symplectitic augite evidence for discontinuous precipitation as an exsolution mechanism in ca rich clinopyroxene part of the augite in the artfjallet gabbro consists of symplectitic intergrowths between augite and blebs or lamellae of orthopyroxene mineral compositions are consistent with formation of these symplectites by exsolution of orthopyroxene from magmatic augite at a temperature of ca 900 1000 c the microstructures indicate that the exsolution mechanism is discontinuous precipitation whereby the boundary of an augite grain sweeps through a neighbouring augite leaving the symplectite in its wake the formation of this symplectitic augite is catalysed by the presence of an intergranular water rich fluid phase which promotes grain boundary mobility

experiments on the heat discharge at the dynamic magma water interface 1 compared to dry atmospheric eruption of magma or dry magma rock contact intensity and time scale of heat discharge from magma to the surroundings is strongly modified by an effective coolant water or water sediment mixes in the case of subaqueous or subglacial eruptions magma water contact must take place and can result in phreatomagmatic explosions even if no explosions occur rapid cooling results in the formation of pyroclasts by thermal granulation to study this process in detail a short term calorimeter was built for the direct measurement of the heat flux from a magmatic melt to a coolant volcanic rocks from recent eruptions in iceland were remelted and used to produce jets of melt poured into a coolant filled container particles could be produced in a non explosive process that are practical identical to those from natural hyaloclastites the process fragmentation energy is about 10 of the total heat transferred from melt to coolant

experimental studies of heat transfer at the dynamic magma ice water interface application to subglacially emplaced lava experiments simulating processes operating in volcano ice interactions were carried out to explain and quantify lava thermal properties and processes of heat transfer from pure lava melt to water and ice and from hot crystalline lava to water the samples used 70 200 g were obtained from an intermediate lava flow benmoreite trachyte that was emplaced under and within the outlet glacier gigjokull in the 2010 eruption of eyjafjallajokull experiments involved settings with direct contact between ice and lava and settings where melt and ice were separated by a few centimeters direct contact involved melt being emplaced on ice and ice placed on melt the direct contact experiments provided initial heat flux of up to 900 kw m 2 at an initially lava melt surface temperature of 1100 c declining to 100 kw m 2 at 200 300 c within 1 2 min while the experiments without melt ice contact yielded an initial maximum of 100 180 kw m 2 dropping to 50 80 kw m 2 in 2 3 min in other experiments where cubes of hot crystalline lava were subjected to forced convection of water at initial temperature of 20 30 c initial heat fluxes of 400 770 kw m 2 were followed by fast decline to 100 kw m 2 in 15 35 s the rate depending on cube size the hot rock experiments provided thermal conductivity values of 1 2 1 7 w m 1k 1 and diffusivity of about 9 10 7 m2s 1 values for heat flux obtained in these experiments are in the same range as those obtained from field observations of the lava emplacement in the eyjafjallajokull 2010 eruption

water magma interaction some theory and experiments on peperite formation experiments using molten thermite as a magma analog produce peperite when the melt interacts with wet sand these experiments also show explosive behavior developing strombolian and surtseyan like bursts the results demonstrate that the application of fuel coolant interaction fci theory is appropriate for interpretation of peperites the theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma rs which determine thermal equilibrium temperature limits and contact interface dynamics the dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation a vapor film layer develops at the interface and acts both as an insulating barrier promoting passive quenching as well as a potential energy reservoir that can cause magma fragmentation mingling of the magma with wet sediments and explosive quenching when the vapor film becomes unstable an important parameter in determining the behavior of the vapor film is the value of rs which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma if rs 1 for fully saturated sediments there is enough water in the sediments to make convective heat flow effective in quenching the magma but below that value there is the potential that the vapor film will be unstable producing highly dynamic phenomena including explosive fragmentation at values of rs 0 1 there is insufficient water to allow the escalation of explosive fragmentation

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

a tem investigation of shock metamorphism in quartz from the vredefort dome south africa the origin of the vredefort structure in south africa is still debated several causes have been discussed namely asteroid impact internal gas explosion or tectonic processes evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains shatter cones veins of pseudotachylite and occurrence of coesite and stishovite high pressure quartz polymorphs a number of these characteristics is widely believed to support an impact origin however the planar features in quartz which are generally considered as one of the strongest indicators of impact are in the vredefort case considered as anomalous when compared with those from accepted impact structures we have investigated by optical and transmission electron microscopy tem the defect microstructures in quartz grains from different lithologies sampled at various places at the vredefort structure whatever the locality only thin mechanical brazil twin lamellae in the basal plane are observed by tem so far such defects have only been found in quartz from impact sites but always associated with sets of thin glass lamellae in rhombohedral planes 10 1n with n 1 2 3 and 4 at the scale of the optical microscope brazil twins in 0001 are easily detected in vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them similar alignments of tiny fluid inclusions parallel to other planes are also detected optically but at the tem scale no specific shock defects are detected along their traces if these inclusion alignments initially were shock features they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae fine grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen but stishovite was not found even in areas where its occurrence was previously reported finally definite evidence of high temperature annealing was observed in all the samples these observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the vredefort structure most of the original shock defects are now overprinted by an intense post shock annealing episode only the thin mechanical twin lamellae in the basal plane have survived

discontinuous precipitation of rutilated quartz grain boundary migration induced by changes to the equilibrium solubility of ti in quartz we experimentally investigated changes to the titanium concentrations of quartz rich rocks caused by grain boundary migration synthesis experiments were performed to make high ti quartz starting materials for usage in recrystallization experiments we isothermally recrystallized quartz starting material in separate experiments at 10 25 kbar to reduce the solubility of ti in quartz microstructural and geochemical results indicate that the high ti quartz starting material recrystallized to form low ti quartz with acicular rutile inclusions during recrystallization high ti quartz was consumed to form low ti quartz and acicular rutile crystals precipitated behind moving grain boundaries via the discontinuous precipitation unmixing process experimentally imposed changes to the equilibrium solubility concentration of ti in quartz by amounts as low as 0 02 weight 200 g g or ppm by weight created a free energy driving force of 8 7 kj mol that was sufficient to drive large scale recrystallization of the quartzites microstructures of relict grains of the high ti quartz starting materials are characterized by cuspate and irregular grain boundaries whereas new quartz grains exhibit subhedral to euhedral grain morphologies and abundant three grain junctions discontinuous precipitation can significantly change mineral compositions and in the absence of a free fluid phase can operate much faster than other types of mass transfer processes commonly associated with solid state reactions

unusual fluidal behavior of a silicic magma during fragmentation in a deep subaqueous eruption havre volcano southwestern pacific ocean magma responds to applied stresses in either a viscous or elastic manner depending on the time scales over which strain is accommodated for silicic magmas high strain rates of explosive volcanism cause brittle fragmentation and produce abundant small particles ash the a d 2012 havre kermadec arc southwestern pacific ocean eruption at 900 m water depth deposited a unit of silicic ash with features indicative of syn and or post fragmentation viscous deformation viscously deformed ash makes up 3 35 of the two main ash subunits s1 and s2 with the remaining ash formed by brittle fragmentation viscous behavior of melt during production of fine ash is unexpected for the silicic havre magma and for the high strain rates typical of fine fragmentation the occurrence together of viscous and brittle ash grains suggests local and or short term variations in eruption conditions we infer an explosive eruption mechanism modified by magma water interaction during which multi source steam veiled fragmentation sites permitted rapid viscous deformation of ash prior to contact with water

asymmetrical quartz crystallographic fabrics formed during constrictional deformation numerical simulations predict unique quartz crystallographic fabric patterns for plane strain flattening and constriction multiple studies support the predictions for plane strain and flattening to test predictions for constriction this paper analyzes five examples of quartz crystallographic fabrics from a 1km wide domain of l tectonites in the pigeon point high strain zone klamath mountains california u s a these samples were deformed under greenschist to amphibolite facies conditions quartz c axis fabrics are similar to the predicted double girdle fabrics except that amphibolite facies samples exhibit caxis maxima and are distinctly asymmetrical about the elongation lineations activation of different slip systems combined with small deviations from pure constriction account for the c axis maxima and noncoaxial flow accounts for the fabric asymmetry the simple shear component is randomly oriented in geographic coordinates throughout the domain of l tectonites these data confirm that numerical simulations predict the quartz c axis fabric geometry developed during constriction for some deformation conditions and they confirm the quartz a axis patterns predicted for constriction for the first time these data also demonstrate that the relationship between quartz crystallographic fabrics and strain geometry is not straightforward and they indicate that a axis fabrics may be more useful indicators of strain geometry variations

generation of volcanic ash by basaltic volcanism 1 the recent eruptions of eyjafjallajokull and grimsvotn volcanoes in iceland demonstrate the importance of a better understanding of processes leading to the formation of volcanic ash specifically of fine volcanic ash that poses a threat to air traffic continuous deformation and brittle type experiments were carried out to better constrain these processes the studies on short time continuous deformation behavior of basaltic melt showed viscoelastic properties deviating from hydrodynamic newtonian models by more than 5 orders of magnitude high temperature deformation experiments on basaltic rock samples revealed an increase of elastic strengths as approaching the melting regime also pointing to a very complex behavior at the solid ductile boundary understanding magma fragmentation from the liquid side is a challenge but meanwhile we propose a pragmatic solution a thermodynamic model based on fracture mechanics this model is in agreement with experiments and observations that show that fine volcanic ash is produced by brittle type fragmentation of magma a critical material property was defined characterizing the conditions for brittle fragmentation the fracture surface energy density which represents the critical fragmentation energy short term fracture experiments using silicate glass have been performed to investigate the formation of ash sized particles by brittle failure and to extract this critical physical property which was found to range between 40 and 130 j m2 this value is in good agreement to fragmentation energies determined from experiments using remelted volcanic rocks now there is a tool to define critical conditions for the production of volcanic ash of a specific magma type

a record of impacts preserved in the lunar regolith abstract the absolute chronology of meteoroid impacts on the moon is largely quantified by only a few 40 ar 39 ar plateau ages of rocks thought to be associated with specific impact events stoffler et al 2006 we demonstrate a more broadly applicable approach by using high resolution 40 ar 39 ar thermochronometry to investigate the physical conditions responsible for partially reset k ar systems in lunar rocks seven rocks from apollo 16 regolith sample 63503 have plateau ages of either 3 9 billion yr ga or 4 2 ga and all experienced varying degrees of partial resetting concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3 3 ga ago that lasted between 10 3 s at 600 c and 20 yr at 300 c we conclude that partial resetting of the k ar systems in these samples record an impact event 3 3 ga ago that mixed several preexisting ejecta units in the cayley plains if partially reset 40 ar 39 ar ages of other lunar highland samples also constrain the timing of late stage reheating associated with impact events they constitute an additional record of impacts preserved in the lunar regolith a review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common and most likely related to dynamical events in the asteroid belt or outer solar system

subaqueous basaltic magmatic explosions trigger phreatomagmatism a case study from askja iceland abstract sequences of basaltic pillow lavas that transition upward with systematic gradation from pillow fragment breccias to fluidal bomb bearing breccias to bomb bearing lapilli tuffs are common at askja volcano iceland based on the detailed textural investigation of three of these sequences we argue that they record temporally continuous transitions from effusive to explosive products that were erupted from and deposited at or near a single subaqueous vent the recognition of such sequences is important as they provide evidence for controls on the onset of explosive activity in subaqueous environments such investigations are complicated by the interplay of magmatic gas expansion and phreatomagmatic and mechanical granulation fragmentation mechanisms in the subaqueous eruptive environment all of the sequences studied at askja have textural componentry and sedimentological characteristics suggestive of a close genetic and spatial relationship between the pillow lavas and all of the overlying glassy clastic deposits the identification of magma fragmentation signatures in pyroclasts was accomplished through detailed textural studies of pyroclasts within the full range of grain sizes of a given deposit i e bomb blocks lapilli and fine ash these textural characteristics were compared and evaluated as discriminators of fragmentation in pyroclastic deposits the presence of angular vitric clasts within the breccia and lapilli tuff displaying fragile glassy projections indicates little or no post depositional textural modification a shift in vesicle and clast textures between the pillow lavas and the large concentration of fluidal bombs in the breccia indicates that the phreatomagmatic explosions were initially triggered by magmatic vesiculation the initial magmatic gas expansion may have been triggered by depressurization caused by the drainage of the ice confined lake surrounding askja the fuel coolant interactions fcis of the more efficient phreatomagmatic explosions were enabled by the increase in the surface area to volume ratio of the fluidal bombs in the water producing a premix of magma and water the onset and increasing influence of phreatomagmatic fragmentation is preserved in the presence of very fine blocky ash particles and diminished presence of larger particles such as fluidal bombs the textural sedimentological and environmental characteristics of these deposits suggest that phreatomagmatic explosions can be triggered by initial magmatic gas expansion but that it is likely one of many mechanisms for triggering such explosions

peperite as a record of early stage phreatomagmatic fragmentation processes an example from the hopi buttes volcanic field navajo nation arizona usa abstract peperite is the product of near surface magma wet sediment mingling some peperites are inferred to record frozen coarse mixing stage fuel coolant interactions fcis and hence can be used in attempts to determine specific parameters related to phreatomagmatic fragmentation such as water magma mass ratios confining pressures and vent region hydrology two lamprophyric dikes in the miocene pliocene hopi buttes volcanic field arizona contain ellipsoidal regions of dilation buds associated with peperite these peperite rimmed zones occur within identifiable stratigraphic horizons and at known depths beneath the elevation of the local pre eruptive surface the presence of both fluidal and blocky juvenile clasts combined with physical aspects of the sediment lamprophyre contacts suggest that coarse mixing stage fci was a potential fragmentation mechanism within these systems confining pressures calculated for sites of single buds range from 0 6 to 1 0 mpa average inferred water magma ratios within peperite bodies at the time they formed vary from 0 09 to 0 16 consistent with experimental values for water melt mixing ratios necessary for explosive heat transfer a number of factors may have inhibited further explosive activity in these peperite forming mixing events including anisotropic saturation of the host sediment low confining pressures and water melt ratios and damped explosive interactions resulting from increased density viscosity and surface tension of sediment water coolants nevertheless these deposits record localized thermohydraulic interactions between magma and wet sediment and provide valuable information about the geometry and mixing ratios present during peperite formation

impact induced hydrothermal activity within the haughton impact structure arctic canada generation of a transient warm wet oasis field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot impact generated rocks following formation of the 24 km diameter 23 ma haughton impact structure hydrothermal alteration is recognized in two settings within polymict impact breccias overlying the central portion of the structure and within localized pipes in impact generated concentric fault systems the intra breccia alteration comprises three varieties of cavity and fracture filling a sulfide with carbonate b sulfate and c carbonate these are accompanied by subordinate celestite barite fluorite quartz and marcasite selenite is also developed particularly in the lower levels of the impact breccia sheet the fault related hydrothermal alteration occurs in 1 7 m diameter subvertical pipes that are exposed for lengths of up 20 m the pipes are defined by a monomict quartz carbonate breccia showing pronounced fe hydroxide alteration associated sulfides include marcasite pyrite and chalcopyrite we propose three distinct stages in the evolution of the hydrothermal system 1 early stage 200 c with the precipitation of quartz vapor phase dominated 2 main stage 200 100 c with the development of a two phase vapor plus liquid zone leading to calcite celestite barite marcasite and fluorite precipitation and 3 late stage 100 c with selenite and fibroferrite development through liquid phase dominated precipitation we estimate that it took several tens of thousands of years to cool below 50 c following impact during this time haughton supported a 14 km diameter crater lake and subsurface water system providing a warmer wetter niche relative to the surrounding terrain the results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems

paleomagnetic and rock magnetic study of the vredefort impact structure and the johannesburg dome kaapvaal craton south africa implications for the apparent polar wander path of the kaapvaal craton during the mesoproterozoic abstract new paleomagnetic and rock magnetic results are presented for various pre syn and post impact lithologies from the region of the 2023 4 ma originally ca 250 km diameter vredefort impact structure on the kaapvaal craton of south africa after removal of a viscous remanent magnetization vrm component the characteristic remanent magnetization chrm component from 16 vredefort granophyre and pseudotachylitic pt breccia samples d 18 3 i 54 8 95 8 1 was isolated and a paleomagnetic pole at 25 1 n and 43 5 e a 95 10 6 obtained since such a chrm component was isolated also from the archean basement rocks of the vredefort dome we consider its nature as a primary component of the impactites proven the pole falls onto the paleoproterozoic part of the apparent polar wander path of the kaapvaal craton rock magnetic analysis and scanning electron microscopy sem revealed that the pseudotachylitic breccia and granophyre samples contain two distinct magnetite phases one ultra small micrometer size and one altered and larger 50 m phase the ultra small magnetite is interpreted as the carrier of the chrm and the larger grains as the carriers of the vrm component additionally rock magnetic and petrophysical data reveal unusually high koenigsberger ratios q values in all pre impact lithologies in some vredefort impactite samples and in the much younger 1 1 ga anna s rust sheet ars gabbro samples as the high q values which had also been reported by previous studies of vredefort lithologies are now also seen in samples from the johannesburg dome a direct link to the vredefort impact can be ruled out this is also supported by the hysteresis data of this study as the observed magnetization is rather hard and shows multiple components of remanent magnetization we exclude lightning as a cause for all observed high q values except in case of ars gabbros it is instead suggested that the cause of the high q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid crustal original setting and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence moreover paleomagnetic analysis of the rocks around the ars gabbro intrusion in the northern part of the vredefort structure revealed the presence of either a shallow north or a shallow south direction which is tentatively related to emplacement of the umkondo large igneous province analysis of all rocks including the vredefort impactites yields occasionally distinct great circle paths towards these shallow directions a likely explanation for this overprint direction is the heating caused by now eroded ars type gabbro in the area or by regional ca 1 1 1 0 ga orogenic effects no evidence of karoo type 0 18 ga overprint is seen

hyperspectral imaging of drill core from the steen river impact structure canada implications for hydrothermal activity and formation of suevite like breccias

heat source or heat sink what dominates behavior of non explosive magma water interaction 1 much of the volcanism on earth takes place in subaqueous settings where magma has direct contact with a water reservoir of restricted or quasi unrestricted volume in order to assess the intensity and timescale of non explosive interaction of magmatic melts and water experiments representing these settings were performed natural volcanic samples were remelted and poured as a continuous jet into a water filled calorimeter where the melt interacts with its coolant the rapid cooling results in granulation i e brittle failure of the material granulation needs energy which is taken from the thermal input of the hot melt energy used in granulation was found to require 5 20 of the melt s initial heat content this energy loss fraction is insensitive to variations in coolant and melt temperatures but instead depends on the melt s thermomechanical properties however analysis of the experimentally produced granulate indicates a strong correlation between the initial coolant temperature i e the heat sink and the grain size distribution but also shows variations due to material properties the maximum of the grain size distribution was determined to change from a diameter of 1 mm up to 4 mm due to coolant temperature increase properties of the heat source melt dominate the efficiency of the process whereas both heat sink and source characteristics determine the products

heat flow in the witwatersrand basin and environs and its significance for the south african shield geotherm and lithosphere thickness the late archaean early proterozoic witwatersrand basin is situated in the center of the kaapvaal craton south africa two new heat flow determinations and 71 new heat flow estimates permit excellent definition of the heat flow pattern nonvertical heat conduction explains much of the observed local heat flow variability regionally the heat flow in the basin is uniform and the overall mean 51 6 mw m 2 is higher than the mean heat flow for archaean cratons published data indicate that heat generated in the basin s strata 7 5 mw m 2 is largely responsible for the excess heat flow the subsediment heat flux is 43 5 mw m 2 seven new heat flow and heat production determinations in archaean granite domes northwest of the basin average 33 2 mw m 2 and 2 3 0 8 w m 3 respectively contrasted with the heat flux across the floor of the basin the lower heat flow in the domes probably reflects a smaller radiogenic contribution from the crystalline crust below the domes published heat production data from upturned granitic basement exposed in the vredefort dome in the center of the basin indicate that the radioactive contribution from the crust below the basin is 26 5 m w m 2 subtracting this value and the contribution from the strata 7 5 mw m 2 from the basin s surface heat flow 51 mw m 2 indicates that the heat flux through the moho is approximately 17 mw m 2 calculated lithospheric temperatures agree closely with pressure temperature regimes inferred from studies of mantle inclusions in kimberlites in the kaapvaal craton both data sets indicate that the cratonic lithosphere is considerably cooler and thicker than that below an equilibrium ocean implications of the cold thick cratonic lithosphere for understanding the distribution of southern african diamondiferous kimberlites and the density and buoyancy of the lithosphere are discussed

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

contrasting mechanisms of magma fragmentation during coeval magmatic and hydromagmatic activity the hverfjall fires fissure eruption iceland growing evidence for significant magmatic vesiculation prior to magma water interaction mwi has brought into question the use of diagnostic features such as low vesicularities and blocky morphologies to identify hydromagmatic pyroclasts we address this question by quantifying co variations in particle size shape and texture in both magmatic and hydromagmatic deposits from the hverfjall fires fissure eruption iceland overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth with substantial vesiculation prior to mwi hydromagmatic fragmentation occurred principally by brittle mechanisms where the length scale and geometry of fracturing was controlled by the bubble population this suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven at least in part by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling in this way the shape and size distributions of hydromagmatic pyroclasts both critical input parameters for ash dispersion models are strongly influenced by the dynamics of vesiculation prior to mwi this result underlines the need to analyse multiple grain size fractions to characterise the balance between magmatic and hydromagmatic processes during the hverfjall fires eruption the external water supply was sufficient to maintain mwi throughout the eruption with no evidence for progressive exhaustion of a water reservoir we suggest that both the longevity and the spatial distribution of mwi were determined by the pre existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas

hydrocode simulations of chicxulub crater collapse and peak ring formation we use hydrocode modeling to investigate dynamic models for the collapse of the chicxulub impact crater our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide angle velocity data to further our understanding of the formation of large impact craters in our simulations we model the collapse of a 100 km diameter bowlshaped cavity formed in comprehensively fractured crustal material to facilitate wholesale collapse we require that the strength of the target be significantly weakened in the present model we achieve this using acoustic fluidization where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target at times and positions where the overburden pressure is sufficiently counteracted the frictional resistance is reduced enabling the rock debris to flow our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at chicxulub in particular we observe a topographic peak ring formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim this model for peak ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned c 2002 elsevier science usa

constraints on the size of the vredefort impact crater from numerical modeling the vredefort structure in south africa was created by a meteorite impact about two billion years ago since that time the crater has been deeply eroded so to estimate its original size researchers have had to rely heavily upon comparison to other terrestrial impact structures recent estimates of the original crater diameter range from 160 km to as much as 400 km in this study we combined the capabilities of both hydrocode and finite element modeling using the former to predict where the pressure of an impact generated shock wave would have been high enough to form planar deformation features pdfs and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater we established constraints on the sizes of the projectile and the transient crater and thus on the size of the final crater by comparing the observed locations of pdfs around vredefort to the results of our simulations of impacts by projectiles of various sizes these simulations indicate that a rocky projectile with a diameter of 10 km impacting vertically at a velocity of 20 km s generates shock pressures that are consistent with the distribution of pdfs around vredefort these projectile parameters correspond to a transient crater 80 km in diameter or a final crater 120 160 km in diameter allowing for uncertainties in our modeling procedures we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of pdfs at vredefort the shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface making the locations of these features less useful constraints on the crater size however they may provide a constraint on the amount of erosion that has occurred since the impact

mid crustal granulite facies metamorphism in the central kaapvaal craton the bushveld complex connection abstract the numerous greenstone remnants which occur as inclusions in the archaean gneissic granitoid basement exposed in the core of the vredefort dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three stage metamorphic history 1 in metapelites migmatites and refractory restites were produced in a mid crustal 0 5 gpa m1 anatectic event at temperatures in excess of 900 c in the restites 2 in the migmatites the peak metamorphic assemblages are variably overprinted by a high grade retrograde reaction which involved crystallizing anatectic melts m2 3 both the peak metamorphic assemblages and the retrogression textures are cross cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the vredefort dome and 4 the peak metamorphic assemblages and the m2 retrogression textures are overprinted by new generations of remarkably fine grained post shock high grade phases which crystallised at a pressure some 0 25 gpa lower than that of both the peak metamorphic conditions and the retrogression m3 these data indicate an anticlockwise p t evolution where mid crustal anatexis most probably resulted from the intraplating of bushveld complex related ultramafic magmas at 2 06 ga a period of approximately isobaric cooling and retrogression was followed by the vredefort catastrophe which resulted from the impact of a large meteorite into the terrane at 2 02 ga this shock event halted the natural mid crustal metamorphic progression prior to the terrane cooling below 650 c and resulted in the rapid exhumation of the terrane by some 9 km in the process it provided a window into the deep levels of the central kaapvaal craton not seen elsewhere in the region

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

an experimental study of hydromagmatic fragmentation through energetic non explosive magma water mixing abstract in this paper we report the first experimental investigation of non explosive hydromagmatic fragmentation during energetic mixing with water we mix magma and water by two methods 1 pouring a basaltic melt between two converging water sprays and 2 jetting basaltic melt at high pressure 3 mpa through a nozzle into a tank of stagnant water these experiments involved shear at relative velocities of 5 16 m s and vigorous mixing for less than a second providing sufficient time for glassy rinds to grow but insufficient time for clot interiors to cool in resulting fragments we examined the gross morphology which reflects fluid deformation during mixing and surface textures which reflect the growth and disruption of glassy rinds we find major differences in both fragment morphology and surface texture between experiments water spray experiments produced pele s hair thin bubble shards melt droplets and angular fracture bound droplet pieces melt jet experiments produced mostly coarse 1 mm diameter wavy fluidal fragments with broken ends fluidal surfaces of fragments produced by water spray experiments were generally shiny under reflected light and in microscopic examination smooth down to micron scale implying no disruption of glassy rinds except for a rare flaking on pele s hair that was bent prior to solidification or b cracking and alligator skin textures on segments of melt balls that had expanded before complete cooling in contrast textures of fluidal surfaces on fragments produced by melt jet experiments are dull in reflected light and in scanning electron images exhibit ubiquitous discontinuous skins rinds that are flaked peeled or smeared away in stripes adhering to these surfaces are flakes blocks and blobs of detached material microns to tens of microns in diameter in the water spray fragments we interpret the scarcity of disrupted surface rinds to result from lack of bending after surfaces formed in the melt jet fragments the ubiquity of partially detached rinds and rind debris likely reflects repeated bending scraping impact and other disruption through turbulent velocity fluctuations when extrapolated to jets of surtseyan scale where velocity fluctuations reach tens of meters per second and turbulent mixing persists for tens of seconds rind disintegration could fragment a large fraction of the erupted material

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

mineral chemistry and crystallization conditions of the artfjallet gabbro and dolerites central swedish caledonides abstract on the basis of plagioclase zoning and other considerations the compositions of the silicate minerals in the artfjallet gabbro are judged to be close to their original magmatic compositions they can be used therefore to estimate the crystallization conditions pyroxene temperatures are 1280 1210 c for picrites 1240 1180 c for olivine gabbros and 1180 1050 c for norites olivine spinel augite thermometry yields ca 1200 c for the picrites comparison with data from melting experiments on basalts suggests that the artfjallet gabbro is one of the few intrusions where pyroxene compositions still record magmatic temperatures the pressure is constrained between 6 and 10 kbar by the country rock metamorphism and the presence of plagioclase in the gabbro no combination of the anorthite ca tschermakite an cats and forsterite enstatite reaction formulations gives a correct pressure most yielding overestimates the an cats reaction is unreliable especially for low alumina clinopyroxenes because of the small cats activities and consequent large errors 5 10 kbar during crystallization of the gabbro the silica activity probably was nearly constant and the oxygen fugacity near the qfm buffer for the dolerites which occur in and are related to the gabbro temperatures are considered to reflect their liquidus solidus interval pyroxene ca 1200 950 c brown hornblende 1025 975 c and biotite roughly 1000 900 c in quartz dolerites temperatures for hornblende 820 660 c and biotite ca 800 c are lower the primary water content of the gabbro and dolerite magmas is estimated to have been between 0 5 and 1 5 wt

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the kermadec arc implications for eruption processes and dredge sampling pyroclastic deposits from four caldera volcanoes in the kermadec arc have been sampled from subaerial sections raoul and macauley and by dredging from the submerged volcano flanks macauley healy and the newly discovered raoul sw suites of 16 32 mm sized clasts have been analyzed for density and shape and larger clasts have been analyzed for major element compositions density spectra for subaerial dry type eruptions on raoul island have narrow unimodal distributions peaking at vesicularities of 80 85 whereas ingress of external water wet type eruption or extended timescales for degassing generate broader distributions including denser clasts submarine erupted pyroclasts show two different patterns healy and raoul sw dredge samples and macauley island subaerial emplaced samples are dominated by modes at 80 85 implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts a minor proportion of healy and raoul sw clasts also show a pink oxidation color suggesting that hot clasts met air despite 0 5 to 1 km of intervening water in contrast macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine eruptive style that is not highly explosive macauley dredged pyroclasts are also the mixed products of multiple eruptions as shown by pumice major element chemistry and the sea floor deposits reflect complex volcanic and sedimentation histories the kermadec calderas are composite features and wide dispersal of pumice does not require large single eruptions when coupled with chemical constraints and textural observations density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment

monitoring of the plume from the basaltic phreatomagmatic 2004 grimsvotn eruption application of weather radar and comparison with plume models the grimsvotn eruption in november 2004 belongs to a class of small to medium sized phreatomagmatic eruptions which are common in iceland the eruption lasted 6 days but the main phase producing most of the 0 02 km3 of magma erupted was visible for 33 h on the c band weather radar of the icelandic meteorological office located in keflavik 260 km to the west of the volcano the plume rose to 8 12 km high over sea level during 33 h the long distance between radar and source severely reduces the accuracy of the plume height determinations causing 3 5 km steps in recorded heights moreover an apparent height overestimate of 1 5 km in the uncorrected radar records occurs possibly caused by wave ducting or super refraction in the atmosphere the stepping and the height overestimate can be partly overcome by averaging the plume heights and by applying a height adjustment based on direct aircraft measurements adjusted weather radar data on plume height are used to estimate the total mass erupted using empirical plume models mostly based on magmatic eruptions and to compare it with detailed in situ measurements of the mass of erupted tephra the errors arising because of the large radar plume distance limit the applicability of the data for detailed comparisons however the results indicate that the models overestimate the mass erupted by a factor of three to four this supports theoretical models indicating that high steam content of phreatomagmatic wet plumes enhances their height compared to dry plumes

distribution of fe2 and mg between coexisting garnet and hornblende in synthetic and natural systems an empirical calibration of the garnet hornblende fe mg geothermometer abstract multiple regression analysis of a compilation of the fe2 mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions n 22 and coexisting garnet clinopyroxene hornblende from natural intermediate to basaltic rocks n 43 has been performed to define ln kd fe2 mg grt hbl as a function of temperature and garnet composition the regression of data covering a large span in pressure 5 16 kbar temperature 515 1025 c and composition yields the ln kd fe2 mg grt hbl p t compositional relationship r2 0 93 t c 1504 1784 x ca grt x mn grt ln k d fe 2 mg grt hbl 0 720 273 where k d fe 2 mg grt hbl fe 2 mg grt fe 2 mg m 1 m 3 hbl x ca grt ca ca mn fe 2 mg in garnet x mn grt mn ca mn fe 2 mg in garnet application of this expression to natural garnet hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

age and p t evolution of the neoproterozoic turkel anorthosite complex eastern ghats province india abstract massif type anorthosite complexes constitute a distinct component of the proterozoic eastern ghats province in india they intruded the poly deformed and poly metamorphic granulite facies terrane near its western and northern tectonic contacts with the archaean bastar and singhbhum cratons respectively following their emplacement the complexes were variously affected by high grade metamorphism and deformation the turkel anorthosite complex comprises an anorthosite leuconorite diapir encircled by voluminous crust derived quartz monzonite intrusions hosted within an expansive batholithic complex of megacrystic k feldspar granite and remnants of an older gneissic unit this contribution evaluates the pressure temperature history of the anorthosite complex based on textural relations paragenetic characteristics and mineral chemistry and provides for the first time constraints on the timing of emplacement and subsequent metamorphism of the rocks through in situ u pb dating of zircon from several members of the suite emplacement of the turkel anorthosite complex occurred after the fabric defining late mesoproterozoic deformation d 1 d 3 and high grade metamorphism of the country rocks and was shortly preceded by the surrounding expansive intrusion of the megacrystic k feldspar granite the leuconorite anorthosite suite crystallized at 980 8 ma while the other components including the monzonite quartz monzonite and ferrodiorite were emplaced later between 956 6 and 945 5 ma the post intrusion evolution of the complex is characterized by partial re equilibration of the igneous parageneses and textures during an initial episode of cooling and decompression from the magmatic crystallization stage 1100 950 c 12 7 kbar towards the ambient thermobaric regime and following prolonged slow cooling by an episode of renewed compressional deformation late d 3 ca 750 c 6 5 7 0 kbar that caused the thorough high grade recrystallization of the rocks and produced the monophase planar fabric of the complex u pb spot ages from metamorphic domains of zircon grains constrain the age of this metamorphism to 901 879 ma the complex was moderately affected by the late neoproterozoic to early palaeozoic pan african tectonothermal event which led to internal tectonic segmentation and westward thrusting of the khariar and rampur domains of the northern eastern ghats province onto the bastar craton

thermal metamorphic signature of an impact event in the vredefort dome south africa pseudotachylitic breccias and shock deformation features related to the 2 02 ga formation of the vredefort dome by meteorite impact were overprinted by a static metamorphic event the intensity of which decreased from granulite facies t 700 c in the center of the dome to greenschist facies t 400 c around its margins geobarometric estimates of 0 2 0 3 gpa for the metamorphic parageneses indicate some 8 11 km of erosion since the impact event the strong lateral thermal gradient implied by these p t results is attributed to the combined effects of differential uplift of mid crustal rocks heated along a pre impact geotherm and increased shock heating of the target crust toward the center of the impact structure we suggest that the exceptionally high grade of metamorphism in the center of the dome may in part reflect an elevated regional geothermal gradient of 25 c km in the target crust due to lingering thermal effects related to the 2 05 2 06 ga bushveld magmatic event

the initial cooling of pahoehoe flow lobes in this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows the accurate modeling of this initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation this model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not an empirical fit to field data we find that the only significant processes are a heat loss by thermal radiation b heat loss by atmospheric convection c heat transport within the flow by conduction with temperature and porosity dependent thermal properties and d the release of latent heat during crystallization the numerical model is better able to reproduce field measurements made in hawai i between 1989 and 1993 than other published thermal models by adjusting one parameter at a time the effect of each of the input parameters on the cooling rate was determined we show that a the surfaces of porous flows cool more quickly than the surfaces of dense flows b the surface cooling is very sensitive to the efficiency of atmospheric convective cooling and c changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures these model results provide a quantitative explanation for the recently observed relationship between the surface cooling rate of pahoehoe lobes and the porosity of those lobes jones 1992 1993 the predicted sensitivity of cooling to atmospheric convection suggests a simple field experiment for verification and the model provides a tool to begin studies of the dynamic crystallization of real lavas future versions of the model can also be made applicable to extraterrestrial submarine silicic and pyroclastic flows

numerical modeling of impact heating and cooling of the vredefort impact structure large meteorite impacts such as the one that created the vredefort structure in south africa 2 ga ago result in significant heating of the target the temperatures achieved in these events have important implications for post impact metamorphism as well as for the development of hydrothermal systems to investigate the post impact thermal evolution and the size of the vredefort structure we have analyzed impact induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the vredefort structure when compared with the extent of estimated thermal shock metamorphism observed at different locations around vredefort our model results support our earlier estimates that the original crater was 120 160 km in diameter based on comparison of predicted to observed locations of shock features the simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0 3 myr but no more than 30 myr

impact crater lakes on mars the robotic search for life on mars centers on identifying accessible environments where the biological catalyst water has existed the formation of large impact craters on mars 65 km diameter may have resulted in the creation of ice covered impact crater lakes which would not freeze for thousands of years even under present climatic conditions water could be supplied from deep confined aquifers penetrated by the impact craters without the need for surface melt water freezing of the lakes is postponed owing to heat from impact generated melt bearing deposits from impact related uplift of hotter rocks from depth and from the latent heat of freezing of a deep crater lake abundant morphologic evidence for ancient crater lakes has not been found in viking images except for craters associated with outflow channels however ice covered crater lakes could have formed and further searches for evidence of these lakes are warranted the lake deposits from dissected impact craters may represent one of the best targets for future surface exobiology investigations or sample return missions from mars

re os isotope and geochemical study of the vredefort granophyre clues to the origin of the vredefort structure south africa the origin of granophyric rock dikes the vredefort granophyre at the 2 ga vredefort structure in south africa has been controversial they have been interpreted as either impact melt or igneous intrusions most vredefort granophyre samples have considerably higher os contents than the country rocks and 187re 188os and 187os 188os ratios that scatter about a 2 ga isochron their initial 187os 188os ratios at 2 ga overlap the meteoritic data range indicating that all the granophyre samples contain some meteoritic os the re os isotopic composition of the granophyre is significantly different from that of local precursor rocks and indicates that up to 0 2 of a chondritic component could be contained in the granophyre our results confirm the interpretation that the vredefort granophyre represents an impact melt rock

recognition of eroded astroblemes abstract the morphology of lunar craters degrades only slowly by the formation of new impacts and their ejecta it is not the same on earth because of weathering and tectonism generally the typical crater topography has disappeared and the term impact structure or astrobleme is applied on planetary surfaces as on the moon two classes of craters are recognized based on their depth to diameter ratio simple bowl shaped and complex readjusted flat bottomed craters with a sharp transition from simple to complex pike 1977 1980 on earth small simple craters retain some of their typical rimmed pit morphology because they are generally recent many are associated with meteorite fragments large impact structures or astroblemes in addition to their specific circular features are recognized as such only by the presence of shock metamorphism criteria such as dense minerals coesite stishovite diaplectic minerals and glass maskelynite planar deformation features in minerals principaly in quartz and feldspars and shattercones which are commonly used these criteria are abundant in allochthonous breccia particularly in fall back breccia among which there are glass fragments such as tectite or microtectite glass spherules shards droplets rods etc but on the floor of the astroblemes the criteria of shock metamorphism are restricted to the central part which is often covered or eroded using the measurements of nine astroblemes visited a scaling model is presented for two types of craters readjusted along listric slip surfaces a well readjusted model based on data from charlevoix and manicouagan canada and a partly readjusted model based on data from the ries germany both possess a central uplift and a ring graben there is also on the floor of the astrobleme a specific type of breccia mylolisthenite and pseudotachylite associated with the displacement along the autochthonous masses of rocks in very large impact structures the shock metamorphism is able to melt a part of the floor grieve and cintala 1992 the heat resulting from the superheated impact melt destroys a part of the shock metamorphism criteria

observations on lava snowpack and their interactions during the 2012 13 tolbachik eruption klyuchevskoy group kamchatka russia abstract observations made during january and april 2013 show that interactions between lava flows and snowpack during the 2012 13 tolbachik fissure eruption in kamchatka russia were controlled by different styles of emplacement and flow velocities a a lava flows and sheet lava flows generally moved on top of the snowpack with few immediate signs of interaction besides localized steaming however lavas melted through underlying snowpack 1 4 m thick within 12 to 24 h and melt water flowed episodically from the beneath flows pahoehoe lava lobes had lower velocities and locally moved beneath within the snowpack even there the snow melting was limited snowpack responses were physical including compressional buckling and doming and thermal including partial and complete melting maximum lava temperatures were up to 1355 k 1082 c type k thermal probes and maximum measured meltwater temperatures were 335 k 62 7 c theoretical estimates for rates of rapid e g radiative and slower conductive snowmelt are consistent with field observations showing that lava advance was fast enough for a a and sheet flows to move on top of the snowpack at least two styles of physical interactions between lava flows and snowpack observed at tolbachik have not been previously reported migration of lava flows beneath the snowpack and localized phreatomagmatic explosions caused by snowpack failure beneath lava the distinctive morphologies of sub snowpack lava flows have a high preservation potential and can be used to document snowpack emplacement during eruptions

a melt through model for chaos formation on europa the character of chaotic terrain on europa is consistent with its formation by the melting of a thin conducting ice shell from below tidal dissipation can provide adequate energy for such a process for example only a few percent of europa s predicted tidal heat spread over a region 200 km in diameter can lead to large melt regions within a few tens of thousands of years stronger more localized concentrations result in melt through in significantly shorter times i e a few hundred years the time scale for melt through is shorter than the time scale for the solid state viscous inflow of ice by several orders of magnitude in general modest concentrations of tidal heat can melt ice away faster than viscous inflow leading to melt through a mechanism to transmit these heat concentrations through the ocean is required for this model such heat transport could be the result of convective plumes in the ocean driven by seafloor volcanism or by the destabilization of a stratified ocean

occurrence of low ca clinopyroxene and the role of deformation in the formation of pyroxene fe ti oxide symplectites pyroxene fe ti oxide symplectites in a norite from the leuconoritic phase of the bjerkreim sokndal lopolith sw norway have been studied using empa electron microprobe analysis and tem transmission electron microscopy techniques textural and mineralchemical data indicate that the symplectites formed under subsolidus conditions at t 720 736 c and fo2 10 17 bars solidus temperatures are estimated as 981 1060 c two models of formation are proposed non isochemical replacement of olivine and growth at boundaries between exsolving grains of orthomagmatic high ca pyroxene and fe ti oxide tem reveals the presence of low ca clinopyroxene and this formed in response to strain and or shear stress comparison with published experimental data indicates that strain rates of up to 10 12 s 1 are necessary to explain the occurrence of low ca clinopyroxene the transition from orthopyroxene to low ca clinopyroxene may be related to deformation which accompanied intrusion of the leuconoritic phase of the bjerkreim sokndal lopolith or to local post intrusive faulting and or shearing selected area diffraction shows that the fe ti oxide lamellae are oriented with respect to the pyroxene such that 111 oxide 100 pyroxene the planes of closest oxygen packing in the constituent phases are thus adjacent to one another which leads to minimal misfit between the structures and to low interfacial strain energy the same topotactic relationship exists in both lamellar and vermicular parts of symplectites but is not continuous along the total length of the interface indicating that the form of the intergrowths is a primary growth feature that to some extent can be explained in terms of growth models for duplex cells deformation during growth is probably necessary to account for the lattice distortion observed in the symplectites

no depth limit to hydrovolcanic limu o pele analysis of limu from lō ihi seamount hawai i deep sea limu o pele are shards of basaltic glass commonly described as bubble walls when first identified they were inferred to form in submarine fire fountains but were then reinterpreted as the products of hydrovolcanic volcanism formed when submarine lava flows entrapped and vaporised seawater limu discovered below the c 3 km critical depth of seawater where superheated water exists as a supercritical fluid instead of a vapour led to the hydrovolcanic model of limu o pele formation being discarded in favour of a magmatic co2 driven strombolian like model this revised magmatic mechanism has been widely accepted by the scientific community we describe a newly discovered limu o pele rich deposit at 1 052 mbsl on the northeast summit plateau region of l ihi seamount hawai i the limu at this site is concentrated in a chemically monomict ash lens interbedded with thin lava sheets that are separated from overlying volcaniclastic material by a discontinuity the geometry and geochemistry of the deposit provide compelling evidence for a hydrovolcanic sheet flow related origin the exceptional abundance and preservation of limu at this site allows 4 morphologic subtypes of limu thin film plateau border convex film and pele s hair to be identified and linked to portions of the isolated rupturing bubbles from which they are derived we extend our discussion to beyond this new l ihi deposit by including a review of limu o pele occurrences and thermodynamic considerations that demonstrate the hydrovolcanic model of limu formation to be more tenable than the magmatic model at all depths including below the critical depth of seawater

experimental interaction of magma and dirty coolants the presence of water at volcanic vents can have dramatic effects on fragmentation and eruption dynamics but little is known about how the presence of particulate matter in external water will further alter eruptions volcanic edifices are inherently dirty places where particulate matter of multiple origins and grainsizes typically abounds we present the results of experiments designed to simulate non explosive interactions between molten basalt and various coolants ranging from homogeneous suspensions of 0 to 30 mass bentonite clay in pure water to heterogeneous and or stratified suspensions including bentonite sand synthetic glass beads and or naturally sorted pumice four types of data are used to characterise the interactions 1 visual video observations 2 grainsize and morphology of resulting particles 3 heat transfer data from a network of eight thermocouples and 4 acoustic data from three force sensors in homogeneous coolants with 20 sediment heat transfer is by forced convection and conduction and thermal granulation is less efficient resulting in fewer blocky particles larger grainsizes and weaker acoustic signals many particles are droplet shaped or and vesicular containing bubbles filled with coolant both of these particle types indicate significant hydrodynamic magma coolant mingling and many of them are rewelded into compound particles the addition of coarse material to heterogeneous suspensions further slows heat transfer thus reducing thermal granulation and variable interlocking of large particles prevents efficient hydrodynamic mingling this results primarily in rewelded melt piles and inefficient distribution of melt and heat throughout the coolant volume our results indicate that even modest concentrations of sediment in water will significantly limit heat transfer during non explosive magma water interactions at high concentrations the dramatic reduction in cooling efficiency and increase in mingling help to explain globular peperite and provide information relevant to analyses of premixing associated with highly explosive molten fuel coolant interactions in debris filled volcanic vents