixing cycle that has not been noticed in previous measurements deep cycle mixing is triggered by instabilities formed as the descending shear layer merges with the marginally unstable shear of the equatorial undercurrent in the morning turbulence decays and the upper ocean restratifies wind accelerates the near surface flow to form a new unstable shear layer and the cycle begins again

wavenumber frequency spectra of inertia gravity and mixed rossby gravity waves in the equatorial pacific ocean abstractin the 1970s and 1980s there was considerable interest in near equatorial variability at periods of days to weeks associated with oceanic equatorial inertia gravity waves and mixed rossby gravity waves at that time the measurements available for studying these waves were much more limited than today most of the available observations were from scattered island tide gauges and a handful of short mooring records more than a decade of the extensive modern data record from the tropical atmosphere ocean tao triangle trans ocean buoy network triton mooring array in the pacific ocean is used to reexamine the internal wave climate in the equatorial pacific with a focus on interpretation of the zonal wavenumber frequency spectrum of surface dynamic height relative to 500 decibars at periods of 3 15 days and zonal wavelengths exceeding 30 of longitude to facilitate interpretation of the dynamic height spectrum and identification of equatorial wave modes the spectrum is decomposed into separate

modulation of equatorial turbulence by a tropical instability wave 1 strong modulation of turbulent mixing by a westward propagating tropical instability wave tiw was observed in the stratified shear layer between the equatorial undercurrent euc and the surface mixed layer during october and november 2008 at 0 n 140 w the unique deep diurnal cycle mixing in the stratified layer beneath the equatorial cold tongue was observed where nighttime turbulent mixing was a factor of 10 greater than during daytime the turbulent kinetic energy dissipation rate e was 10 6 w kg 1 and the turbulent heat flux was 500 w m 2 at least 5 10 times greater than observed previously in the central equatorial pacific turbulence mixing varied significantly during the four distinct phases of the meridional flow associated with the tiw observations during the northward to southward transition recorded the largest values of reduced shear squared the thickest nighttime surface mixed layer the deepest penetration of the deep cycle turbulence and the largest turbulent heat flux and largest integrated e in the deep cycle layer dcl during steady southward flow the depth of the bases of the nighttime surface mixed layer and of the dcl were the shallowest a 50 m thick layer of strong turbulence was observed immediately above the euc core during the northward to southward and steady southward phases here the average e exceeded 10 6 w kg 1 the eddy diffusivity exceeded 10 3 m2 s 1 and the turbulent heat flux was 500 w m 2 to parameterize mixing in the central equatorial pacific accurately numerical models must simulate the enhancement of mixing associated with tiws and also the variability of mixing in different tiw phases

petrological differentiation patterns and geomorphic distribution of ferricretes in central africa geomorphic distribution and petrological differentiation patterns of ferricretes widespread on landsurfaces were studied in the dembia zemio area southeastern central african republic four types of ferricretes are distributed on high plateaux hillslopes and low plateaux the main contrast corresponds to the differentiation between ferricretes of high plateaux rich in poorly hydrated minerals hematite and kaolinite and those covering hillslopes and low plateaux richer in hydrated minerals goethite and gibbsite a principal component analysis of geochemical and mineralogical data was used to characterize the main differentiation patterns of ferricretes in relation with their petrographic facies and the geomorphic features of the landscape the distribution of secondary minerals and their relationships to trace elements within and between the ferricrete types reflects differences in weathering and erosion processes our results document that the petrological patterns and the distribution of ferricretes are related to geomorphic features depending on hydroclimatic variations that govern the landscape evolution of many tropical shields

the northeast southwest oscillating equatorial mode of the tropical instability wave and its impact on equatorial mixing the tropical instability waves tiws in the eastern pacific consist of waves with central periods of about 33 and 17days while the former manifest as vortices north of the equator and are known to modulate diapycnal mixing at their southern edge the latter remain largely unexplored here the structure of the 17 day tiws and the mechanism through which they may influence equatorial mixing are investigated based on long term in situ measurements and reanalysis data different from the well recognized meridional velocity oscillation the 17 day tiws are found to induce northeast southwest ne sw velocity oscillations they are confined within but asymmetric about the equator differing from free yanai waves which they were commonly assumed to resemble the vertical shear associated with the westward anomalous velocity superimposes on the shear of the mean flow resulting in the strongest shear in the upper thermocline that is expected to facilitate diapycnal mixing therein plain language summary observed from satellites the tropical instability wave tiw in the eastern tropical pacific ocean is a 1 000 km long gigantic combination of waves and vortexes it emerges between the energetic zonally interleaving equatorial currents impacts the atmosphere and transfers enormous energy to the western pacific ocean and to the deep ocean as wellone of the research foci for both physical oceanographers and meteorologists however the structures of its huge body and the associated complicated smaller scale processes remain unrevealed primarily because the measurements that have been conducted usually by research vessels are too limited to derive a full picture here based on long term observations at a hotspot of tiw and 4 d numerical model outputs we identify the structure of tiw at the equator which manifests as a pair of slanted clockwise and anticlockwise vortexes and induces strong east west oscillations we also find the most efficient mechanism for the tiw to strengthen vertical velocity shear shear instability and ocean turbulent mixing the findings therefore are important for understanding the tiw dynamics tiws signatures in atmosphere ocean interactions and their impact on vertical heat transport from warmer sea surface to colder subsurface layers of the ocean

kimberlite u th he dating links surface erosion with lithospheric heating thinning and metasomatism in the southern african plateau documenting the surface response to processes at depth is a central challenge in continental tectonics kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts pipe erosion levels and mantle xenolith suites here we combine new apatite u th he ahe data from four south africa kimberlites ranging in emplacement age from ca 143 ma to ca 74 ma with a wealth of other geologic information from the pipes to resolve the timing patterns and causes of erosion across 200 km of the southern african plateau comparison of the ahe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca 117 90 ma which was most intense from ca 100 to 90 ma this unroofing phase is synchronous with warming thinning and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites these thermochemical changes could be related to still deeper dynamic mantle processes our ahe data appear to directly document the erosional response to surface uplift induced by these various events the ahe results also detect as much as 1 5 km of cenozoic unroofing that is spatially centered on a proposed mid tertiary paleotributary to the orange river suggesting drainage patterns as the cause our results not only tightly refine the surface history of the interior of the southern african plateau but show that kimberlite ahe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links

turbulent kinetic energy and its dissipation rate in the equatorial mixed layer abstract the authors apply a one dimensional mixed layer model based on second moment closure of turbulence to study the characteristics of turbulence in the equatorial mixed layer in particular they focus on characteristics such as the tke and the dissipation rate in the upper layers of the equatorial ocean especially the phenomenon of deep episodic penetration well below the mixed layer of elevated dissipation rates commonly observed in microstructure measurements in the central tropical pacific model simulations for the tropic heat experiments in 1984 and 1987 and tropical instability wave experiment in the central pacific as well as the tropical oceans global atmosphere coupled ocean atmosphere response experiment toga coare intensive operating period iop observations in the western pacific warm pool are performed modeled dissipation rates are found to be in reasonable agreement with those measured by microstructure profilers during these observations sensitivity studies are carried out to

area selection for diamond exploration using deep probing electromagnetic surveying abstract previously proposed methods of area selection for diamond prospective regions have predominantly relied on till geochemistry airborne geophysics and or an appraisal of tectonic setting herein we suggest that a novel deep probing geophysical technique electromagnetic studies using the natural source magnetotelluric mt method can contribute to such an activity essentially diamondiferous regions must have 1 old lithosphere 2 thick lithosphere and 3 lithosphere that contains high concentrations of carbon deep probing mt studies are able to address all three of these the second and the third of these can be accomplished independently using mt but for the first the geometries produced from modelling the mt observations must be interpreted with appropriate interaction with geologists geochemists and other geophysicists examples are given from the slave and superior cratons in north america with a brief mention of an area of the rae craton and general speculations about possible diamondiferous regions

mineralogy and geochemical dispersion in tropical residual soils overlying a talc deposit in southwestern nigeria abstract soil profiles overlying a talc deposit and associated amphibolite country rock have been investigated in order to determine the mineralogy and nature of geochemical dispersion as a potential tool of industrial mineral prospecting in tropical terraines results of mineralogical analysis show that in soils overlying the talc body talc still persists as a major component of the soil in association with quartz chlorite illite and goethite whereas in the amphibolite soil most primary minerals have decomposed to kaolinite quartz and goethite chemical variations in the soils are characterized by general enrichment in fe 2 o 3 and al 2 o 3 and depletion in na 2 o k 2 o cao mgo and feo relative to the parent rocks trace element abundances show inconsistent patterns while ni and co are depleted relative to the parent rocks zn and cu show considerable enhancement in spite of the intense weathering characteristics of tropical environments high mgo contents of the soils seem to be the only reliable geochemical indicator of the existence of talc deposits beneath the soil cover this parameter in conjunction with x ray diffraction analysis of the soils may constitute an invaluable exploration tool for detecting buried talc deposits

equatorial ocean response to rapidly translating wind bursts abstract the response of the ocean at low latitude to idealized westerly wind bursts can be described as a wave wake composed of equatorial gravity and rossby gravity modes the excited waves are those with phase speeds that match the zonal translation speed of a wind burst typically 10 m s 1 these modes sum to produce oscillations near the local inertial frequency at each latitude analogous to near inertial internal gravity waves generated by moving storms at midlatitude linear theory predicts that typical wind burst amplitudes stresses of 0 1 pa will generate substantial current oscillations o 1 m s 1 in the upper ocean response is initially confined to the region directly beneath a wind burst after which the wake descends and refracts equatorward as a propagating beam waves are of sufficient amplitude to dominate shear and vertical strain in the upper ocean phase differences between oscillations at neighboring latitudes induce motion in the meridional vertical plane at ever decreasing mer

tectonic setting of kimberlites kimberlites can be viewed as time capsules in a global plate tectonic framework their distribution illustrates clustering in time and space kimberlite ages span the assembly and break up of a number of supercontinents such as rodinia and gondwana these supercontinents show time lines with i broad periods devoid of kimberlite magmatism corresponding to times of continent stability and ii narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations this episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents the onset of kimberlite magmatism is closely associated with thermal perturbations thermal insulation mantle upwelling beneath a stagnant or sluggish supercontinent these perturbations may have caused uplift and the onset of continental break up through fracture zones propagating into the supercontinent subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths resultant strain is accommodated along trans lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration the corridors are manifest as discontinuities in the lithosphere mantle measured as geophysical gradients and as changes in mantle lithosphere composition within the crust these corridors are expressed as a terrane boundaries b incipient continental rifts c fracture zones or d major dyke swarms some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters the association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults such conditions may be attained during different stages in the evolution of continents a supercontinent formation b incipient rifting driven by far field stresses and onset of continental break up and c strain accommodation along the continental continuation of oceanic fracture zones during spreading type c may show concomitant kimberlite magmatism in separate continents after break up

diamond provenance studies from 40ar 39ar dating of clinopyroxene inclusions an example from the west coast of namibia abstract the west coast of namibia is host to substantive detrital diamond deposits located in onshore and offshore beach gravels desert deflation deposits and lower orange river terraces the origin of the namibian diamonds is controversial with some studies favouring derivation from distal cretaceous jurassic kimberlites on the kaapvaal craton and others arguing that most diamonds originated from proximal dwyka glacial deposits 300 ma which incorporated diamonds from older 500 ma pre karoo kimberlites previous studies have demonstrated that clinopyroxene inclusions extracted from their host diamonds give 40ar 39ar ages approaching the time of source kimberlite eruption this behaviour is attributed to diffusion of argon to lattice defect sites and the diamond inclusion interface region during mantle residence with subsequent loss of the latter component on cleaving of the diamond to release the inclusion s in this study we measured 40ar 39ar ages of extracted clinopyroxene inclusions from namibian detrital diamonds in order to determine potential kimberlite sources craton erosion histories and palaeo drainage evolution in southern africa 40ar 39ar step heating data were obtained for eclogitic and peridotitic clinopyroxene inclusions from 50 namibian diamonds low temperature steps produced older apparent ages than high temperature fusion steps consistent with partial retention of pre eruption argon in defect sites with one exception fusion steps yielded younger ages ranging from 62 30 ma to 1441 700 ma the majority 80 of inclusions have fusion ages based on the ages of known southern african kimberlites the current results suggest that very few

on the formation of a subsurface weakly sheared laminar layer and an upper thermocline strongly sheared turbulent layer in the eastern equatorial pacific interplays of multiple time scale equatorial waves

internal waves and mixing in the upper equatorial pacific ocean microstructure measurements in the equatorial pacific at 140 w in late 1984 show a pronounced diurnal variation in both high frequency internal wave energy and kinetic energy dissipation rate observations indicated that after sunset internal waves presumably generated by convective overturns in the mixed layer propagate downward and increase turbulence levels in the pycnocline it is proposed that large mixed layer eddies in the south equatorial current interact with the large shear caused by the equatorial undercurrent to generate a westward going anisotropic wave field the momentum transport in the radiated wave field results in a drag force on the equatorial mean flow field the observed mean wind stress at 140 w during tropic heat i which is twice as large as the annual mean wind is closer to the estimated radiation stress 10 4 m2 s 2 at the base of the mixed layer 30 m than to the estimated turbulent stress 10 5 m2 s 2 a wave dissipation model based on the observed turbulent kinetic energy dissipation rate is introduced in order to estimate the wave momentum flux divergence in the stratified region above the undercurrent core the model predicts that most of the downward wave momentum flux penetrates through the undercurrent core it is hypothesized that when the wind stress is strong the equatorial pacific ocean responds by generating a westward traveling internal wave field which transports much of the surface wind stress below the actively mixing surface layer

internal gravity waves in the upper eastern equatorial pacific observations and numerical solutions on the basis of data from a towed thermistor chain collected near 140 w on the equator during april 1987 the zonal wavenumber and vertical structure of internal gravity waves were observed to vary significantly between wave events our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal velocity and density assuming that the observed waves were the fastest growing modes for shear instability we solve the taylor goldstein equation using different analytical basic states including a zonal and meridional flow to simulate the different conditions during 4 nights of intense internal wave activity we find that while the observed waves are of finite amplitude linear shear instability is sufficient to explain the wavelength and vertical structure of vertical displacement for most of the waves the fastest growing unstable mode one solutions have e folding growth times of less than 10 min these solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m near the base of the mixed layer at a flow inflection point the kinetic energy of the mean flow is extracted for wave growth wave vertical displacement is maximum near this inflection point zonal phase speeds range from 0 8 to 0 1 m s the propagation direction of waves with growth rates of 75 of the maximum growth rate can range from about 45 north to 45 south of the zonal direction the vertical wave induced reynolds stress divergence could explain a discrepancy in zonal momentum budgets of the upper 90 m of this region estimates of this stress divergence show that only about 100 days of intense internal wave activity is needed per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50 to 100 m depth

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

cretaceous mantle of the congo craton evidence from mineral and fluid inclusions in kasai alluvial diamonds abstract alluvial diamonds from the kasai river democratic republic of the congo drc are sourced from cretaceous kimberlites of the lucapa graben in angola analysis of 40 inclusion bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the congo craton silicate inclusions permitted us to classify diamonds as peridotitic containing fo 91 95 and en 92 94 23 diamonds 70 of the suite and eclogitic containing cr poor pyrope and omphacite with 11 27 jadeite 6 diamonds 18 of the suite fluid inclusion compositions of fibrous diamonds are moderately to highly silicic matching compositions of diamond forming fluids from other drc diamonds regional homogeneity of congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of cretaceous diamond forming fluids within the congo lithospheric mantle in situ cathodoluminescence secondary ion mass spectrometry and fourier transform infrared spectroscopy reveal large heterogeneities in n n aggregation into b centers n b and 13 c indicating that diamonds grew episodically from fluids of distinct sources peridotitic diamonds contain up to 2962 ppm n show 0 88 n b and have 13 c isotopic compositions from 12 5 to 1 9 with a mode near mantle like values eclogitic diamonds contain 14 1432 ppm n n b spanning 29 68 and wider and lighter 13 c isotopic compositions of 17 8 to 3 4 fibrous diamonds on average contain more n up to 2976 ppm and are restricted in 13 c from 4 1 to 9 4 clinopyroxene garnet thermobarometry suggests diamond formation at 1350 1375 c at 5 8 to 6 3 gpa whereas n aggregation thermometry yields diamond residence temperatures between 1000 and 1280 c if the assumed mantle residence time is 0 9 3 3 ga integrated geothermobaromtery indicates heat fluxes of 41 44 mw m 2 during diamond formation and a lithosphere asthenosphere boundary lab at 190 210 km the hotter than average cratonic mantle may be attributable to contemporaneous rifting of the southern atlantic multiple post archean reactivations of the craton and or proximal cretaceous plumes

rossby gravity waves in the central equatorial pacific ocean during the norpax hawaii to tahiti shuttle experiment spectra of sea level and current taken from the central pacific during the norpax experiment contain broad peaks centered around 10 day periods zonal dispersion shows these peaks to be caused by rossby gravity waves an empirical orthogonal function eof analysis indicates that 44 of the variance can be attributed to these rossby gravity waves and they show clear temporal modulation in amplitude there is no evidence that the rossby gravity waves are generated locally but coherence between the eof and a modeled response to winds from kapingamarangi island allowing for dispersion suggests that these waves are generated by meridional wind stress in the west pacific

subduction history reveals cretaceous slab superflux as a possible cause for the mid cretaceous plume pulse and superswell events abstract subduction is a fundamental mechanism of material exchange between the planetary interior and the surface despite its significance our current understanding of fluctuating subducting plate area and slab volume flux has been limited to a range of proxy estimates here we present a new detailed quantification of subduction zone parameters from the late triassic to present day 230 0 ma we use a community plate motion model with evolving plate topologies to extract trench normal convergence rates through time to compute subducting plate areas and we use seafloor paleo age grids to estimate the thickness of subducting lithosphere to derive the slab flux through time our results imply that slab flux doubled to values greater than 500 km3 yr from 180 ma in the jurassic to 130 ma in the mid cretaceous subsequently halving again towards the cretaceous paleogene boundary largely driven by subduction zones rimming the pacific ocean basin the 130 ma spike can be attributed to a two fold increase in mid ocean ridge lengths following the break up of pangea and a coincident increase in convergence rates with average speeds exceeding 10 cm yr with one third of the total 230 0 ma subducted volume entering the mantle during this short 50 myr period we suggest this slab superflux drove a surge in slab penetration into the lower mantle and an associated increase in the vigour of mantle return flow this mid cretaceous event may have triggered or at least contributed to the formation of the darwin rise mantle superswell dynamic uplift of the south african plateau and the plume pulse that produced the ontong java hikurangi manihiki and kerguelen plateaus among others the models presented here contribute to an improved understanding of the time evolving flux of material consumed by subduction and suggest that slab superflux may be a general feature of continental dispersal following supercontinent breakup these insights may be useful for better understanding how supercontinent cycles are related to transient episodes of large igneous province and superswell formation and the associated deep cycling of minerals and volatiles as well as leading to a better understanding of tectonic drivers of long term climate and icehouse to greenhouse transitions

turbulence and internal waves at the equator part i statistics from towed thermistors and a microstructure profiler abstract high correlations between turbulent dissipation rates and high wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the equatorial undercurrent an extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high wavenumber internal waves in the low richardson number ri thermocline below the mixed layer but above the core of the equatorial undercurrent and when winds were moderate and steadily westward it was found that the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band corresponding to 150 250 m zonal wavelength of internal waves both turbulence and internal waves varied diurnally hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

buoyant gravity currents released from tropical instability waves abstracttwo extremely sharp fronts with changes in sea surface temperature 0 4 c over lateral distances of 1 m were observed in the equatorial pacific at 0 140 w and at 0 75 n 110 w in both c

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

distribution of kimberlite and aillikite in the diamond province of southern west greenland a regional perspective based on groundmass mineral chemistry and bulk compositions a regional study of dike and sill occurrences of kimberlitic rocks in terms of xenolith and megacryst assemblages from the diamond province in southern west greenland demonstrates very systematic variation in the composition of melts across the margin of the archaean core of the north atlantic craton melt derived from the diamond window in the lithospheric mantle below the archean core in greenland is kimberlitic toward the palaeoproterozoic nagssugtoqidian orogen melts also derived from the diamond window gradually change to ultramafic lamprophyre compositions evolving toward an end member akin to the aillikites of the penecontemporaneous torngat region in canada the identification of the types of melts is based on the groundmass paragenesis including the presence or absence of clinopyroxene and chromite and the compositions of groundmass ilmenite spinel and phlogopite the systematic regional variation is also shown although somewhat blurred by the bulk rock major and trace element compositions the change in melt composition is not a reflection of change in depth of initial melting but relates to the composition of the deep sclm kimberlite melts in the archaean craton are suggested to be reaction products between a carbonate rich astenospheric melt and an orthopyroxene bearing and phlogopite poor depleted sclm whereas the aillikite type melts across the craton margin are the products of reaction between a similar carbonate rich asthenospheric melts and a subducted sclm that had previously been subjected to metasomatism and veining

sea surface cooling at the equator by subsurface mixing in tropical instability waves changes in the sea surface temperature of equatorial waters have critical effects on the large scale atmospheric circulation shipboard measurements of turbulence kinetic energy dissipation rate indicate that seasonal surface cooling in the central equatorial pacific may be largely caused by mixing induced by tropical instability waves

patterns and controls on the distribution of diamondiferous intrusions in australia abstract the distribution of kimberlite lamproite and related alkaline volcanism in australia can be broadly related to the structure of the australian continent and lithosphere diamondiferous kimberlites and lamproites with the apparent exception of the weakly diamondiferous orrorro kimberlites in the adelaide fold belt lie within the large precambrian shield where seismic tomographic models and heat flow data indicate the presence of relatively cold high seismic wave speed lithosphere tectosphere typically some 200 km thick or more beneath the archaean cratons and up to 300 km in parts of central australia many of the diamondiferous intrusions appear to lie at the margins rather than in the centre of the lithosphere domains the highest concentration of diamondiferous intrusions kimberlites and lamproites is on and around the kimberley craton where seismic data indicate crustal thicknesses of 35 40 km and a lithosphere up to 275 km thick that is distinct from proterozoic northern australia many but clearly not all of the intrusions show evidence of regional and local structural controls some are spatially associated with known crustal structures especially regional faults others are aligned either singly or in clusters along or near discontinuities and or gradients evident in regional scale potential field data especially the total horizontal gradients of gravity data continued upward tens to hundreds of kilometres many of these features are not evident in the original datasets as their signatures are masked by shorter wavelength near surface anomalies in some cases the kimberlites and associated rocks lie within crustal blocks and domains defined by discontinuities in the potential field data rather than at domain boundaries our overview suggests that analysis of potential field data especially horizontal gradients in upwardly continued potential field data at all scales can assist definition of crustal and potentially lithospheric structures that may influence the distribution of diamond pipes however more definitive mapping of australia s diamond prospective regions requires the integration of data on crustal structures especially trans lithospheric faults and geodynamic settings with high resolution tomographic models and other geophysical petrologic and isotopic information on the nature of the lithosphere beneath the australian continent

bayesian noise reduction in arabia somalia and nubia arabia finite rotations since 20 ma implications for nubia somalia relative motion knowledge of nubia somalia relative motion since the early neogene is of particular importance in the earth sciences because it i impacts on inferences on african dynamic topography and ii allows us to link plate kinematics within the indian realm with those within the atlantic basin the contemporary nubia somalia motion is well known from geodetic observations precise estimates of the past 3 2 myr average motion are also available from paleo magnetic observations however little is known of the nubia somalia motion prior to 3 2 ma chiefly because the southwest indian ridge spread slowly posing a challenge to precisely identify magnetic lineations this also makes the few observations available particularly prone to noise here we reconstruct nubia somalia relative motions since 20 ma from the alternative plate circuit nubia arabia somalia we resort to trans dimensional hierarchical bayesian inference which has proved effective in reducing finite rotation noise to unravel the arabia somalia and arabia nubia motions we combine the resulting kinematics to reconstruct the nubia somalia relative motion since 20 ma we verify the validity of the approach by comparing our reconstruction with the available record for the past 3 2 myr obtained through antarctica results indicate that prior to 11 ma the total motion between nubia and somalia was faster than today furthermore it featured a significant strike slip component along the nubia somalia boundary it is only since 11 ma that nubia diverges away from somalia at slower rates comparable to the present day one kinematic changes of some 20 might have occurred in the period leading to the present day but plate motion steadiness is also warranted within the uncertainties

emplacement of ultramafic carbonatite intrusions along reactivated north american mid continent rift structures abstract understanding the various emplacement pathways of ultramafic carbonatite intrusive complexes and mapping of their global distribution are in an early stage discerning the pattern of their emplacement can reveal information about earth s dynamics and guide exploration for associated economic deposits here we apply multiple spatial analysis methods and synthetic aperture radar sar data to assess the correlation between outcrops of ultramafic carbonatite intrusives and regional craton scale fractures and assign targets for further exploration kernel density cluster two point azimuth and the fry method two dimension spatial analyses are applied to 58 known outcrops of intrusives in the avon volcanic district avd southeast missouri usa the results of the spatial analysis show linear trends among the outcrops with three primary orientations northwest northeast and east west and a strong correlation with regional fault zones sar data processing and analysis guided by the spatial results reveals rhombic lineament patterns with sides that parallel the northwest northeast emplacement patterns of the avd these rhombic structures are related to possible extensional stepover duplexes likely formed during precambrian north american plate rifting along transfer faults reactivation of the precambrian structures during high angle dip slip and wrench faulting that was penecontemporaneous with emplacement of the avd in the devonian likely provided pathways for the mantle derived intrusives these findings can guide exploration efforts and help us understand the emplacement of a unique intracontinental intrusive complex

reconstruction of the southwestern african continental margin by backward modeling abstract as the evolution of the southwestern african passive continental margin and in particular the vertical movements in this area are still a matter of debate this study aims to quantify the past vertical movements by reconstructing the paleobathymetric evolution of the southwestern african margin during the post rift phase to quantify the vertical movements inferred from paleobathymetries different subsidence components that have affected the passive continental margin are calculated and interpreted with respect to the generating processes our investigation takes advantage of information on the present day sedimentary thicknesses and the crustal configuration along the sw african margin by using this detailed information and modeling backward in time we attempt to separate different subsidence components such as the load induced subsidence and the thermal subsidence for different time intervals during the post rift phase calculating the amounts of each subsidence component and subtracting them from the present day configuration yields paleobathymetries which are interpreted with respect to the evolution of the southwestern african passive margin these restored paleobathymetries for different time steps during the post rift phase indicate spatial and temporal variations of vertical movements including phases of seafloor uplift as a result our investigation allows to give minimum estimates of the vertical movements that have occurred during the post rift phase in addition to predominant subsidence about 1000 m of seafloor uplift are deduced for the northern and southern parts of the sw african margin quantifying the amounts of vertical movements cannot finally clarify the generating processes behind these movements but the timing and magnitude of seafloor uplift indicate a deep mantle mechanism

distinct 17 and 33 day tropical instability waves in subsurface observations abstract tropical instability waves tiws within a half degree of the equator in the pacific ocean have been consistently observed in meridional velocity with periods of around 20 days on the other hand near 5 n tiws have been observed in sea surface height ssh thermocline depth and velocity to have periods near 30 days tropical atmosphere ocean tao project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of tiws during 3 years of la nina conditions from 1998 through 2001 along 140 w where the tiw temperature and velocity variabilities are at their maxima these variabilities include two distinct tiws with periods of 17 and 33 days rather than one broadbanded process as predicted by modeling studies the 17 day tiw variability is shown to occur not only in meridional velocity at the equator but also in subsurface temperature at 2 n and 2 s while the 33 day tiw variability is observed primarily in subsurface temperature at 5

turbulence variability at the equator in the central pacific at the beginning of the 1991 1993 el nino a 38 day 5990 cast microstructure study on the equator performed during the onset of the 1991 1993 el nino shows the effect on small scale activity at 140 w of an equatorial kelvin wave by using two ships data were taken continuously from november 4 to december 12 1991 near the national oceanic and atmospheric administration pacific marine environmental laboratory mooring at o n 140 w the ships occupied the station sequentially with a 3 5 day overlap for intercalibration variability in currents was observed on tidal periods and periods of 4 days presumably equatorially trapped internal gravity waves 8 days cause unknown 20 days tropical instability waves and longer kelvin waves variation in water structure occurred most prominently on the timescale of kelvin waves the diurnal cycle typical of that location was observed nocturnal deepening of the surface mixed layer was accompanied by a deep cycle bursts of turbulence penetrating into the stratified region below the nighttime mixed layer during the observational period one kelvin wave trough and one crest passed through the site changes accompanying the phase change in the kelvin wave included a reversal of the near surface current a deepening of the thermocline and a change of water mass changes in small scale activity included a tenfold decrease of the thermal dissipation rate and a fourfold decrease of the rate of heat transport downward from the mixed layer the nighttime mixed layer deepened from 30 to 60 m the thickness of the stratified region in which nocturnal turbulence bursts occurred the deep cycle region thinned from 40 to 20 m because it was confined between the bottom of the nighttime mixed layer and the low shear region near the core of the undercurrent the decrease in downward heat flux observed at this passage of the downwelling kelvin wave front could explain the rapid sea surface temperature sst increase seen at el nino onsets the magnitude of the change in vertical flux is similar to the magnitude of the change in horizontal advection this process would produce a warmer sst much more quickly than could the advection of warm waters eastward

rapid south atlantic spreading changes and coeval vertical motion in surrounding continents evidence for temporal changes of pressure driven upper mantle flow the south atlantic region displays 1 a topographic gradient across the basin with africa elevated relative to south america 2 a bimodal spreading history with fast spreading rates in late cretaceous and eo oligocene and 3 episodic regional uplift events in the adjacent continents concentrated in late cretaceous and oligocene here we show that these observations can be linked by dynamic processes within earth s mantle through temporal changes in asthenosphere flow beneath the region the topographic gradient implies westward pressure driven mantle flow beneath the basin while the rapid spreading rate changes on order 10 million years require significant decoupling of regional plate motion from the large scale mantle buoyancy distribution through a mechanically weak asthenosphere andean topographic growth in late miocene can explain the most recent south atlantic spreading velocity reduction arising from increased plate boundary forcing associated with the newly elevated topography but this mechanism is unlikely to explain the late cretaceous tertiary spreading variations as changes in andean paleoelevation at the time are small we propose an unsteady pressure driven flow component in the asthenosphere beneath the south atlantic region to explain the late cretaceous tertiary spreading rate variations temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions we find that this prediction from our models agrees with geologic and geophysical observations of the south atlantic region including episodes of passive margin uplift regional basin reactivation and magmatic activity

post break up tectonic inversion across the southwestern cape of south africa new insights from apatite and zircon fission track thermochronometry the south west african margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning recent attention focused on this margin has included theoretical modelling studies of rift processes plate kinematic studies of the opening geometry and timing and empirical studies focused on documenting the crustal structure and offshore sedimentary record here we examine the onshore geomorphic and tectonic response to rifting and breakup with a specific focus on the sw cape of south africa we present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of south africa the data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the early cretaceous 150 130 ma and late cretaceous 100 80 ma respectively both periods of enhanced erosion are regional in extent involved km scale erosion and extend well inland of the current escarpment zone albeit with spatially variable intensity and style the late cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km scale differential displacement and erosion the new aft data do not exclude the possibility of modest surface uplift occurring during the cenozoic but they restrict the depth of regional cenozoic erosion on the western margin to less than c 1 km the inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore orange and bredasdorp basins it is suggested that the late cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the sw cape by local tectonic forces arising from dextral displacement of the falkland plateau along the falkland agulhas fracture zone

dating kimberlite emplacement with zircon and perovskite u th he geochronology kimberlites provide rich information about the composition and evolution of cratonic lithosphere accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution but kimberlites can sometimes be difficult to date with available methods we explored whether u th he dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages we obtained zircon and or perovskite u th he zhe phe dates from sixteen southern african kimberlites most samples with abundant zircon yielded reproducible zhe dates 15 dispersion that are in good agreement with published eruption ages the majority of dated zircon were xenocrystic zircons with reproducible dates were fully reset during eruption or resided at temperatures above the zhe closure temperature prior to entrainment in the kimberlite magma not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and or partial he loss during the eruptive process all seven kimberlites dated with phe yielded reproducible 15 dispersion and reasonable results we conducted two preliminary perovskite 4he diffusion experiments which suggest a phe closure temperature of 300 c perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that phe dates will typically record emplacement rather than post emplacement processes zhe and phe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques this article is protected by copyright all rights reserved

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

in situ upb age determination and srnd isotopic analysis of perovskite from the premier cullinan kimberlite south africa abstract south africa is characterized by multiple episodes of kimberlite intrusion however the age and sr nd isotopic compositions of the primary magma of most of the earlier emplacement events remain unresolved using in situ secondary ion mass spectrometry and laser ablation multi collector inductively coupled plasma mass spectrometry the u pb ages and sr nd isotopic compositions were obtained for perovskite from the premier cullinan kimberlite one of the oldest diamondiferous kimberlites the u pb perovskite ages are 1150 16 1151 9 and 1156 12 ma for the brown black and green kimberlites respectively demonstrating that the different phases of the kimberlite were emplaced contemporaneously at 1150 ma in situ sr nd isotopic analyses of perovskite yield initial sr isotopic ratios and e nd t 1150 values of 0 7031 4 and 1 9 0 9 for the brown 0 7025 1 and 1 8 0 4 for the black and 0 7030 2 and 0 8 0 7 for the green kimberlites these are essentially identical within analytical uncertainties and suggest that the kimberlites were derived from a moderately depleted mantle source in contrast whole rock data yield variable sr nd isotopic compositions confirming that whole rock isotopic data cannot be used to constrain the character of the source of kimberlitic magma due to crustal contamination later alteration and or weathering our data also indicate that kimberlite emplacement might be considered as a prelude to mantle plume activity beneath a craton

synchroneity of cratonic burial phases and gaps in the kimberlite record episodic magmatism or preservational bias abstract a variety of models are used to explain an apparent episodicity in kimberlite emplacement implicit in these models is the assumption that the preserved kimberlite record is largely complete however some cratons now mostly devoid of phanerozoic cover underwent substantial phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the slave craton superior craton and cratonic western australia a similar pattern exists in the kaapvaal craton although its magmatic deposition and erosion history differs in key ways from the other localities one explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts even if kimberlites occurred during cratonic burial phases the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced in this model kimberlite magmatism was more continuous than the preserved record suggests implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects either way the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns

factors that affect the amplitude of el nino in global coupled climate models historically el nino like events simulated in global coupled climate models have had reduced amplitude compared to observations here el nino like phenomena are compared in ten sensitivity experiments using two recent global coupled models these models have various combinations of horizontal and vertical ocean resolution ocean physics and atmospheric model resolution it is demonstrated that the lower the value of the ocean background vertical diffusivity the greater the amplitude of el nino variability which is related primarily to a sharper equatorial thermocline among models with low background vertical diffusivity stronger equatorial zonal wind stress is associated with relatively higher amplitude el nino variability along with more realistic east west sea surface temperature sst gradient along the equator the sst seasonal cycle in the eastern tropical pacific has too much of a semiannual component with a double intertropical convergence zone itcz in all experiments and thus does not affect nor is it affected by the amplitude of el nino variability systematic errors affecting the spatial variability of el nino in the experiments are characterized by the eastern equatorial pacific cold tongue regime extending too far westward into the warm pool the time scales of interannual variability as represented by time series of nino3 ssts show significant power in the 3 4 year enso band and 2 2 5 year tropospheric biennial oscillation tbo band in the model experiments the tbo periods in the models agree well with the observations while the enso periods are near the short end of the range of 3 6 years observed during the period 1950 94 the close association between interannual variability of equatorial eastern pacific ssts and large scale sst patterns is represented by significant correlations between nino3 time series and the pc time series of the first eofs of near global ssts in the models and observations

mineralogy and geochemical dispersion in lateritic soil profiles of northern nigeria abstract six lateritic soil profiles overlying metasedimentary and crystalline basement rocks of northern nigeria have been investigated in order to study the mineralogical and geochemical relationships between bedrock and weathering products a total of 96 samples have been collected including 26 from exposed quarry and road cut faces and 70 from auger holes to a depth of 8 m two selective leaching methods cold extraction with 0 1 m hci and hot extraction with aqua regia have been applied to the m fraction and up to sixteen elements determined by aas conclusions are based largely on the auger hole samples where element distributions have not been modified by subaerial exposure quartz is the dominant mineral in the upper part of the soil profile while kaolinite increases strongly with depth at the base of some profiles mica is also abundant elements are distributed through the soil profile according to their association with clays iron oxides and to a lesser extent carbonates while intercalations of lateritic crusts obscure general trends with depth dispersion and accumulation processes appear to be similar within each profile in spite of the intensive weathering and secondary accumulation processes characteristic of the savanna belt the lateritic soils within surface sampling depth appear to retain a geochemical imprint of the underlying bedrock criteria and potential problems in the application of soil geochemistry to bedrock mapping are discussed

mapping the mantle lithosphere for diamond potential using teleseismic methods abstract recent developments in seismic magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres to map its structures and through geological interpretations to assess its potential as a diamond reservoir several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns for example beneath the central slave craton seismic discontinuities at depths of 38 110 140 and 190 km appear to bound two distinct anisotropic layers tomographic 3 d inversions of seismic wave travel times and 2 d inversions of surface or scattered waves use arrays of stations and provide lateral coverage in combination and by correlation with electrical conductivity and xenolith petrology studies these techniques provide maps of key physical properties within parts of the cratons known to host diamonds beneath the slave craton the discontinuity at 38 km is the base of the crust the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite to date only some of these techniques have been applied to the slave and kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not as yet generally agreed

profiles of elemental concentrations in precambrian paleosols on basaltic and granitic parent materials abstract profiles of elemental concentrations are interpreted for four precambrian paleosols two developed on basalt and two on granodiorite all four paleosols appear to be the erosional remnants of originally thick soil saprolite regoliths the granitic paleosols are in south africa where they underlie the 2 9 3 0 ga pongola supergroup and the 2 8 2 9 ga dominion reef conglomerate one of the basaltic paleosols also occurs in south africa where it caps the ventersdorp basalt and underlies the 2 3 ga black reef quartzite the other basaltic paleosol underlies sandstone of the 2 7 ga timiskaming group in the abitibi belt of the canadian shield all four paleosols exhibit pronounced upward loss of sodium but no consistent loss of the heavier alkali elements rubidium and cesium iron decreases upward above iron rich basaltic parent rocks but there is no consistent loss of iron above iron poor granitic parents the rare earth elements are less fractionated than during intensive modern weathering uranium locally has been fractionated from thorium possibly due to oxidative dissolution during precambrian weathering

recurrent magmatic activity on a lithosphere scale structure crystallization and deformation in kimberlitic zircons kimberlites are not only the most economically important source of diamonds they also carry unique information encapsulated in rock fragments entrained as the magma traverses the whole thickness of the lithosphere the nurbinskaya pipe in the siberian kimberlite province russia is one of several intruded along the vilyui rift a major terrane boundary the pipe contains three populations of mantle derived zircon xenocrysts archean mean age 2709 9 ma devonian mean age 371 2 3 ma and a subset of grains with evidence of brittle deformation and rehealing and a range of ages between 370 and 450 ma the hf isotope o isotope and trace element signatures of the last group provide a link between the archean and devonian events indicating at least three episodes of magmatic activity and zircon crystallization in the lithosphere beneath the pipe the emplacement of the nurbinskaya pipe ca 370 ma ago was only the youngest activity in a magma plumbing system that has been periodically reactivated over at least 2 7 billion years controlled by the lithosphere scale structure of the vilyui rift

asthenospheric kimberlites volatile contents and bulk compositions at 7 gpa abstract during ascent kimberlites react with the lithospheric mantle entrain and assimilate xenolithic material loose volatiles and suffer from syn and post magmatic alteration consequently kimberlite rocks deviate heavily from their primary melt experiments at 7 gpa 1300 1480 c 10 30 wt co 2 and 0 46 wt h 2 o on a proposed primitive composition from the jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300 1350 c for a carbonatitic melt with 2 and 35 wt co 2 at asthenospheric temperatures of 1400 c where the jericho melt stays kimberlitic this composition saturates only in low ca pyroxene garnet and partly olivine we hence forced the primitive jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite saturation in olivine low and high ca pyroxene and garnet was obtained at 1400 1650 c 7 gpa melts are kimberlitic with 18 29 wt sio 2 al 2 o 3 22 1 24 6 wt mgo 15 27 wt co 2 and 0 4 7 1 wt h 2 o with a trade off of h 2 o vs co 2 and temperature melts in equilibrium with high ca pyroxene with typical mantle compositions have 2 5 wt na 2 o much higher than the commonly proposed 0 1 0 2 wt the experiments allow for a model of kimberlite origin in the convective upper mantle which only requires mantle upwelling that causes melting at the depth where elemental carbon in metal diamond or carbide converts to co 2 at 250 km if primary melts leading to kimberlites contain a few wt h 2 o then adiabatic temperatures of 1400 1500 c would yield asthenospheric mantle melts that are kimberlitic 18 wt sio 2 al 2 o 3 but not carbonatitic 2 al 2 o 3 in composition carbonatites only forming 100 200 c below the adiabat these kimberlites represent small melt fractions concentrating co 2 and h 2 o and then acquire part of their chemical signature by assimilation fractionation during ascent in the subcratonic lithosphere

geophysical signatures of some recently discovered large 40 ha kimberlite pipes on the alto cuilo concession in northeastern angola abstract this paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the alto cuilo concession in the diamond fields of northeastern angola in the years 2005 to 2008 several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory delineation and bulk sample drilling in order to rapidly identify and evaluate the kimberlite bodies the kimberlites were emplaced through karoo supergroup sandstones and shales have eruption ages contemporaneous with the sand dominated cretaceous age calonda formation and are covered by sand dominated poorly consolidated sediments of the kalahari group given that sand dominated non kimberlite lithologies are magnetically transparent a low level high resolution helicopter borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets even for low amplitude anomalies e g 1 2 nt the helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area most kimberlites take the form of well preserved crater edifices containing a full range of crater related kimberlite lithologies ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow up investigation geophysical responses were ground truthed against magnetic susceptibility and density measurements which were routinely collected on all drill cores the geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials which demonstrate characteristically variable lithologies geophysical interpretations guided the drill targeting at all stages of the program at alto cuilo from exploration to evaluation combined with geochemical and mineralogical prioritization techniques the geophysical signatures in magnetic gravity and electromagnetic data provide a sound basis to guide exploratory delineation and mini bulk sample drilling of kimberlites buried under 10 70 m of overburden the significant variability in the geophysical responses from kimberlites that are similar in size structure and geometry highlights the importance of applying all three independent geophysical methods in order to effectively achieve kimberlite exploration and evaluation goals the effectiveness of a multidisciplinary approach to kimberlite evaluation is demonstrated in the rapid assessment of a cluster of large kimberlites discovered at project alto cuilo

perovskite and baddeleyite from kimberlitic intrusions in the tarim large igneous province signal the onset of an end carboniferous mantle plume several tens of kimberlitic pipes and dykes are exposed in the wajilitag area in the western tarim large igneous province here we report for the first time secondary ion mass spectrometric u pb age data on perovskite and baddeleyite grains in a kimberlitic pipe and a kimberlitic dyke from the tarim craton the perovskite yielded a well defined intercept age of 299 87 4 3 ma which is consistent with its corresponding concordia and 206 pb 238 u ages corrected for the common pb contribution of about 300 ma the baddeleyite separated from two kimberlitic samples from a dyke display identical concordia u pb ages of 300 87 4 7 ma and 300 57 4 4 ma our age data show that the kimberlitic intrusions were emplaced at ca 300 ma rather than in the late permian as previously regarded these new ages are slightly older than the eruption ages of tarim flood basalts 291 273 ma offering a critical regional time marker for the onset of permo carboniferous magmatism in the tarim craton detailed petrographic observations did not reveal any ultrahigh pressure mineral assemblage in the wajilitag kimberlitic intrusions phlogopites from these intrusions show end t values of 3 7 to 4 2 the baddeleyites which are texturally primary and therefore inferred to have crystallized directly from the kimberlitic magma yield a range of ehf t from 4 8 to 8 7 these results combined with previously reported geochemical data suggest that the wajilitag kimberlitic intrusions were most likely derived from a moderately refractory and depleted subcontinental lithosphere mantle metasomatized by subduction components associated with an early middle paleozoic convergent regime the kimberlitic magma was generated by small degree partial melting of the lithospheric mantle in response to the impingement of the tarim mantle plume thus our new geochronological data suggest the arrival of the mantle plume beneath the tarim lithosphere at least 10 million years before the onset of tarim flood basalt volcanism the end carboniferous wajilitag kimberlitic intrusions the oldest known phase associated with carboniferous magmatism in the tarim craton signals the initial magmatic pulse triggered by mantle plume impingement

carbonatite associated with ultramafic diatremes in the avon volcanic district missouri usa field petrographic and geochemical constraints abstract here we report field petrographic and geochemical analyses of the southeast missouri avon volcanic district intrusive rocks and present the first combined textural and geochemical evidence for the presence of a primary magmatic carbonatite phase among ultramafic dikes pipes and diatremes of olivine melilitite alnoite and calciocarbonatite the 13 c vpdb values measured for primary calciocarbonatite as well as carbonates in olivine melilitite and alnoite rocks range from 3 8 to 8 2 which are within the typical range of mantle values and are distinct from values of the carbonate country rocks 0 0 to 1 3 the carbonate oxygen isotope compositions for the intrusive lithologies are in the range of 21 5 to 26 2 vsmow consistent with post emplacement low temperature hydrothermal alteration or kinetic fractionation effects associated with decompression and devolatilization metasomatized country rock and breccia contaminated igneous lithologies have carbonate 13 c vpdb values gradational between primary carbonatite values and country rock values unaltered sedimentary dolomite breccia and mafic spheroids entrained by calciocarbonatite and the lack of microstratigraphic crystal growth typical of carbonate replacement also exclude the possibility of hydrothermal replacement as the cause of the magmatic textured carbonates rare earth element ree patterns for the alnoite olivine melilitite and carbonatite are similar to each other with strong light ree enrichment and heavy ree depletion relative to morb these patterns are distinct from those of country rock rhyolite and sedimentary carbonate these data suggest that rocks of the avon volcanic district represent a single ultramafic carbonatite intrusive complex possibly derived from a single mantle source

diamonds exploration mines and marketing abstract the beauty value and mystique of exceptional quality diamonds such as the 603 carat lesotho promise recovered from the letseng mine in 2006 help to drive a multi billion dollar diamond exploration mining and marketing industry that operates in some 45 countries across the globe five countries botswana russia canada south africa and angola account for 83 by value and 65 by weight of annual diamond production which is mainly produced by four major companies de beers alrosa rio tinto and bhp billiton bhpb which together account for 78 by value and 72 by weight of annual diamond production for 2007 during the last twelve years 16 new diamond mines commenced production and 4 re opened in addition 11 projects are in advanced evaluation and may begin operations within the next five years exploration for diamondiferous kimberlites was still energetic up to the last quarter of 2008 with most work carried out in canada angola democratic republic of the congo drc and botswana many kimberlites were discovered but no new economic deposits were outlined as a result of this work except for the discovery and possible development of the bunder project by rio tinto in india exploration methods have benefitted greatly from improved techniques of high resolution geophysical aerial surveying new research into the geochemistry of indicator minerals and further insights into the formation of diamonds and the relation to tectonic structural events in the crust and mantle recent trends in diamond marketing indicate that prices for rough diamonds and polished goods were still rising up to the last quarter of 2008 and subsequently abruptly sank in line with the worldwide financial crisis most analysts predict that prices will rise again in the long term as the gap between supply and demand will widen because no new economic diamond discoveries have been made recently the disparity between high rough and polished prices and low share prices of publicly traded diamond companies may be due to investors losing patience with the slow pace or absence of new promising discoveries and switching into shares of base metals and fertilizers for agriculture potash and phosphates

a late cretaceous ca 90 ma kimberlite event in southern india implication for sub continental lithospheric mantle evolution and diamond exploration abstract we report groundmass perovskite u pb sims ages perovskite nd isotopic la icpms composition and bulk rock geochemical data of the timmasamudram diamondiferous kimberlite cluster wajrakarur kimberlite field in the eastern dharwar craton of southern india the kimberlite pipes gave similar mesoproterozoic ages of 1086 19 ma tk 1 microcrystic variant and 1119 12 ma tk 3 however a perovskite population sampled from the macrocrystic variant of tk 1 gave a much younger late cretaceous age of ca 90 ma this macrocrystic kimberlite phase intrudes the mesoproterozoic microcrystic phase and has a distinct bulk rock geochemistry the nd isotope composition of the 1100 ma perovskites in the cluster show depleted end t values of 2 1 0 6 to 6 7 0 3 whereas the 90 ma perovskites have enriched end t values of 6 3 1 3 the depleted mantle dm model age of the cretaceous perovskites is 1 2 ga whereas the dm model age of the proterozoic perovskites is 1 2 to 1 5 ga bulk rock incompatible trace element ratios la sm gd lu la nb and th nb of all timmasamudram kimberlites show strong affinity with those from the cretaceous group ii kimberlites from the bastar craton india and kaapvaal craton southern africa as the late cretaceous age of the younger perovskites from the tk 1 kimberlite is indistinguishable from that of the marion hotspot linked extrusive and intrusive igneous rocks from madagascar and india we infer that all may be part of a single madagascar large igneous province our finding constitutes the first report of cretaceous kimberlite activity from southern india and has significant implications for its sub continental lithospheric mantle evolution and diamond exploration programs

the thabazimbi murchison lineament of the kaapvaal craton south africa 2700 ma of episodic deformation the thabazimbi murchison lineament is a prominent tectonic feature extending for approximately 500 km in a nne ssw direction across the kaapvaal craton the lineament has a tectonic history of more than 2 5 ga because it appears to form and or influence the boundary between the archaean granite greenstone terrain of the kaapvaal craton and the southern margin of the high grade terrain of the limpopo mobile belt the northern margin of the archaean proterozoic transvaal basin the southern margin of the mesoproterozoic waterberg basin and has probably also influenced the emplacement of the mesoproterozoic bushveld complex mesozoic karoo basalts and sediments formed the springbok flats these lavas erupted into half graben structures formed by the normal reactivation of the previously strike slip zebedelia fault part of the thabazimbi murchison relay system the lineament is cut by a mesozoic dyke swarm associated with the break up of gondwana

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record we assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 1200 m and by a comparison with intraplate monogenetic basaltic volcanic fields eroded kimberlite fields are composed of pipes or diatremes and dikes and within any one kimberlite field regardless of erosion level kimberlite bodies vary in area at earth s surface over 2 3 orders of magnitude typically 60 70 of the bodies are

trace elements and their relation to the mineral phases in the lateritic bauxites from southeast guinea bissau abstract fourteen trace elements sr ba la ca mg cu zn v ni co mn ga cr zr were determined in 120 auger and core drill samples from one bauxite plateau situated in the se part of guinea bissau following the quantitative determination of minerals identified in the samples all geochemical and mineralogical data were statistically treated the use of several statistical tools enabled us to identify two distinct populations of samples which were found to be related to the shale and dolerite parent rocks respectively the dispersion patterns of the elements mn zr la zn cu cr and ni yield strong contrasts between these two populations bauxites developed on devonian shales were found to contain more cr and ni than bauxites derived from cretaceous dolerites which were richer in cr and ni it is postulated that the difference in duration of the bauxitization processes which affected the two parent bedrocks and or the different resistance to weathering of the primary mineral host phases in the two rock types are responsible for this inversion r mode factor analysis revealed the conspicuous affinity of the mn zn ni cu co mg group and v to goethite gibbsite boehmite and anatase were found to be associated with resistate trace elements like zr and to a lesser extent cr gibbsite also reveals a certain affinity to more mobile strongly depleted elements like sr and ba the study proved the existence of numerous statistically significant correlations between the trace elements like the groups zr ti zn cu ni co mn and sr ba la

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling

seasonality of deep cycle turbulence in the eastern equatorial pacific abstractthe seasonal cycles of the various oceanic and atmospheric factors influencing the deep cycle of turbulence in the eastern pacific cold tongue are explored moored observations at 140 w have shown seasonal variability in the stratification velocity shear and turbulence above the pacific equatorial undercurrent euc in boreal spring the thermocline and euc shoal and turbulence decreases marginal instability clustering of the local gradient richardson number around the critical value of 1 4 evident throughout the rest of the year has not been detected during spring while the daily averaged turbulent energy dissipation in the euc is weakest during the spring it is not clear whether the diurnal fluctuations that define the deep cycle cease large eddy simulations are performed using climatological initial and boundary conditions representative of january april july and october deep cycle turbulence is evident in all cases the mechanism remains the same and the maximum turbulence leve