prompt penetration electric fields ppefs and their ionospheric effects during the great magnetic storm of 30 31 october 2003 1 we explore the ionospheric effects of prompt penetration electric fields ppefs for a variety of interplanetary magnetic field directions we use the great magnetic storm of 30 31 october as an example of ppef effects for intense southward interplanetary magnetic fields imfs inward plasma sheet convection occurs with the result of magnetospheric ring current formation and an intense magnetic storm concurrent with the above positive phase ionospheric storms occur in the dayside and negative phase ionospheric storms occur on the nightside the topics of this paper the dayside ionospheric storms due to ppefs are characterized by transport of near equatorial plasma to higher altitudes and latitudes forming a giant plasma fountain these features are part of what is called the dayside ionospheric superfountain dis for these southward imfs dusk and dawn plasma are predicted to be transported toward the dayside for northward imfs negative phase ionospheric storms are expected on the dayside if the ppefs indeed reach that region of space imf by components are expected to have weak or neglible ionospheric effects on the basis of ppef arguments intervals of imf by should not be related to geomagnetic storms they are not imf by intervals should however cause a shearing of the magnetotail a feature that has been previously reported in the literature

evidence for olivine deformation in kimberlites and other mantle derived magmas during crustal emplacement this paper highlights published and new field and petrographic observations for late stage crustal level deformation associated with the emplacement of kimberlites and other mantle derived magmas thus radial and tangential joint sets in the competent 183 ma karoo basalt wall rocks to the 5 ha lemphane kimberlite blow in northern lesotho have been ascribed to stresses linked to eruption of the kimberlite magma further examples of emplacement related stresses in kimberlites are brittle fractures and close spaced parallel shears which disrupt olivine macrocrysts in each of these examples there is no evidence of post kimberlite regional tectonism which might explain these features indicating that they reflect auto deformation in the kimberlite during or immediately post emplacement on a microscopic scale these inferred late stage stresses are reflected by fractures and domains of undulose extinction which traverse core and margins of some euhedral and anhedral olivines in kimberlites and olivine melilitites undulose extinction and kink bands have also been documented in olivines in cumulates from layered igneous intrusions our observations thus indicate that these deformation features can form at shallow levels crustal pressures which is supported by experimental evidence undulose extinction and kink bands have previously been presented as conclusive evidence for a mantle provenance of the olivines i e that they are xenocrysts the observation that these deformation textures can form in both mantle and crustal environments implies that they do not provide reliable constraints on the provenance of the olivines an understanding of the processes responsible for crustal deformation of kimberlites could potentially refine our understanding of kimberlite emplacement processes

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

the influence of volcanological and sedimentological processes on diamond grade distribution in kimberlites examples from the ekati diamond mine nwt canada the distribution of diamonds within individual kimberlite pipes is poorly documented in the public domain due to the proprietary nature of the data the study of the diamond distribution within two pipes fox and koala from the ekati diamond mine nwt canada in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution in both pipes the lithological facies represent grade units which can be distinguished from each other in terms of relative size and abundance of diamonds a positive relationship between olivine grain size and abundance with diamond size and abundance is observed indicating that sorting of fragmental kimberlites influences diamond distribution though surface geological processes do not control the diamond potential of the erupting magma they can be responsible for concentrating diamonds into economically significant proportions a good understanding of the eruption transport and depositional processes responsible for the individual lithological units and the diamond distribution within them is important for successful resource estimation this may lead to recognition of areas suitable for selective mining making a marginal deposit economic

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

crustal structure of the lofoten continental margin off northern norway from ocean bottom seismographic studies summary in 1988 an extensive geophysical experiment was performed on the lofoten passive continental margin which was formed by the continental break up between greenland and northern norway at 58 ma the geological units of the outer lofoten margin are characterized by seaward dipping reflectors sdrs and landward flood basalt which extends up to 100 km landwards of sdrs in this study we obtain the p wave velocity structure beneath the lofoten basin the sdrs and the landward flood basalt by use of ocean bottom seismograph refraction profiling and we also discuss the formation of the northern norwegian passive continental margin in the lofoten basin the crust is of oceanic type consisting of sedimentary layers oceanic laver 2 4 9 5 5 km s 1 layer 3a 6 3 6 8 km s 1 and layer 3b 7 0 7 1 km s 1 beneath the sdrs the crustal layers are identical to those of the lofoten basin but the thickness of the lower crust which represents the same velocity as layer 3b increases to 5 km towards the continent side and a high velocity lower crustal layer 7 3 km s 1 is formed at the base of the crust the ocean continent transition zone is situated between the landward side of the sdrs and the northward continuation of the voring plateau escarpment in this region the velocity of the lower crust gradually decreases and approaches the lower crustal velocity beneath the lofoten islands 6 8 km s 1 the model also indicates that the high velocity layer disappears in this region comparing our model with the crustal structure on the voring margin it is clear that the lower crustal body 7 km s 1 thickens southwards along the northern norwegian continental margin recent results from petrological and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenospheric material increases the thickness of the oceanic crust we interpret this as that the oceanic crust in the southern area in the voring lofoten margin was generated by hotter material than that of the northern area

seismic attenuation in faroe islands basalts we analysed vertical seismic profiling vsp data from two boreholes at glyvursnes and vestmanna on the island of streymoy faroe islands to determine the magnitude and causes of seismic attenuation in sequences of basalt flows the work is part of seifaba a major project integrating data from vertical and offset vsp surface seismic surveys core samples and wireline log data from the two boreholes values of effective seismic quality factor q obtained at glyvursnes and vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image this observation is consistent with results from other vsp experiments in the north atlantic region we demonstrate that the most likely cause of the low values of effective q at glyvursnes and vestmanna is a combination of 1d scattering and intrinsic attenuation due to seismic wave induced fluid flow within pores and microcracks tests involving 3d elastic wave numerical modelling with a hypothetical basalt model based on field observations indicate that little scattering attenuation is caused by lateral variations in basalt structure

imaging and regional distribution of basalt flows in the faeroe shetland basin we demonstrate that the use of long offset seismic data allows wide angle reflections and diving waves to be recorded and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the faeroe shetland basin crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray tracing methods finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals we show that prestack depth migration of selected wide angle arrivals of known origin such as the base basalt reflection using the crustal velocity model allows us to build a composite image of the structure down to the pre rift basement this has the advantage that the wide angle first arriving energy must be primary and not from one of the many multiples or mode converted phases that plague near offset seismic data this allows us to tag these primary arrivals with confidence and then to identify the same arrivals on higher resolution prestack migrations that include data from all offsets examples are drawn from the faeroe shetland basin with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre rift basement the maps show how the basalts thin to the southeast away from their presumed source west of the present faeroe islands and also show the extent to which the structure of the pre rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows

progressive infilling of a kimberlite pipe at diavik northwest territories canada insights from volcanic facies architecture textures and granulometry abstract processes responsible for the formation of crater facies kimberlite remain poorly understood because there are few occurrences of well preserved deposits new open pit exposures of the a154n kimberlite pipe at diavik in the lac de gras region of the northwest territories canada reveal well preserved crater facies deposits here we describe the geometries structures and componentry of deposits from the uppermost 130 m of the a154n pipe we then interpret the processes responsible for their emplacement on the basis of these characteristics the base of the section features a poorly sorted primary massive volcaniclastic kimberlite deposit mvk1 grading upward into a better sorted and weakly stratified volcaniclastic kimberlite deposit mvk2 which is itself overlain by 45 m of stratified volcaniclastic kimberlite svk the mvk svk contact is sharp and marks an abrupt transition from sustained eruption of pyroclastic kimberlite to episodic sedimentation from collapse of an unstable volcanic edifice into the open crater the svk deposit is overlain dis conformably by a poorly to well sorted diffusely bedded and graded pyroclastic kimberlite pk1 and pk2 that erupted from another younger kimberlite volcano the pk deposits show a variety of textures supporting a pyroclastic origin including vesiculated juvenile pyroclasts a grain and clast supported fabric and the absence of mud size particles in the matrix vertical clast size grading over 40 50 m and the absence of bedding in the pk2 suggest hydraulic sorting during en masse deposition of pyroclastic kimberlite into a deep water column granulometric analysis shows the pk2 deposit was originally transported as a pyroclastic density current that was sourced from one of the many adjacent pipes in the diavik area and inundated the a154n pipe these observations have implications for kimberlite volcanic processes and grade distribution

interpretation of refraction experiments in the north sea before 1977 only three long range seismic refraction profiles had been shot in the north sea basin with somewhat inconsistent results in terms of a description of the mohorovicic discontinuity moho in the summer of 1977 the marine group of cambridge university fired three refraction lines with the use of pull up shallow water seismometers the longest reversed profile extending for 400 km along the 0 5 e meridian from east of the shetland islands to the latitude of dundee although records varied in quality a time term interpretation of the results was made with the use of detailed velocity depth information supplied by the oil industry this interpretation indicates that there is an anticlinal form to the moho beneath the deepest sediments of the moray firth basin as demanded by gravity data from the area it is further suggested that the seismic evidence supports an extensional model to explain the origin of the north sea basin

controls on the location of compressional deformation on the nw european margin the distribution of cenozoic compressional structures along the nw european margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling and with the distribution of high velocity lower crust and or partially serpentinized upper mantle detected by seismic experiments only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle notably partially serpentinized upper mantle is well documented beneath the central part of the southern rockall basin but compressional features are sparse in that area where compressional structures have formed but the upper mantle is not serpentinized simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling the presence of pre existing weak zones associated with the properties of the gouge and overpressure in fault zones and local stress magnitude and orientation are important contributing factors

the economic implications of kimberlite emplacement abstract the economic potential of kimberlite bodies is defined by the volume and diamond content of all their internal units which are summarised in three dimensional geological models canadian geological models reveal distinct types of kimberlite bodies characterised by fundamentally different emplacement processes and products data for representative samples of canadian kimberlites are used to show that qualitative and quantitative macroscopic petrography olivines and xenoliths are powerful and practical techniques in assessing the economic implications of emplacement within individual phases of kimberlite kimberlite bodies result from the near surface emplacement of multiple discrete batches of mantle derived magmas each carrying different diamond populations and in some instances subsequent resedimentation processes magmas containing 25 modal of olivine macrocrysts the average maximum load of solids that can be carried to surface by a kimberlite magma have the greatest potential to be significantly diamondiferous modifications to the abundance and size distribution of the olivine macrocrysts in each batch of magma commonly occur during emplacement the modifications to the olivine macrocrysts which vary with different emplacement processes and products are mirrored in the macrodiamond content dilution by xenoliths is important in predicting diamond contents but it is also reflected in reduced olivine contents thus olivine macrocrysts can act as a proxy for macrodiamonds the abundance and size distribution data for olivine can improve the prediction of diamond grade and distribution within and between kimberlite units summary geological models and macroscopic petrography together improve the evaluation of newly discovered bodies resulting in enhanced resource estimates and increased degrees of confidence

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

the composition and structure of volcanic rifted continental margins in the north atlantic further insight from shear waves abstract imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non volcanic rifted and convergent margins massive volcanism occurred during break up at 70 of the passive margins bordering the atlantic ocean the causes and dynamics of which are still debated this paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four component ocean bottom seismometers along two north atlantic continental margin profiles this traveltime tomography was performed using two different approaches the first a flexible layer based parameterisation enables the quality control of traveltime picks and investigation of the crustal structure the second with a regularised grid based parameterisation requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via monte carlo analyses the velocity models indicate high lower crustal velocities and sharp transitions in both velocity and vp vs ratios across the continent ocean transition the velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins interpretation of the high quality seismic reflection profile on the faroes margin confirms that this mixing is through crustal intrusion converted shear wave data also provide constraints on the sub basalt lithology on the faroes margin which is interpreted as a pre break up mesozoic to paleocene sedimentary system intruded by sills

depth imaging of basalt flows in the faeroe shetland basin summary we use a grid of long offset seismic profiles to map the thickness of subsurface basalt flows that extend southeastward across the faeroe shelf offsets of up to 38 000 m were achieved using two seismic acquisition vessels towing multichannel streamers and shooting alternately wide angle reflections and refractions from the basalt and underlying strata allow us to build a velocity model down to the basement lewisian crust the velocity models give a first order picture of the geology of low velocity sediments of presumed early palaeocene and mesozoic age beneath the basalt flows they also provide a control for pre stack depth migration of the entire seismic data set thus allowing us to produce good depth images of the structure pre stack depth migration of selected wide angle arrivals such as those from the base of the basalt and the underlying basement produce strong reflection images that allow these interfaces to be identified unambiguously on the higher resolution images produced from the entire data set in particular we can distinguish between the primary and multiple reflections we thus use the very long offset arrivals to tag the velocities of the geological interfaces from which they are returned and hence to improve both the imaging and the interpretation of the basalt flows and the underlying structure

basin evolution in southern east greenland an outcrop analog for cretaceous paleogene basins on the north atlantic volcanic margins the cretaceous paleogene kangerlussuaq basin in southern east greenland represents a unique outcrop analog for the frontier petroleum provinces along the deep water volcanic margins of the northern north atlantic the 1 km thick sedimentary succession comprises six facies associations 1 alluvial plain and shallow marine late aptian 2 fluvio estuarine late aptian early albian 3 offshore marine late cretaceous early paleocene 4 submarine fan and channel levee early paleocene 5 fluvial mid paleocene and 6 volcanic late paleocene sedimentation was terminated in the late paleocene by extrusion of flood basalts related to continental breakup the basin fill is divided into two depositional megasequences related to regional tectonic events and sea level changes the oldest megasequence sq1 spans the late aptian to the earliest paleocene with sea level rise in the late aptian and maximum flooding in the late albian cenomanian followed by sea level highstand in the late cretaceous early paleocene sq1 is truncated by a basin wide unconformity related to regional uplift and basin reorganization in the mid paleocene the upper megasequence sq2 spans the mid to late paleocene and comprises sediments deposited during early sea level rise extensive volcanic deposits and continental flood basalts overlie it deep burial 6 km and middle eocene early oligocene uplift excludes the kangerlussuaq basin as a petroleum basin in itself however to evaluate the petroleum potential of similar volcanic influenced offshore basins such as the west shetland faeroe more and voring basins we discuss three conceptual play models based on the kangerlussuaq basin 1 early albian stratigraphic trap 2 early paleocene structural trap and 3 paleocene stratigraphic trap

crustal constraint through complete model space screening for diverse geophysical datasets facilitated by emulation deep crustal constraint is often carried out using deterministic inverse methods sometimes using seismic refraction gravity and electromagnetic datasets in a complementary or joint scheme with increasingly powerful parallel computer systems it is now possible to apply joint inversion schemes to derive an optimum model from diverse input data these methods are highly effective where the uncertainty in the system is small however given the complex nature of these schemes it is often difficult to discern the uniqueness of the output model given the noise in the data and the application of necessary regularization and weighting in the inversion process means that the extent of user prejudice pertaining to the final result may be unclear we can rigorously address the subject of uncertainty using standard statistical tools but these methods also become less feasible if the prior model space is large or the forward simulations are computationally expensive we present a simple monte carlo scheme to screen model space in a fully joint fashion in which we replace the forward simulation with a fast and uncertainty calibrated mathematical function or emulator this emulator is used as a proxy to run the very large number of models necessary to fully explore the plausible model space we develop the method using a simple synthetic dataset then demonstrate its use on a joint data set comprising first arrival seismic refraction mt and scalar gravity data over a diapiric salt body this study demonstrates both the value of a forward monte carlo approach as distinct from a search based or conventional inverse approach in incorporating all kinds of uncertainty in the modelling process exploring the entire model space and shows the potential value of applying emulator technology throughout geophysics though the target here is relatively shallow the methodology can be readily extended to address the whole crust

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

long duration penetration of the interplanetary electric field to the low latitude ionosphere during the main phase of magnetic storms 1 it is well known that the interplanetary electric field can penetrate to the low latitude ionosphere it is generally believed that the penetration of electric fields can last only for 30 min because of the shielding effect in the ring current in this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field imf in six cases the eastward electric field in the dayside equatorial ionosphere measured by the jicamarca incoherent scatter radar was enhanced for 2 3 hours after the imf turned southward and remained continuously southward in one case the eastward electric field in the dayside midlatitude ionosphere measured by the millstone hill incoherent scatter radar was continuously enhanced for 10 hours during southward imf since millstone hill is close to the equatorward boundary of the auroral zone during magnetic storms the penetration electric field there may be different from that at the equatorial ionosphere the most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay the electric field enhancements in the middle and low latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms the observations show that the interplanetary electric field can continuously penetrate to the low latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

seismic attenuation of atlantic margin basalts observations and modeling paleogene basalts are present over much of the northeastern atlantic european margin in regions containing significant thicknesses of layered basalt flows conducting seismic imaging within and beneath the volcanic section has proven difficult largely because the basalts severely attenuate and scatter seismic energy we use data from a vertical seismic profile vsp from well 164 07 1 that penetrated 1 2 km of basalt in the northern rockall trough west of britain to measure the seismic attenuation caused by the in situ basalts the effective quality factor qeff of the basalt layer is found from the vsp to be 15 35 which is considerably lower more attenuative than the intrinsic attenuation measured on basalt samples in the laboratory we then run synthetic seismogram models to investigate the likely cause of the attenuation full waveform 1d modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the vsp can b

duskside enhancement of equatorial zonal electric field response to convection electric fields during the st patrick s day storm on 17 march 2015 the equatorial zonal electric field responses to prompt penetration of eastward convection electric fields ppef were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 march 2015 at dusk sector indian longitudes a rapid uplift of equatorial f layer to 550 km and development of intense equatorial plasma bubbles epbs were observed these epbs were found to extend up to 27 13 n and 25 98 s magnetic dip latitudes indicating their altitude development to 1670 km at apex in contrast at few degrees east in the premidnight sector thailand indonesian longitudes no significant height rise and or epb activity has been observed the eastward electric field perturbations due to ppef are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo idd fields whereas they were mostly counter balanced by the idd fields in the premidnight sector in situ observations from swarm a and swarm c and communication navigation outage forecasting system satellites detected a large plasma density depletion near indian equatorial region due to large electrodynamic uplift of f layer to higher than satellite altitudes further this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator this study brings out the significantly enhanced equatorial zonal electric field in response to ppef that is uniquely confined to dusk sector the responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field bz

a review of the ne atlantic conjugate margins based on seismic refraction data the ne atlantic region evolved through several rift episodes leading to break up in the eocene that was associated with voluminous magmatism along the conjugate margins of east greenland and nw europe existing seismic refraction data provide good constraints on the overall tectonic development of the margins despite data gaps at the ne greenland shear margin and the southern jan mayen microcontinent the maximum thickness of the initial oceanic crust is 40 km at the greenland iceland faroe ridge but decreases with increasing distance to the iceland plume high velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the east greenland ridge and the lofoten margin with the exception of the vestbakken volcanic province at the sw barents sea margin south of the narrow lofoten margin the european side is characterized by wide margins the opposite trend is seen in greenland with a wide margin in the ne and narrow margins elsewhere the thin crust beneath the basins is generally underlain by rocks with velocities of 7 km s 1 interpreted as serpentinized mantle in the porcupine and southern rockall basins while off norway alternative interpretations such as eclogite bodies and underplating are also discussed

subsidence breccias in kimberlite pipes an application of fractal analysis abstract a particular variety of volcanogenic country rock breccia is described a contact breccia that has been identified at venetia river ranch and wimbledon kimberlite pipes the contact breccia is clast supported with no juvenile kimberlite component has tightly packed angular fragments with occasional rounding of smaller particles and has a shear fabric dipping towards the center of each kimberlite pipe or volcanic event clasts have preferred orientations parallel to the fabric photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis the fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism clast size distribution fractal dimensions in the coarser particles greater than circa 3 cm range from greater than 3 for nonsheared breccia down to circa 2 3 for the sheared breccia breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high energy explosions followed by collapse and abrasion by subsequently gravity induced rockmass subsidence all studied contact breccias produced a distinctive fractal signature in the finer particles

spatial and temporal evolution of kimberlite magma at a154n diavik northwest territories canada abstract the a154n kimberlite pipe is part of the diavik kimberlite cluster in the northwest territories canada the pipe is filled by deposits that record a sequence of eruptive and intrusive events for which we have established relative times and modes of emplacement as such this sequence of kimberlite deposits provides a unique opportunity to explore the connections between magma properties pipe or conduit geometry and eruption style deposits within the a154n pipe represent four discrete volcanic events including 1 an array of pre cursor dykes of coherent kimberlite that intrude the adjoining country rock ck 1 2 pyroclastic deposits recording the explosive eruption of a154n pk 1 3 late dykes of coherent kimberlite intruding the main infill of the pipe ck 2 and 4 a sequence of water laid pyroclastic deposits that sourced from another kimberlite volcano pk 2 we use observations on polished slabs and thin sections to characterize the magma that erupted to produce each deposit image analysis techniques are used to quantify olivine content abundance and size distributions and abundances of vesicles within deposits and within individual juvenile pyroclasts our analysis shows that during the eruption cycle ck 1 to pk 1 to ck 2 olivine content increased and volatile content decreased lastly we estimate physical properties for the magmas that produced each deposit the magma properties are combined with 3 d models for the geometry of the conduit at the time of eruption to constrain the mass flux and duration of each eruptive event steady state assumptions for velocity and magma flux yield total eruption durations of minutes to hours the diversity of textures and compositions recorded in pyroclastic kimberlite pk 1 and dykes of coherent kimberlite ck 1 and ck 2 are a possible manifestation of separated three phase flow involving kimberlite melt and crystals and an ever increasing proportion of exsolved co 2 h 2 o fluid specifically we suggest that the early dykes ck 1 pyroclastic kimberlite pk 1 and late stage dykes ck 2 are representative of three separate regimes of kimberlite magma created by the ascent of a vigorously degassing magma we envisage this being composed of a the gas charged front of the magma created by decoupling the more buoyant gas phase from the silicate melt b a transient gas rich body of magma in which later stage exsolved fluids gases are essentially coupled to the melt and c a fluid depleted tail of magma which remains buoyant within the lithosphere but ascends more slowly than the fluid and exsolving gas phases

alteration styles in the orion central volcanic complex fort a la corne kimberlite field saskatchewan and their effects on primary volcaniclastic textures implications for facies mapping and diamond exploration diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes which are generally determined from volcaniclastic facies characteristics however the effects and distribution of secondary alteration styles which usually mask primary kimberlite facies have been given little attention at the fort a la corne kimberlite field saskatchewan complex overprinting alteration textures and mineral phases are recognized this study documents the characteristics overprinting relationships and spatial distribution of alteration styles within a crater filling volcaniclastic deposit body 219 of the orion central volcanic complex data were acquired from macroscopic drill core observations microscopic petrography sem imaging eds element mapping and xrd analyses olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite late stage serpentine has migrated into the interstitial medium bulk rock alteration textures include 1 fine grained crystal rich domains which are alteration pseudoclasts and or relict juvenile pyroclasts 2 a heterogeneous interstitial medium varying from early stage disseminated calcite rich serpentine to amorphous serpentine and crystalline calcite and 3 overprinting serpentine carbonate and magnetite veins and serpentine nodules alteration styles defined by unique alteration characteristics are spatially zoned through the body 219 volcaniclastic deposit and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit recognition of alteration processes their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies and by consequence the distribution of diamond grades

continent ocean transition and voluminous magmatic underplating derived from p wave velocity modelling of the east greenland continental margin summary deep seismic refraction data were gathered across the entire east greenland rifted margin north of the jan mayen fracture zone between 72 n and 75 n in 2003 investigations of the deep crustal structure of this continental margin provide constraints on the formation of the margin and its structural evolution during and after late cretaceous early tertiary rifting and continental break up we present here the results along two profiles located in the prolongation of the godthab gulf and the kejser franz joseph fjord regional p wave velocity models were derived from forward traveltime modelling of land stations and ocean bottom hydrophone obh recordings for the first time long deep seismic sounding transects off east greenland provide a full insight into the crustal architecture of the transition from continental to oceanic crust a mean result is the identification of voluminous magmatic underplating which is wider and thicker than previously thought p wave velocities of the underplated material range between 7 1 and 7 4 km s 1 and the horizontal extents on the profiles are 225 and 190 km the maximum thickness of the underplated material is 15 16 km furthermore the p wave velocity models reveal a 120 130 km wide continent ocean transition zone cot based on an interpretation of the extent of cretaceous syn rift sediments mixed with basaltic intrusions and the lateral increase of velocities in the crustal layers excess magmatism must have been present during a long term rifting process accompanying the extension of the continental crust and giving rise to the voluminous magmatic underplating a consequence of our interpretation of the seismic refraction data is a likely rift propagation in the greenland sea from north to south additionally a comparison of p wave velocity models of the east greenland margin and voring margin reveals significantly asymmetric crustal architectures the voluminous magmatic underplating and asymmetrical conjugate margins formations are considered as a mirror of complex pre and syn rift processes

imaging low velocity anomalies with the aid of seismic tomography abstract theoretical considerations snell s law suggest that low velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray tracing methods a synthetic study considering a 40 20 m low velocity anomaly 300 m s placed at the center of a 400 50 m block with gradient background velocity model from 3000 m s at the surface to 4000 m s at the base indicates that the low ray density in ray tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low velocity anomalies combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first arrival travel times form an efficient scheme to resolve low velocity anomalies such as fracture zones furthermore the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface weathering layer and provide seismic reflection images of fracture zones low velocity anomaly within a granitic pluton

imaging igneous rocks on the north atlantic rifted continental margin summary extruded basalt flows with thicknesses of several kilometres occur ubiquitously along the rifted continental margins of the northern north atlantic their total volume exceeds 1 million km 3 and may reach several million cubic kilometres intruded igneous rock comprising the complementary melt fraction to that extruded at the surface should exist in the lower crust with a volume at least as large as that of the extrusive basalts to image the extrusive and intrusive igneous rocks on the continental margin near the faroe islands in 2002 we shot a 375 km long deep penetration seismic profile across the margin using 85 ocean bottom seismometers for wide angle acquisition and shot a separate reflection profile using a 12 km streamer for optimal imaging by using large airgun sources tuned to produce low frequency energy we were able to constrain the seismic velocity structure of the whole crust we imaged not only layering and structure within the extrusive basalts but also lower crustal intrusions on the continent ocean transition cot combination of good velocity control on the lower crust together with direct imaging of the extrusive basalts enables us to constrain the volumes of extrusive and intrusive igneous rocks produced near the faroe islands to be 340 420 and 560 780 km 3 per kilometre along strike respectively the cot marked both by high velocity intrusions and lower crustal layering is surprisingly narrow with only 50 km separating unstretched continental crust from fully oceanic crust in contrast the extruded basalts flow up to 150 km landward at the paleo surface

seismic characterization of basalt flows from the faroes margin and the faroe shetland basin high quality seismic profiles across the faroes volcanic continental margin and the adjacent continental shelf show contrasting styles of basalt emplacement across two major structural boundaries at the rifted continental margin the basalts form prominent seaward dipping reflector sequences up to 5 km thick the same basalts after they had travelled in a landward direction across the elevated subaerial faroes shelf produced prominent landward dipping foresets where they flowed across the paleo coastline into the deep water of the faroe shetland basin in both cases we are able to image details of the internal structure of the basalts as a result of using a low frequency airgun source capable of penetrating through the basalt sequence together with long offset recordings from ocean bottom seismometers and a 12 km long deep towed hydrophone streamer

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

origin of olivine in kimberlite phenocryst or impostor abstract kimberlite hosts two populations of olivine that are distinguished on the basis of grain size and morphology the populations are commonly described genetically as xenocrysts and phenocrysts olivine xenocrysts or macrocrysts are thought to derive from disaggregation of mantle xenoliths whereas the smaller euhedral olivine crystals are presumed entirely cognate to the kimberlite melt recent studies of zoning patterns of euhedral olivine in kimberlite have however cast doubt on the actual origins of the smaller olivine crystals here we elucidate the nature and origins of the textural and chemical zonation that characterize both populations of olivine macrocrysts olivine i and euhedral crystals olivine ii specifically we show that both olivine i and olivine ii feature chemically distinct overgrowths resulting from heterogeneous crystallization onto pre existing olivine xenocrysts our analysis limits the total volume of olivine crystallized during transport to 5 in contrast to previous estimates of 25 the reduced extent of olivine crystallization allows for closer reconciliation of crystallized olivine compositions and estimates of mg s for primitive kimberlite melts it also places constraints on processes involving orthopyroxene assimilation by kimberlite melt if olivine crystallization and orthopyroxene assimilation are coupled then orthopyroxene assimilation is limited to 7 larger masses of orthopyroxene assimilation i e 25 are possible only if kimberlite magmas originate at super liquidus 100 oc conditions and sub equal amounts of olivine crystallization occurs

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

a new approach to kimberlite facies terminology using a revised general approach to the nomenclature of all volcanic rocks and deposits descriptive to genetic although kimberlite pipes bodies are usually the remains of volcanic vents in vent deposits and subvolcanic intrusions the terminology used for kimberlite rocks has largely developed independently of that used in mainstream volcanology existing kimberlite terminology is not descriptive and includes terms that are rarely used used differently and even not used at all in mainstream volcanology in addition kimberlite bodies are altered to varying degrees making application of genetic terminology difficult because original components and depositional textures are commonly masked by alteration this paper recommends an approach to the terminology for kimberlite rocks that is consistent with usage for other volcanic successions in modern terrains the eruption and emplacement origins of deposits can often be readily deduced but this is often not the case for old variably altered and deformed rock successions a staged approach is required whereby descriptive terminology is developed first followed by application of genetic terminology once all features including the effects of alteration on original texture and depositional features together with contact relationships and setting have been evaluated because many volcanic successions consist of both primary volcanic deposits as well as volcanic sediments terminology must account for both possibilities

response of the equatorial ionosphere in the indian midnight sector to the severe magnetic storm of july 15 2000 1 during the severe magnetic storm of july 15 2000 an impulsive and remarkably large downward movement of f region occurred simultaneously at locations throughout the equatorial region dip 0 3 20 n in the indian sector in close association with episodes of rapid ring current intensification the abnormal midnight descent of equatorial f region maximum amplitude close to the magnetic equator 215 km hr indicative of a short lived westward electric field disturbance peak amplitude 4 6 mv m is interpreted as the signature of prompt penetration electric fields associated primarily with impulsive ring current injections the westward electric field disturbance in the indian midnight sector occurred near simultaneous with the eastward electric field disturbances in the dusk sector reported by basu et al 2001a moreover the prompt electric field penetration to the magnetic equator both in the dusk and midnight sectors occurred in an environment already under the influence of ionospheric disturbance dynamo idd electric fields illustrating the profound manner in which the equatorial f region plasma dynamics can get modified globally during the main phase of severe magnetic storms

rates of continental breakup magmatism and seafloor spreading in the norway basin iceland plume interaction 1 in year 2000 an ocean bottom seismometer obs profile was acquired across the more margin to the aegir ridge an extinct seafloor spreading axis the margin is an early eocene volcanic passive margin located between the faeroe iceland ridge fir and the east jan mayen fracture zone ejmfz the p wave data were modeled by ray tracing to give a crustal transect showing a 10 11 km thick igneous crust created by breakup magmatism tapering off to magma starved seafloor spreading by c23 time 51 4 ma the location of the ejmfz was reinterpreted from a satellite derived gravity map and spreading direction in the norway basin reevaluated no other fracture zones were confirmed and both thin oceanic crust 4 5 km and lack of fracture zones resemble ultraslow spreading on the arctic gakkel ridge magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the obs profile and half spreading rates were derived early seafloor spreading was slow 15 32 mm yr 1 approaching ultraslow 6 8 mm yr 1 by c20 time 42 7 ma a v shaped pattern seen in the gravity field located only around the northern part of the aegir ridge corresponds to increased crustal thickness in the seismic model recording northeast transport 3 6 mm yr 1 of more melt fertile asthenosphere zones the magma starved character of the norwegian basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the iceland plume constructed the fir to the south as the asthenosphere is subsequently transported into the norway basin

the t reflection and the deep crustal structure of the voring margin offshore mid norway seismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust we have completed a detailed 2d seismic interpretation of the deep crustal structure of the voring margin offshore mid norway where high quality seismic data allow the identification of high amplitude reflections locally referred to as the t reflection using a dense seismic grid we have mapped the geometry of the t reflection in order to compare it with filtered bouguer gravity anomalies and seismic refraction data the t reflection is identified between 7 and 10 s sometimes it consists of one single smooth reflection however it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries amplitudes and contact relationships the t reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults the spatial correlation between the filtered positive bouguer gravity anomalies and the deep dome shaped reflections indicates that the latter represent a high impedance boundary contrast associated with a high density and velocity body in 50 of the outer voring margin the depth of the mapped t reflection is found to correspond to the depth of the top of the lower crustal body lcb which is characterized by high p wave velocities 7 km s we present a tectonic scenario where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high grade metamorphic rocks deep intrusions into the faulted crustal blocks are responsible for the rough character of the t reflection whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km s the result is a heterogeneous lcb that likely represents a complex mixture of pre to syn breakup mafic and ultramafic rocks cumulates and sills and old metamorphic rocks such as granulites and eclogites an increasing degree of melting towards the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

geology of the victor kimberlite attawapiskat northern ontario canada cross cutting and nested craters abstract the pipe shapes infill and emplacement processes of the attawapiskat kimberlites including victor contrast with most of the southern african kimberlite pipes the attawapiskat kimberlite pipes are formed by an overall two stage process of 1 pipe excavation without the development of a diatreme sensu stricto and 2 subsequent pipe infilling the victor kimberlite comprises two adjacent but separate pipes victor south and victor north the pipes are infilled with two contrasting textural types of kimberlite pyroclastic and hypabyssal like kimberlite victor south and much of victor north are composed of pyroclastic spinel carbonate kimberlites the main features of which are similar clast supported discrete macrocrystal and phenocrystal olivine grains pyroclastic juvenile lapilli mantle derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices these partly bedded juvenile lapilli bearing olivine tuffs appear to have been formed by subaerial fire fountaining airfall processes the victor south pipe has a simple bowl like shape that flares from just below the basal sandstone of the sediments that overlie the basement the sandstone is a known aquifer suggesting that the crater excavation process was possibly phreatomagmatic in contrast the pipe shape and internal geology of victor north are more complex the northwestern part of the pipe is dominated by dark competent rocks which resemble fresh hypabyssal kimberlite but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias volcaniclastic kimberlite current evidence suggests that the hypabyssal like kimberlite is in fact not intrusive and that the northwestern part of victor north represents an early formed crater infilled with contrasting extrusive kimberlites and associated breccias the remaining main part of victor north consists of two macroscopically similar but petrographically distinct pyroclastic kimberlites that have contrasting macrodiamond sample grades the juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically the nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different indicating that they derive from different magma pulses or phases of kimberlite and thus represent separate eruptions the initial excavation of a crater cross cutting the earlier northwestern crater was followed by emplacement of phase i a low grade olivine phenocryst rich pyroclastic kimberlite and the subsequent eruption of phase ii a high grade olivine phenocryst poor pyroclastic kimberlite as two separate vents nested within the original phase i crater the second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade thus the final pyroclastic pipe infill of the main part of the victor north pipe appears to consist of at least three geological macrodiamond grade zones in conclusion the victor kimberlite was formed by several eruptive events resulting in adjacent and cross cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal like kimberlite which is now interpreted to be of probable extrusive origin within the pyroclastic kimberlites of victor north there are two nested vents a feature seldom documented in kimberlites elsewhere this study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

direct observational evidence for disturbance dynamo on the daytime low latitude ionosphere a case study based on the 28 june 2013 space weather event a case of the westward disturbance dynamo dd electric field influencing the daytime equatorial and low latitude ionosphere during a geomagnetic storm that occurred on 28 29 june 2013 is presented the gps total electron content tec observations from a network of stations in the indian equatorial low and middle latitude regions along with the radio beacon tec ionosonde and magnetic field observations are used to study the storm time behavior of the ionosphere negative ionospheric storm effects were seen over the low and middle latitudes during the storm time due to the presence of a westward dd electric field observations show that the suppression of the equatorial ionization anomaly eia from the morning hours itself on 29 june 2013 took place due to the prevailing westward dd electric field providing evidence for the model calculations by balan et al 2013 simulations using the gitm model also agree well with our results the present study gains importance as the direct observational evidences for disturbance dynamo effects on the daytime low latitude ionosphere and the eia are sparse as it has been difficult to delineate it from the compositional disturbances

integrated wide angle and near vertical subbasalt study using large aperture seismic data from the faeroe shetland region acquiring large aperture seismic data 38 km maximum offset along a profile crossing the faeroe shetland basin in the north atlantic enables us to use wide angle reflections and refractions in addition to conventional streamer data 0 6 km for subbasalt imaging the wideangle results are complemented and confirmed by images obtained from the conventional near vertical offset range traveltime tomography applied to the wide angle data shows a low velocity layer 3 5 4 5 km s underneath southeastward thinning lava flows suggesting a 2 5 3 0 km thick sedimentary layer the velocity model obtained from traveltime tomography is used to migrate wide angle reflections from large offsets that arrive ahead of the water wave cone the migrated image shows base basalt and sub basalt reflections that are locally coincident with the tomographic boundaries application of a new multiple suppression technique and controlled stacking of the conventional streamer data produces seismic sections consistent with the wide angle results prestack depth migration of the near vertical offsets shows a continuous base basalt reflection and a clearly defined termination of the basalt flows

use of fractal analysis for discrimination of particles from primary and reworked jokulhlaup deposits in se iceland abstract the morphology of sideromelane particles from jokulhlaup deposits in southeastern iceland has been studied by fractal analysis to assess post depositional changes associated with reworking fractal dimensions of particle boundaries were calculated from data obtained by the caliper and dilation methods most particles exhibit multifractal relationships with a textural d1 and structural d2 fractal dimension particles from primary jokulhlaup deposits have significantly greater textural and structural fractal dimensions compared to reworked deposits differences in the fractal dimensions indicate that reworking in the littoral zone has reduced both the amount of fine scale features on particle surfaces and the complexity of the overall particle shapes the caliper and dilation methods yield similar values of textural fractal dimension for each particle for the same particles the dilation method yields higher values of structural fractal dimension reworked particles used in this study can be effectively discriminated from primary particles using a combination of d1 and d2 fractal dimensions the results indicate that fractal analysis provides a useful quantitative characterization of complex volcanic particles and can be utilized to examine aspects of particle origin transport and deposition

the stratigraphy and structure of the faroese continental margin this paper presents a summary of the stratigraphy and structure of the faroese region as the faroese area is mostly covered by volcanic material the nature of the pre volcanic geology remains largely unproven seismic refraction data provide some indications of the distribution of crystalline basement which probably comprises archaean rocks with the overlying cover composed predominantly of upper mesozoic cretaceous and cenozoic strata the cenozoic succession is dominated by the syn break up faroe islands basalt group which crops out on the faroe islands where it is up to 6 6 km thick and shelf areas post break up sediments are preserved in the adjacent deep water basins including the faroe shetland basin seismic interpretation of the post volcanic strata shows that almost every sub basin in the faroe shetland basin has been affected by structural inversion particularly during the miocene these effects are also observed on the faroe platform the munkagrunnur ridge and the fugloy ridge where interpretation of low gravity anomalies suggests a large scale fold pattern the structure of the iceland faroe ridge which borders the nw part of the faroe area remains ambiguous the generally thick crust together with the absence of well defined seawards dipping reflectors may indicate that much of it is underlain by continental material

combining quantitative textural and geochemical studies to understand the solidification processes of a granite porphyry shanggusi east qinling china igneous rock textures reflect the cooling history of the parental magma combined with chemical data they can provide physical and chemical information about the evolution of a magma body the petrographic textures and chemical compositions of 21 coarse and fine grained granite samples along an 250 m horizontal outcrop of the shanggusi granite porphyry are presented in this case study the coarse grained granite porphyry is an early intrusion and the fine grained granite dykes mostly intruded into the granite porphyry are later intrusions the studied samples have nearly homogeneous major element bulk rock and mineral compositions but show large variations in their trace element compositions and textural characteristics the trace element data suggest the influence of hydrous fluids possibly enriched in co2 f and cl in the evolution of the plutonic body textural analysis of the coarse grained granite porphyry indicates that the crystal size distribution csd slopes intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals lmax average length of the four largest quartz crystals for each sample however most fine grained samples and the groundmass of the coarse grained samples show concave down csds indicating textural coarsening quartz csds in the coarse grained samples are kinked with a steep sloped log linear section representing small crystals 5 1mm and a shallow sloped log linear section representing large crystals 4 1mm these two crystal populations are interpreted as resulting from a shift in cooling regime the straight csds of two fine grained samples may be due to a different cooling history in general the spatial variation of the csd patterns can be attributed to various degrees of overgrowth and mechanical compaction the quartz phenocrysts in several coarse grained samples exhibit a high degree of alignment which may be the result of magmatic flow by integrating the field geology geochemistry and quantitative textural data from the horizontal profile of the shanggusi granite porphyry it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history fluid migration controlled undercooling can explain the solidification processes in the shanggusi intrusion and may also be prevalent in other fluid rich shallow intrusions quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system and provide relatively comprehensive insights into the petrogenesis of granites

seismic structure of basalt flows from surface seismic data borehole measurements and synthetic seismogram modeling we use the wide angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave this wide angle wavefield contains arrivals that are returned from within and below the basalt flows including the diving wave through the basalts as the first arrival and p wave reflections from the base of the basalts and from subbasalt structures the velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide angle base basalt reflection this can be refined by using the amplitude variation with offset avo of the basalt diving wave synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below the diving wave amplitudes of the

fragmentation in kimberlite products and intensity of explosive eruption the explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes shapes and textures however all pyroclastic deposits of kimberlite comprise two main types of pyroclasts 1 pyroclasts of kimberlite with or without enclosed olivine crystals and 2 olivine crystals which lack coatings of kimberlite here we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption 1 olivine crystals break during kimberlite eruption and 2 kimberlite melt can be efficiently separated from crystals during eruption these ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at diavik nt olivines are expected to break because of rapid pressure changes during the explosive eruption disruption of kimberlite magma and pyroclast production is driven by ductile deformation processes rather than by brittle fragmentation the extent to which melt separates from olivine crystals to produce kimberlite free crystals is a direct consequence of the relative proportions of gas melt and crystals lastly the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions a fragmentation index is proposed for kimberlite eruption based on a crystal size distributions of olivine and on b ratios of selvage free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals

passive kimberlite intrusion into actively dilating dyke fracture arrays evidence from fibrous calcite veins and extensional fracture cleavage abstract calcite veins are invariably associated with en echelon kimberlite dyke fracture arrays a detailed microstructural study of veining indicates four vein types type i stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins type i vein mineralogy indicates syn dilational crystallization of vein minerals in local tensional areas within the kimberlite vein types ii stretched to syntaxial elongate blocky and iii antitaxial indicate late crystallization vein mineral growth during subsequent or repeated dilation calcite fibers in type i to type iii veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins type iv calcite veins with blocky or mosaic polycrystalline textures are attributed to minor post intrusion extension which was potentially accompanied by repeated kimberlite intrusion within a given dyke array syn crystallization syn intrusion type i veins and an ubiquitous dyke parallel fracture cleavage in a zone up to 4 m on either side of dyke contacts suggest that en echelon kimberlite dyke fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust type ii and iii veins indicate that extension or dilation continued independently of an occupying kimberlite fluid phase after initial intrusion arrested mobile hydrofracturing under low differential stress within the upper brittle or seismic carapace of the continental crust followed by repeated dilation of the dyke fracture system is proposed as a mechanism for producing the features observed in this study the conditions constrained in this study indicate passive dyke intrusion into dilating fracture arrays during crustal extension

kinetic processes for plastic deformation of olivine in the poyi ultramafic intrusion nw china insights from the textural analysis of a 1700 m fully cored succession abstract the ubiquitous presence of undulose extinction and subgrain boundaries in olivine crystals is commonly perceived as originating in the mantle however these plastic deformation features are also well developed in the poyi ultramafic intrusion nw china in this case olivine was deformed through kinetic processes in a crustal magma chamber rather than by deformation processes in the upper mantle moreover accumulation and textural coarsening were critical to the characteristics of crystal size distributions csds of olivines in the poyi intrusion the axial deformational compaction of crystal mush was revealed by virtue of other quantitative textural analyses e g spatial distribution patter alignment factor and aspect ratio additionally based on the contrast of density between crystal matrix and interstitial melt adequate stress was generated by the km scale crystal framework in poyi body 2 11 mpa which triggered the distortion of grain lattice in olivine the deformation mechanisms of olivine primarily are dislocation creep and dislocation accommodated grain boundary sliding disgbs while diffusion creep is subsidiary this study has revealed various kinetic processes in a magmatic system by first demonstrating the genetic relationship between mineral deformation and axial compaction of crystal mush while highlighting the uncertainty of employing the deformation features of olivine in peridotite xenoliths as an indicator for a mantle origin in contrast to the olivine populations of xenocrysts that underwent fragmentation during ascent the deformed primitive olivines in compaction exhibit a distinct shortage of small grains which is conducive to delimiting these two types of deformed grains

seasonal and longitudinal dependence of equatorial disturbance vertical plasma drifts 1 we used equatorial measurements from the rocsat 1 satellite to determine the seasonal and longitudinal dependent equatorial f region disturbance vertical plasma drifts following sudden increases in geomagnetic activity the prompt penetration vertical drifts are upward during the day and downward at night and have strong local time dependence at all seasons the largest prompt penetration drifts near dusk and dawn occur during june solstice the daytime disturbance dynamo drifts are small at all seasons they are downward near dusk with largest smallest values during equinox june solstice the nighttime drifts are upward with the largest magnitudes in the postmidnight sector during december solstice during equinox the downward disturbance dynamo drifts near sunset are largest in the eastern hemisphere while the late night upward drifts are largest in the western hemisphere the longitudinal dependence of the disturbance dynamo drifts is in good agreement with results from simulation studies

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

crystal size distribution analysis of plagioclase in experimentally decompressed hydrous rhyodacite magma article i nfo this study presents crystal size distributions csd of plagioclase forming during decompression experiments of hydrous rhyodacite magma samples were annealed at 130 mpa subjected to continuous decompression at either 2 mpa h 1 or 0 5 mpa h 1 and then quenched along their respective decompression paths at 20 mpa intervals samples display concave up curved csd plots which result from a combination of two crystal populations those which formed during the anneal period and those which formed during decompression the csds also display a decrease in crystal number density at 3 7 m a size range that is easily resolvable with the imaging strategy employed the downturn at small sizes is presumably due to insufficient compensation for the intersection probability effect in converting 2d measurements to 3d size distributions crystal nucleation and growth rates derived from csds using standard assumptions are compared with values obtained using 2d measurements of bulk crystal populations batch methods csd calculated nucleation rates are substantially low relative to the batch values in fact csd derived volumetric nucleation rates may underestimate actual nucleation rates by up to two orders of magnitude in contrast growth rates from csds are consistently higher than batch rates although plagioclase growth rates are relatively constant during decompression at a given rate the average growth rate in the rapidly decompressed series is approximately five times faster than the crystal growth rate in the slowly decompressed series because crystal growth rate depends on decompression rate csds are incapable of revealing decompression timescales or magma ascent rates without independent knowledge of crystal growth rate published by elsevier b v

olivine crystal size distributions in kimberlite olivine is the dominant component in coherent kimberlite rocks and related pyroclastic rocks quantitative characterization of olivine crystals in kimberlite rocks may be used to better understand kimberlite emplacement and eruption here we construct the first complete olivine crystal size distribution csd for magmatic or coherent kimberlite using two dimensional image analysis techniques with a new method for scale integration crystal size and frequency data are collected from polished slabs and thin sections normalized to the largest scale of observation and stereologically corrected to create a complete csd covering the full range of observed olivine crystal sizes resulting csds show the following properties for olivine in crystal rich interiors of intrusive coherent kimberlite in the a154n kimberlite pipe diavik northwest territories 1 0 03 10 0 mm in diameter size range 2 46 7 51 2 modal crystal content 3 modal dominance by crystals 1 mm 4 frequency dominance by crystals 1 mm and 5 equant axial crystal shapes that cannot be easily described by a single external morphology for any size fraction measured size ranges and modal crystal content are consistent with previous reported data relative frequency and area ratios between crystals of different sizes and shape properties differ from previous assessments olivine csds are consistent with a power law distribution over at least two orders of magnitude and statistical parameters power law exponents inman parameters suggest that apparent characteristic ranges are independent of crystal frequencies modal abundance area or sampling locale these properties suggest olivine crystals may be a single population with a shared paragenesis and not the end product of a combination of two populations of olivine on this basis olivine csds and population parameters have the following uses 1 to compare and discriminate among kimberlite facies and deposit types and 2 to test models for olivine paragenesis

what can crystal size distributions and olivine compositions tell us about magma solidification processes inside kilauea iki lava lake hawaii abstract lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers here we present olivine compositions and crystal size distributions csds from scoria and drill core samples from kilauea iki lava lake which formed during the 1959 eruption of kilauea volcano hawaii three chemically distinct olivine populations were distinguished in the basis of their forsterite fo content 1 a high fo population fo86 90 2 an intermediate fo population fo78 82 and 3 a minor low fo population fo74 78 populations 1 and 2 both have deformed and undeformed crystals the third population may be the result of rejuvenation olivine in the lower 60 m of lake has a less fo rich composition and more crystals are deformed the csd analysis yields estimates of the average olivine residence time 1 60 years the shape of the olivine csds is fairly uniform with respect to depth curved csds are considered to be evidence of hybrid populations partly or totally involving crystal or magma mixing the turndown at the smallest sizes of most foundered crust and lake csds may be the result of coarsening making this process active both before and after eruption our csd modelling does not support significant crystal settling and overall convection in the lava lake although small advective currents are known to have occurred the olivine vertical stratification cannot be an original feature which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases it is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification

variations of olivine abundance and grain size in the snap lake kimberlite intrusion northwest territories canada a possible proxy for diamonds abstract the snap lake hypabyssal kimberlite is a 15 inclined intrusion that intrudes archaean rocks of the slave craton mapping has identified two main lithofacies within the intrusion olivine rich kimberlite ork and olivine poor kimberlite opk extensive bulk sampling has demonstrated that diamonds vary in both abundance and size across the intrusion since fresh forsteritic olivine and diamond possess similar densities they can be expected to be hydraulically equivalent this study investigates the local scale variation of olivine grain size and content using image analysis and whether olivine can be used as a proxy for diamond concentration and size direct measurement of diamonds is made difficult by extremely low concentrations of around 0 4 ppm and by the inability of the sampling methodology to sample selectively olivines from the two kimberlite varieties possess distinctly different crystal size distributions and this together with contrasting groundmass textures mineralogy and phlogopite compositions indicates that the opk and ork represent two distinct batches of kimberlite magma the general lack of sorting of olivine grains and the presence of decussate phlogopite textures in the ork indicate that this magma crystallized rapidly but was probably preceded by the opk which had a more complex crystallization history as revealed by complex zoning patterns of its phlogopite grains a methodology for comparing olivine and diamond size distributions has been developed and a strong correlation between diamond and olivine size distributions is demonstrated the correlation is used to demonstrate that diamond size distributions can be predicted from the proportions of opk and ork that comprise each bulk sample it can be concluded that the two textural varieties of kimberlite possess distinctly different olivine and diamond populations and that olivines are a good proxy for understanding diamond distributions at a local scale in the snap lake intrusion

imaging the pulsing iceland mantle plume through the eocene the temperature of mantle plumes may vary on geologic time scales from a few million years to tens of millions of years in special circumstances such as near iceland in the north atlantic where the plume underlies an oceanic spreading center temporal variations in the oceanic crustal thickness provide a sensitive proxy for the mantle temperature if as is likely the crustal thickness is controlled primarily by passive decompression of mantle rising beneath the spreading center we show from both seismic reflection imaging and wide angle ocean bottom seismometer data from the norwegian sea that the temperature of the iceland mantle plume decreased by 50 c over the first 5 m y following continental breakup and then oscillated by 25 c over an 3 m y period similar temperature variations on a 3 6 m y time scale creating strong lineations in the gravity field are inferred from the regional north atlantic they occur both in the period immediately following breakup and at the present day reykjanes ridge south of iceland where they create v shaped ridges as the mantle thermal anomalies propagate away from the center of the plume beneath iceland we propose that mantle plume temperature variations of 25 c have occurred in the iceland plume with a similar amplitude and frequency since at least 49 ma and are likely to be a feature of all mantle plumes

the ascent of kimberlite insights from olivine olivine xenocrysts are ubiquitous in kimberlite deposits worldwide and derive from the disaggregation of mantle derived peridotitic xenoliths here we provide descriptions of textural features in xenocrystic olivine from kimberlite deposits at the diavik diamond mine canada and at igwisi hills volcano tanzania we establish a relative sequence of textural events recorded by olivine during magma ascent through the cratonic mantle lithosphere including xenolith disaggregation decompression fracturing expressed as mineral and fluid inclusion rich sealed and healed cracks grain size and shape modification by chemical dissolution and abrasion late stage crystallization of overgrowths on olivine xenocrysts and lastly mechanical milling and rounding of the olivine cargo prior to emplacement ascent through the lithosphere operates as a kimberlite factory wherein progressive upward dyke propagation of the initial carbonatitic melt fractures the overlying mantle to entrain and disaggregate mantle xenoliths preferential assimilation of orthopyroxene opx xenocrysts by the silica undersaturated carbonatitic melt leads to deep seated exsolution of co2 rich fluid generating buoyancy and supporting rapid ascent concomitant dissolution of olivine produces irregular shaped relict grains preserved as cores to most kimberlitic olivine multiple generations of decompression cracks in olivine provide evidence for a progression in ambient fluid compositions e g from carbonatitic to silicic during ascent numerical modelling predicts tensile failure of xenoliths disaggregation and olivine cracks over ascent distances of 2 7 km and 15 25 km respectively at velocities of 0 1 to 4 m s 1 efficient assimilation of opx during ascent results in a silica enriched olivine saturated kimberlitic melt i e sio2 20 wt that crystallizes overgrowths on partially digested and abraded olivine xenocrysts olivine saturation is constrained to occur at pressures 1 gpa an absence of decompression cracks within olivine overgrowths suggests depths 25 km late stage 25 km resurfacing and reshaping of olivine by particle particle milling is indicative of turbulent flow conditions within a fully fluidized gas charged crystal rich magma

the size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions in an explosive eruption the atmospheric column dynamics depend strongly on the mass fraction of gas in the erupting mixture which is fixed by fragmentation in the volcanic conduit at fragmentation gas present in vesicular magmatic liquid gets partitioned between a continuous phase separating magma clasts and a dispersed phase in individual bubbles within the clasts as regards flow behavior it is the former continuous gas phase which matters and we show that its amount is determined by the fragment size analysis of 25 fall deposits and 37 flow deposits demonstrates that ash and pumice populations follow a power law size distribution such that n the number of fragments with radii larger than r is given by n r d d values range from 2 9 to 3 9 and are always larger than 3 0 in fall deposits d values for pyroclastic flow deposits are systematically smaller than those of fall deposits we show that at fragmentation the amount of continuous gas phase is an increasing function of the d value large d values cannot be attributed to a single fragmentation event and are due to secondary fragmentation processes laboratory experiments on bubbly magma and on solid pumice samples demonstrate that primary breakup leads to d values of 2 5 0 1 and that repeated fragment collisions act to increase the d value a model for size dependent refragmentation accounts for the observations we propose that in a volcanic conduit fragmentation proceeds as a sequence of events primary breakup releases a small amount of gas and is followed by fragment collisions due to refragmentation and decompression both the mass and volume fractions of continuous gas increase the final d value and hence the mass fraction of continuous gas at the vent depends on the time spent between primary fragmentation and eruption out of the vent

overthickening of sedimentary sequences by igneous intrusions the identification of extensive intrusive igneous complexes both in subsurface data and in field studies has resulted in quantification of the volumes of igneous material despite this research there is still little connection established between the amount of igneous material intruded into a basin and its effect on subsequent basin evolution in terms of burial and loading to understand how additional igneous material may influence basin evolution we present work from the faroe shetland basin fsb utilising subsurface data this study highlights that the total estimated thickness of sediment during cretaceous is likely an overestimate as the sedimentary fill consists of significant quantities of igneous material which was emplaced 10 55 ma years after deposition previously this additional igneous material has not been accounted for in estimates of sedimentation rates and the burial history of the fsb importantly petroleum system modelling to understand generation and expulsion of hydrocarbons benefits from correct estimates of basin fill the overthickening of basins by igneous material will affect the timing of hydrocarbon generation and subsequently the proper evaluation of exploration targets in order to fully understand basin evolution the volumes of igneous material and when this material was emplaced must be acknowledged and considered

the transition from sedimentation to flood volcanism in the kangerlussuaq basin east greenland basaltic pyroclastic volcanism during initial palaeogene continental break up primary basaltic pyroclastic deposits compose volumetrically c 35 50 of the lower palaeogene vandfaldsdalen mikis and haengefjeldet formations within the lower volcanic series of the east greenland flood basalt province the eruption and emplacement mechanisms of these deposits can be used to evaluate the volcanotectonic evolution of continental break up and initiation of flood basalt volcanism we present a revised stratigraphy for the pre volcanic sedimentary rocks and synvolcanic pyroclastic and epiclastic deposits and lavas from the kangerlussuaq basin east greenland primary basaltic pyroclastic deposits range from airfall tuffs to bomb beds surge deposits and vent sites with cinder cones early volcanism was apparently restricted to localized basins and formed shield volcano structures 30 40 km in diameter that are characterized by a heterogeneous sequence of intercalated hyaloclastites sediments pyroclastic deposits and lavas the lateral variation of deposit morphology reflects distance from source and indicates a migrating locus of volcanism along the axis of rifting towards the ne paired with continued down dropping of inactive volcanic centres the complex stratigraphic architecture of the initial volcanic deposits reflects local tectonic and sedimentary processes interacting with regional flood basalt volcanism and continental rifting

climatology of f region zonal plasma drifts over jicamarca 1 we use extensive incoherent scatter radar observations made at the jicamarca radio observatory between 1970 and 2003 to study and model empirically the equatorial zonal plasma drifts near the f region peak using bernstein polynomials as base functions our quiet time model results confirm that the daytime drifts are westward and are nearly season and solar cycle independent the nighttime drifts are eastward have larger magnitudes and increase strongly with solar flux particularly near equinox and december solstice enhanced geomagnetic activity drives small eastward perturbation drifts during the day and much larger westward disturbance drifts at night the nighttime drift perturbations are largest near midnight and increase strongly with solar flux near equinox and december solstice but are essentially absent near june solstice the jicamarca zonal disturbance drifts can be largely accounted for by disturbance dynamo electric fields with a dominant time delay of about 3 15 hours following enhanced geomagnetic activity in the postmidnight sector there are also smaller westward disturbance drifts associated with time delays of about 15 24 hours and perhaps even longer our results strongly suggest that the longitudinal dependence of both the quiet and disturbed equatorial nighttime zonal drifts varies with season

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

transport survival and modification of xenoliths and xenocrysts from source to surface abstract a wide variety of magmas entrain transport and erupt mantle material in the form of xenoliths and xenocrysts the host magmas are often low viscosity in nature and range from basalt to more esoteric compositions such as kimberlite nephelinite and basanite here we focus on kimberlite magmas which are particularly successful at transporting deep mantle cargo to the surface including economically important quantities of diamond collections of mantle derived xenoliths and xenocrysts are critical to our understanding of the structure stability composition thermal state age and origin of the lithosphere however they also inform on magma transport conditions through a series of scaled analogue experiments we document the relative mechanical stability of olivine garnet orthopyroxene clinopyroxene and diamond xenocrysts during magma ascent our experiments fluidized these mantle minerals at a constant gas flux for variable amounts of time approximating transport in a high velocity turbulent fluid rich supercritical fluid or gas depending on depth magma the evolution of mineral surface features morphology and grain size distributions is analyzed as a function of residence time we show that on timescales consistent with magma ascent each mantle mineral is subject to mechanical modification resulting in mass loss and reshaping rounding by grain size reduction and surface pitting we further discuss the chemical consequences of producing fine particle chips that are highly susceptible to dissolution lastly we utilize an empirical model that relates textural observations e g impact pit size on xenocrysts to differential particle velocities our approach applied to natural kimberlitic olivine and garnet xenocrysts indicates differential velocities of 4 m s 1 the first direct estimate for velocity in an ascending kimberlite magma

physical mechanisms of the ionospheric storms at equatorial and higher latitudes during the recovery phase of geomagnetic storms 1 the paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes which are generally opposite both during the main phase mp and recovery phase rp of geomagnetic storms the mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable the paper first illustrates the recent developments in the understanding of the mechanisms during daytime mps when generally negative ionospheric storms in nmax and tec develop at equatorial latitudes and positive storms occur at higher latitudes including why the storms are severe only in some cases the paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes within 15 during daytime rps when negative storms occur at higher latitudes using champ ne and gps tec data and sheffield university plasmasphere ionosphere model the results indicate that the mechanical effect of the storm time equatorward neutral winds that causes plasma convergence at equatorial f region could be a major source for the positive storms with the downwelling effect of the winds and zero or westward electric field if present acting as minor sources

radar and satellite global equatorial f region vertical drift model we present the first global empirical model for the quiet time f region equatorial vertical drifts based on combined incoherent scatter radar observations at jicamarca and ion drift meter observations on board the atmospheric explorer e satellite this analytical model based on products of cubic b splines and with nearly conservative electric fields describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values our results indicate that during solar minimum the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15 20 m s is observed only in the american sector during december solstice with amplitudes of about 5 10 m s and is absent at all longitudes during june solstice the solar minimum evening reversal times are fairly independent of longitude except during december solstice during solar maximum the evening upward vertical drifts and reversal times exhibit large longitudinal variations particularly during the solstices in this case for a solar flux index of 180 the june solstice evening peak drifts maximize in the pacific region with drift amplitudes of up to 35 m s whereas the december solstice velocities maximize in the american sector with comparable magnitudes the equinoctial peak velocities vary between about 35 and 45 m s the morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle the daytime drifts have largest amplitudes between about 0900 and 1100 lt with typical values of 25 30 m s we also show that our model results are in good agreement with other equatorial ground based observations over india brazil and kwajalein

low latitude ionospheric disturbance electric field effects during the recovery phase of the 19 21 october 1998 magnetic storm 1 low latitude ionospheric electric fields and currents are often strongly disturbed during periods of enhanced geomagnetic activity these perturbations can last for several hours after geomagnetic quieting we use incoherent scatter radar measurements from jicamarca and arecibo during 19 21 october 1998 to study for the first time the low latitude disturbance electric fields during the recovery phase of a large magnetic storm on 19 october the jicamarca data showed initially large and short lived time scale of about 10 20 min upward and westward drift perturbations in the early afternoon sector due to the penetration of strong magnetospheric electric fields probably driven by an increase in the solar wind dynamic pressure following the decrease of auroral activity very strong afternoon and nighttime zonal and vertical disturbance dynamo drifts were observed over jicamarca but surprisingly only small and sporadic perturbation drifts were present over arecibo the latitudinal variation of the daytime zonal disturbance dynamo drifts during this day is in good agreement with that previously observed for thermospheric disturbance winds the disturbance dynamo drifts also indicate significantly different local storm time and possibly latitudinal dependences for their vertical perpendicular and zonal drift components suggesting large variations in both the amplitude and direction of the disturbance dynamo electric field vector in the next day during moderately disturbed conditions the daytime perturbation drifts were small but following transient increases in geomagnetic activity large upward perpendicular drifts were observed near dusk over jicamarca and arecibo where simultaneous westward perturbation drifts were also seen these perturbations are consistent with the occurrence of strong prompt penetration electric fields reaching the magnetic equator the measured prompt penetration drift patterns are generally in good agreement with predictions from global convection models later at night under moderately disturbed conditions relatively large zonal and meridional disturbance dynamo electric fields were observed by the two radars our results illustrate the large variability of the low latitude perturbation electric fields relative to their climatological values after large storms probably due to the importance of additional disturbance processes they also indicate that a much deeper understanding of solar wind magnetospheric ionospheric processes is required for accurate predictions of these electric fields

on the latitudinal changes in ionospheric electrodynamics and composition based on observations over the 76 77 e meridian from both hemispheres during a geomagnetic storm the relative contributions of the composition disturbances and the disturbance electric fields in the redistribution of ionospheric plasma is investigated in detail by taking the case of a long duration positive ionospheric storm that occurred during 18 21 february 2014 gps total electron content tec data from the indian antarctic station bharti 69 4 s 76 2 e geographic the northern midlatitude station hanle 32 8 n 78 9 e geographic northern low latitude station lying in the vicinity of the anomaly crest ahmedabad 23 04 n 72 54 e geographic dip latitude 17 n and the geomagnetic equatorial station trivandrum 8 5 n 77 e geographic dip latitude 0 01 s are used in the study these are the first simultaneous observations of tec from bharti and hanle during a geomagnetic storm the impact of the intense geomagnetic storm dst 130 nt on the southern hemisphere high latitude station was a drastic reduction in the tec negative ionospheric storm starting from around 0330 indian standard time ist on 19 february which continued till 21 february the maximum reduction in tec at bharti being 35 tec units on 19 february in the northern hemisphere midlatitude and equatorial stations a positive ionospheric storm started on 19 february at around 0900 ist and lasted for 3 days the maximum enhancement in tec at hanle was about 25 tecu on 19 february while over trivandrum it was 10 tecu this long duration positive ionospheric storm provided an opportunity to assess the relative contributions of disturbance electric fields and composition changes latitudinally the results indicate that the negative ionospheric storm over bharti and the positive ionospheric storm over hanle are the effect of the changes in the global wind system and the storm induced composition changes at the equatorial latitudes the positive ionospheric storm was due to the interplay of prompt penetration electric field and disturbance dynamo electric field

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

the role of fluidisation in the formation of volcaniclastic kimberlite grain size observations and experimental investigation abstract deep steep sided kimberlite pipes in southern africa are normally partially filled with multiple units of a characteristically massive structureless volcaniclastic rock here termed massive volcaniclastic kimberlite mvk these units can be arranged in a pseudo concentric nested pipes within pipes structure several key observations on typical mvk from the venetia kimberlite pipes south africa indicate that fluidisation may have been an important process during its emplacement these include 1 the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and 2 a matrix composed of void filling minerals related to hydrothermal metamorphism mainly diopside and serpentine which indicates that the primary deposit had a high porosity 50 and 3 particle size distributions poor in fine ash 10 cm analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of mvk high pressure gas fed through the bed from a point source resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a gaussian distribution of vertical velocity in the cross flow direction a decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the pipes within pipes structures in kimberlite pipes the results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes we propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep wide conduit this bed is subject to quasi sustained fluidisation by the erupting gas particle flow which imparts on it the distinctive characteristics of a massive well mixed poorly sorted deposit

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

motions of the equatorial ionization anomaly crests imaged by formosat 3 cosmic 1 the equatorial ionization anomaly eia structures and evolutions are imaged using radio occultation observation of the newly launched formosat 3 cosmic f3 c satellite constellation three dimensional ionospheric images provide unprecedented detail of the eia structure globally this paper presents images of the eia structures during july august 2006 and discusses the development and subsidence of the eia clear seasonal asymmetries in both ionospheric electron density and layer height are observed two dimensional cross section maps at a meridian provide dynamic variations and motions of the northern and southern eia crests results suggest that in addition to the asymmetric neutral composition effect interactions between the summer to winter transequatorial neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the eia crests as imaged by the f3 c