stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite lac de gras field northwest territories canada abstract the oxygen and carbon isotope compositions of carbonates from thirteen late cretaceous to paleogene hypabyssal intrusions from the lac de gras kimberlite field indicate that the carbonates are well preserved and have a complex paragenetic formation history kimberlites at ten localities studied are calcite bearing based on the presence of phenocrysts microphenocrysts and or millimeter scale calcite only segregations of sr rich calcite the calcite bearing kimberlites also contain large cm scale sr poor calcite serpentine segregations that are texturally distinct from the smaller sr rich calcite only segregations kimberlites at three other localities studied are dolomite bearing based on the presence of calcite dolomite segregations some of which preserve complex oscillatory and banded textures of calcite dolomite and magnesian calcite 18 o values for whole rock calcite in calcite bearing kimberlite and for micro samples of calcite from individual segregations in calcite and dolomite bearing kimberlite generally vary from 6 to 9 consistent with their formation at magmatic temperatures e g 750 c from a kimberlite melt in contrast higher 18 o values of 9 to 14 characterize micro samples of calcite from calcite serpentine segregations in calcite bearing kimberlite as well as whole rock dolomite and micro samples of dolomite from segregations in dolomite bearing kimberlite these data are consistent with formation from deuteric fluids late stage magmatic at sub solidus temperatures e g 500 to 100 c these higher 18 o values for whole rock calcite also correlate with increased intensity of forsterite alteration increased abundance of calcite serpentine segregations textural overprint of primary groundmass minerals by anhedral calcite and serpentine and development of atoll spinels all interpreted as a result of deuteric fluids in contrast whole rock calcite in dolomite bearing kimberlite has notably lower 18 o values of 1 5 to 5 5 interpreted to have formed from or been overprinted by locally derived high co 2 h 2 o deuteric fluids at 500 to 100 c 13 c values of the various types of calcite and dolomite 8 to 3 have no systematic variation with 18 o values and therefore there is no isotopic record of volatile degassing from these hypabyssal bodies

miocene phreatomagmatic volcanism at tihany pannonian basin hungary abstract a late miocene 7 56 ma maar volcanic complex tihany maar volcanic complex tmvc is preserved in the pannonian basin and is part of the bakony balaton highland volcanic field base surge and fallout deposits were formed around maars by phreatomagmatic explosions caused by interactions between water saturated sediments and alkali basalt magma carrying peridotite lherzolite xenoliths as well as pyroxene and olivine megacrysts subsequently nested maars functioned as a sediment trap where deposition built up gilbert type delta sequences at the onset of eruption magma began to interact with a moderate amount of groundwater in the water saturated sand as eruption continued phreatomagmatic blasts excavated downward into limestones providing access to abundant karst water and deeper to sandstones and schist both providing large amount of fracture filling water at the surface this wet eruption led to the emplacement of massive tuff breccias by fall surge mudflow and gravity flow deposition the nature of the tmvc maar eruptions and their deposits appears to depend on the hydrological condition of the karst and or fracture filling aquifer which varies seasonally with rainfall and spring runoff the west and east maar volcanoes of tmvc are interpreted to represent low water input from the karst and or fracture filling aquifer summer vent whereas the east maar is interpreted to have formed when abundant karst and or fracture filling water was available spring vent

migration direction of sand wave structures in pyroclastic surge deposits implications for depositional processes sand wave structures of both progressive downstream migrating and regressive upstream migrating types occur within the same pyroclastic surge deposit at both roccamonfina volcano southern italy and sugarloaf mountain arizona united states both wet and dry surge deposits are described that contain examples of regressive and progressive sandwave structures at sugarloaf mountain progressive and regressive sand waves occur within metres of each other within the same bedset indicating that penecontemporaneous deposition of the two different sand wave structures occurred in close proximity on the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic surge deposits is controlled by the flow regime of the surge cloud rather than the temperature and or moisture content

effects of explosion energy and depth to the formation of blast wave and crater field explosion experiment for the understanding of volcanic explosion we made field explosion experiments as an analogue of volcanic explosion to understand the relationship between the explosion condition and the resultant surface phenomena the main parameters we employed were explosion depth and explosion energy through the experiments we confirmed that scaled depth which is the depth divided by cube root of energy is the main parameter determining the properties of explosive volcanism the energy assigned to blast wave decreased exponentially against the scaled depth the scaled crater diameter became maximum when the scaled depth was about 4 10 3 m j scaled crater diameters by nuclear chemical subsurface and some volcanic explosions were almost the same from the scaling law the overpressure at crater rim was estimated to be several mpa which corresponded to typical rock strength probably the ground forming materials were broken inside the area where overpressure exceeded their strength

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

multiphase flow above explosion sites in debris filled volcanic vents insights from analogue experiments abstract discrete explosive bursts are known from many volcanic eruptions in maar diatreme eruptions they have occurred in debris filled volcanic vents when magma interacted with groundwater implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material no details are available regarding the structure of the disturbed debris between the explosion site and the surface field studies of diatreme deposits reveal cross cutting steep sided zones of non bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by debris jets driven by explosions in order to determine the general processes and deposit geometry resulting from discrete explosive injections of entrained particles through a particulate host we ran a series of analogue experiments specific volumes of compressed 0 5 2 5 mpa air were released in bursts that drove gas particle dispersions through a granular host the air expanded into and entrained coloured particles in a small crucible before moving upward into the host white particles each burst drove into the host an expanding cavity containing air and coloured particles total duration of each run recorded with high speed video was approximately 0 5 1 s the coloured beads sedimented into the transient cavity this same behaviour was observed even in runs where there was no breaching of the surface and no coloured beads ejected a steep sided body of coloured beads was left that is similar to the cross cutting pipes observed in deposits filling real volcanic vents in which cavity collapse can result not only from gas escape through a granular host as in the experiments but also through condensation of water vapour a key conclusion from these experiments is that the geometry of cross cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the intrusions that formed them an additional conclusion is that complex structures can form quickly from discrete events

climatic change evidence and lacustrine varves from maar lakes germany annually laminated non glacial lake sediments from lake holzmaar eifel western germany were investigated using large merkt thin sections the absolute age of varve intervals with variations in thickness and composition were correlated to climatic changes recorded by glacier fluctuations in the alps back to 8800 years vt varve time varve years before 1950 glacier advances coincide with sedimentation rate minima prior to 8800 years vt they coincide with sedimentation rate maxima the early and middle holocene sediments suggest a periodicity of about 1000 years for cold warm cycles a sequence of 512 varve thickness measurements was subjected to spectral analysis these provide apparent evidence for a 11 year sun spot cycle

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

geological evolution of the coombs allan hills area ferrar large igneous province antarctica debris avalanches mafic pyroclastic density currents phreatocauldrons the jurassic ferrar large igneous province of antarctica comprises igneous intrusions flood lavas and mafic volcaniclastic deposits now lithified the latter rocks are particularly diverse and well exposed in the coombs allan hills area of south victoria land where they are assigned to the mawson formation in this paper we use these rocks in conjunction with the pre ferrar sedimentary rocks beacon supergroup and the lavas themselves kirkpatrick basalt to reconstruct the geomorphological and geological evolution of the landscape in the early jurassic the surface of the region was an alluvial plain with perhaps 1 km of mostly continental siliciclastic sediments underlying it after the fall of silicic ash from an unknown but probably distal source mafic magmatism of the ferrar province began the oldest record of this event at allan hills is a 180 m thick debris avalanche deposit member m of the mawson formation which contains globular domains of mafic igneous rock these domains are inferred to represent dismembered ferrar intrusions emplaced in the source area of the debris avalanche shallow emplacement of ferrar magmas caused a slope failure that mobilized the uppermost beacon supergroup and the silicic ash deposits into a pre existing valley or basin the period which followed mawson time was the main stage for explosive eruptions in the ferrar province and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years phreatomagmatic explosions were the dominant fragmentation mechanism with magma water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris at coombs hills a vent complex or phreatocauldron was formed by coalescence of diatreme like structures at allan hills member m of the mawson formation consists mostly of thick coarse grained poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents meanwhile at carapace nunatak mafic clasts were mixed with detrital material to form the carapace sandstone in alluvial and eventually lacustrine environments eruptions then became largely effusive producing hundreds of metres of flood lavas that covered the landscape kirkpatrick time in places lava flowed into ephemeral lakes to form pillow palagonite breccias base of sequence carapace nunatak or pillow lavas top of sequence coombs hills several generations of ferrrar intrusions were emplaced during the course of these events at least three can be distinguished based on field relations new geochemical data indicates that for the ferrar province magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas we also note the possibility that flood lavas were fed by plugs cross cutting the mawson formation at coombs hills rather than by major dikes extending to the surface finally we infer that eruption plumes were limited to the troposphere and that direct environmental impacts were thus likely restricted to the southern hemisphere

high frequency paleoclimate signals from foulden maar waipiata volcanic field southern new zealand an early miocene varved lacustrine diatomite deposit abstract a freshwater diatomite deposit near dunedin south island new zealand provides an exceptional archive of mid latitude seasonal climate variation during latest oligocene early early miocene time the diatomite accumulated in a small 1 5 km diameter maar floored lake that formed during basaltic volcanism except for exposures of basanite dated at 23 2 ma by 40 ar 39 ar and remnants of a paleogene sandstone and conglomerate cover the diatomite body is surrounded by otago schist geophysical profiles indicate that the total diatomaceous sediment thickness may exceed 100 m two depositional facies are described from 15 5 m of weakly consolidated fresh diatomite accessible in two pits a thinly laminated facies comprising 60 of the section consists of dark brown and white couplets of average thickness 0 5 mm both brown and white laminae are composed essentially of frustules of a single pennate diatom along with 1 2 of siliceous sponge spicules dark laminae also contain abundant 5 7 5 m diameter siliceous chrysophycean stomatocysts and organic matter decalcified complete skeletons of galaxias a southern hemisphere group of freshwater or diadromous fish are found throughout diverse well preserved leaves some with attached scale insects are dominated by lauraceae rare flowers are also present trace fossils preserved in the laminated comprise 1 1 5 mm wide pale diatomaceous strings of probable faecal origin and isolated 20 40 mm diameter sand lenses interpreted as faecal remains of swimming waterfowl the second diatomite facies group comprises dark brown speckled beds interpreted as sediment gravity flows up to 14 cm thick and interspersed throughout the study section they incorporate abundant laminated diatomite flecks and leaves and minor woody plant matter and terrigenous silt almost all are capped by 1 8 mm of white diatomite that is interpreted as post flow fall out of resuspended diatom frustules associated breccia and swirly beds up to 8 cm thick are composed of variably contorted clasts of laminated diatomite in a speckled matrix nodules spheroids and films of authigenic framboidal pyrite are commonly associated with plant and fish remains as in modern lakes white laminae likely formed from diatom blooms when light flux temperature or nutrients were optimal during spring early summer dark laminae accumulating during autumn winter months laterally continuous pinstripe lamination absence of bioturbation and excellent fish and plant preservation indicate that the profundal lake and floor were anoxic periodic variations in couplet thickness of 3 10 years compare with quaternary records of el nino southern oscillation variability and indicate that the early early miocene low altitude new zealand climate was seasonal and strongly ocean influenced as it is today

miocene to recent history of the western altiplano in northern chile revealed by lacustrine sediments of the lauca basin 18 15 18 40 s 69 30 69 05 w the intramontane lauca basin at the western margin of the northern chilean altiplano lies to the west of and is topographically isolated from the well known plio pleistocene lake system of fluvio lacustrine origin that covers the bolivian altiplano from lake titicaca to the north for more than 800 km to the salar de uyuni in the south the lauca basin is filled by a sequence of some 120 m of mainly upper miocene to pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin volcanic rocks partly pyroelastic provide useful marker horizons in the first period 6 4 ma of its evolution the lago lauca was a shallow ephemeral lake evaporites indicate temporarily closed conditions after 4 ma the lake changed to a perennial water body surrounded by alluvial plains in the late pleistocene and holocene 2 0 ma there was only marginal deposition of alluvial and glacial sediments the basin formed as a half graben or by pull apart between 10 and 15 ma tectonic displacement of the basal ignimbrite sequence during the quechua phase and 6 2 ma maximum k ar ages of biotites of tuff horizons in the deepest part of the basin apart from this early basin formation there has been surprisingly little displacement during the past 6 ma close to the western cordillera of the altiplano also climate indicators pollen evaporites sedimentary facies suggest that an arid climate has existed for the past 6 ma on the altiplano together these pieces of evidence indicate the absence of large scale block faulting tilt and major uplift during the past 5 6 ma in this area

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

complex bombs of phreatomagmatic eruptions role of agglomeration and welding in vents of the 1886 rotomahana eruption tarawera new zealand 1 basaltic bombs ejected from a rotomahana crater during the 1886 tarawera eruption in new zealand are of two fundamental types cored bombs which have single large fragments generally of rhyolite wall rock material as their cores and loaded bombs which contain abundant small fragments dispersed through the bombs similar lithic bearing bombs are common products of other phreatomagmatic eruptions worldwide cored bombs originate when a large wall rock fragment is entrained in basaltic melt and then ejected with an adhering skin of basalt in contrast loaded bombs are constructed by agglomeration of multiple fragments of basalt among which smaller rhyolitic wall rock fragments are dispersed different styles and rates of bomb loading are reflected in the bombs textures and compositions these textures provide insight into the nature of fragmentation during the eruption the processes of recycling that produce multiply ejected clasts versus recapitulated ones those where early formed clasts were reingested in magma and then formed the cores of new bombs the degree and timescale of interaction of magma with vent walls and the timescales over which lithic material from the vent walls is incorporated into complex bombs prior to their ejection because fragments of rhyolitic pumice are readily refused their condition constrains estimates of the timing of fragmentation and agglomeration the presence of nonfused rhyolite pumice within many loaded bombs implies that the processes forming and quenching the bombs cannot have exceeded several seconds in contrast other bombs contain patches of compositionally modified basalt with isolated rhyolite derived feldspar xenocrysts that indicate complete fusing melting and assimilation of entrained rhyolitic debris the bomb population overall indicates that conditions within this rotomahana vent were thermally and physically heterogeneous and that recycling of debris was ubiquitous both as recycling of large individual clasts and by fallback and aggregation of material which was reingested in magma before being reejected as new bombs

premixing of magma and water in mfci experiments experimental studies have been performed to evaluate pre explosive water melt mixes with respect to explosive volcanic molten fuel coolant interaction mfci i e phreatomagmatic explosion remolten ultrabasic volcanic rock was used as a magma simulant measurement of the explosion intensity was used to determine optimal premixing conditions a well defined optimal range was found for the hydrodynamic mixing energy differential flow speed of 4 2 m s as well as for the water melt mass ratio 0 03 to 0 04 under experimental conditions the mass flux of water had a minor influence on the explosion intensity additionally transparent mixing experiments with silicon oil and inked water were carried out they indicate a direct dependence of the pre explosive water melt interface area on the explosion intensity the experimental results show that the contact conditions of water and melt required for explosive mfci may easily be established in natural volcanic systems thus explosive mfci is a probable mechanism of explosive volcanism

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

facies variation of eruption units produced by the passage of single pyroclastic surge currents hopi buttes volcanic field usa abstract base surges are a significant volcanic hazard associated with phreatomagmatism but their transport and depositional dynamics are incompletely understood stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from haskie tuff ring hopi buttes volcanic field usa extend to 1 km from the vent and show similar downcurrent changes in lithofacies within 200 m of the vent eruption units are primarily disorganized lapilli tuff that is rich in juvenile pyroclasts at distances 200 m from the vent eruption units transform into a couplet of juvenile rich lapilli that grades upward into bedform bearing ash tuff at distances 900 m upper and lower beds of the eruption unit couplet transform into planar stratified beds at distances 1100 m couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar bedded ash tuff the collapse of dense eruption columns following explosive magma wet sediment interaction generated turbulent surge currents as the surges traveled away from the haskie vent they expanded and partitioned into a basal juvenile rich traction carpet and overlying accidental rich ash cloud surge near the limit of their runout the surges cooled and lost their pyroclastic load leaving planar beds of accidental rich ash the deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics

estimation of electric fields in the conducting earth s crust for oscillating electric current dipole sources and implications for anomalous electric fields associated with earthquakes abstract electromagnetic radiation possibly associated with earthquake occurrences in the earth s crust can be treated as a problem of electromagnetic induction within a conducting half space representing the crust we estimated the magnitude of the electric field near the earth s surface for oscillating electric current dipole sources located at various depths this shows that the electric field highly attenuates as expected and the source must be stronger than 10 5 a m even if it is located at a depth of several kilometers for practically detecting signals at frequencies in the ulf ultra low frequency range we also examined the electric field near the earth s surface for a box car type source in uniform and non uniform crust models we found that the wave form of the source is rather well preserved in both models but signal amplitudes are very small and unlikely to be drastically enhanced even in a non uniform crustal structure although this discussion is based only on a specific non uniform model

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

gold nugget morphology and geochemical environments of nugget formation southern new zealand abstract gold nuggets centimetre scale have formed in a supergene alteration zone on hydrothermal gold deposits and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals and along fractures in quartz and host rocks the supergene alteration was driven by groundwater driven water rock interaction near to a regional unconformity beneath fluvial sediments and involved clay alteration and oxidation that extended up to 50 m below the unconformity oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals the nuggets have some crystalline form and internally they consist of anhedral grains elongated gold plates and intimate intergrowths of gold and iron oxyhydroxide nugget surfaces have further micron scale overgrowths of microparticulate gold gold plates and gold crystals nuggets were eroded and recycled into nearby proximal miocene quartz pebble conglomerates where they concentrated in placers near the basal unconformity later recycling transferred gold into pleistocene fluvial channels gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles with little change in mass all groundwater maintained high ph throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation hence gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets albeit with lower dissolved sulphur concentrations despite aridification of the climate in the late cenozoic with resulting localised high dissolved chloride concentrations chloride complexation did not contribute to gold mobility

effect of explosion energy and depth on the nature of explosion cloud a field experimental study field explosion experiments using dynamite were carried out to investigate the effect of explosion energy and depth on the nature of explosion cloud the scaling law of explosion was established among the energy depth and the nature of explosion cloud shape height and duration of flow out the shape of explosion cloud changed systematically from funnel type via elongated to short types with increasing the scaled depth of explosion at the shallow scaled depth both scaled height and scaled duration increased with increasing scaled depth both values reached the maximum at 0 003 0 004 m j1 3 of cube root scaled depth and then decreased with increasing scaled depth if the cube root scaled depth was over about 0 01 m j1 3 we could not observe any explosion cloud on the ground surface these experimental results should be applicable to the understanding of volcanic explosions if the buoyant effect of internal heat in the explosion cloud was negligible and ground medium is identical we applied these results to the explosion clouds of usu phreatic explosion at 14 53 h on 17 april 2000 and estimated that the explosion energy and the depth were 4 109 j and 6 m respectively errors caused by uncertainty of scaling factors are less than one order of magnitude for the energy and depth

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

late pleistocene basaltic ash and volcanic eruptions in the bonneville basin utah microprobe analyses of glass and radiocarbon dating of samples associated with three basaltic volcanic ashes in the deposits of lake bonneville make the ashes useful stratigraphic markers for correlation within the bonneville basin two of the ashes were derived from tuff cones pavant butte and tabernacle hill that erupted into lake bonneville in what is now the black rock desert of south central utah near kanosh utah the pavant butte ash is interbedded with barrier beach lagoon marl that is slightly less than 16 000 yr old five radiocarbon dates on carbonate materials stratigraphically associated with the ash elsewhere in the sevier desert average 15 300 yr b p lake bonneville was in its transgressive phase about 15 m below the bonneville shoreline when the pavant butte eruption began at the tabernacle hill tuff cone a basalt flow that has pillows wave rounded cobbles and tufa on its outer margins was erupted into lake bonneville shortly after the tuff cone at or near the provo shoreline the tabernacle hill tuff cone and basalt flow are older than a radio carbon date of about 14 300 yr b p on tufa collected from the outer margin of the basalt flow but younger than the bonneville flood at about 14 500 yr b p the third ash informally termed the thiokol ash has been found interbedded with lake bonneville deposits in two exposures in northern utah and in sediment cores from the great salt lake it is about 25 000 yr old and may have been erupted from a volcanic field on the northwest shore of great salt lake or from the snake river plain microprobe analyses of glass from samples of the black rock desert ashes show them to have similar chemical compositions but glass from tabernacle hill can be distinguished from pavant butte glass by its higher concentrations of cao and p 2 o 5 systematic differences in chemical composition between samples of pavant butte glass can be explained by comagmatic processes thiokol glass has less sio 2 ai 2 o 3 mgo and na 2 o but greater total iron and p 2 o 5 than do the black rock desert glasses

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

recent advances in the geology of koffiefontein mine free state province south africa abstract the complex internal geology of the koffiefontein pipe has contributed to the marginal nature of the mine the key to this is the presence of a large zone dominated by down rafted country rock karoo sediment and dolerite xenoliths recent work indicates that the kimberlite pipe at koffiefontein consists of precursor dykes the west and east fissures and the main pipe in which two main eruptive phases have been recognized groundmass spinel compositions have been used to provide a chemical fingerprint of each lithology there is evidence for at least three magma batches each with its own chemical signature cross cutting contact relationships were used to determine the emplacement sequence the characterization of the different internal geological units permitted the development of a three dimensional 3d model of the pipe both main eruptive phases viz the speckled west kimberlite and the speckled east kimberlite comprise volcaniclastic kimberlite they are separated by a large irregular mass of kimberlite that contains abundant country rock xenoliths comprising varying proportions of karoo mudstone and dolerite as well as probable bedded crater facies fragments this zone of contamination dilutes the grade of the kimberlites affects the geotechnical stability and adversely affects the economics of the mine

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

post eruptive development of the ukinrek east maar since its eruption in 1977 a d in the periglacial area of south west alaska maar eruptions form small initially steep walled basins that contain important archives for the climatic and palaeoenvironmental history in continental areas the two ukinrek maars in south western alaska erupted between 30 march and 9 april 1977 and are the best documented maars that have erupted in historical time this study presents a preliminary analysis of geomorphology hydrology magnetic susceptibility geochemistry and sedimentology data of a field study in august 2004 these results photographs and topographic surveys are combined for reconstructing the post eruptive evolution of ukinrek east maar within less than 30 years the initially polygonal shape of the crater with nearly vertical crater walls has developed into an almost elliptic form with slopes of 35 inclination on debris fans between a few escarpments the water table and the crater floor have risen significantly and the crater diameter depth ratio increased from 3 4 to 5 7 whereas the average height of the crater rim remained almost constant the main sub aerial resedimentation process is formation of rock falls rock slumps and scree resulting in debris flows and turbidites within the lake that is ice covered throughout about half of the year distal lake sediments consist of laminated minerogenic clayey sandy silts that document frequent turbidity currents from the linear sedimentation rate of only ca 5 mm year 1 in 17 cm long cores it may be concluded that the largest portion of the crater sediments formed within the first few months of the maar history however this has to be confirmed by future studies

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

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions

origin of the joya honda maar san luis potosi mexico abstract joya honda is a quaternary maar of unusual type from the mexican basin and range province its 300 m deep crater is excavated in cretaceous limestones the surrounding tephra deposit which in places is 100 m thick begins with a series of weakly indurated pyroclastic surge and fall layers that we interpret as dry surge deposits these are overlain by the main sequence of strongly indurated massive tuff breccias that we interpret as wet surge deposits joya honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths local aquifer characteristics controlled the style of eruption and the nature of the deposits groundwater in the limestone hosted aquifer beneath joya honda was apparently contained within solution enhanced fractures at the onset of the eruption magma began to interact with a moderate amount of groundwater producing the dry surge deposits which are typical of deposits found at many maars and tuff rings as the eruption continued the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci a marked increase in the water magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry surge stage at the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout surges and mudflows these steeply dipping tuff breccias are similar to deposits found at many tuff cones juvenile clasts in the near vent deposits show marked upward increases in both hydration palagonitization and vesicularity the increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time correlating with greater carbonate cementation and lithification in the upper part of the deposit the transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation perhaps related to unroofing during progressive excavation of the overlying maar crater thus joya honda does not support maar formation models that invoke downward displacement of explosion foci caused by formation of a cone of depression in the aquifer in order to maintain the confining pressure for the hydromagmatic blasts