unbedded diatreme deposits reveal maar diatreme forming eruptive processes standing rocks west hopi buttes navajo nation usa diatremes provide partial records of how dyke delivered magma periodically interacting with water produces the largest known cylindrical conduit structures of any volcano type we address how pre eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at standing rocks west and the volumes and sources of wall rock within the diatreme and in a complementary tephra ring at teshim both in the hopi buttes volcanic field the unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns interpreted as ephemeral conduit deposits of well mixed poorly sorted composite and juvenile pyroclast rich deposits that truncate marginal layered deposits plus a peripheral heterolithic country rock breccia wall rock clasts are scarce in the massif and were mainly sourced from 75 to 245 m depth below the paleosurface by contrast the tephra ring has abundant wall rock fragments dominantly from the upper 75 m of the pre eruptive sedimentary sequence the diatreme structure is interpreted to have been formed by many small volume explosions fed by a low flux of basaltic magma late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure this resulted in damped and shifting intra diatreme explosions and jets that facilitated gradual mixing recycling and remobilization of debris in the diatreme with incremental addition of juvenile material and possibly a local rise in the crater floor

criteria for interpreting kimberlite as coherent insights from the muskox and jericho kimberlites nunavut canada the jurassic muskox and jericho kimberlites northern slave province nunavut canada contain a variety of facies exhibiting different geometries contact relationships internal organisation country rock abundance and olivine shapes although many have similar matrix groundmass mineralogies and textures five facies are examined that either have characteristics consistent with coherent rocks in general i e intrusive and extrusive non fragmental rocks or are mineralogically and texturally similar to kimberlite described as coherent or apparent coherent three facies are interpreted as coherent on the basis of 1 geological setting 2 apparent porphyritic texture 3 sharp contacts with fragmental kimberlite 4 relative abundance of elongate and unbroken olivine crystals and 5 paucity of country rock xenoliths while the remaining two facies are interpreted as fragmental on the basis of 1 the gradational contacts with demonstrably fragmental kimberlite 2 relative abundance and range of sizes of country rock lithic clasts and 3 numerous broken olivine crystals comparisons are made with coherent and apparent coherent kimberlite from the literature our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock ckd in terms of internal organisation low abundance of country rock xenoliths and apparent porphyritic texture conversely our two fragmental facies share attributes with previously described pipe filling coherent and apparent coherent kimberlite ckpf in terms of geometry internal organisation and abundance of country rock xenoliths we conclude that ckd and most ckpf although similar in matrix groundmass mineralogy and texture can be distinguished on the basis of internal organisation country rock lithic clast abundance texture e g apparent porphyritic texture and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported

apparent welding textures in altered pumice rich rocks fiamme and eutaxitic texture are common in ancient non welded pumice breccias the fiamme are phyllosilicate rich lenses that define a bedding parallel foliation resembling eutaxitic texture in welded ignimbrites pumice breccias in the cambrian mount read volcanics australia are composed of tube pumice clasts bubble wall shards plagioclase crystal fragments and volcanic lithic clasts and contain dark green fiamme and stylolites these sericite or chlorite sericite fiamme are aligned roughly parallel to regional bedding and are enclosed in pale pink feldspar quartz altered domains of uncompacted randomly oriented tube pumice clasts and bubble wall shards similarly pumice breccias in the miocene green tuff belt in the hokuroku basin japan comprise tube pumice clasts fiamme bubble wall shards crystal fragments and non vesicular volcanic clasts the dark green wispy smectite or chlorite fiamme are aligned parallel to bedding and are enclosed in domains of variably clay zeolite and sericite altered uncompacted pumice clasts and shards the fiamme in these examples vary in length 0 5 mm to 1 m length to height ratio 3 1 to 40 1 shape e g flame like feathery branching bow tie wedge and internal texture porphyritic massive vesicular or stylolitic the fiamme define an anastomosing bedding parallel pre tectonic eutaxitic texture delicate feathery terminations and locally preserved vesicles indicate that the fiamme are altered compacted pumice clasts the presence of bedding parallel stylolites and solution seams undeformed shards and pumice clasts adjacent to the fiamme and the overprinting of fiamme by the regional cleavage suggest that the fiamme formed during diagenesis prior to tectonic deformation and that compaction did not involve welding fiamme and eutaxitic texture in these examples resulted from the alteration dissolution and mechanical compaction of cold tube pumice clasts during diagenesis and lithification

formation of tuffisitic kimberlites by phreatomagmatic processes the rock type tuffisitic kimberlite tk occurs in the deeper diatremes of some southern african canadian and siberian kimberlite pipes and has been considered a unique rock type related to a particular emplacement process the key features of this rock type are its fragmental nature its massive well mixed appearance and a specific matrix mineralogy characterized by the presence of serpentine and microlitic clinopyroxene and the absence or scarcity of carbonate historically all these features were thought to be related to a highly specific intrusive extrusive magmatic emplacement process in this process the expansion of magmatic volatiles drives the fluidization of a pre existing shallow crustal vertical magma reservoir the embryonic pipe and its consequent evolution into the final diatreme after breakthrough to the surface the specific matrix mineralization is explained by the dissociation of carbonate into co2 and cao while the expanding co2 drives the fluidization process cao reacts with sio2 released from xenolith and olivine alteration to form microlitic clinopyroxene with the phreatomagmatic process chain we offer an alternative model that can readily explain this specific rock type ongoing explosions in a downward penetrating root zone excavate a pipe consisting of a rather regular cone shaped diatreme underlain by the irregular root zone at this stage the pipe in its majority is infilled by warm to hot pyroclastic tephra during thermohydraulic explosions in the root zone the expansion of water vapor homogenizes the overlying diatreme tephra and mixes it with the adjacent country rock lithologies this result in the massive well mixed volcaniclastic rock type typical for tkbs post emplacement hydrothermal alteration and mineralization under epi to mesothermal conditions are considered responsible for the observed specific matrix mineralogy carbonate is frequently present in coherent root zone rocks but is rare or absent due to its dissolution in hot hydrothermal fluids ascending from the lower diatreme which in turn precipitate their dissolved carbonate higher up in the cooler bedded upper diatreme region clinopyroxene also grows under epi to mesothermal conditions for the crystallization of serpentine a x h2o x co2 ratio of 20 1 is required which suggests both a high availability of groundwater and a low abundance of co2 after and likely during emplacement isotope studies h c o sr of matrix minerals suggest participation of groundwater in the formation of these minerals

complex evolution of volcanic arcs the lithofacies and palaeogeography of the cambrian stavely arc delamerian fold belt western victoria abstract the mount stavely volcanic complex is a cambrian greenstone belt in western victoria australia which formed as a continental arc during subduction of the proto pacific plate under gondwana while the msvc has been broadly described in literature its lithofacies and palaeogeography remains unconstrained the lithofacies that occur throughout the msvc were characterised using field work and diamond drill core from the geological survey of victoria the majority of the msvc consists of fragmental facies that were deposited in a deep marine environment these deposits were subsequently intruded by melts which formed the coherent facies some became either shallow intrusive or even extrusive the volcanic processes that formed the mount stavely volcanic complex are similar to those currently occurring in the present day kermadec arc furthermore the submarine nature of these volcanic facies is similar to the cambrian mount read volcanics in tasmania however in timing and cause of volcanic activity not all of these ancient volcanic terranes are related these findings help constrain the depositional environment in which the mount stavely volcanic complex was emplaced which in turn provides important constraints for the tectonic reconstruction of gondwana

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

reconstruction of a multi vent kimberlite eruption from deposit and host rock characteristics jericho kimberlite nunavut canada abstract the jericho kimberlite 173 1 1 3 ma is a small 130 70 m multi vent system that preserves products from deep 1 km portions of kimberlite vents pit mapping drill core examination petrographic study image analysis of olivine crystals grain size distributions and shape studies and compositional and mineralogical studies are used to reconstruct processes from near surface magma ascent to kimberlite emplacement and alteration the jericho kimberlite formed by multiple eruptions through an archean granodiorite batholith that was overlain by mid devonian limestones 1 km in thickness kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone at least in part by locally brecciating the host rocks after the first explosive breakthrough to surface vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption by gravitationally induced failures as portions of the vent margins slid into the vent and in the deeper portions of the vent 1 km by scaling as thin slabs burst from the walls into the vent at currently exposed levels coherent kimberlite ck dykes

mantle transition zone beneath eurasia mantle pds fig 1 converted phases are detected in the records of 11 seismograph stations in easternmost russia and china these data reveal neither a strong depression on the 660 km discontinuity nor a layer of partial melting atop the 410 km discontinuity that were found previously beneath this region in studies using long period underside sh reflections and multiple scs reverberations apparently a significant deepening of the 660 km discontinuity occurs only if the subducted plate penetrates the discontinuity which is not the case in the study region most estimates of thickness of the mantle transition zone mtz in eurasia based on the pds data are a few kilometres larger than the standard value 250 km and suggest that the average thickness of the mtz beneath eurasia is about 10 kilometres more than beneath the surrounding oceans

on the first experimental phreatomagmatic explosion of a kimberlite melt abstract detailed field investigations of kimberlite pipes of the upper cretaceous gibeon kimberlite field in southern namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation in the pipe hanaus 2 vesicle free magmatic kimberlite intruded the diatreme as a plug here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural non fragmental magmatic kimberlite from the hanaus 2 central plug as a best fit model kimberlite magma system after water was injected into the melt under controlled conditions the resulting water melt mix was triggered by a low energy shock wave 400 m s

merging field mapping and numerical simulation to interpret the lithofacies variations from unsteady pyroclastic density currents on uneven terrain the case of la fossa di vulcano aeolian islands italy abstract in order to obtain results from computer simulations of explosive volcanic eruptions one either needs a statistical approach to test a wide range of initial and boundary conditions or needs using a well constrained field case study via stratigraphy here we followed the second approach using data obtained from field mapping of the grotta dei palizzi 2 pyroclastic deposits vulcano island italy as input for numerical modeling this case study deals with impulsive phreatomagmatic explosions of la fossa cone that generated ash rich pyroclastic density currents interacting with the topographic high of the la fossa caldera rim one of the simplifications in dealing with well sorted ash one particle size in the model is to highlight the topographic effects on the same pyroclastic material in an unsteady current we demonstrate that by merging field data with 3d numerical simulation results it is possible to see key details of the dynamical current terrain interaction and to interpret the lithofacies variations of the associated deposits as a function of topography induced sedimentation settling rate results suggest that a value of the sedimentation rate lower than 5 kg m 2 s at the bed load can still be sheared by the overlying current producing tractional structures laminae in the deposits instead a sedimentation rate higher than that threshold can preclude the formation of tractional structures producing thicker massive deposits we think that the approach used in this study could be applied to other case studies both for active and ancient volcanoes to confirm or refine such threshold value of the sedimentation rate which is to be considered as an upper value as for the limitations of the numerical model

factors controlling the internal facies architecture of maar diatreme volcanoes most if not all kimberlite pipes show a multitude of facies types which imply that the pipes were emplaced under an episodic re occurrence of eruptive phases often with inter mittent phases of volcanic quiescence the majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur ing sedimentation of volcaniclastic deposits as well as their alteration and compaction after deposition an additional factor controlling pipe facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar diatreme volcano in a growing pipe the root zone moves downward and with that movement the overlying diatreme enlarges both in size and diameter however during the life span of the volcano the explosion chamber can also move upward backintothelower diatreme where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures nextto vertical shifts of explosion chambers the loci of explosions can also move laterally along the feeder dyke or dyke swarm this mobility of explosion chambers results in a highly com plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam ated crosscutting diatreme and maar crater with several lobes pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag ma recent mapping and logging results of canadian and african kimberlite pipes suggest that kimberlite magma frag mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter like production with scarce xenoliths on occasions spatter may even reconstitute and form a texturally coherent deposit on the crater floor in addition ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface respectively this highly variable emplacement behaviour is typical for basaltic maar diatreme volcanoes and since similar deposits can also be found in kimberlites it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes

tectonically controlled vent migration during maar diatreme formation an example from a miocene half graben basin in se korea abstract understanding the roles of external controls such as hydrology and tectonics is crucial for interpreting the evolution of a phreatomagmatic volcano and its subsurface structure called the diatreme the yangpori diatreme in a miocene terrestrial half graben basin of korea provides an opportunity to assess the roles of synvolcanic fault movement in forming a maar diatreme volcano the exposed part of the diatreme fill consists of two offset stacked sequences with contrasting lithofacies characteristics a lower sequence early diatreme fill of locally stratified or bedded basaltic lapilli tuff and tuff in the northeastern part and an upper sequence later diatreme fill of dacite clast rich and disorganized tuff breccia in the southwestern part the boundary between these sequences is abrupt suggesting eruption of the volcano in two distinct phases the first phase of eruption was driven by phreatomagmatic explosions mainly within the basin fill some of the diatreme fill was deposited in a local pond by debris flows and suspension settling of fines whereas the rest by fallout from eruption plumes and jets the second phase of eruption was driven by phreatomagmatic explosions at a fracture aquifer within the basement accompanying sudden lowering of the explosion locus the later diatreme fill therefore contains abundant basement derived dacite clasts its deposition was accomplished by en masse fallout of volcanic debris from dense collapsing columns a number of faults and shear fractures were generated within and in the vicinity of the diatreme together with penecontemporaneous basaltic intrusions structural and geochemical analyses show that many of these structures are genetically related with the formation of the diatreme the early diatreme fill was also tilted toward the southwest suggesting southwestward shift of the locus of explosion and diatreme excavation the explosion locus of the yangpori volcano is therefore interpreted to have migrated both vertically and laterally the abrupt lithofacies boundary between the early and later diatreme fills and the proximity of the diatreme to the basin margin suggests that the migration was caused by an abrupt tectonic activity near the basin margin most probably the feeder dike was dislocated by movement along the basin margin fault which created abundant fractures deep within the basement that redirected ascending magma and opened a new hydraulically active zone in the basement synvolcanic faulting is therefore interpreted to have played a crucial role in forming the offset stacked architecture of the yangpori diatreme sequences the faulting is also inferred to have influenced significantly the morphology and internal facies architecture of unpreserved ejecta rimbeds

pyroclastic kimberlite deposits from the victor northwest pipe ontario canada the transition from phreatomagmatic to magmatic explosivity magmas of all compositions including kimberlites may undergo both magmatic and phreatomagmatic fragmentation during emplacement in this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the victor northwest kimberlite northern ontario canada detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles each consisting of three repeated eruptive phases pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine grained poorly sorted deposits containing broken olivine crystals angular country rock fragments accretionary lapilli and variably vesicular irregular shaped juvenile pyroclasts all observations indicate these deposits formed by phreatomagmatism subsequent clastogenic coherent kimberlite deposits were formed as a result of hawaiian fire fountaining during the second phase of both cycles these phases were followed by mass wastin

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

eruption processes and facies architecture of the orion central kimberlite volcanic complex fort a la corne saskatchewan kimberlite mass flow deposits in a sedimentary basin abstract the fort a la corne diamondiferous kimberlite field consists of at least 70 bodies of volcaniclastic kimberlite hosted within a contemporaneous non volcanic sedimentary succession this study of the three dimensional stratigraphy and facies architecture across the orion central kimberlite volcanic complex highlights the variations in upper and extra vent processes the sedimentary succession consists of continental to marginal marine quartz sandstones and mudstones overlain by marginal to deep marine dark mudstone and muddy sandstones and siltstones relatively thin conformable volcaniclastic kimberlite packages are interbedded throughout the host rock stratigraphy extremely thick up to at least 211 m thick discordant to concordant volcaniclastic packages series infill at least three elongate northwest trending craters 145a 145b and 219 craters and contain laterally equivalent conformable extra crater deposits bound by marine mudstones indicative of a prevailing dominantly marine environment the volcaniclastic deposits within the 145a and 145b craters respectively are separated by a considerable hiatus whereas the deposit infilling the 219 crater was formed around the same time as 145b crater deposit multiple depositional units of massive to stratified olivine rich sand to pebble sized volcaniclastic facies infill craters and were emplaced by megaturbidite pulses fed by crystal rich eruption fountains which interacted with the crater relief stacked normally graded thick to very thick bedded matrix supported olivine rich facies characterized by brief depositional breaks between some beds represent syn to post eruptive turbidite pulses associated with the early eruptive event in the 145a crater thin layers of light grey kimberlitic mudstone underlie or occur near the base of and above the main volcaniclastic packages associated with the 145b and 219 eruptions crater infilling volcaniclastic deposits were later reworked by storm induced currents into thin to medium graded moderately sorted fine to coarse olivine rich sandstone with intercalated discontinuous muddy laminae laminated to cross laminated olivine rich silt to sand sized volcaniclastic facies constitutes a small volume volcaniclastic turbidite which marks the last preserved kimberlite emplacement event at orion central

explosive volcanic eruptions ix the transition between hawaiian style lava fountaining and strombolian explosive activity the commonest eruption styles of basaltic volcanoes involve hawaiian lava fountaining or intermittent strombolian explosions we investigate the ways in which magma rise speed at depth magma volatile content and magma viscosity control which of these eruption styles takes place we develop a model of the degree of coalescence between gas bubbles in the magma which allows us to simulate the transition between the two extreme styles of activity we find that magma rise speed is the most important factor causing the transition with gas content and viscosity also influencing the rise speed at which the transition occurs counter to intuitive expectations a decrease in gas content does not cause a transition from hawaiian to strombolian activity but instead causes a transition to passive effusion of vesicular lava rather a change from hawaiian to strombolian style requires a significant reduction in magma rise speed

competence and lithostratigraphy of host rocks govern kimberlite pipe morphology we use analogue experimentation to test the hypothesis that host rock competence primarily determines the morphology of kimberlite pipes which are subdivided into three classes class 1 pipes are steep sided diatremes emplaced into crystalline rock class 2 pipes have a wide shallow crater emplaced into sedimentary rock overlain by unconsolidated sediments class 3 pipes comprise a steep sided diatreme with a shallow angled crater emplaced into competent crystalline rock overlain by unconsolidated sediments we use different configurations of three analogue materials with varying cohesions to model the contrasting geological settings proposed to explain kimberlite pipe morphology pulses of compressed air representing the energy of the gas rich head of a kimberlitic magma are used to disrupt the experimental substrate in the case of an eruption of a gas containing no particles or fluids the organization of the analogue materials controls the excavation processes as well as the final shape of the ana

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

identification of maars and similar volcanic landforms in the west eifel volcanic field through image processing of dtm data efficiency of different methods depending on preservation state the west eifel volcanic field comprises 98 maars tuff rings and scoria rings of volcanoes younger than 700 ka digital terrain models dtms allow to automatically measure morphologic parameters of volcanic edifices such as slope angles diameters elevations floor and slope surface areas based on their morphological characteristics we subdivided the west eifel volcanoes into five morphometric groups which reflect different stages of erosion group i ii and iv comprise clear ring shaped structures the difference between these groups is that a tephra ring is well preserved in group i partially preserved in group ii and absent in group iv the original shapes of group iii maars have been lost more substantially than in groups i ii or iv but they nevertheless retain a negative shape a depression and have characteristic channel systems which can be used as search criteria maar diatremes of group v are eroded down to their feeder pipes and form hills in order to locate potential volcanic depressions that are likely to be maar volcanoes we defined common search criteria such as circular negative landforms or particular drainage system patterns for all groups except the least well preserved group v these criteria were taken as the basis for further processing of the dtm data the first processing step consisted of constructing a residual relief calculated as the difference between a filtered smoothed topographic surface and the original dtm data this identifies local topographic features we propose a method for regulating the degree of smoothing which is based on filtering of local maxima according to their distance from a surface constructed from local minima the previously defined search criteria for groups i to iv such as specific ranges of curvature slope circularity density of the drainage network were then applied to the residual relief in order to extract maar shapes not all criteria work equally well for all morphological groups combinations of multiple search criteria therefore yield the best results and efficiently identify most known maars they also separate some probable new hitherto unrecognized maars from a large number of other local depressions we also compared the erosional state of maars to their absolute ages published estimates of erosion rates for maars in the french massif central suggest a general trend of erosion rates decreasing with time elapsed since eruption however this cannot explain the strongly varying ages for maars of the same morphometric group i e similar preservation state in the west eifel volcanic field the spatial distribution of the morphometric groups shows some regularity for example strongly eroded maars are concentrated in the gerolstein area where maar density is highest whereas most well preserved maars are located east of the eifel north south depression ensd most maars affected by fluvial erosion lie near the kyll and kleine kyll streams these observations suggest differential recent uplift of the west eifel volcanic field with stronger uplift occurring west of the ensd

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

reply to discussion by brown et al on in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada

identifying magma water interaction from the surface features of ash particles the deposits from explosive volcanic eruptions those eruptions that release mechanical energy over a short time span1 are characterized by an abundance of volcanic ash2 3 this ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre existing crust host rocks interactions between rising magma and the hydrosphere oceans lakes and ground water play an important role in explosive volcanism4 5 because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy although the relative proportion of magma to host rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2 3 4 5 here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments we find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water the abundance of such ash grains can then be related to the water magma mass ratios during their interaction

tuffisitic kimberlites from the wesselton mine south africa mineralogical characteristics relevant to their formation abstract tuffisitic kimberlites from the wesselton mine consist in order of formation of the following primary components chloritized olivine macrocrysts and phenocrysts magmaclasts cryptocrystalline diopside phlogopite rich mantles and a smectite chlorite interclast matrix magmaclasts consist of one to several crystals of chloritized olivine set in a microcrystalline groundmass of diopside apatite perovskite spinel and chloritized and fresh phlogopite the latter commonly rimming chloritized olivines magmaclasts have some similarities to holocrystalline hypabyssal kimberlite but lack monticellite carbonate carbonate serpentine segregations and atoll spinels spinels in the magmaclasts show only a limited compositional evolution relative to spinels in spatially associated hypabyssal kimberlite pre existing solids including discrete olivine grains magmaclasts and most xenoliths are mantled by acicular diopside and phlogopite the interclast matrix is now represented by mixed layer phyllosilicates chlorite smectites that are poorer in alumina and iron than chlorite pseudomorphing olivine and microlitic phlogopite and diopside the interclast chlorite smectite is considered to represent former phlogopite which has undergone late stage deuteric hydrothermal like modification the interclast matrix crystallized from the volatile rich remnants of the magma none of the constituents of tuffisitic kimberlites including the chlorite or chlorite smectites represent material formed from externally derived fluids these primary textures are unique to kimberlites and only form in certain circumstances tuffisitic kimberlites formed by progressive crystallization and volatile exsolution induced segregation and or disruption within a kimberlite magma magmaclasts mantles interclast matrix during a continuum of rapidly changing conditions in a subsurface subvolcanic magmatic system this continuum represents a transition from a degassing magmatic system to a magmatic derived hydrothermal like fluid rapid drops in temperature with extensive exsolution of co 2 and incorporation of large amounts of locally derived country rock xenoliths resulted in explosive diatreme formation and within diatreme modification of the crystallization and deuteric replacement sequence of typical kimberlite magma resulting in a distinctive mineralogy and texture

the basalt pipes of the tunguska basin siberia russia high temperature processes and volatile degassing into the end permian atmosphere abstract a number of mechanisms have been proposed to explain the end permian crisis many of them explore the link between this catastrophe and the siberian traps we test the hypothesis that eruption of thermogenic gas generated in contact aureoles around igneous sills intruded into evaporite sequences of the tunguska basin triggered the crisis in particular we test the idea that the aspect that breccia pipes represent conduits for voluminous gas migration from the deep basins to the atmosphere this contribution sheds new light on the pipe formation based on new field and borehole observations and electron microscopy analyses of more than three hundred mapped magnetite bearing basalt pipes 43 are classified as diatremes the diatremes are usually circular or elliptical with multiple zones of brecciation reaching the surface sometimes with preserved in filled crater lakes the pipe diameter on the surface varies from a few tens of meters for small single diatremes to about a kilometer the largest crater lake area is 2 7 km 2 we have conducted a detailed study of the breccias in the sholokhovsk basalt pipe located within the nepa potash deposit in the tunguska basin siberia russia about n 59 and e 107 and find that the breccias are cemented by carbonate matrix calcite dolomite and halite breccia clasts are altered at various temperatures evidenced by growth of albite and garnet from basaltic glass and diopside garnet magnetite and chlorine bearing amphibole up to 1 8 cl in altered magmatic clasts these mineral assemblages suggest high temperature interactions with evaporites within the pipe conduits the large number of pipes support that degassing of halogen rich volatiles was a widespread and violent process with implications for the end permian crisis

the influence of complex intra and extra vent processes on facies characteristics of the koala kimberlite nwt canada volcanology sedimentology and intrusive processes the koala kimberlite northwest territories canada is a small pipe like body that was emplaced into the archean koala granodiorite batholith and the overlying cretaceous to tertiary sediments at 53 ma koala is predominantly in filled by a series of six distinct clastic deposits the lowermost of which has been intruded by a late stage coherent kimberlite body the clastic facies are easily distinguished from each other by variations in texture and in the abundance and distribution of the dominant components from facies analysis we infer that the pipe was initially partially filled by a massive poorly sorted matrix supported olivine rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe forming eruption this unit is overlain by a granodiorite cobble boulder breccia and a massive poorly sorted mud rich pebbly sandstone these deposits represent post eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim the crater then filled with water within which 20 m of non kimberlitic wood rich silty sand accumulated representing up to 47 000 years of quiescence the upper two units in the koala pipe are both olivine rich and show distinct grain size grading these units are interpreted to have been deposited sub aqueously from pyroclastic flows sourced from one or more other kimberlite volcanoes the uppermost units in the koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

volcanology of tuzo pipe gahcho kue cluster root diatreme processes re interpreted abstract the middle cambrian 540 ma gahcho kue kimberlite field is situated about 275 km ene of yellowknife nwt canada the kimberlites were emplaced into 2 6 ga archean granitic rocks of the yellowknife supergroup four larger kimberlite bodies 5034 tesla tuzo and hearne as well as a number of smaller pipes and associated sheets occur in the field in plan view the tuzo pipe has a circular outline at the surface and it widens towards deeper levels the pipe infill consists of several types of coherent and fragmental kimberlite facies coherent or apparent coherent possibly welded kimberlite facies dominate at depth but also occur at shallow levels as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts the central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as tuffisitic kimberlites the definition geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant the fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi plastic state or even of welded material ragged clasts at low levels are inferred to form part of a complex peperite like system that intrudes the base of the root zone a variable often high abundance of local wall rock xenoliths between and within the kimberlite phases is observed varying in size from sub millimeter to several tens of meters wall rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt like geometry between 120 and 200 m depth in the pipe steeply outward dipping bedded deposits made up of wall rock fragments occur in deep levels of the pipe and are especially common under the downward widening roof segments the gradational contact relationships of these deposits with the surrounding kimberlite bearing rocks as well as their location suggest that they formed more or less in situ different breccia facies inside the pipe suggest an origin by slumping grain flows rock fall or pyroclastic deposition the shape and facies architecture of the tuzo pipe suggests that the studied section of the pipe lies at a root zone diatreme transitional structural level composite coherent kimberlite clasts imply that recycling processes were active over time while reworked wall rock rich deposits and ductily deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone the emplacement of the tuzo pipe did not occur in a single violent explosion but involved repetitive volcanic explosions alternating with periods of relative quiescence the observed features are typical of phreatomagmatic processes which may include phases of less explosive magmatic activity

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

arrested diatreme development standing rocks east hopi buttes navajo nation usa abstract maar diatreme volcanoes defined by their relatively large pyroclastic debris filled subsurface structures and craters that cut into the pre eruptive land surface are typically found in small volume mafic to ultramafic monogenetic volcanic fields diatremes are associated with strong explosions throughout most of their development focused along feeder dikes and generally attributed to magma water interaction or high magmatic volatiles detailed mapping of the magnificently exposed standing rocks east sre diatreme shows evidence of additional eruptive complexity and offers new insights into how the plumbing and vent structures of small volume volcanoes evolve during an eruption sre is part of a larger basanitic volcanic complex that includes several diatremes formed along a series of irregular offset nw se trending dikes exposed 300 m below the pre eruptive land surface its similarly oriented elliptical shaped diatreme structure comprises predominantly country rock lithic rich breccia of coarse inhomogeneously mixed wall rock blocks sourced from above and below the current surface plus sparse juvenile material domains of pyroclastic deposits crosscut the country rock breccia deposits and the best exposed is the nw massif rising 35 m above the current erosional surface it represents a cross section of an evolving crater floor and comprises matrix rich lapilli tuff and spatter deposits cut by irregularly distributed dikes some with very complex textures the most significant deposit in terms of volume is an unbedded lapilli tuff that is poorly sorted and has a well mixed population of wall rock and juvenile clast varieties thus resembling deposits typical of diatremes it is overlain by and locally intercalated with spatter deposits and this irregular contact demarcates the base of what was during eruption an uneven evolving crater floor the generally massive variably welded spatter deposits constitute mostly lapilli sized juvenile clasts with fluidal folded over shapes and ropy surfaces subordinate thermally altered wall rock and variegated domains of lapilli tuff sre shows a progressive transition from fissure to diatreme and overall evolution from more explosive to weakly explosive eruption styles recorded at the conduit crater transition diatreme development was initiated by deep quarrying explosive eruptions along a fissure to form the country rock rich breccia only parts of the fissure remained active as magma feeding the highly explosive eruptions along the fissure localized into discrete point sources forming the matrix rich lapilli tuff deposits these superimposed deposits record the passage of multiple debris jets and subvertical fallback from shallow cratering arising from explosions triggered by magma water interaction at numerous discrete sites however instead of continuing to build a well formed diatreme the system switched to weak spattering with intermittent explosive activity and near surface dike emplacement into the unconsolidated anisotropic pyroclastic debris of the crater floor dominant spatter from strombolian style bursts accumulated on the topographically varied evolving unstable syn eruptive crater floor and led to local failure and remobilization this study demonstrates how the combination of fissure behavior and sensitivity of the shallow plumbing system to local conditions during an eruption can lead to a decrease in eruptive footprint within the diatreme structure and an overall decrease in explosivity resulting in the arrested development of an immature diatreme

gps measurements of postseismic deformation in the andaman nicobar region following the giant 2004 sumatra andaman earthquake 1 we report gps measurements of postseismic deformation from 22 campaign mode and one continuous gps sites in the andaman nicobar region following the great sumatra andaman earthquake of 26 december 2004 large horizontal displacements toward west to southwest varying in magnitude from 10 to 40 cm and with uplift reaching 16 cm occurred in the region in the first year after the earthquake the observed motion decreased logarithmically in the subsequent year we suggest that in the andaman region frictional afterslip occurred farther downdip of the coseismic rupture while in the little andaman and nicobar regions the coseismic rupture and afterslip patch partly overlapped the afterslip was mostly aseismic and did not contribute to the aftershocks the aftershocks and postseismic displacements appear to follow a similar relationship although with different decay times the temporal dependence of the two differs only by a term linear in time thus the temporal evolution of the afterslip seems to be consistent with a mechanism governed by frictional afterslip available rates of interseismic and postseismic deformation and coseismic static offsets allow us to approximately estimate a return period of about 400 years for great earthquakes in the andaman region

experimental aqueous fluidization of ignimbrite abstract experiments were carried out on the aqueous fluidization behaviour of ignimbrite and the associated formation of fluid escape pipes the starting material was an ignimbrite that had been saturated with water under vacuum until 80 15 of the vesicles were filled this aimed to reproduce water logging conditions of a hot pyroclastic flow in contact with water such as in the case of a lahar or of a pyroclastic flow entering the sea the ignimbrite sample was fluidized by water at vertical velocities from 0 005 to 7 mm s 1 for durations of 10 180 min channelling occurred almost immediately at even the lowest velocities and pipe channel size increased slightly with time the pipes had the form of elongated upwardly flared funnels and grew downwards and sideways with time even under decreasing flow conditions pipe nucleation and growth generated irregular pressure fluctuations in the sample showing that the standard p u plots commonly used in fluidization studies are not useful for coarse grained poorly sorted samples each pipe was stratified internally with a basal layer rich in dense lithic and crystals an intermediate layer rich in pumice and an upper layer rich in fine components as much as 30 of the initial sample mass was elutriated including platy mica crystals at the highest flow rates at velocities exceeding 2 mm s 1 duration of experiment 10 min single pipes grew and coalesced rapidly either forming a single large pipe or causing the entire sample to become segregation layered in natural water lain sediments pipes may form near the end of deposition and during compaction because during transport shear may reduce channelling by water we also measured the degree of crystal enrichment in pipes we conclude that the presence of fluid escape pipes in ignimbrite like sediment cannot be used to infer a gas fluidized origin of the deposit since the geometry granulometry and degree of crystal enrichment in water generated pipes are similar to those in pipes formed by gas under dry conditions

false pyroclastic textures in altered silicic lavas with implications for volcanic associated mineralization there is strong emphasis in recent literature that zn cu pb massive sulfide deposits associated with felsic volcanism the kuroko type deposits are directly related to submarine calderas formed by large submarine pyroclastic eruptions altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits volcaniclastic debris flows and massive to thin bedded tuffaceous rocks form a major component of a silurian succession that hosts zn cu pb massive sulfide deposits at benambra southeastern australia however critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures including apparent pumice fiamme and glass shards intercalated sediments indicate that most of the silicic volcanics were emplaced in subaqueous settings ranging from relatively shallow water to deep marine the false textures were produced mainly in originally glassy lava by the combined effects of devitrification perlitic fracture and pervasive hydrothermal alteration these processes are related to the emplacement and cooling history of individual lavas and to a regional essentially synvolcanic hydrothermal system tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics thereby further enhancing clastic appearance subsequent to recognition of widespread false pyroclastic textures at benambra similar features have been identified in three other australian massive sulfide districts these results suggest that a reinvestigation of host rocks is warranted at other deposits where submarine pyroclastics are documented noncaldera settings combining subsidence in an extensional volcano tectonic basin passive submarine lava effusion within the basin and ambient sedimentation derived from basin margins including resedimented pyroclastic debris may be more relevant models in a number of massive sulfide districts

a practical guide to terminology for kimberlite facies a systematic progression from descriptive to genetic including a pocket guide kimberlite terminology remains problematic because both descriptive and genetic terms are mixed together in most existing terminology schemes in addition many terms used in existing kimberlite terminology schemes are not used in mainstream volcanology even though kimberlite bodies are commonly the remains of kimberlite volcanic vents and edifices we build on our own recently published approach to kimberlite facies terminology involving a systematic progression from descriptive to genetic the scheme can be used for both coherent kimberlite i e kimberlite that was emplaced without undergoing any fragmentation processes and therefore preserving coherent igneous textures and fragmental kimberlites the approach involves documentation of components textures and assessing the degree and effects of alteration on both components and original emplacement textures this allows a purely descriptive composite component textural and compositional petrological rock or deposit name to be constructed first free of any biases about emplacement setting and processes then important facies features such as depositional structures contact relationships and setting are assessed leading to a composite descriptive and genetic name for the facies or rock unit that summarises key descriptive characteristics emplacement processes and setting flow charts summarising the key steps in developing a progressive descriptive to genetic terminology are provided for both coherent and fragmental facies deposits rock units these can be copied and used in the field or in conjunction with field e g drill core observations and petrographic data because the approach depends heavily on field scale observations characteristics and process interpretations only the first descriptive part is appropriate where only petrographic observations are being made where field scale observations are available the progression from developing descriptive to interpretative terminology can be used especially where some petrographic data also becomes available

the role of fluidization in the emplacement of pyroclastic flows 2 experimental results and their interpretation abstract although often quoted as being important in the emplacement of pyroclastic flows little relevant experimental fluidization work has been done fluidization processes are first reviewed for a simple system against which ignimbrite samples can be compared results from fluidization experiments on ignimbrite samples show that their behaviour differs radically from any simple system principally because of their extremely poor sorting at a certain gas velocity u ie whose value cannot be predicted some ignimbrite samples begin to expand whilst at a higher gas velocity u mp the samples begin to show segregation structures the minimum fluidization velocity u mf in simple systems is replaced by u mp the value of the latter in pyroclastic materials cannot be easily or reliably predicted from published u mf correlations and a new empirical method for determining u mp is presented from the experimental data during fluidization ignimbrite samples expand much less than more conventional materials due to the bypassing of gas through segregation channels the total amount of expansion is also limited e g a 100 m thick pyroclastic flow will from fairly high gas velocities deflate to form an ignimbrite which is not less than 70 85 m thick during fluidization the poor sorting causes only part of the weight of the ignimbrite samples to be supported by the gas flow the degree of support increasing with an improvement in sorting segregation structures and grain size and compositional variations within fluidized ignimbrite samples show varying behaviour depending on the sorting of the sample after subjecting a sample to high gas velocities the net result is a grain size and compositionally zoned bed which becomes richer upwards in pumice and fine material once formed these features cannot be destroyed under laboratory conditions implying that some segregation structures should be present in any ignimbrite where u mp was exceeded

an integrated model of kimberlite ascent and eruption diatremes are carrot shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock these unusual enigmatic and complex features are famous as the source of diamonds here we present a new model of kimberlite ascent and eruption emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes dyke initiation in a deep co2 rich source region in the mantle leads to rapid propagation of the dyke tip below which co2 fluid collects with a zone of magmatic foam beneath when the tip breaks the surface of the ground gas release causes a depressurization wave to travel into the magma this wave implodes the dyke walls fragments the magma and creates a ringing fluidization wave together these processes form the diatreme catastrophic magma chilling seals the dyke no precursor to the eruption is felt at the surface and the processes are complete in about an hour

tephra ring interpretation in light of evolving maar diatreme concepts stracciacappa maar central italy abstract maar diatreme volcanoes in their simplest form are thought to result primarily from discrete subsurface phreatomagmatic explosions tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward ejecting progressively deeper seated lithic clasts that contribute to the tephra rings additionally variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation which are in turn related to variations in magma water ratios at the explosion sites recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime rather than being limited to simple downward migration experiments numerical modeling and field data indicate that most explosions deeper than about 200 250 m will not vent to the surface but instead contribute to mixing within the diatremes arrival of deep seated lithics at the surface is related to this mixing process experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings even in the absence of variations in explosion mechanisms fragmentation efficiency here we document the tephra ring of stracciacappa maar central italy and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations because most elements of both conceptual models are viable the work points to an important need to develop criteria to distinguish between the various inferred mechanisms

sedimentologic and stratigraphic constraints on emplacement of the star kimberlite east central saskatchewan abstract diamond bearing kimberlites in the fort a la corne region east central saskatchewan consist primarily of extra crater pyroclastic deposits which are interstratified with lower cretaceous albian and cenomanian marine marginal marine and continental sediments approximately 70 individual kimberlite occurrences have been documented the star kimberlite occurring at the southeastern end of the main fort a la corne trend has been identified as being of economic interest and is characterized by an excellent drill core database integration of multi disciplinary data sets has helped to refine and resolve models for emplacement of the star kimberlite detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex micropaleontologic and biostratigraphic analysis of selected sedimentary rocks and u pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at star the oldest kimberlite in the star body erupted during deposition of the predominantly continental strata of the lower mannville group cantuar formation kimberlites within the cantuar formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate successive eruptive events occurred contemporaneous with deposition of the marginal marine upper mannville group pense formation kimberlites within the pense formation consist primarily of terrestrial airfall deposits fine to medium grained cross stratified kimberlitic olivine dominated sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression the location of the source feeder vents of the cantuar and pense kimberlite deposits has not been identified the youngest and volumetrically most significant eruptive events associated with the star kimberlite occur within the predominantly marine lower colorado group joli fou and viking formations kimberlite beds which occur at several horizons within these units consist of subaerial and marine fall deposits the latter commonly exhibiting evidence of wave reworking black shale encased resedimented kimberlite beds likely deposited as subaqueous debris flows and turbidites are particularly common in the lower colorado group during its multi eruptive history the star kimberlite body is interpreted to have evolved from a feeder vent and overlying positive relief tephra ring into a tephra cone initial early joli fou volcanism resulted in formation of a feeder vent 200 m diameter and tephra ring subsequent eruptions dominated by subaerial deposits partly infilled the crater and constructed a tephra cone a late joli fou eruption formed a small 70 m diameter feeder pipe slightly offset to the nw of the early joli fou feeder vent deposits from this event further infilled the crater and were deposited on top of early joli fou kimberlite proximal to the vent and sediments of the joli fou formation distal to the vent the shape of the tephra cone was modified during multiple marine transgression and regression cycles coeval with deposition of the lower colorado group resulting in wave reworked kimberlite sand along the fringes of the cone and kimberlitic event deposits tempestites turbidites debris flows in more distal settings

distinct kimberlite pipe classes with contrasting eruption processes abstract field and scott smith field m scott smith b h 1999 contrasting geology and near surface emplacement of kimberlite pipes in southern africa and canada proc 7th int kimb conf eds gurney et al 1 214 237 propose that kimberlite pipes can be grouped into three types or classes classical or class 1 pipes are the only class with characteristic low temperature diatreme facies kimberlite in addition to hypabyssal and crater facies kimberlite class 2 and 3 pipes are characterized only by hypabyssal and crater facies kimberlite in an increasing number of class 1 pipes a new kimberlite facies transitional facies kimberlite is being found in most cases this facies forms a zone several metres wide at the interface between the hypabyssal and diatreme facies the transitional facies exhibits textural and mineralogical features which are continuously gradational between the hypabyssal and the diatreme types the textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside both transitional and diatreme facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially co 2 rich volatiles from the volatile rich kimberlite magma the transitional facies is initiated by volatile exsolution at depths of about 3 km below the original surface with subsequent cracking through to the surface and resultant rapid decompression the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies thus diatreme facies kimberlite and class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism distinctly different petrographic features characterize crater facies kimberlite in each of the three pipe classes in crater facies kimberlites of class 1 pipes small pelletal magmaclasts and abundant microlitic diopside are characteristic these features appear to reflect the derivation of the crater facies material from the underlying diatreme zone most class 2 pipes have shallow craters and the crater facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli which are sometimes welded and have vesicles as well as glass possible kimberlite lava also occurs at two class 2 pipes in n angola the possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form class 2 pipes most class 3 kimberlites have very steep craters and crater facies rocks are predominantly resedimented volcaniclastic kimberlites in some cases characterized by the presence of abundant angular magmaclasts which are petrographically very similar to typical hypabyssal facies kimberlite found in class 1 pipes the differences in crater facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition specifically volatile composition kimberlite forming pipe classes 1 and 3 is thought to be relatively water rich and is emplaced by processes involving magmatic exsolution of volatiles the kimberlite magma forming class 2 pipes is co 2 rich can rise to shallow levels and can initiate phreatomagmatic emplacement processes

low temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic mt gambier volcanic complex south australia and their relevance for understanding some deposits in diatremes the occurrence of pyroclastic flow deposits is not restricted to polygenetic and or felsic volcanic systems but can also occur at mafic monogenetic volcanic centres at the c 5 ka mt gambier volcanic complex australia two small volume pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the blue lake east and the valley lake maar craters the facies descriptions of the two massive poorly sorted lapilli tuff and tuff breccia deposits are given low grade carbonized wood fragments in the deposits mark low emplacement temperatures 180 270 c for these deposits with only c 12 23 of the total thermal energy of the original magma preserved at the time of deposition the blue lake east vent erupted by forming a short lived vertical eruption column which could not be sustained and collapsed the valley lake pyroclastic flow deposit has a highly asymmetric dispersal to the west indicating that this deposit was formed during an eruptive phase with a large westward directed lateral component such massive poorly sorted deposits outside the maar crater indicate that some similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets

phreatomagmatism and its relevance abstract phreatomagmatic explosive volcanism results from the interaction of magma with external water groundwater or surface water close to or at the earth s surface magma of any composition acid to ultrabasic may be involved in this type of explosive activity at present the relevance of phreatomagmatism is realized increasingly because it takes place in many diverse environments and affects not only small monogenetic volcanoes but also large volcanoes at some volcanoes as for example at lake taupo new zealand a volume of several tens of km 3 of magma may have been mixed with several km 3 of external water phreatomagmatism has thus become an important aspect in volcanic hazard assessments other fields involving the interaction of water with a melt are the study of nuclear and other industrial safety hazards of magma mixing of diamond carrying volcanism and of hydrothermal ore deposits as well as geothermal and groundwater reservoirs experiments on the dynamics of magma water interactions provide increasing insight into the principles and hazards of phreatomagmatic explosions this type of research will become a new field in experimental petrology

finding eruptive mechanisms through pyroclasts from the current eruption 1984 present at nevado del ruiz volcanic complex colombia abstract the dominant fragmentation mechanisms in the magma involved in a volcanic eruption can be understood from the compositional and textural analysis of the fragments emitted as pyroclasts individual pyroclasts can be juvenile accessory and accidental and detailed analyses of their relative proportions as well as the juvenile clast features can shed light into the fragmentation processes involved in the eruption despite the multiple hundreds eruptive pulses since the nevado del ruiz re activation in 1984 following a quiescence of nearly 100 years such analyses have not been carried out for the ash emissions preceding the recently emplaced plug here four deposits related to the recent emissions were analysed from their pyroclasts the samples correspond to eruptive pulses from the years 1985 1986 1988 and 2012 their characterisation allowed proposing differences between likely juvenile fragments i e representative of involved magma and accessory fragments i e pre existing co magmatic the results indicate that alteration can be used as a criterion to distinguish them although this could not account for syn eruptive hydrothermal alteration of juvenile glass or for the preservation of accessory glass emplaced in glacial conditions this criterion could serve as a foundation to direct fresh glass fragment analyses following this criterion the fresh glass fragments which are assumed to be juvenile fragments are highly vesicular they exhibit spongy and fibrous textures that are related to the predominant vesicle shape their predominance suggests a magmatic fragmentation mechanism while blocky shaped poorly vesicular cracked glass shards suggest a phreato magmatic origin specifically november of 1985 and january of 1986 glassy ashes are primarily highly vesicular suggesting magmatic explosive events by contrast samples from the march of 1988 and may of 2012 events are primarily characterised by low fresh glass content they also exhibit quenched surfaces indicating that the magma water interaction acted as an important factor for triggering the eruptive pulses real time componentry petrography and glass characterisation in fine ash could significantly complement traditional geophysical monitoring and importantly improve the hazard assessments on the civil aviation during the ongoing nevado del ruiz activity

experimental birth of a maar diatreme volcano abstract maar diatreme eruptions are hazardous to people and infrastructure and are also linked to the formation of the kimberlitic variety of diatremes which is important economically processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood we conducted field scale experiments using analog materials to shed more light on these processes especially the formation of the proto diatreme during the first explosions of a maar eruption specifically a series of buried explosions in a prepared layered substrate pad produced craters extra crater deposits and sub crater deposits analogous to volcanic maar craters tephra rings and incipient diatremes post explosion substrate excavation revealed that single large explosions produce sub crater deposits extending nearly to the crater rim crest the same energy divided into three blasts either co located or at different depths with the same epicenter produced narrower and sometimes deeper sub crater deposits even though the final sizes of the craters were similar to that produced by the single large blast the sub crater deposits have an upper zone with domains from different substrate depths and an underlying zone distinguished primarily by being more loosely packed than the original substrate videos show surface motion extending beyond the post shot crater rim and largely vertical ejection and fallback of material into the footprint of these deposits especially for the explosions that occurred below optimal depth of burial we infer that much of the loosely packed material was disassembled vertically transported to different heights during the explosions then fell back without significant relative lateral movement of grains however subvertical fallback did produce apparent cross cutting structures in shallow sub crater deposits one explosion ejected material from the deepest substrate horizon but it was redeposited only within the crater and is unrepresented in the ejecta rim implications of the experiments for maar diatreme volcanoes including some kimberlite pipes are as follows 1 vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes 2 direct vertical fallback is possibly an important process forming diatreme deposits especially during the earliest stages and 3 even in our limited simulation the number and scaled depth of explosions clearly affect proto diatreme size and structure

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the muskox pipe nunavut canada abstract the matrix of volcaniclastic kimberlite vk from the muskox pipe northern slave province nunavut canada is interpreted to represent an overprint of an original clastic matrix muskox vk is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents temperature of formation and nature of the altering fluids using whole rock x ray fluorescence xrf whole rock x ray diffraction xrd microprobe analyses back scatter electron bse imaging petrography and core logging we find that most matrix minerals serpentine phlogopite chlorite saponite monticellite fe ti oxides and calcite lack either primary igneous or primary clastic textures the mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools or in the case of calcite by precipitation from ca bearing fluids into a secondary porosity the first mineral assemblage consists largely of serpentine phlogopite calcite fe ti oxides and monticellite and occurs in vk with relatively fresh framework clasts alteration reactions driven by deuteric fluids derived from the juvenile constituents promote the crystallisation of minerals that indicate relatively high temperatures of formation 400 c lower temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures thus shielding the facies from further interaction with fluids the other two matrix mineral assemblages consist largely of serpentine phlogopite calcite diopside and chlorite they form in vk that contains more country rock which may have caused the deposit to be cooler upon emplacement most framework components are completely altered suggesting that larger volumes of fluids drove the alteration reactions these fluids were likely of meteoric provenance and became heated by the volcaniclastic debris when they percolated into the vk infill most alteration reactions ceased at temperatures 200 c as indicated by the absence or paucity of lower temperature phases in most samples such as saponite recognition that muskox vk contains an original clastic matrix is a necessary first step for evaluating the textural configuration which is important for reconstructing the physical processes responsible for the formation of the deposit

hydrothermal breccias in vein type ore deposits a review of mechanisms morphology and size distribution abstract breccias are among the most widely distributed rock textures found in hydrothermal vein type deposits previous studies have mainly been interested in developing qualitative descriptive approaches leading to a confusing profusion of terms brecciation originates in numerous ways resulting in highly complex classification systems and frequent misinterpretations of facies field observations are difficult to reconcile with physical theories of fragmentation partly due to the fact that few satisfactory quantitative tools have been developed a review of the main brecciation processes occurring in hydrothermal vein type deposits allows for the discrimination between chemical and physical mechanisms including tectonic comminution wear abrasion two types of fluid assisted brecciation hydraulic and critical volume expansion or reduction impact and collapse each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry including fragment morphology size distribution of the fragments fabric and dilation ratio the first two parameters are especially important because 1 the morphology of the fragments allows chemical and physical mechanical breccias to be distinguished and 2 the particle size distribution psd is a function of the energy input during breccia formation the slope of the cumulative psd fractal dimension ranges from high values for high energy brecciation processes to low values for low energy processes indicated by an isometric distribution the evolution of a vein system can be divided into three stages propagation wear and dilation these stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity these two thresholds also mark the transition between different types of brecciation mineralization occurs during all three stages and may display different textures due to pressure variations the use of quantitative parameters in fault related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment including structural setting and crustal level as well as fluid rock interactions recognition of the different breccia types could also be important during the early stages of mineral exploration

early volcanological research in the vulkaneifel germany the classic region of maar diatreme volcanoes the years 1774 1865 the vulkaneifel volcanic west eifel in the western part of the rhenish slate mountains germany played a major role in the early history of volcanology in general and especially so with respect to the recognition and volcanology of maar diatreme volcanoes in 1819 the volcano term maar was introduced into the scientific literature by johann steininger a teacher from trier treves who established the vulkaneifel as the type region for this kind of volcano at that time only a few pioneers in the earliest days practically all of them were amateurs had visited this part of western central europe in the late eighteenth and earliest nineteenth century despite many making important contributions to knowledge of volcanoes not all were appreciated in consequence only in the second half of the twentieth century did new ideas and concepts concerning volcanism in general and phreatomagmatic activity in particular arise that were not previously presented by these pioneers their pathbreaking ideas have so far been mostly ignored we feel because of the old literature s limited availability this paper tries to shed new light on these trailblazers a selection of early geological maps of the vulkaneifel and its neighbouring regions demonstrates the enormous advancements that had been achieved during those early days to a large extent in this part of western europe

the geology of kimberlite pipes of the ekati property northwest territories canada abstract this paper reviews key characteristics of kimberlites on the ekati property nwt canada to date 150 kimberlites have been discovered on the property five of which are mined for diamonds the kimberlites intrude archean basement of the central slave craton numerous proterozoic diabase dykes intrude the area the precambrian rocks are overlain by quaternary glacial sediments no phanerozoic rocks are present however mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late cretaceous and tertiary cover likely

problems with an in vent column collapse model for the emplacement of massive volcaniclastic kimberlite a discussion of in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada by porritt et al j volcanol geotherm res 174 90 102

volcanic initiation of the eocene heart mountain slide wyoming usa abstractthe eocene heart mountain slide of northwest wyoming covers an area of as much as 5000 km2 and includes allochthonous paleozoic carbonate and eocene volcanic rocks with a run out distance of as much as 85 km recent geochronologic data indicated that the emplacement of the slide event occurred at 48 9 ma using laser ablation inductively coupled plasma mass spectrometry la icpms extracted from u pb zircon ages from basal layer and injectite carbonate ultracataclasite cuc we now refine that age with u pb results from a lamprophyre diatreme that is temporally and spatially related to the cuc injectites the ages for the lamprophyre zircons are 48 97 0 36 ma la icpms and 49 19 0 02 ma chemical abrasion isotope dilution thermal ionization mass spectrometry thus the lamprophyre and cuc zircons are identical in age and we interpret that the zircons in the cuc were derived from the lamprophyre during slide emplacement moreover the intrusion of the lamprophyre diatreme provided the trigg

the miocene costa giardini diatreme iblean mountains southern italy model for maar diatreme formation on a submerged carbonate platform in this paper we present a model for the growth of a maar diatreme complex in a shallow marine environment the miocene age costa giardini diatreme near sortino in the region of the iblei mountains of southern sicily has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions vesicular lava clasts accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas rich magma abundant xenoliths in the upper late deposited beds of the ring suggest rapid magma ascent and deepening of the eruptive vent is shown by the change in slope of the country rock the interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount the occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity intrusions of nonvesicular magma forming plugs and dikes occur on the western side of the diatreme and at the margins close to the contact between breccia deposits and country rock they indicate involvement of volatile poor magma possibly during late stages of activity we propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment this allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center a subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water at which time the hyaloclastite was deposited emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated locally forming peperites the sheltered low energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts ultimately lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions

the importance of the transport system in shaping the growth and form of kimberlite volcanoes transport of kimberlite magma from the mantle to the surface is a complex process that involves many often sudden and dramatic changes in transport style the pace and magnitude of changes in the transport system acclerate as the magma nears the surface culminating in eruptions during which multiple transport events take place simultaneously as well as single transport events that change significantly from their point of origin to final deposition understanding the range of transport styles recorded by kimberlite deposits is key to describing the type and style of eruptions the growth of cones and craters and the distribution geometry and structure of the deposits that make up kimberlite volcanoes in an economic context building a clear picture of the processes that shape deposits is essential for selecting exploration targets and evaluating the grade and value of diamond bearing kimberlites variations in diamond grade reflect differences in the diamond content of different magma parcels erupted during the lifetime of the kimberlite volcano sorting during transport of eruption products or re working of diamonds during crater growth cone collapse and erosion deep transport systems control delivery of magma and diamonds to eruption sites the initial stages of kimberlite transport involve rise of magma in narrow interconnected networks of fluidfilled fractures there is compelling evidence that kimberlite magmas are extracted from their source region and erupted without significant residence times in the crust many single kimberlite volcanoes have significant variations in diamond grade between different kimberlite phases suggesting eruption of distinct perhaps unrelated magma parcels at the same site

geology of the bk9 kimberlite damtshaa botswana implications for the formation of dark volcaniclastic kimberlite the bk9 kimberlite consists of three overlapping pipes it contains two dark varieties of massive volcaniclastic kimberlite informally termed dark volcaniclastic kimberlite dvk dvk ns is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes dvk c is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments the features observed within the dvk units of the bk9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material these include the massive interior of the pipes marginal epiclastic units well dispersed country rock xenoliths and small scale heterogeneities in lithic clast abundance the upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater after the eruption has ceased the wall rock of the bk9 kimberlite dips inwards and is interpreted as post pipe fill subsidence of the adjacent country rock the two dvk units contain interstitial silt sized pyroclasts the dvk ns has a higher fraction of former melt and displays evidence of incipient welding as a result of differences in eruption dynamics these units demonstrate that whilst dvk is comparable in many respects to mvk and forms part of a spectrum of volcaniclastic rocks formed by fluidisation it differs in frequently containing silt sized particles and including agglutinated and welded varieties with a high melt fraction the dvk varieties studied here also have a distinctive hydrothermal assemblage resulting from the abundance of low silica accidental lithic clasts both the hydrothermal alteration and the abundance of silt sized particles contribute to the dvks distinctive dark colour

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

the crust and upper mantle discontinuity structure beneath alaska inferred from receiver functions in this study three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south central alaska we used teleseismic waveform data recorded by 36 stations in the broadband experiment across the alaska range beaar and 4 permanent stations in alaska h stacking method using p to s converted wave and its multiply reflected waves between the earth s surface and the moho discontinuity is adopted to estimate the crustal thickness h and average crustal vp vs ratio in this region the receiver function results for 24 stations show that the crustal thickness under alaska ranges from 26 0 to 42 6 km with an average value of 33 8 km and the vp vs ratio varies from 1 66 to 1 94 with an average value of 1 81 which corresponds to an average poisson s ratio of 0 277 with a range from 0 216 to 0 320 high poisson s ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle common converted point ccp stacking results of receiver functions along three lines show clear moho and slab images under this subduction zone the depths of the slab from our ccp stacking images are consistent with those estimated from the wadati benioff zone wbz in the area between two stations dh2 147 8 w 63 3 n and dh3 147 1 w 63 0 n a moho depth offset of about 10 km is found by both the h and ccp stacking techniques common depth point cdp stacking of receiver functions shows not only the 410 520 and 660 km discontinuities but also significant variations 30 to 15 km in the transition zone thickness under the southwest and southeast parts of the study region the transition zone becomes thinner by 20 30 km indicating that the temperature there is 150 200 k higher than that of the normal mantle 2004 elsevier b v all rights reserved

emplacement and reworking of cretaceous diamond bearing crater facies kimberlite of central saskatchewan canada in central saskatchewan canada kimberlites were emplaced into cretaceous marine and nonmarine clastic sediments core recovered from one drill hole that intersects kimberlite smeaton fac uk core 169 8 was selected for an integrated study involving sedimentology volcanology mineralogy geochemistry palynology micropaleontology organic petrology and radiometric age determination only crater facies kimberlite has been observed there is no indication of the locations of feeder dikes four varieties of kimberlite occur all originating from subaerial volcanism 1 fluvial reworked kimberlite 2 diamondiferous kimberlite lapillistone air fall deposits 3 kimberlite olivine crystal tuff air fall deposits and 4 diamondiferous marine wave reworked kimberlite within the multiple primary eruptive phases of the kimberlite air fall deposits the volcanic style changed upward with time from violent strombolian to more explosive volcanism the bulk of the volcanism formed conformable air fall deposits on terrestrial sediments of the cantuar formation resulting in the development of positive relief tephra cones subsequent marine transgression associated with the westgate formation partially beveled the top of the cone the kimberlite air fall deposits contain microdiamonds 5 to 25 m in diameter the maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits although originally derived from magma at high temperatures did not thermally affect entrained surficially derived clasts or the country rock during emplacement the chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile at least two episodes of kimberlite volcanism occurred in the middle and late albian paleontologically assigned a u pb perovskite radiometric age of 101 1 2 2 ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post middle albian and pre late albian to late albian

geology and diamond distribution of the 140 141 kimberlite fort a la corne central saskatchewan canada abstract the cretaceous age fort a la corne falc kimberlite province comprises at least 70 bodies which were emplaced near the edge of the western canadian interior seaway during cycles of marine transgression and regression many of the bodies were formed during a marine regression by a two stage process firstly the excavation of shallow but wide craters and then subsequent infilling by xenolith poor crater facies subaerial primary pyroclastic kimberlite the bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin but high volume pyroclastic kimberlite deposits each body is composed of contrasting types of kimberlite reflecting different volcanic histories and therefore are considered separately the 140 141 kimberlite is the largest delineated body in the province estimated to have an areal extent below glacial quaternary sediments in excess of 200 ha the infilling of the 140 141 crater is complex resulting from multiple phases of kimberlite the central part of the infill is dominated by several contrasting phases of kimberlite one of these phases is a primary pyroclastic airfall mega graded bed up to 130 m in thickness the constituents grade in size from very fine to coarse macrocrystic kimberlite through to a basal breccia the mega graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe a second different phase of kimberlite is interpreted as representing a younger nested crater within the mega graded bed centrally located thicker intersections 450 m of this younger kimberlite may indicate a vent for the kimberlite crater the thickness of the mega graded bed increases with proximity to the younger kimberlite in the study area macrodiamond minibulk sample grades from the mega graded bed have been obtained from nine large diameter drill holes located within the northwest part of the body from an area of 20 ha which represents approximately 10 of the currently modeled kimberlite outline diamond grade increases with depth within the mega graded bed and also increases within the same unit towards the centrally positioned younger kimberlite macrodiamond sample grades vary from low at the top of the mega graded bed to considerably higher grades near the base total sample grade per drill hole varies from moderate near the vent feature to lower grades 200 300 m from the vent feature macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega graded bed there is a strong correlation between depth and increased stones per tonne and a similar correlation between stones per tonne and proximity to the vent feature the data supports the emplacement model of the mega graded bed and in turn this information is useful in understanding the macrodiamond distribution within this bed

seismic reflection imaging of thin kimberlite dykes and sills exploration and deposit characterization of the snap lake dyke canada abstract seismic reflection techniques are for the first time used to image a thin diamondiferous kimberlite dyke from subcrop to depths greater than 1300 m exploration for vertical kimberlite pipes generally utilizes potential field techniques that often fail to reveal subhorizontal or shallow dipping intrusions in contrast seismic reflection methods are well suited for imaging targets with this geometry therefore in order to evaluate seismic reflection as a tool for subhorizontal kimberlite dyke sill exploration and mine planning a feasibility study and subsequent seismic survey were undertaken on the diamondiferous snap lake dyke northwest territories canada a substantial drilling program has mapped the dyke as a gently dipping sheet that averages 2 3 m in thickness the detailed structural and composition data available at snap lake provides a unique opportunity to test reflection techniques on a well sampled deposit the feasibility study involved measuring p velocities and densities of cores drilled from the kimberlite and host rocks these data were used to model reflection amplitudes evaluate resolution limitations and determine the acquisition parameters for the reflection survey two 2 d lines were acquired that provide comparative datasets for different sources explosive and vibroseis and ground types land and lake ice in addition the exploration scale survey incorporated high fold 40 260 nominal and long offsets 3260 m the explosive source profile recorded on land yielded a superb image of the dyke from depths of 60 m to more than 1300 m over a lateral distance of 5700 m the seismic image correlates well with adjacent drill hole data and adds considerable detail to the topography of the kimberlite sheet determined by drilling the vibroseis source also imaged the dyke but only when sources and geophones were on land the dyke was not imaged beneath the ice due to reverberation and attenuation effects the frequency response and unusually strong reflection amplitudes from the dyke indicate the importance of tuning effects and multiples for this type of target and acquisition environment apparent correlations between reflection amplitudes and dyke structure e g thickness feathering 3 d geometry suggest that seismic reflection data may be valuable for guiding drilling programs the results demonstrate that in the appropriate situation seismic methods have great potential for use in kimberlite exploration subsurface mapping and detailed imaging for mine development purposes

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective and some suggestions five significant problems hinder advances in understanding of the volcanology of kimberlites 1 kimberlite geology is very model driven 2 a highly genetic terminology drives deposit or facies interpretation 3 the effects of alteration on preserved depositional textures have been grossly underestimated 4 the level of understanding of the physical process significance of preserved textures is limited and 5 some inferred processes and deposits are not based on actual modern volcanological processes these issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits the traditional steep sided southern african pipe model class i consists of a steep tapering pipe with a deep root zone a middle diatreme zone and an upper crater zone if preserved each zone is thought to be dominated by distinctive facies respectively hypabyssal kimberlite hk descriptively called here massive coherent porphyritic kimberlite tuffisitic kimberlite breccia tkb descriptively here called massive poorly sorted lapilli tuff and crater zone facies which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite rvk porphyritic coherent kimberlite may however also be emplaced at different levels in the pipe as later stage intrusions as well as dykes in the surrounding country rock the relationship between hk and tkb is not always clear sub terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis modern in vent volcanological processes should initially be considered to explain observed deposits crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes indicating that the pipe was largely empty at the end of the eruption and subsequently began to fill in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents classes ii and iii canadian kimberlite models have a more factual descriptive basis but are still inadequately documented given the recency of their discovery the diversity amongst kimberlite bodies suggests that a three model classification is an over simplification every kimberlite is altered to varying degrees which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in vent context few preserve original textures the effects of syn to post emplacement alteration on original textures have not been adequately considered to date and should be back stripped to identify original textural elements and configurations applying sedimentological textural configurations as a guide to emplacement processes would be useful the traditional terminology has many connotations about spatial position in pipe and of process perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies mining terminological scheme because it is so entrenched however for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics important variations in these and the process origins for example every deposit of tkb is different in componentry texture or depositional structure however because so many deposits in many different pipes are called tkb there is an implication that they are all similar and that similar processes were involved which is far from clear

internal vs external forcing in shallow marine diatreme formation a case study from the iblean mountains se sicily central mediterranean a model of diatreme evolution in a shallow marine setting is based on a multi disciplinary analysis of diatremes in the iblean mountains sicily the approach includes stratigraphic volcanological structural petrologic and compositional data we invoke a complex interplay of internal rapid ascent and pyroclastic fragmentation of a volatile co2 rich nephelinitic magma at depth and external factors these comprise hydroclastic explosions due to near surface interaction of the rising particle volatile mixture with seawater and water saturated lime mud other external factors contributing to diatreme formation include regional and local tectonics graben formation in pull apart motion combined with lateral pipe enlargement by bedrock spalling and radial block subsidence into the diatreme pipe we suggest that fragmentation of volatile rich magma due to internal eruption forcing was fundamental in the formation of the iblean shallow marine diatremes internal and external factors may act to a variable degree however during diatreme evolution in general

earliest eocene cold period and polar amplification insights from δ2h values of lignin methoxyl groups of mummified wood abstract three well preserved mummified wood specimens have been excavated from three earliest eocene 55 5 55 2 and 53 3 ma kimberlite pipes in the subarctic northwest territories canada 64 n each specimen contained multi decadal length tree ring series and allowed measurements of stable hydrogen isotope ratios 2h values of the lignin methoxyl groups commonly used cellulose was largely degraded we used these 2h signatures for the reconstruction of three representative 2h values of the local precipitation by applying calibrated isotope fractionations to investigate deep time paleoclimatology our reconstructions indicate unprecedented low values for the subarctic early eocene which however show within 2 2 myr an increasing trend from 206 17 202 17 to 168 17 these values were interpreted along with other arctic subarctic and mid latitudinal proxy records of that time period indicating in total a period of low stable hydrogen and oxygen isotope ratios in precipitation onset between 55 7 and 54 9 ma and a recovery at 53 3 0 6 ma which is assumed to have been primarily caused by a continental cold period an increased magnitude of this cold period was noted for our subarctic reconstructions pointing to a polar amplification where surface air temperature changes in the arctic exceed the global trend in response to climate forcing in an attempt to quantify the polar amplification magnitude we estimated arctic temperature changes using our 2h results in combination with existing relationships between early eocene temperatures and stable water isotopes comparing the arctic temperature change with a global estimation proposes a polar amplification of magnitude

on the volcanology of the gibeon kimberlite field namibia kimberlite volcanism in the upper cretaceous gibeon kimberlite field southern namibia consisting of at least 42 diatremes and a number of associated dykes is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the field the volcanology of the diatremes and dykes as well as their structural setting is reported here because of the paleohydrogeological setting and since juvenile kimberlite occurring in dykes intrusive plugs and spherical lapilli is devoid of vesicles a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes karoo dolerite basalt and sediment xenoliths in the diatremes provide evidence for the former extent of karoo strata at the time of eruption

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

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

eruption of kimberlite magmas physical volcanology geomorphology and age of the youngest kimberlitic volcanoes known on earth the upper pleistocene holocene igwisi hills volcanoes tanzania the igwisi hills volcanoes ihv tanzania are unique and important in preserving extra crater lavas and pyroclastic edifices they provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes cosmogenic 3he dating of olivine crystals from ihv lavas and palaeomagnetic analyses indicates that they are upper pleistocene to holocene in age this makes them the youngest known kimberlite bodies on earth by 30 ma and may indicate a new phase of kimberlite volcanism on the tanzania craton geological mapping global positioning system surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices craters and lavas the volcanoes stand 102 to 106 pa s for kimberlite attributed to degassing and in vent cooling each volcano is inferred to be the result of a small volume short lived days to weeks monogenetic eruption the eruptive processes of each igwisi volcano were broadly similar and developed through three phases 1 fallout of lithic bearing pyroclastic rocks during explosive excavation of craters and conduits 2 fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent and 3 effusion of degassed viscous magma as lava flows these processes are similar to those observed for other small volume monogenetic eruptions e g of basaltic magma

in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada abstract the origins of massive poorly sorted fragmental kimberlite and kimberlite breccias in the diatremes of kimberlite volcanic pipes is currently poorly understood studies of the textural features of the major infill of the fox kimberlite ekati diamond mine nwt canada show that it may have formed as a result of the collapse of the explosive eruption column above the vent into the deep open pipe during the climactic stage of the eruption as the eruption intensity increased the column became critically overloaded with dense particles and rapidly collapsed onto itself unlike deposits typically preserved from column collapse events such as ignimbrites some portion of the collapsing mass was trapped in the deep pipe forming a distinct type of pyroclastic deposit in vent vent fill column collapse lapilli tuff this deposit has previously been tentatively identified as tuffisitic kimberlite tk or massive volcaniclastic kimberlite mvk based on broadly similar textural characteristics with these lithologies though every individual deposit will have formed under slightly different circumstances mass emplacement from column collapse is proposed as a viable model for massive volcaniclastic kimberlite emplacement based on extrapolation of modern volcanological processes